Arenaviral Haemorrhagic Fevers

D . Cummins

SUMMARY . Three arenaviruses-Lassa, Junin and Machupo-cause severe haemorrhagic disease in humans : Lassa fever, Argentine haemorrhagic fever and Bolivian haemorrhagic fever, respectively . These conditions are a source of considerable economic hardship in endemic areas and remain a worldwide concern for public-heakh officials . They are characterised by an insidious onset of influenza-like symptoms followed, in severe cases, by a generalised bleeding diathesis, encephalopathy and death . Central to studies of their pathogenesis is evidence for cellular dysfunction disproportionate to overt histopathology . Recent studies of patients with Lassa fever indicate that platelet and possibly endothelial-cell dysfunction play an important role in the bleeding tendency . The platelet defect appears to be mediated by an inhibitory factor in plasma ; the nature of this is uncertain but it seems to be neither viral protein nor virus antibody . A similar inhibitor has since been demonstrated in patients with Argentine haemorrhagic fever . Plasma from patients with Lam fever also profoundly modulates the amount of superoxide generated by normal neutropbils in response to the cbemotactic peptide f-met-leu-phe, suggesting the inhibitor(s) has global effects on cellular function. These findings may have important implications for future therapeutic strategies .

Arenaviruses are single-stranded RNA viruses which derive their name from arenosus-latin for sandyon the basis of characteristic granules seen inside the virion on ultrathin section .' Four cause disease in humans : lymphocytic choriomeningitis (LCM), Lassa, Junin and Machupo (Table 1) . LCM virus, the first described,' has a worldwide distribution but rarely causes severe disease in man . The others cause viral haemorrhagic fevers in West Africa (Lassa virus) and South America (Argentina : Junin virus; Bolivia : Machupo virus). These conditions have several features in common: (1) a viral reservoir in wild rodents ; (2) a haemorrhagic diathesis; (3) an acute encephalopathy ; and (4) a 10-20% mortality rate in untreated hospitalised patients . With increasing intercontinental travel, sporadic imported cases of arenaviral haemorrhagic fevers are a major global concern Senior Registrar, Department of Haematology, Edgware General Hospital, Edgware, London, UK . D. Cuoumins MD,

Blood Reviews (1991) 5, 129-137

1991 Longman Group UK Ltd

for public-health officials, and thus greater awareness of their clinical features and pathophysiology is needed worldwide . Epidemiology Lassa fever was first described in 1969 following the mysterious deaths of two medical missionaries in northern Nigeria .' A virus isolated from these patients was named after the town of Lassa, Nigeria, where the index case occurred .' Subsequently, the disease was reported in several West African countries,` and in 1972 the multimammate rat, Mastomys natalensis, was identified as the natural host .' The mode of rodent-to-human transmission is unknown but may involve contamination of superficial abrasions with rat excreta . Human-to-human transmission may also occur but is probably of lesser importance . In parts of Sierra Leone up to 50% of individuals have serological evidence of past exposure to Lassa

130

ARENAVIRAL HAEMORRHAGIC FEVERS

Table I

Recognised arenavirus diseases of humans

Virus

Disease

Locality

Haemorrhage

Lymphocytic choriomeningitis Lassa

Grippe, aseptic meningitis, encephalomyelitis Lassa fever

Rare but documented

Junin

Argentine haemorrhagic fever

Machupo

Bolivian haemorrhagic fever

Probably originated in Europe ; now worldwide West Africa ; esp . Nigeria, Liberia and eastern Sierra Leone Circumscribed area of Argentina : Buenos Aires to NE Beni region of Bolivia

virus .' It has been estimated that there are about 250 000 human Lassa virus infections in West Africa each year, with 5000 deaths .' Argentine haemorrhagic fever (AHF), in contrast to Lassa fever, shows marked geographical restriction . First described in detail by Arribalzaga in 1955, 10 its infective aetiology was established when a previously unknown virus was isolated from a patient in Junin, Buenos Aires Province ." AHF occurs in a well-circumscribed area of the central Pampas in the heart of the richest farmlands of Argentina : the population density and economic significance of the endemic zone make the disease an important health problem ." Junin virus has its natural reservoir in several species of Calomys rodents 13 whose predominantly rural habitat explains a high incidence of AHF in male agricultural workers ; human-to-human transmission is rare . AHF is far less common than Lassa fever, with about 300-500 cases each year ." Bolivian haemorrhagic fever (BHF) is caused by Machupo virus, first isolated in 1963 . 15 Few cases have been seen in recent years, but large outbreaks in the Beni Province of Bolivia in the early 1960s were traced to the infected mouse, Callomys callosus . 16 Person-to-person spread is rare ."

17% of hospitalized cases, usually mild ; esp . oral mucosal Common and severe; esp. G U and GI tracts and skin Common and severe esp. GI tract and skin

munity-based studies, however, indicate that most Lassa virus infections are mild or subclinical, with an overall fatality rate of 1-2% .' Clinical Presentation . Lassa fever starts insidiously,

after an incubation period of 7-18 days, with fever and malaise, followed 1-2 days later by generalised myalgia, back pain, headache and sore throat .20 In more severe cases, complete prostration can occur by the seventh day of illness . Clinical signs may include exudative pharyngitis and conjunctivitis ; pleural and pericardial effusions are seen occasionally . Signs of heart failure are rare, though subclinical myocarditis is probably common ." For reasons that are unclear, a proportion of Lassa fever patients show a rapid and dramatic clinical deterioration between the seventh and twelfth day of illness, developing facial and neck oedema, respiratory distress, encephalopathy, haemorrhage and shock (Fig. 1) . 3,20,22 Haemorrhage . The bleeding tendency warrants special

mention . About one-fifth of hospitalised Lassa fever patients develop overt haemorrhage .2' This is usually from the gingival mucosae, but can occur at almost

Clinical Features Human arenavirus infections exhibit a wide range of clinical manifestations and disease severity . The reasons for this variability in disease expression are illunderstood . LCM Virus Infection

LCM virus causes three distinct clinical entities : influenza-like illness, meningitis and encephalomyelitis ." Although severe disease in humans is uncommon, one fatal case was associated with a prominent haemorrhagic diathesis more typical of other arenavirus infections.' 9 Lassa Fever

Lassa virus causes a spectrum of manifestations in humans ranging from asymptomatic infection to lifethreatening disease . In eastern Sierra Leone about half of all hospital admissions and 30% of adult medical deaths are caused by Lassa fever.20 Com-

Fig . 1 Facial oedema and oral mucosal haemorrhage in a patient with severe Lassa fever .

BLOOD REVIEWS

any site ." , " Petechiae and echymoses, however, are not seen ." Although rarely sufficient, in itself, to cause shock, haemorrhage is an ominous sign and frequently heralds death ." Clinical Laboratory Tests . Standard laboratory tests

show no specific abnormalities . The haematocrit is often raised, reflecting dehydration . The white cell count is usually normal, though a marked neutrophiha may occur in severe cases . In contrast to many other viral haemorrhagic fevers, the platelet count is normal or only slightly depressed ." , ' Coagulation tests including prothrombin time, partial thromboplastin time and the levels of fibrinogen and fibrin degradation products (FDPs) show no specific changes ." Mild elevation of blood urea, raised hepatic enzymes and mild proteinuria occur in some cases . Complications . There are two main complications of Lassa fever, abortion and deafness . In the third trimester of pregnancy, Lassa virus infection is associated with a high fetal (> 85%) and maternal (> 25%) mortality, 25 abortions often being associated with catastrophic vaginal haemorrhage ." Thus, in endemic areas, Lassa fever is a principle cause of maternal death ." Approximately one third of Lassa fever patients develop acute sensorineural deafness, 26 an incidence which is considerably greater than that seen with any other post-natally-acquired infection . The onset of deafness occurs in convalescence and is preceded by seroconversion, suggesting an immune-mediated injury . Its severity and prognosis are variable but appear unrelated to the severity of Lassa fever . Thus, in areas where Lassa virus infection is highly endemic, deafness is common ; indeed, the prevalence of virusinduced hearing loss in eastern Sierra Leone is greater than that reported from anywhere else in the world ."

Argentine Haemorrhagic Fever

The clinical presentation of AHF is similar to that of Lassa fever . After a 1- to 2-week incubation period, patients develop fever, retro-ocular pain, myalgia and diffuse, erythematous skin rash . 27 Early in the disease, petechial haemorrhages are common, usually most marked in the axillae. At 6-10 days of illness many patients deteriorate, developing predominantly neurological or haemorrhagic manifestations . Bleeding tends to be more severe than that seen in Lassa fever : epistaxis, melaena, haematemesis and haematuria are common . After 10-15 days over 80% of patients improve while the remainder deteriorate, with total mortality approaching 16% in the absence of convalescent-plasma treatment (see below) ." Thrombocytopenia is more severe in AHF than in Lassa fever ; it is rare, however, for the platelet count to fall below 40 x 103 /111 . 29 Leucopenia and proteinuria are common .

131

Bolivian Haemorrhagic Fever

BHF closely resembles AHF . 30 One third of patients develop a haemorrhagic diathesis with petechiae on the trunk and palate and bleeding from the gastrointestinal tract, nose, gums and uterus ; nearly half of patients develop an intention tremor of the tongue and hands . The acute disease lasts 2-3 weeks and convalescence may be protracted . Reported mortality rates have varied from 5-30% . Pathology One might expect that the wide spectrum of clinical manifestations in human arenavirus infections, and the wide range of disease severity, would be associated with extensive variation in histopathological changes . This is not the case . 31 The most common macroscopic abnormality in fatal cases of Lassa fever is widespread haemorrhage, particularly from gastric mucosa, kidneys and conjunctivae. Evidence of increased vascular permeability, including serous effusions and facial, intestinal and laryngeal oedema, is found occasionally . Microscopic lesions are remarkably subtle .'' 32 The most consistent feature is multifocal hepatocellular necrosis ." However, jaundice is rarely if ever seen and neither measures of hepatic function nor the extent of tissue necrosis are adequate to implicate liver failure as the cause of death . . .-33 Lesions can often be found in the spleen, adrenals, kidney, heart and lung, but the changes are of unimpressive magnitude . 3, 31-33 Generally, cell necrosis and immunohistochemical evidence of cellular infection are focally dispersed and not concentrated at vital sites ." Microscopic haemorrhagic diathesis is rare, and fibrin thrombi are not seen . In fatal AHF, abnormalities include widespread haemorrhage, hepatocellular necrosis, myocarditis, renal papillary necrosis and secondary bacterial lung infection ." Sites of tissue necrosis generally correspond to sites of viral antigen accumulation ." Although the abnormalities tend to be more marked than those found in patients dying of Lassa fever, they are sufficient to account for death .31 .34 Few fatal cases of BHF have been studied at necropsy . Gastrointestinal and intracranial haemorrhage have been the commonest macroscopic findings ; histological lesions have included interstitial pneumonia and hepatocellular necrosis ." Pathogenesis The close histopathological and clinical similarities between Lassa fever, AHF and BHF might be taken as indicating a common pathogenesis . It is apparent, however, that there are striking differences between Lassa fever and the South American haemorrhagic fevers, particularly with regard to the immune re-



IJ2 ARENAFIRAL fnrNIVRlct7AGll, FEVERS

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appropriate to briefly review their immunology .

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Virology and Immune Response Lassa virus can be isolated from most tissues and hndv fluids of infected hnmanQ and fatality rates are closely related to virus titre : the mortality in patients where this exceeds 103 .6 tisgrne-cnlmre infection dose (TCID 50 /ml) in serum is over 75% . 37 Virus has been recovered from the cerehrospinal fluid (CSF) of a few cases, but titres are generally lower than those in blood ." Production of IgG and IgM Lassa-specific antibodies is almost invariable and can be detected by immunofluorescent antibodies in over 50% of patients at the time of their admission to hospital ." However, viraemia is unaffected by this antibody response and virus may cocirculate with high titres of antibody for up to three weeks . Indeed, neutralising antibodies do not appear in significant quantity at any stage of illness, suggesting that cell-mediated immunity is important in viral clearance and protection against reinfection . Infections with Junin and Machupo viruses differ from Lassa virus infection in two main ways : first, viraemia is sporadic or of low concentration and does not predict outcome; second, high-quality neutralising immunoglobulins are demonstrable in serum within a few days of the resolution of fever .3s-41 It is clear, therefore, that clinical recovery from arenavirus infections may be mediated by both cellular and Immoral immune responses . In Lassa fever, cellular mechanisms would appear dominant, whereas in the South American haemorrhagic fevers there is greater potential for antibody participation .

Immunosuppressive Effects of Infection The acute phase of illness in Lassa fever and the South American haemorrhagic fevers is associated with significant depression of host immunity . Perhaps the most obvious example of this is the delayed immune reactivity to the viruses themselves, with 10-20 days required before effective immune responses are achieved . In Lassa fever, immunosuppression may underlie the high, persistent viraemia and disseminated viral replication in host tissues . Limited studies of Lassa virus infection in experimentally infected primates have demonstrated impaired lymphocyte responses to mitogenic stimuli in vitro ; 42 and in both humans 24 and animal models 42 lymphocytopenia appears to be a consistent feature of the disease . Impairment of immune function has been studied more closely in AHF . The disease is associated with a profound decrease in recall of delayed hypersensitivity, reduced responsiveness of lymphocytes to nonspecific mitogens, decreased levels of circulating Band t-cells, and inversion of CD4/CD8 lymphocyte ratios . 41-4 ' Abnormalities reported in animal models

. .C . .,

. . .,h ., ..e.. T

..A R_ 1 1,

cyte depletion, and anergy after established tuberculin .e 46,47

Unlike Lassa fever, AHF is associated with a high inr:denre of sernnclarv nvnaenir infertnn in hnmans 30. and experimentally infected primates .' This may he related to nenlrnnenia 27 and imnaired nentrnphil function . 49

Genesis of Haemorrhage The pathogenesis of haemorrhage in patients with arenaviral haemorrha2ic fevers has remained unclear : neither the degree of thrombocytopenia nor the mild abnormalities reported in coagulation tests have been sufficient to account for the catastrophic bleeding seen in some cases .23,24,29,42,50 This is an unusual state of affairs : few disorders are complicated by an acute, generalised haemorrhagic diathesis where one or other of these tests, either alone or in combination, is not markedly abnormal . Thus there is no evidence, from these observations or from histopathological studies, that disseminated intravascular coagulation plays any significant role in the bleeding tendency . 20,22-24,29,31,3Z34,42,50 Haemorrhage in patients with Lassa fever is associated with a poor prognosis, even in cases where it is insufficient, in itself, to cause shock . Indeed, in a prospective case-control study of 441 hospitalised patients in Sierra Leone, haemorrhage was identified as the single best predictor of a fatal outcome ." A better understanding of the haemostatic defect, therefore, might well shed light on the pathogenesis of the disease in general. Coagulation Studies . In patients 2 o .23,24 and monkeys 24.5° with Lassa virus infection platelet counts usually remain above 100 x 10 3/gl, and prothrombin times, partial thromboplastin times and the levels of fibrinogen and FDPs are either normal or show minor abnormalities . Platelet survival and fibrinogen turnover are normal, 42,50 Human infection with Junin virus is associated with moderate thrombocytopenia, a mild prolongation of partial thromboplastin time and a slight decrease in factors VIIIc, IX, XI and XII (levels usually remain above 40IU/dl) . 27,29 Fibrinogen levels show a progressive rise after the fourth day of illness, FDPs remain undetectable ." Similar findings have been reported in experimental infection with Junin 51 and Machupo viruses." Although these minor abnormalities may contribute to the bleeding manifestations, it seems unlikely that they are the major cause of the haemorrhagic tendency ." Endothelial Cell Function . It has been suggested that the tissue oedema, serous effusions and shock observed in severe arenaviral infections may be due to impaired endothelial cell function . One study of Lassa virus infection in rhesus monkeys demonstrated loss of prostacyclin production from post-mortem endo-



BLOOD thelium .42 This would appear to be an indirect effect of the virus, since primate endothelial tissue does not show generalised histological destruction from Lassa virus replication . Elevated Factor VIII VWF :Ag occurs commonly in patients with AHF, 53 but this may reflect the acute-phase response rather than endothelial injury. Platelet Function . A study of Lassa virus infection in rhesus monkeys indicated that platelet dysfunction may play an important role in the bleeding tendency ." It was postulated that the platelet abnormalities might be due to an acquired storage pool disorder similar to that found in conditions such as haemolytic uraemic syndrome ." The platelet defect has since been investigated in more detail ." , " In a recent study in Sierra Leone, 23 patients with severe Lassa virus infection were shown to have a marked depression of platelet aggregation to ADP, collagen and sodium arachidonate in vitro (Table 2), some showing absent responses to all three agonists . However, when plasma from patients with severe Lassa fever was mixed with platelets from normal people, a marked inhibition of agonist-induced response was observed (Fig. 2) . Subsequently, acute-phase plasma from Lassa fever patients was shown to inhibit the aggregation, secretion and thromboxane generation of normal platelets to a wide range of agonists, including ADP, collagen, thrombin, the endoperoxide analogue U44619, the calcium ionophore A23187 and, to a lesser extent, sodium arachidonate . 23,55 The inhibitory effect was almost immediate in onset and unaffected by pre-incubation of platelets with plasma for periods of up to 30 min . No significant change in inhibitory activity occurred when the plasma was heated at 56°C for 30 min, left at room temperature for 48 h, or exposed to doses of gamma radiation sufficient to inactivate Lassa virus . The inhibitory effect could not be reproduced by supernatants from Lassa virus-infected tissue cultures or by concentrated IgG fraction from convalescent serum . Inhibitor was demonstrable in 80% of patients with overt haemorrhage, but in only 16% of those without obvious bleeding (P=0.03), and was strongly

REVIEWS

CONTROL LASSA

/

IPM 046619

1pM

2pM

ADP Mg/ml

5µM

I

54 (19-101) 51 (25-84) 43 (1-103) 5 (0-25)

112 (62-140)

114 (53-135) 25 (0-90) 13 (0-130) 0

0.002 0.0008

0 .0006

110

0.0004

> 0 .05 0 .0003

Collagen 4 Mglml

SA I mM

137 (101-163) 103 (92-116) 93 (0-146) 6 (0-40)

103 (0-140) 56 (0-109) 54 (0-142)

0 .0008

> 0 .05 0.0015

0 .0004

11

(0-47)

Shown are the mean heights of the aggregation responses in millimetres ; ranges are in parentheses . S A = sodium arachidonate ; WRST = Wilcoxon rank sum test ; MLF = mild Lassa fever ; SLF = severe Lassa

fever;

HC =healthy

control .

-ti imin

associated with disease severity (P=0 .007) . Significantly, inhibitory activity was detected only in the plasma of patients whose own platelets had previously been shown to have markedly depressed responses . Furthermore, a progressive rise in inhibitory activity accompanied clinical deterioration, while a fall coincided with clinical improvement and recovery of platelet responsiveness in vitro . The platelet defect was shown to be, at least partially, reversible, since platelets from Lassa fever patients which were washed and resuspended in normal plasma showed a marked improvement in aggregation response . Recently, plasma from patients with AHF has been shown to contain an inhibitor of platelet function ." The inhibitor has similar effects in vitro to that found in patients with Lassa fever, but may have greater thermal lability . Its effects are not neutralised by convalescent plasma containing a high titre of (neutralising) antibodies to Junin virus . The platelet inhibition appears to be reversible in vitro ." Thus, the available evidence suggests that abnormal platelet function in patients with arenaviral infections

IPM

(92-144) 93 (40-148) is (0-60)

Aggregation

Fig . 2 U46619-induced aggregation and ATP release of normal platelets following 1 :1 mixes with plasma from a Lassa fever patient and plasma from a healthy control . Platelet-rich plasma from a healthy control subject was mixed 1 :1 with control plasma (left) and plasma from a Lassa fever patient (centre and right) . Aggregation and ATP release were assessed in a lumiaggregometer.

Table 2 Platelet aggregation responses in Lassa fever patients and controls

Healthy controls (n=12) Febrile controls (n=5) Mild Lassa fever (n=16) Severe Lassa fever (n = 8) P value (WRST) SLF vs MLF SLF vs HCs

133



1 34

ARENAVIRAL HAEMORRHAGIC FEVERS

is not due to an intrinsic platelet defect, but to inhibition by an extrinsic factor in plasma . Neutrophil Studies The paucity of major histopathological lesions in fatal cases of arenaviral haemorrhagic fevers has led many to speculate that widespread disturbance of cellular function rather than direct virus-induced cellular damage is important in their pathogenesis . 22-24,31-33,42,49,16 This concept of altered cell function without overt histopathology is well developed for LCM virus in experimental animal models ." To assess the potential for the platelet inhibitory factor(s) to play a role in mediating such global cellular dysfunction a recent study 49 investigated the effects of plasma from Lassa fever patients on neutrophil function . (Neutrophils were chosen simply because their function is relatively easy to assess in vitro .) Samples from patients in the acute phase of Lassa fever profoundly inhibited the amount of superoxide generated by normal neutrophils in response to the chemotactic peptide f-met-leu-phe (FMLP) . In contrast, samples from febrile controls and patients in the convalescent phase of Lassa fever enhanced the neutrophil response (Figs 3 & 4) . All samples which had been shown to inhibit platelet aggregation inhibited neutrophils . Unlike the effect on platelets, however, inhibition of neutrophils was only apparent when they were stimulated within 5 min of exposure to the plasma. The neutrophil inhibition was not due moo

0

0 a

a

tso a

a

O

0 0 a a

a

IN

a

too either interference with the binding of FMLP to its neutrophil receptor or to an effect on the NADPH oxidase, suggesting a suppression of signal transduction processes. It is unclear whether the inhibition of neutrophil function is of clinical significance, since intercurrent pyogenic infections are not a feature of Lassa fever . The prominence of secondary bacterial infections in AHF, 34,4a however, and the suggestion that neutrophil function may be impaired in the disease, 49 raises the possibility that such a mechanism may be important in the South American haemorrhagic fevers . The nature of the inhibitory factor(s) in patients with Lassa fever and AHF is also unclear . Its physicochemical stability excudes live virus, and the failure to reproduce the inhibition with tissue culture supernatants mitigates against a viral protein . Virus antibody also appears an unlikely candidate, since the effect is not seen with convalescent samples containing high titre antibodies . Could the inhibitory factor be a cytokine? Cytokines are known to exert powerful modulatory effects on cellular function, and there is increasing evidence to implicate them in the evolution of human arenaviral infections . Involvement of Cytokines Serum concentrations of alpha interferon in patients with AHF are among the highest reported for any infectious disease, with levels as high as 64000 IU/ m160,61 Unlike the situation with most acute viral infections, interferon concentrations show no association with viraemia . However, they correlate strongly with several clinical manifestations of the disease and are strong predictors of a fatal outcome . (Although alpha interferon inhibits platelet aggreagation in vitro,62 its effects are quite different to those of the inhibitory factor in AHF plasma .") The mechanism by which Junin virus induces such massive levels of interferon in vivo is unknown, but it is interesting that the virus is relatively insensitive to interferon in vitro . Recent studies have also reported significantly elevated levels of tumour necrosis factor in AHF patients, with normal levels of interleukin 1 . 63 Although these alterations in cytokine levels appear to be closely associated with the clinical severity of AHF, their precise role in the pathogenesis of the disease remains to be established. Encephalopathy

0 Acute

Convalescent

LASSA FEVER

WAIrlan

Caucasian

FEBRILE CONTROL

Fig . 3 Effect of plasma from Lassa fever patients and febrile controls on FMLP-induced neutrophil superoxide generation . Shown is the superoxide generated with each sample expressed as a percentage of that obtained with plasma from a healthy control . The final concentration of plasma in the assay was 10% (v/v) . FMLP (10 - 'M) was added immediately after addition of plasma

Encephalopathy is common in arenaviral disease, and occurs in the great majority of patients with AHF and BHF . 17,49 Its pathogenesis is unclear. In Lassa fever patients it typically presents as a subtle alteration in mental state, followed rapidly by confusion, drowsiness, convulsions and coma; focal neurological signs and clinical evidence of raised intracranial pressure are not seen ." CSF contains neither IgM Lassa-



BLOOD REVIEWS

100

C e G

1 35

A

'5a

as too

v`0 M

a

0 0

0

10

10

20

Days post-admission

25 T000 -

Fig . 4 Effects of serial samples from a patient who recovered from severe Lassa fever on (A) FMLP-induced superoxide generation by normal neutrophils and (B) ADP-induced aggregation responses of normal platelets . (A) Superoxide production in the presence of serial plasma samples from a Lassa fever patient is expressed as a percentage of that obtained in the presence of control plasma . The final plasma concentration in the assay was 10% (v/v) . The data given are those from a single experiment with plasma obtained on day 1 and the mean ±the range of duplicate experiments with plasma obtained on all other days. FMLP (10 - 'M) was added immediately after addition of plasma . (B) Aggregation responses of normal platelets to 4 pM ADP are shown following 1 :1 mixes of platelet-rich plasma from a healthy control with plasma samples obtained from the patient on various days after admission .

specific antibodies nor a mononuclear cell response ; Lassa virus is often undetectable in both CSF and post-mortem brain tissue ; and histological lesions in the central nervous system are minimal or absent ." It therefore appears that neither direct cytopathic nor immune-mediated mechanisms contribute significantly to the neurological dysfunction . The possible role of soluble mediators remains to be explored . Treatment A large trial has shown that the antiviral drug ribavarin is effective treatment for Lassa fever ." The greatest benefit was in patients with high viraemia treated within 6 days of disease onset (P=0 .006) . Improvement in survival was less marked if treatment was not started until after the sixth day of illness, but in patients with high viraemias the results were still significant (P=0 .035) . Although ribavarin is welltolerated and free of serious side effects, its expense will ensure it remains unavailable to the vast majority of West Africans . Convalescent plasma does not improve outcome in patients with Lassa fever ." This is hardly surprising since, as mentioned, Lassa fever can easily be isolated from humans in the presence of high titres of circulating antibody, and antibody production does not correlate with clinical improvement ." However, convalescent plasma is extremely effective in AHF, the mortality rate failing to less than 1 % in patients treated within the first 8 days of illness ." In 10% of immune-plasma recipients, a late neurological syndrome of unknown pathogenesis develops 4-6 weeks after the acute phase of illness ." Mortality rates in patients with late, severe Lassa fever remain poor despite ribavarin . The demon-

stration of a circulating inhibitor of platelet and neutrophil function in such patients suggests a potential role for plasma exchange therapy . One patient was recently described who showed a dramatic, early response to such treatment . 66 Further studies will be required to evaluate this therapeutic approach . Concluding Remarks It will be apparent from this review that much of the pathogenesis of arenaviral haemorrhage fevers remains unclear. Recent work, however, has defined more accurately their clinical presentation, evolution and pathophysiology, and has indicated important areas for future study . Global immunosuppression is a prominent feature of severe arenaviral disease, and failure to mount an effective virus-specific response is characteristic of fatal infections . The mechanism of the immunosuppression is uncertain but may involve direct viral effects, cytokines or other factors . Haemorrhage appears to result from platelet and possibly endothelial cell dysfunction, with thrombocytopenia and reduced coagulation factors contributing particularly in the South American haemorrhage fevers ; disseminated intravascular coagulation is not important in the pathogenesis . The platelet dysfunction appears to be mediated by an inhibitor in plasma, and this same inhibitory factor may also be responsible for inhibition of neutrophil function . Characterising the inhibitor(s) and determining its precise mode of action may provide insight into the mechanisms underlying more-global cellular dysfunction in these disorders, and may suggest new approaches to treatment . Vaccines for Lassa fever and AHF must be the prime goal for future therapeutic intervention . A

1 36

ARENAVIRAL HAEMORRHAGIC FEVERS

genetically engineered vaccinia virus expressing viral glycoproteins has been developed for Lassa fever and appears to prevent the disease in experimentally infected primates ; human trials will hopefully begin soon . A live attenuated vaccine has been developed for AHF and is currently being tested in humans . Before long, these diseases may not only be treatable, but preventable .

Acknowledgments I am grateful to the Wellcome Trust, Schering Health Care and the British Society for Haematology for financial support; Dr Simon Ardeman for his helpful advice ; and the British Journal of Haematology for permission to reproduce illustrations from previously published work .

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