J. Comp. Path. 2015, Vol. 152, 9e21
Available online at www.sciencedirect.com
ScienceDirect www.elsevier.com/locate/jcpa
INFECTIOUS DISEASE: REVIEW ARTICLE
An Update on the Epidemiology and Pathology of African Swine Fever J. M. S anchez-Vizcaıno*, L. Mur*, J. C. Gomez-Villamandos† and L. Carrasco† * Centro VISAVET and Animal Health Department, Universidad Complutense de Madrid, Av. Puerta de Hierro SN 28040, Madrid and † Department of Comparative Pathology, Universidad de Cordoba, Cordoba, Spain
Summary African swine fever (ASF) is one of the most important infectious diseases of swine and has major negative consequences for affected countries. ASF is present in many sub-Saharan countries, Sardinia and several countries of eastern and central Europe, where its continuous spread has the swine industry on heightened alert. ASF is a complex disease for which no vaccine or treatment is available, so its control is based on early detection and rapid control of spread. For a robust and reliable early detection programme it is essential to be able to recognize the clinical signs and pathological changes of ASF, keeping in mind that in most cases the first introductions don’t show high mortality nor characteristic clinical signs or lesions, but fever and some hemorrhagic lymph nodes. Knowledge of the main characteristics of this infection, including its current distribution and routes of transmission, is also essential for preventing and controlling ASF. This review addresses each of these topics and aims to update knowledge of the disease in order to improve early detection of ASF in the field and allow implementation of public health programmes. Ó 2014 Elsevier Ltd. All rights reserved. Keywords: African swine fever; clinical signs; differential diagnosis; lesions
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . African Swine Fever Epidemiology and Main Routes of Introduction . . . . . . . . . . . . . . . . . . . . . . . . . Clinical Presentation and Lesions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peracute African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acute African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subacute African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chronic African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Differential Diagnosis between African Swine Fever and Other Haemorrhagic Diseases . . . . . . . . . . . . Classical Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porcine Reproductive and Respiratory Syndrome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Swine Erysipelas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Septicaemic Salmonellosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porcine Dermatitis and Nephropathy Syndrome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laboratory Diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Early Detection, Prevention and Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
10 10 11 11 11 12 13 14 14 16 16 17 17 17 18 18 18
Correspondence to: J. M. Sanchez-Vizcaıno (e-mail: [email protected]). 0021-9975/$ - see front matter http://dx.doi.org/10.1016/j.jcpa.2014.09.003
Ó 2014 Elsevier Ltd. All rights reserved.
J.M. S anchez-Vizcaıno et al.
10
Introduction African swine fever (ASF) is one of the most important infectious diseases of swine and is present in many African countries, some eastern and central European countries and Sardinia. ASF must be notified to the World Organization of Animal Health (OIE) and its presence leads to immediate restrictions on the pig and pork trade. ASF is caused by infection with a complex DNA virus, the ASF virus (ASFV), which is the only member of the family Asfarviridae (Dixon et al., 2005). ASFV is a large enveloped virus of approximately 200 nm in diameter that contains double-stranded DNA of 170e193 kilobase pairs (Dixon et al., 2013). ASFV is composed of more than 50 structural proteins and produces more than 150 proteins in infected macrophages (Salas and Andres, 2013), many of which are highly immunogenic. Hence, infection causes a strong humoral immune response that persists for long periods of time. Nevertheless, the antibodies produced are not able to neutralize ASF infection effectively (Neilan et al., 2004) and serotyping is not possible. Consequently, classification is based on genotyping procedures through the analysis of some genome regions, such as the C-terminal region of the gene encoding vp72 (Bastos et al., 2003). Based on the differences observed in this region, circulating isolates of ASFV have been classified into 22 genotypes (Boshoff et al., 2007). ASFV replicates in mononuclear phagocytic cells of both domestic and wild swine. The virus infects monocytes and macrophages (Malmquist and Hay, 1960), but infection of T or B lymphocytes has never been observed (Minguez et al., 1988). The virus also replicates in endothelial cells (Carrasco et al., 1996a), hepatocytes, renal tubular epithelial cells (G omez-Villamandos et al., 1995a) and neutrophils. After initial replication in these primary sites, ASFV spreads through the blood or the lymphatic system, where it persists for long periods of time in the absence of neutralizing antibodies and moves towards secondary sites of replication. ASFV also replicates in soft ticks of the genus Ornithodoros, which act as virus reservoirs. These ticks are involved in the epidemiological cycle of ASF in eastern and southern Africa (Ornithodoros moubata) and have also been observed during infection on the Iberian Peninsula (Ornithodoros erraticus). Other Ornithodoros spp. have been demonstrated to be susceptible to ASF infection (European Food Safety Authority, 2010). ASFV strain Georgia 2007/1 can also replicate in O. erraticus ticks (Diaz et al., 2012), but the potential role of these ticks in virus transmission is still unknown in other European regions.
Despite efforts made over past decades, there is no vaccine available for preventing and controlling ASFV infection. Several strategies have been studied; however, the lack of neutralizing antibodies, genetic variability and presence of some gaps in knowledge about ASF pathogenesis and immune modulation make discovery-based approaches difficult.
African Swine Fever Epidemiology and Main Routes of Introduction ASF was first described in Kenya in 1921 (Montgomery, 1921) and since then it has spread rapidly to other African countries. ASF first escaped from Africa in 1957 to reach Portugal via contaminated waste containing infected pig products that were used to feed pigs. After this incursion, which was rapidly controlled, ASFV re-entered Portugal in 1960 and this time it spread to the whole Iberian Peninsula, where it persisted for more than 30 years (Arias and Sanchez-Vizcaıno, 2002). During this period (1960e1995), ASFV spread sporadically to other countries in Europe and America (i.e. Brazil, Dominican Republic, Cuba and Haiti). ASF has been eradicated from all of these countries except from the Italian island of Sardinia, which has been affected since 1978 (Sanchez-Vizcaıno and Arias, 2012; Costard et al., 2013a, b). ASF has also continued to spread on the African continent and reached West African countries and some islands that were previously free of the disease. Some authors have stated that this spread started in 1994 due to a combination of factors including: (1) increasing pig production on the continent, (2) presence of non-symptomatic pigs that could act as reservoirs to spread the disease without being noticed and (3) globalization. These factors, together with the economic crisis of the present century, could be the origin of the spread of ASFV to Eastern Europe (Penrith and Vosloo, 2009; Sanchez-Vizcaıno et al., 2013). In 2007, ASFV entered Georgia through the port of Poti (Beltran-Alcrudo et al., 2008), potentially via contaminated food used to feed pigs. From this region, ASFV spread rapidly through the country and affected neighbouring countries including Armenia, Azerbaijan and the Russian Federation. In this geographical region, ASFV affects domestic and wild boar and has spread to the north and west. In 2012, the first outbreaks were declared in Ukraine, followed by Belarus in 2013 (Sanchez-Vizcaıno et al., 2013). From these countries, ASF continued to spread before reaching the European Union (EU) borders in 2014, when several dead wild boars were found in Lithuania and Poland. Since then, several cases have been reported in Estonia and
Epidemiology and Pathology of African Swine Fever
Latvia and repeatedly in Lithuania and Poland (OIE, 2014). At this time, the critical situation of ASF in East and Central Europe poses a serious risk to other EU countries, which could suffer the severe consequences of trade restrictions, as has already happened with Russian Federation bans. However, it is important to remember that the main source of ASFV is presently Africa and from there, it could spread to other disease-free countries/regions and could potentially spread and cause high-impact consequences, as have occurred previously in Europe. The main routes for the spread of ASF involve the transport of infected animals and, more frequently, the transport of infected products (Table 1). These infected products could be imported illegally to make profit or could involve catering waste containing infected pig products. Indeed, this was the means by which ASF was introduced into many disease-free countries in the 1970s (Sanchez-Vizcaıno and Arias, 2012) and the cause of the last introduction in 2007 (Beltran-Alcrudo et al., 2008). Other sources of infection are related to contaminated fomites and transport, including livestock vehicles or infected material. Natural ranging of wild boar is one of the most serious concerns for the EU due to the proximity of affected territories, as demonstrated by the recent cases detected at EU borders and risk assessment analyses (De la Torre et al., in press). However, this route is not likely to cause distant outbreaks in disease-free territories such as Asia without prior suspicion or detection. Other factors, such as infected ticks or airborne transmission, are also important, but only at a local level and should be ruled out as potential sources for ASF infection in distant disease-free territories. Table 1 Main sources of ASF infection into free areas of Europe Transmission route
Raw pork waste at airport/port
Movement of pork or pig product
Natural ranging of infected wild boar
Infected ticks
Location and date of occurrence Lisbon, 1957 Malta, 1978 Sardinia, 1978 Georgia, 2007 Portugal, 1960 Spain, 1960 Italy, 1983 Belgium, 1985 Russian Federation, 2007 Lithuania, 2014 Poland, 2014 Estonia, 2014 Portugal, 1999
11
Clinical Presentation and Lesions ASF can have different clinical presentations and pathological lesions, depending on the virulence of the virus isolate, the route and dose of infection and the host characteristics. The clinical presentation of ASF in a naive farm with a low dose of virus infection, will not cause high mortality nor characteristic clinical signs, except fever and deaths with some hemorrhagic lymph nodes. Usually, few days later, due to the increased viral circulation, more explosive infections could be observed with more mortality and characteristic clinical signs and lesions. Therefore, every dead animal that present fever in a high risk area, should be tested for ASF. ASFV strains are usually classified as highly virulent, moderately virulent and low virulent (Pan and Hess, 1984). Highly virulent strains are usually responsible for the peracute (pigs dead 1e4 days post infection [dpi]) and acute forms (animals dead 3e8 dpi) of the disease, while moderately virulent strains are involved in the acute (pigs dead 11e15 dpi) and subacute (animals die after 20 dpi) forms. In clinical terms, acute ASF develops over a 7-day period, compared with 10e20 days for the subacute form of the disease. Chronic ASF has been associated with infection by moderate-to-low virulence isolates (Mebus and Dardiri, 1979; McVicar, 1984), which were only described in Spain, Portugal and the Dominican Republic when ASF infection in these areas was endemic. The main lesions observed in the different forms of ASF are summarized in Table 2. Peracute African Swine Fever
This form is induced by highly virulent ASFV strains and is characterized by high fever (body temperature 41e42 C), loss of appetite, inactivity, hyperpnoea and cutaneous hyperaemia. Animals usually die suddenly 1e4 days after the onset of clinical signs and no lesions are evident in organs. Acute African Swine Fever
This is the most usual form of the disease and is induced by highly or moderately virulent virus strains. Animals with acute ASF display fever (body temperature 40e42 C) and a tendency to crowd together; they also exhibit loss of appetite, inactivity, apathy and early leucopenia, induced by lymphopenia and changes in monocyte numbers (Colgrove et al., 1969; Pan and Hess, 1984; GomezVillamandos et al., 1995a; Carrasco et al., 1996b; Sanchez-Vizcaıno and Arias, 2012). Severe pulmonary oedema, giving rise to respiratory changes, is a characteristic finding in pigs infected
J.M. S anchez-Vizcaıno et al.
12
Table 2 Main lesions observed in the different forms of ASF
Fever Thrombocytopenia Skin Lymph nodes
Peracute ASF
Acute ASF
High Absent Erythema e
High Absent or slight (late) Erythema Gastrohepatic and renal with marbled aspect Hyperaemic splenomegaly
Spleen
e
Kidney
e
Lung Gall bladder Heart
e e e
Tonsils Reproductive alteration
e e
Subacute ASF
Petechial haemorrhages, mainly in cortex Severe alveolar oedema Petechial haemorrhages Haemorrhages in epicardium and endocardium e e
with highly virulent strains of ASF. Affected animals die in shock, usually 1 week after fever begins and foam is generally observed around the mouth and nose (Sierra et al., 1990; Carrasco et al., 1996a, 2002). Affected pigs show erythema, which particularly affects the skin of the ears, tail, distal extremities, chest, abdomen and perianal area. Cyanosis may also be observed 1e2 days prior to death on the skin of the ears, abdomen and perianal areas. Small foci of cutaneous necrosis (more characteristic of infections with strains of moderate virulence) and subcutaneous haematomas also occur. Other clinical signs include mucoid nasal discharge, epistaxis, vomiting, abdominal pain, constipation or diarrhoea, which is initially mucoid, but may later become bloody (melaena) (Moulton and Coggins, 1968; Mebus and Dardiri, 1979; Gomez-Villamandos et al., 2013). Abortion (caused by fever) may occur in pregnant sows; in some cases, abortion may be the first sign of an outbreak. Almost 90e100% of pigs with these signs will die within 7 days. Together with erythema and cyanosis of the skin, the animals that develop acute ASF present with characteristic hyperaemic splenomegaly in which the spleen can be up to six times larger than normal. An affected spleen may have rounded edges, be friable in consistency and purpleeblack in colour and may occupy the entire abdominal cavity from one side to the other (Fig. 1A). Lymph nodes, mainly the gastrohepatic and renal nodes, display haemorrhages in the medulla (Fig. 1B), which is why sections of affected lymph nodes sometimes have a marbled appearance (Fig. 1C). Kidneys usually show petechial haemorrhages in the cortex (Fig. 1D) and renal pelvis. Other lesions that pigs usually show with acute forms of the disease are petechial haemorrhages in the mucosa of the urinary bladder, epicardium, endocardium and pleura (Mebus and
Chronic ASF
Moderate Irregular or absent Transient Absent Erythema Necrotic areas The majority of lymph nodes Swollen resemble a blood clot Partial hyperaemic splenomegaly Enlarged with normal or focal infarct colour Petechial haemorrhages in cortex, e medulla and pelvis; perirenal oedema e Pleuritis and pneumonia Wall oedema e Haemorrhages in epicardium and Fibrinous pericarditis endocardium; hydropericardium e Necrotic foci Abortion Abortion
Dardiri, 1979; Gomez-Villamandos et al., 1995a; Hervas et al., 1996; Carrasco et al., 1997a, b; S anchezVizcaıno and Arias, 2012). Subacute African Swine Fever
This form of the disease is caused by moderately virulent isolates and affected animals show similar clinical signs to those animals with the acute form of the disease, although clinical signs tend to be less marked. However, the vascular changes observed in subacute forms of ASF, mainly haemorrhage and oedema, are more intense than those reported in the acute form of the disease (Gomez-Villamandos et al., 1995a, 2013). Haemorrhages related to the development of intense, although transient, thrombocytopenia are observed in the early and mid stages of the disease (Villeda et al., 1993a, b; Gomez-Villamandos et al., 1998). Abortion is usually the first clinical sign of these forms. Affected pigs usually die within 7e20 days and the mortality rate can range from 30 to 70%. Survivors usually recover within 3e4 weeks. Recovered pigs can still excrete the virus up to 6 weeks after infection. Death of animals may occur at two different stages: in the intense thrombocytopenia/ leucopenia phase or in the recovery phase when haemorrhages appear due to erythrodiapedesis by vasodilation, especially in young animals (G omezVillamandos et al., 1998, 2013). Pigs suffering from the subacute form of ASF display a moderate to high fever, ascites, hydropericardium and characteristic oedema of the wall of the gallbladder and bile duct (Fig. 2A), as well as in the surrounding area of kidneys (perirenal oedema) (Fig. 2B). The spleen usually shows initial partial hyperaemic splenomegaly (Fig. 2C) that progressively reverses, leaving some focal damage, which eventually
Epidemiology and Pathology of African Swine Fever
13
Fig. 1. Acute ASF. (A) An enlarged, purpleeblack spleen crosses the entire abdominal cavity (hyperaemic splenomegaly). (B) Several lymph nodes display degrees of haemorrhage, most intensely in the medulla. (C) Cut sections of lymph nodes with marbled appearance. (D) Petechial haemorrhages in the renal cortex.
disappears, or focal infarction with different stages of organization. Lymph nodes, mainly the gastrohepatic (Fig. 2D) and renal nodes (Fig. 2E), as well as the submandibular, retropharyngeal, mediastinal, mesenteric and inguinal nodes, are haemorrhagic, oedematous and friable, which is why they often look like dark red haematomas (S anchez-Vizcaıno and Arias, 2012; G omez-Villamandos et al., 2013). Renal haemorrhages (Fig. 2F) are more intense (petechiae and ecchymoses) and more extensive (cortex, medulla and pelvis) than in acute forms, but are not linked to the endothelial lesions reported in acute ASF (G omez-Villamandos et al., 1995a; Hervas et al., 1996). Chronic African Swine Fever
The chronic form of ASF is caused by infection by low virulence strains, observed in Spain and Portugal, as well as the Dominican Republic. This form is characterized by necrotic lesions of the skin and by arthritis (S anchez-Botija, 1982). However, this form of the disease has never been described in those countries where ASFV has been present for long periods of time (e.g. Africa and Sardinia). Therefore, it has been hypothesized that the chronic form may have originated
through the natural evolution of the ASFV isolates employed in the vaccination studies performed on the Iberian Peninsula in the 1960s. To confirm this hypothesis, several molecular studies to elucidate the similarity between chronic forms and vaccine isolates are underway (Sanchez-Vizcaıno, unpublished observations). The chronic form of the disease, which has no specific clinical signs, was mainly detected during serological screening to eradicate the disease in Spain, in carrier animals, of which around 10% can spread the virus for long periods of time, which likely plays a key role in the persistence of the disease (Arias and Sanchez-Vizcaıno, 2002). These animals usually have necrotic lesions of the skin and arthritis (Sanchez-Botija, 1982), delayed growth, emaciation, lameness, respiratory signs, abortion and low mortality (Arias et al., 1986). Unlike other forms of ASF, chronic forms are characterized by the absence of vascular lesions and by the presence of lesions in which bacteria are involved, such as fibrinous pleuritis and/or pericarditis, pleural adhesions, necrotic pneumonia, fibrinous arthritis/periarthritis and necrotic skin lesions, as well as necrotic areas on the tonsils and the tongue (Moulton and Coggins, 1968;
14
J.M. S anchez-Vizcaıno et al.
Fig. 2. Subacute ASF. Intense oedema of the wall of the gall bladder (A) and surrounding the kidney (B). (C) Spleen with partial hyperaemic splenomegaly. Haemorrhagic and oedematous gastrohepatic (D) and renal (E) lymph nodes. (F) Kidneys with intense haemorrhage in the cortex, medulla and pelvis.
Arias et al., 1986). This form of the infection is not currently in circulation.
Differential Diagnosis between African Swine Fever and Other Haemorrhagic Diseases As discussed previously, during the early stages of ASF, particularly when low numbers of animals are affected, diagnosis is not straightforward. The presence of non-specific lesions and a small percentage of deaths can easily be confused with other porcine haemorrhagic diseases. Therefore, differential diagnoses based on the gross lesions include the following swine diseases: classical swine fever (CSF), highpathogenic porcine reproductive and respiratory syndrome (HP-PRRS), swine erysipelas, septicaemic salmonellosis and porcine dermatitis nephropathy
syndrome (PDNS). The specific characteristics of these differential diagnoses are summarized in Table 3 and descriptions of each disease, emphasizing the similarities and differences with ASF, are presented below. Classical Swine Fever
CSF, also known as ‘hog cholera’, is a fatal disease caused by a small enveloped RNA virus belonging to the family Flaviviridae, genus Pestivirus (Van Regenmortel et al., 2000). CSF takes various clinical forms (i.e. peracute, acute, chronic and congenital) depending on the virus strain involved, the environment, the time of infection and the individual host response (Trautwein, 1988). However in the acute and chronic forms of the disease, affected pigs show similar clinical signs:
Table 3 Differential diagnosis of African swine fever on the basis of gross lesions ASF (subacute)
CSF
HP-PRRS
Swine erysipelas
Septicaemic salmonellosis
DNS
Erythema Marbled aspect
Erythema Haemorrhagic
Erythema Marbled aspect
Diamond skin lesions Marbled aspect
Cyanosis Swelling
Macules and papules Marbled aspect
Spleen
Hyperaemic splenomegaly
Multiple infarcts at the margin
Hyperaemic splenomegaly
Splenomegaly
e
Kidney
Petechial haemorrhages Petechial haemorrhages e
Partial hyperaemic splenomegaly or focal infarct Petechial haemorrhages; perirenal oedema Wall oedema
Cyanosis Swelling or marbled aspect Scattered infarcts or white spots on the surface Petechial haemorrhages e
Petechial haemorrhages e
Petechial haemorrhages e
Glomerulonephritis
e
e
e
Button ulcers
Swelling or with haemorrhages e
e
Necrotic enterocolitis
e
Intense (early)
Absent
Absent
Absent
Absent
Nervous signs Congenital malformation (congenital form)
Thymic atrophy Interstitial pneumonia
Skin Lymph nodes
Gall bladder Tonsils Intestine Thrombocytopenia Other lesions
Petechial haemorrhages Absence or slight (late) e
e Petechial haemorrhages Transient e
Petechial haemorrhages Petechial haemorrhages Necrotic areas
Nervous signs; congestion Arthritis and vegetative endocarditis of gastric mucosa; necrotic foci in the liver (chronic form) and bronchopneumonia
e
e
Epidemiology and Pathology of African Swine Fever
ASF (acute)
African swine fever, ASF; classical swine fever, CSF; high-pathogenic porcine reproductive and respiratory syndrome, HP-PRRS; dermatitis and nephropathy syndrome, DNS; e, no change.
15
16
J.M. S anchez-Vizcaıno et al.
anorexia, lethargy, conjunctivitis, respiratory signs and constipation followed by diarrhoea. These clinical signs, together with the haemorrhages (petechiae and ecchymoses) of the skin, kidneys, tonsils, gallbladder and ileocaecal junction observed in the acute form, could be confused easily with ASF. Some of the most characteristic lesions of CSF include the presence of multifocal infarction of the margin of the spleen, which takes a triangular shape, and the presence of necrotic foci on the tonsils (Cheville and Mengeling, 1969; Trautwein, 1988; Potier et al., 2006). Although these lesions are not always present together, they represent, together with the neurological signs (induced by non-purulent meningoencephalitis), the best differentiation between CSF and the acute and subacute forms of ASF. Another characteristic lesion of the subacute and chronic forms of CSF, which allows a differential diagnosis to be made with ASF, is the presence of ‘button ulcers’ in the small (mainly ileum and ileocaecal valves) and large intestines (caecum and colon) (Cheville and Mengeling, 1969; Trautwein, 1988; Van Oirschot, 1999; Potier et al., 2006). Additionally, the onset of thrombocytopenia and haemorrhages also differs between CSF and ASF. Haemorrhages in CSF are associated with early intense thrombocytopenia, which coincides with the appearance of fever and the start of viraemia (Cheville and Mengeling, 1969; Trautwein, 1988; Bautista et al., 2002). However, in ASF, slight thrombocytopenia in the late stage can be detected in acute forms or transient thrombocytopenia may be present in the early and mid stages of subacute disease (Villeda et al., 1993a, b). Porcine Reproductive and Respiratory Syndrome
PRRS is caused by two genotypes of the PRRS virus (PRRSV), a member of the family Arteriviridae (Nelsen et al., 1999). PRRS is characterized by respiratory problems in growing and finishing pigs and by reproductive failure in sows, with late-term abortion, stillbirth and increased preweaning mortality. Recently, however, highly pathogenic strains (HPPRRSV) have been isolated in China and southeast Asia and in Eastern Europe (Garcıa-Nicolas et al., 2013; Gomez-Laguna et al., 2013). Infection with HP-PRRSV strains is associated with severe clinical signs, pulmonary lesions and aberrant host immune responses (Gomez-Laguna et al., 2013). Pigs infected with HP-PRRS strains display high mortality accompanied by high fever (body temperature 40e42 C) severe lethargy, anorexia, cough, severe dyspnoea, lameness and cyanosis of the ears, limbs and perineum (Zhou et al., 2009; Han et al.,
2014). The main lesions are found in the lungs, which display interstitial pneumonia (Han et al., 2014), and in lymphoid organs, where severe thymic atrophy and swelling and haemorrhage in the tonsils and lymph nodes may be observed (located in the medulla with a cut surface showing a marbled appearance), together with scattered infarcts or white spots on the surface of the spleen. Petechial haemorrhages may be observed in the renal cortex (Wang et al., 2011). Although haemorrhages in kidneys and lymph nodes are present in HP-PRRSV, the absence of hyperaemic splenomegaly and the intensity of respiratory distress, due to interstitial pneumonia, are used to make the differential diagnosis with ASF. Swine Erysipelas
Swine erysipelas (SE) manifests commonly as acute or subacute septicaemia and chronic proliferative lesions caused by Erysipelothrix rhusiopathiae, a small grampositive bacillus (Sneath et al., 1986). Acute SE is characterized by sudden onset, with occasional sudden death, in which animals have a high fever (body temperature 40e42 C) for 5e7 days and also display severe depression and leucopenia. The subacute forms are less severe than the acute form and abortion may occur in sows that come into contact with acute or subacute SE during pregnancy (Wood and Henderson, 2006). The most characteristic lesion of SE is the presence of rhomboid urticarial lesions (or ‘diamond skin’ lesions) in the acute non-fatal form from days 2e3 of infection, which appear as small square or rhomboid areas with light pink to dark purple colouration that are usually raised and firm to the touch. In the acute fatal form of SE lesions are similar to those of septicaemia, with extensive dark purple discolouration over the ears, tail, abdomen, the posterior aspect of the thighs, and the jowls. The lungs are congested and oedematous and haemorrhages (petechial to ecchymotic) may be seen in the cortex of the kidneys, the heart (particularly in the epicardium and the musculature of the atria) and the serosa of the stomach. The spleen may be congested and markedly enlarged (hyperaemic splenomegaly) (Wood and Henderson, 2006), which is similar to the acute form of ASF. The involvement of lymph nodes depends on the area that they drain; usually the peripheral nodes show marked congestion to haemorrhage of the medulla and the cut surface reveals a marbled appearance (Wood and Henderson, 2006). Despite the similarities between acute SE and ASF (i.e. clinical course, presence of haemorrhages, similar spleen lesions), there are several characteristics that differ between these diseases. Firstly, the presence of
Epidemiology and Pathology of African Swine Fever
diamond skin lesions is typical in acute SE, but not in ASF. Additionally, the peripheral lymph nodes are usually the most affected in acute SE, while the gastrohepatic and renal nodes are mainly affected in acute ASF. Septicaemic Salmonellosis
Swine salmonellosis is caused by Salmonella choleraesuis and has been related to the development of septicaemia or enterocolitis (Baskerville and Dow, 1973). Pigs with septicaemic salmonellosis are inappetent, lethargic, febrile (body temperature 40.5e41.6 C), have a moist cough with slight expiratory dyspnoea, uncoordinated gait, tremor, paralysis, convulsions and recumbency. These clinical signs may result in death. At 3e4 dpi, animals suffer from diarrhoea with watery yellow faeces. In most outbreaks, which are frequently associated with stressful situations, morbidity is variable but usually
Available online at www.sciencedirect.com
ScienceDirect www.elsevier.com/locate/jcpa
INFECTIOUS DISEASE: REVIEW ARTICLE
An Update on the Epidemiology and Pathology of African Swine Fever J. M. S anchez-Vizcaıno*, L. Mur*, J. C. Gomez-Villamandos† and L. Carrasco† * Centro VISAVET and Animal Health Department, Universidad Complutense de Madrid, Av. Puerta de Hierro SN 28040, Madrid and † Department of Comparative Pathology, Universidad de Cordoba, Cordoba, Spain
Summary African swine fever (ASF) is one of the most important infectious diseases of swine and has major negative consequences for affected countries. ASF is present in many sub-Saharan countries, Sardinia and several countries of eastern and central Europe, where its continuous spread has the swine industry on heightened alert. ASF is a complex disease for which no vaccine or treatment is available, so its control is based on early detection and rapid control of spread. For a robust and reliable early detection programme it is essential to be able to recognize the clinical signs and pathological changes of ASF, keeping in mind that in most cases the first introductions don’t show high mortality nor characteristic clinical signs or lesions, but fever and some hemorrhagic lymph nodes. Knowledge of the main characteristics of this infection, including its current distribution and routes of transmission, is also essential for preventing and controlling ASF. This review addresses each of these topics and aims to update knowledge of the disease in order to improve early detection of ASF in the field and allow implementation of public health programmes. Ó 2014 Elsevier Ltd. All rights reserved. Keywords: African swine fever; clinical signs; differential diagnosis; lesions
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . African Swine Fever Epidemiology and Main Routes of Introduction . . . . . . . . . . . . . . . . . . . . . . . . . Clinical Presentation and Lesions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peracute African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acute African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subacute African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chronic African Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Differential Diagnosis between African Swine Fever and Other Haemorrhagic Diseases . . . . . . . . . . . . Classical Swine Fever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porcine Reproductive and Respiratory Syndrome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Swine Erysipelas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Septicaemic Salmonellosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porcine Dermatitis and Nephropathy Syndrome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laboratory Diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Early Detection, Prevention and Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
10 10 11 11 11 12 13 14 14 16 16 17 17 17 18 18 18
Correspondence to: J. M. Sanchez-Vizcaıno (e-mail: [email protected]). 0021-9975/$ - see front matter http://dx.doi.org/10.1016/j.jcpa.2014.09.003
Ó 2014 Elsevier Ltd. All rights reserved.
J.M. S anchez-Vizcaıno et al.
10
Introduction African swine fever (ASF) is one of the most important infectious diseases of swine and is present in many African countries, some eastern and central European countries and Sardinia. ASF must be notified to the World Organization of Animal Health (OIE) and its presence leads to immediate restrictions on the pig and pork trade. ASF is caused by infection with a complex DNA virus, the ASF virus (ASFV), which is the only member of the family Asfarviridae (Dixon et al., 2005). ASFV is a large enveloped virus of approximately 200 nm in diameter that contains double-stranded DNA of 170e193 kilobase pairs (Dixon et al., 2013). ASFV is composed of more than 50 structural proteins and produces more than 150 proteins in infected macrophages (Salas and Andres, 2013), many of which are highly immunogenic. Hence, infection causes a strong humoral immune response that persists for long periods of time. Nevertheless, the antibodies produced are not able to neutralize ASF infection effectively (Neilan et al., 2004) and serotyping is not possible. Consequently, classification is based on genotyping procedures through the analysis of some genome regions, such as the C-terminal region of the gene encoding vp72 (Bastos et al., 2003). Based on the differences observed in this region, circulating isolates of ASFV have been classified into 22 genotypes (Boshoff et al., 2007). ASFV replicates in mononuclear phagocytic cells of both domestic and wild swine. The virus infects monocytes and macrophages (Malmquist and Hay, 1960), but infection of T or B lymphocytes has never been observed (Minguez et al., 1988). The virus also replicates in endothelial cells (Carrasco et al., 1996a), hepatocytes, renal tubular epithelial cells (G omez-Villamandos et al., 1995a) and neutrophils. After initial replication in these primary sites, ASFV spreads through the blood or the lymphatic system, where it persists for long periods of time in the absence of neutralizing antibodies and moves towards secondary sites of replication. ASFV also replicates in soft ticks of the genus Ornithodoros, which act as virus reservoirs. These ticks are involved in the epidemiological cycle of ASF in eastern and southern Africa (Ornithodoros moubata) and have also been observed during infection on the Iberian Peninsula (Ornithodoros erraticus). Other Ornithodoros spp. have been demonstrated to be susceptible to ASF infection (European Food Safety Authority, 2010). ASFV strain Georgia 2007/1 can also replicate in O. erraticus ticks (Diaz et al., 2012), but the potential role of these ticks in virus transmission is still unknown in other European regions.
Despite efforts made over past decades, there is no vaccine available for preventing and controlling ASFV infection. Several strategies have been studied; however, the lack of neutralizing antibodies, genetic variability and presence of some gaps in knowledge about ASF pathogenesis and immune modulation make discovery-based approaches difficult.
African Swine Fever Epidemiology and Main Routes of Introduction ASF was first described in Kenya in 1921 (Montgomery, 1921) and since then it has spread rapidly to other African countries. ASF first escaped from Africa in 1957 to reach Portugal via contaminated waste containing infected pig products that were used to feed pigs. After this incursion, which was rapidly controlled, ASFV re-entered Portugal in 1960 and this time it spread to the whole Iberian Peninsula, where it persisted for more than 30 years (Arias and Sanchez-Vizcaıno, 2002). During this period (1960e1995), ASFV spread sporadically to other countries in Europe and America (i.e. Brazil, Dominican Republic, Cuba and Haiti). ASF has been eradicated from all of these countries except from the Italian island of Sardinia, which has been affected since 1978 (Sanchez-Vizcaıno and Arias, 2012; Costard et al., 2013a, b). ASF has also continued to spread on the African continent and reached West African countries and some islands that were previously free of the disease. Some authors have stated that this spread started in 1994 due to a combination of factors including: (1) increasing pig production on the continent, (2) presence of non-symptomatic pigs that could act as reservoirs to spread the disease without being noticed and (3) globalization. These factors, together with the economic crisis of the present century, could be the origin of the spread of ASFV to Eastern Europe (Penrith and Vosloo, 2009; Sanchez-Vizcaıno et al., 2013). In 2007, ASFV entered Georgia through the port of Poti (Beltran-Alcrudo et al., 2008), potentially via contaminated food used to feed pigs. From this region, ASFV spread rapidly through the country and affected neighbouring countries including Armenia, Azerbaijan and the Russian Federation. In this geographical region, ASFV affects domestic and wild boar and has spread to the north and west. In 2012, the first outbreaks were declared in Ukraine, followed by Belarus in 2013 (Sanchez-Vizcaıno et al., 2013). From these countries, ASF continued to spread before reaching the European Union (EU) borders in 2014, when several dead wild boars were found in Lithuania and Poland. Since then, several cases have been reported in Estonia and
Epidemiology and Pathology of African Swine Fever
Latvia and repeatedly in Lithuania and Poland (OIE, 2014). At this time, the critical situation of ASF in East and Central Europe poses a serious risk to other EU countries, which could suffer the severe consequences of trade restrictions, as has already happened with Russian Federation bans. However, it is important to remember that the main source of ASFV is presently Africa and from there, it could spread to other disease-free countries/regions and could potentially spread and cause high-impact consequences, as have occurred previously in Europe. The main routes for the spread of ASF involve the transport of infected animals and, more frequently, the transport of infected products (Table 1). These infected products could be imported illegally to make profit or could involve catering waste containing infected pig products. Indeed, this was the means by which ASF was introduced into many disease-free countries in the 1970s (Sanchez-Vizcaıno and Arias, 2012) and the cause of the last introduction in 2007 (Beltran-Alcrudo et al., 2008). Other sources of infection are related to contaminated fomites and transport, including livestock vehicles or infected material. Natural ranging of wild boar is one of the most serious concerns for the EU due to the proximity of affected territories, as demonstrated by the recent cases detected at EU borders and risk assessment analyses (De la Torre et al., in press). However, this route is not likely to cause distant outbreaks in disease-free territories such as Asia without prior suspicion or detection. Other factors, such as infected ticks or airborne transmission, are also important, but only at a local level and should be ruled out as potential sources for ASF infection in distant disease-free territories. Table 1 Main sources of ASF infection into free areas of Europe Transmission route
Raw pork waste at airport/port
Movement of pork or pig product
Natural ranging of infected wild boar
Infected ticks
Location and date of occurrence Lisbon, 1957 Malta, 1978 Sardinia, 1978 Georgia, 2007 Portugal, 1960 Spain, 1960 Italy, 1983 Belgium, 1985 Russian Federation, 2007 Lithuania, 2014 Poland, 2014 Estonia, 2014 Portugal, 1999
11
Clinical Presentation and Lesions ASF can have different clinical presentations and pathological lesions, depending on the virulence of the virus isolate, the route and dose of infection and the host characteristics. The clinical presentation of ASF in a naive farm with a low dose of virus infection, will not cause high mortality nor characteristic clinical signs, except fever and deaths with some hemorrhagic lymph nodes. Usually, few days later, due to the increased viral circulation, more explosive infections could be observed with more mortality and characteristic clinical signs and lesions. Therefore, every dead animal that present fever in a high risk area, should be tested for ASF. ASFV strains are usually classified as highly virulent, moderately virulent and low virulent (Pan and Hess, 1984). Highly virulent strains are usually responsible for the peracute (pigs dead 1e4 days post infection [dpi]) and acute forms (animals dead 3e8 dpi) of the disease, while moderately virulent strains are involved in the acute (pigs dead 11e15 dpi) and subacute (animals die after 20 dpi) forms. In clinical terms, acute ASF develops over a 7-day period, compared with 10e20 days for the subacute form of the disease. Chronic ASF has been associated with infection by moderate-to-low virulence isolates (Mebus and Dardiri, 1979; McVicar, 1984), which were only described in Spain, Portugal and the Dominican Republic when ASF infection in these areas was endemic. The main lesions observed in the different forms of ASF are summarized in Table 2. Peracute African Swine Fever
This form is induced by highly virulent ASFV strains and is characterized by high fever (body temperature 41e42 C), loss of appetite, inactivity, hyperpnoea and cutaneous hyperaemia. Animals usually die suddenly 1e4 days after the onset of clinical signs and no lesions are evident in organs. Acute African Swine Fever
This is the most usual form of the disease and is induced by highly or moderately virulent virus strains. Animals with acute ASF display fever (body temperature 40e42 C) and a tendency to crowd together; they also exhibit loss of appetite, inactivity, apathy and early leucopenia, induced by lymphopenia and changes in monocyte numbers (Colgrove et al., 1969; Pan and Hess, 1984; GomezVillamandos et al., 1995a; Carrasco et al., 1996b; Sanchez-Vizcaıno and Arias, 2012). Severe pulmonary oedema, giving rise to respiratory changes, is a characteristic finding in pigs infected
J.M. S anchez-Vizcaıno et al.
12
Table 2 Main lesions observed in the different forms of ASF
Fever Thrombocytopenia Skin Lymph nodes
Peracute ASF
Acute ASF
High Absent Erythema e
High Absent or slight (late) Erythema Gastrohepatic and renal with marbled aspect Hyperaemic splenomegaly
Spleen
e
Kidney
e
Lung Gall bladder Heart
e e e
Tonsils Reproductive alteration
e e
Subacute ASF
Petechial haemorrhages, mainly in cortex Severe alveolar oedema Petechial haemorrhages Haemorrhages in epicardium and endocardium e e
with highly virulent strains of ASF. Affected animals die in shock, usually 1 week after fever begins and foam is generally observed around the mouth and nose (Sierra et al., 1990; Carrasco et al., 1996a, 2002). Affected pigs show erythema, which particularly affects the skin of the ears, tail, distal extremities, chest, abdomen and perianal area. Cyanosis may also be observed 1e2 days prior to death on the skin of the ears, abdomen and perianal areas. Small foci of cutaneous necrosis (more characteristic of infections with strains of moderate virulence) and subcutaneous haematomas also occur. Other clinical signs include mucoid nasal discharge, epistaxis, vomiting, abdominal pain, constipation or diarrhoea, which is initially mucoid, but may later become bloody (melaena) (Moulton and Coggins, 1968; Mebus and Dardiri, 1979; Gomez-Villamandos et al., 2013). Abortion (caused by fever) may occur in pregnant sows; in some cases, abortion may be the first sign of an outbreak. Almost 90e100% of pigs with these signs will die within 7 days. Together with erythema and cyanosis of the skin, the animals that develop acute ASF present with characteristic hyperaemic splenomegaly in which the spleen can be up to six times larger than normal. An affected spleen may have rounded edges, be friable in consistency and purpleeblack in colour and may occupy the entire abdominal cavity from one side to the other (Fig. 1A). Lymph nodes, mainly the gastrohepatic and renal nodes, display haemorrhages in the medulla (Fig. 1B), which is why sections of affected lymph nodes sometimes have a marbled appearance (Fig. 1C). Kidneys usually show petechial haemorrhages in the cortex (Fig. 1D) and renal pelvis. Other lesions that pigs usually show with acute forms of the disease are petechial haemorrhages in the mucosa of the urinary bladder, epicardium, endocardium and pleura (Mebus and
Chronic ASF
Moderate Irregular or absent Transient Absent Erythema Necrotic areas The majority of lymph nodes Swollen resemble a blood clot Partial hyperaemic splenomegaly Enlarged with normal or focal infarct colour Petechial haemorrhages in cortex, e medulla and pelvis; perirenal oedema e Pleuritis and pneumonia Wall oedema e Haemorrhages in epicardium and Fibrinous pericarditis endocardium; hydropericardium e Necrotic foci Abortion Abortion
Dardiri, 1979; Gomez-Villamandos et al., 1995a; Hervas et al., 1996; Carrasco et al., 1997a, b; S anchezVizcaıno and Arias, 2012). Subacute African Swine Fever
This form of the disease is caused by moderately virulent isolates and affected animals show similar clinical signs to those animals with the acute form of the disease, although clinical signs tend to be less marked. However, the vascular changes observed in subacute forms of ASF, mainly haemorrhage and oedema, are more intense than those reported in the acute form of the disease (Gomez-Villamandos et al., 1995a, 2013). Haemorrhages related to the development of intense, although transient, thrombocytopenia are observed in the early and mid stages of the disease (Villeda et al., 1993a, b; Gomez-Villamandos et al., 1998). Abortion is usually the first clinical sign of these forms. Affected pigs usually die within 7e20 days and the mortality rate can range from 30 to 70%. Survivors usually recover within 3e4 weeks. Recovered pigs can still excrete the virus up to 6 weeks after infection. Death of animals may occur at two different stages: in the intense thrombocytopenia/ leucopenia phase or in the recovery phase when haemorrhages appear due to erythrodiapedesis by vasodilation, especially in young animals (G omezVillamandos et al., 1998, 2013). Pigs suffering from the subacute form of ASF display a moderate to high fever, ascites, hydropericardium and characteristic oedema of the wall of the gallbladder and bile duct (Fig. 2A), as well as in the surrounding area of kidneys (perirenal oedema) (Fig. 2B). The spleen usually shows initial partial hyperaemic splenomegaly (Fig. 2C) that progressively reverses, leaving some focal damage, which eventually
Epidemiology and Pathology of African Swine Fever
13
Fig. 1. Acute ASF. (A) An enlarged, purpleeblack spleen crosses the entire abdominal cavity (hyperaemic splenomegaly). (B) Several lymph nodes display degrees of haemorrhage, most intensely in the medulla. (C) Cut sections of lymph nodes with marbled appearance. (D) Petechial haemorrhages in the renal cortex.
disappears, or focal infarction with different stages of organization. Lymph nodes, mainly the gastrohepatic (Fig. 2D) and renal nodes (Fig. 2E), as well as the submandibular, retropharyngeal, mediastinal, mesenteric and inguinal nodes, are haemorrhagic, oedematous and friable, which is why they often look like dark red haematomas (S anchez-Vizcaıno and Arias, 2012; G omez-Villamandos et al., 2013). Renal haemorrhages (Fig. 2F) are more intense (petechiae and ecchymoses) and more extensive (cortex, medulla and pelvis) than in acute forms, but are not linked to the endothelial lesions reported in acute ASF (G omez-Villamandos et al., 1995a; Hervas et al., 1996). Chronic African Swine Fever
The chronic form of ASF is caused by infection by low virulence strains, observed in Spain and Portugal, as well as the Dominican Republic. This form is characterized by necrotic lesions of the skin and by arthritis (S anchez-Botija, 1982). However, this form of the disease has never been described in those countries where ASFV has been present for long periods of time (e.g. Africa and Sardinia). Therefore, it has been hypothesized that the chronic form may have originated
through the natural evolution of the ASFV isolates employed in the vaccination studies performed on the Iberian Peninsula in the 1960s. To confirm this hypothesis, several molecular studies to elucidate the similarity between chronic forms and vaccine isolates are underway (Sanchez-Vizcaıno, unpublished observations). The chronic form of the disease, which has no specific clinical signs, was mainly detected during serological screening to eradicate the disease in Spain, in carrier animals, of which around 10% can spread the virus for long periods of time, which likely plays a key role in the persistence of the disease (Arias and Sanchez-Vizcaıno, 2002). These animals usually have necrotic lesions of the skin and arthritis (Sanchez-Botija, 1982), delayed growth, emaciation, lameness, respiratory signs, abortion and low mortality (Arias et al., 1986). Unlike other forms of ASF, chronic forms are characterized by the absence of vascular lesions and by the presence of lesions in which bacteria are involved, such as fibrinous pleuritis and/or pericarditis, pleural adhesions, necrotic pneumonia, fibrinous arthritis/periarthritis and necrotic skin lesions, as well as necrotic areas on the tonsils and the tongue (Moulton and Coggins, 1968;
14
J.M. S anchez-Vizcaıno et al.
Fig. 2. Subacute ASF. Intense oedema of the wall of the gall bladder (A) and surrounding the kidney (B). (C) Spleen with partial hyperaemic splenomegaly. Haemorrhagic and oedematous gastrohepatic (D) and renal (E) lymph nodes. (F) Kidneys with intense haemorrhage in the cortex, medulla and pelvis.
Arias et al., 1986). This form of the infection is not currently in circulation.
Differential Diagnosis between African Swine Fever and Other Haemorrhagic Diseases As discussed previously, during the early stages of ASF, particularly when low numbers of animals are affected, diagnosis is not straightforward. The presence of non-specific lesions and a small percentage of deaths can easily be confused with other porcine haemorrhagic diseases. Therefore, differential diagnoses based on the gross lesions include the following swine diseases: classical swine fever (CSF), highpathogenic porcine reproductive and respiratory syndrome (HP-PRRS), swine erysipelas, septicaemic salmonellosis and porcine dermatitis nephropathy
syndrome (PDNS). The specific characteristics of these differential diagnoses are summarized in Table 3 and descriptions of each disease, emphasizing the similarities and differences with ASF, are presented below. Classical Swine Fever
CSF, also known as ‘hog cholera’, is a fatal disease caused by a small enveloped RNA virus belonging to the family Flaviviridae, genus Pestivirus (Van Regenmortel et al., 2000). CSF takes various clinical forms (i.e. peracute, acute, chronic and congenital) depending on the virus strain involved, the environment, the time of infection and the individual host response (Trautwein, 1988). However in the acute and chronic forms of the disease, affected pigs show similar clinical signs:
Table 3 Differential diagnosis of African swine fever on the basis of gross lesions ASF (subacute)
CSF
HP-PRRS
Swine erysipelas
Septicaemic salmonellosis
DNS
Erythema Marbled aspect
Erythema Haemorrhagic
Erythema Marbled aspect
Diamond skin lesions Marbled aspect
Cyanosis Swelling
Macules and papules Marbled aspect
Spleen
Hyperaemic splenomegaly
Multiple infarcts at the margin
Hyperaemic splenomegaly
Splenomegaly
e
Kidney
Petechial haemorrhages Petechial haemorrhages e
Partial hyperaemic splenomegaly or focal infarct Petechial haemorrhages; perirenal oedema Wall oedema
Cyanosis Swelling or marbled aspect Scattered infarcts or white spots on the surface Petechial haemorrhages e
Petechial haemorrhages e
Petechial haemorrhages e
Glomerulonephritis
e
e
e
Button ulcers
Swelling or with haemorrhages e
e
Necrotic enterocolitis
e
Intense (early)
Absent
Absent
Absent
Absent
Nervous signs Congenital malformation (congenital form)
Thymic atrophy Interstitial pneumonia
Skin Lymph nodes
Gall bladder Tonsils Intestine Thrombocytopenia Other lesions
Petechial haemorrhages Absence or slight (late) e
e Petechial haemorrhages Transient e
Petechial haemorrhages Petechial haemorrhages Necrotic areas
Nervous signs; congestion Arthritis and vegetative endocarditis of gastric mucosa; necrotic foci in the liver (chronic form) and bronchopneumonia
e
e
Epidemiology and Pathology of African Swine Fever
ASF (acute)
African swine fever, ASF; classical swine fever, CSF; high-pathogenic porcine reproductive and respiratory syndrome, HP-PRRS; dermatitis and nephropathy syndrome, DNS; e, no change.
15
16
J.M. S anchez-Vizcaıno et al.
anorexia, lethargy, conjunctivitis, respiratory signs and constipation followed by diarrhoea. These clinical signs, together with the haemorrhages (petechiae and ecchymoses) of the skin, kidneys, tonsils, gallbladder and ileocaecal junction observed in the acute form, could be confused easily with ASF. Some of the most characteristic lesions of CSF include the presence of multifocal infarction of the margin of the spleen, which takes a triangular shape, and the presence of necrotic foci on the tonsils (Cheville and Mengeling, 1969; Trautwein, 1988; Potier et al., 2006). Although these lesions are not always present together, they represent, together with the neurological signs (induced by non-purulent meningoencephalitis), the best differentiation between CSF and the acute and subacute forms of ASF. Another characteristic lesion of the subacute and chronic forms of CSF, which allows a differential diagnosis to be made with ASF, is the presence of ‘button ulcers’ in the small (mainly ileum and ileocaecal valves) and large intestines (caecum and colon) (Cheville and Mengeling, 1969; Trautwein, 1988; Van Oirschot, 1999; Potier et al., 2006). Additionally, the onset of thrombocytopenia and haemorrhages also differs between CSF and ASF. Haemorrhages in CSF are associated with early intense thrombocytopenia, which coincides with the appearance of fever and the start of viraemia (Cheville and Mengeling, 1969; Trautwein, 1988; Bautista et al., 2002). However, in ASF, slight thrombocytopenia in the late stage can be detected in acute forms or transient thrombocytopenia may be present in the early and mid stages of subacute disease (Villeda et al., 1993a, b). Porcine Reproductive and Respiratory Syndrome
PRRS is caused by two genotypes of the PRRS virus (PRRSV), a member of the family Arteriviridae (Nelsen et al., 1999). PRRS is characterized by respiratory problems in growing and finishing pigs and by reproductive failure in sows, with late-term abortion, stillbirth and increased preweaning mortality. Recently, however, highly pathogenic strains (HPPRRSV) have been isolated in China and southeast Asia and in Eastern Europe (Garcıa-Nicolas et al., 2013; Gomez-Laguna et al., 2013). Infection with HP-PRRSV strains is associated with severe clinical signs, pulmonary lesions and aberrant host immune responses (Gomez-Laguna et al., 2013). Pigs infected with HP-PRRS strains display high mortality accompanied by high fever (body temperature 40e42 C) severe lethargy, anorexia, cough, severe dyspnoea, lameness and cyanosis of the ears, limbs and perineum (Zhou et al., 2009; Han et al.,
2014). The main lesions are found in the lungs, which display interstitial pneumonia (Han et al., 2014), and in lymphoid organs, where severe thymic atrophy and swelling and haemorrhage in the tonsils and lymph nodes may be observed (located in the medulla with a cut surface showing a marbled appearance), together with scattered infarcts or white spots on the surface of the spleen. Petechial haemorrhages may be observed in the renal cortex (Wang et al., 2011). Although haemorrhages in kidneys and lymph nodes are present in HP-PRRSV, the absence of hyperaemic splenomegaly and the intensity of respiratory distress, due to interstitial pneumonia, are used to make the differential diagnosis with ASF. Swine Erysipelas
Swine erysipelas (SE) manifests commonly as acute or subacute septicaemia and chronic proliferative lesions caused by Erysipelothrix rhusiopathiae, a small grampositive bacillus (Sneath et al., 1986). Acute SE is characterized by sudden onset, with occasional sudden death, in which animals have a high fever (body temperature 40e42 C) for 5e7 days and also display severe depression and leucopenia. The subacute forms are less severe than the acute form and abortion may occur in sows that come into contact with acute or subacute SE during pregnancy (Wood and Henderson, 2006). The most characteristic lesion of SE is the presence of rhomboid urticarial lesions (or ‘diamond skin’ lesions) in the acute non-fatal form from days 2e3 of infection, which appear as small square or rhomboid areas with light pink to dark purple colouration that are usually raised and firm to the touch. In the acute fatal form of SE lesions are similar to those of septicaemia, with extensive dark purple discolouration over the ears, tail, abdomen, the posterior aspect of the thighs, and the jowls. The lungs are congested and oedematous and haemorrhages (petechial to ecchymotic) may be seen in the cortex of the kidneys, the heart (particularly in the epicardium and the musculature of the atria) and the serosa of the stomach. The spleen may be congested and markedly enlarged (hyperaemic splenomegaly) (Wood and Henderson, 2006), which is similar to the acute form of ASF. The involvement of lymph nodes depends on the area that they drain; usually the peripheral nodes show marked congestion to haemorrhage of the medulla and the cut surface reveals a marbled appearance (Wood and Henderson, 2006). Despite the similarities between acute SE and ASF (i.e. clinical course, presence of haemorrhages, similar spleen lesions), there are several characteristics that differ between these diseases. Firstly, the presence of
Epidemiology and Pathology of African Swine Fever
diamond skin lesions is typical in acute SE, but not in ASF. Additionally, the peripheral lymph nodes are usually the most affected in acute SE, while the gastrohepatic and renal nodes are mainly affected in acute ASF. Septicaemic Salmonellosis
Swine salmonellosis is caused by Salmonella choleraesuis and has been related to the development of septicaemia or enterocolitis (Baskerville and Dow, 1973). Pigs with septicaemic salmonellosis are inappetent, lethargic, febrile (body temperature 40.5e41.6 C), have a moist cough with slight expiratory dyspnoea, uncoordinated gait, tremor, paralysis, convulsions and recumbency. These clinical signs may result in death. At 3e4 dpi, animals suffer from diarrhoea with watery yellow faeces. In most outbreaks, which are frequently associated with stressful situations, morbidity is variable but usually
