EQUINE VETERINARY JOURNAL Equine vet. J . (1992) 24 (2) 155-158

Fatal, congenitally acquired infection with equine arteritis virus in a neonatal Thoroughbred WENDY E. VAALA, A. N. HAMIF?*, E. J. DUBOVlt, P. J. TIMONEYS and B. RUIZ* Departments of Clinical Studies, and *Pathobiology, New Bolton Center, University of Pennsylvania School of Veterinary Medicine, Kennett Square, PA 19348, USA; t Veterinary Diagnostic Laboratory, New York State College of Veterinary Medicine, Cornell University, Ithaca, N Y 14851, USA; and #Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546, USA.

Introduction EQUINE viral arteritis (EVA), a non-arthropod-borne togavirus (Porterfield et a1 1978), was first identified as a separate viral pathogen of the horse following an epizootic of abortion on a Standardbred breeding farm in Bucyrus, Ohio, in 1953 (Doll, Bryans, McCollum and Crowe 1957a). The 50% abortion rate amongst mares exposed to the virus was the most serious consequence of this outbreak. Other clinical signs commonly observed in adult and weanling horses with EVA are limb, ventral abdominal and periorbital oedema, serous nasal and ocular discharge, conjunctivitis, fever, depression, and skin rash (Bryans, Doll, Crowe and McCollum 1957; Jones 1969; Timoney 1985). Adult horses naturally infected with equine arteritis virus (EAV) make uneventful recoveries, regardless of the severity of clinical illness. Mortality is rare in foals with naturally acquired infection (Golnik and Michalak 1979; Golnik, Michalska and Michalak 1981). Case history A I3-year-old, pregnant, multiparous Thoroughbred mare was purchased at public auction 4 months before parturition, and maintained on a regular deworming and vaccination schedule. The mare had a transabdominal, ultrasound-guided amniocentesis performed on Day 319 of pregnancy and transabdominal ultrasonograms performed on Days 319, 326, 333 and 352 to record foetal heart rate, foetal position, foetal fluid clarity, and foetal movement. No foetal abnormalities were observed. The mare's vital signs were recorded twice daily. At 2 weeks before parturition a non-painful plaque of pitting oedema developed along the mare's ventrum, just cranial to the mammary gland. On Day 346 the mare's rectal temperature increased transiently to 101.4'F (38.6'C), a slight but evident rise above her average temperature of 99.5"F (37.5'C). On Day 349 the mare developed generalised urticaria, consisting of multiple raised nodules, 1-3cm in diameter, that disappeared within 72 h. On Day 352 of pregnancy the mare delivered a 140 Ib (63.6

kg) Thoroughbred filly and passed a grossly normal placenta 30 min post partum. The filly had a 5-min post-partum modified Apgar score (Martens 1982) of 8, indicating an absence of periparturient hypoxia. The foal stood and nursed unassisted within 60 min of birth. Except for a pansystolic machinery murmur auscultated over the left heart base, compatible with a patent ductus arteriosus, there were no abnormalities discovered on physical examination. Routine thoracic radiographs taken at 8 h of age did not reveal nay evidence of pulmonary disease. Clinical pathology Aerobic culture of amniotic fluid, collected via needle and syringe aspiration during parturition before amnion rupture, failed to produce any bacterial growth. Additional amniotic fluid was frozen at -20°C. A presuckle blood sample, taken 30 min post parturn, revealed a mild leukopaenia and moderate lymphopaenia (Table I). A presuckle blood culture was negative for aerobic and anaerobic bacterial growth. The specific gravity of pre-suckle colostrum measured 1.070 (Equine Colostrometer, Lane Manufacturing, Inc., Denver, CO 80222, USA), indicating a high concentration of immunoglobulins. The filly's serum immunoglobulin levels at 13 h of age were 8 g/l, indicating adequate passive transfer of colostral antibodies. Clinical course and treatment The filly remained bright, afebrile and extremely active during the first 3 days post parturn. On the morning of Day 4 the filly was discovered laterally recumbent, unable but struggling to rise, with a laboured respiratory rate of 132 breathshin and a temperature of 103°F (39.4"C). Mucous membranes were gray and injected with a prolonged capillary refill time of 3 sec. Fine moist inspiratory and expiratory crackles were auscultated throughout both lung fields. Marked nostril flaring, expiratory grunting, increased abdominal effort and exaggerated rib retractions indicated severe respiratory compromise. Open mouth breathing ensued and the filly was intubated with

TABLE 1: Blood values in normal horses and In the filly Infected congenitallywith EAV ~

Packed cell volume (litres/litre) White blood cells (x l0Wtre) Segmented neutrophils (x 1OWitre) Non-segmented neutrophils (x 1OVlitre) Lymphocytes (x 1OWitre) Monocytes (x 1OVIitre)

~~

Day 1

Normal values

Day 4

Normal values

42 4.60 4.14 0.046 0.368 0.046

43' 9.5 f 2.44' 7.94 2.22' 0.138k0.198t 1.34 f 0.6' 0.19 f 0.2'

37 6.80 5.37 0.068 1.36 0.0

35' 9.86 f 1.79' 7.45 f 1.55' 0.029 f 0.037t 2.10 f 0.63' 0.27f 0.11'

*Harvey, Asquith and McNulty (1984). tSchalrn and Carlson (1982)

*

~

EQUINE VETERINARY JOURNAL

IS6

a 10 mm diameter nasotracheal tube and started on assisted positive pressure ventilation with an oxygen demand value (Hudson Demand Valve 11; Model 5040 - S.S. Osbom, Media, PA 19063, USA). Despite the delivery of 40 psi inspiratory pressure, chest excursions remained abnormally shallow, suggesting severe reduction in lung compliance. Low doses of diazepam (0.15 mg/kg iv) were administered as needed for sedation. Aggressive intravenous fluid therapy was initiated. Blood analyses revealed a normal leucogram (Table I ) and plasma fibrinogen concentration (2.12 gA), with an elevated packed cell volume (0.4 1/1) compatible with dehydration. Plasma creatinine and electrolyte concentrations were unremarkable. Arterial blood gas analysis, following 45 min of positive pressure ventilation, confirmed hypercapnia. relative hypoxaemia and mixed respiratory/metabolic acidosis (Table 2). The primary differential for the filly's respiratory distress, fever, and abnormal lung sounds was acute pneumonia due to bacterial and/or viral infection. Intravenous antibiotics (amikacin, 6.6 mgkg bwt every 8 h, potassium penicillin G, 40,000 Ukg bwt every 6 h) were administered. Flunixin meglumine (1 .O m a g bwt) was given to reduce fever and inflammation and ameliorate the cardiovascular effects of possible endotoxaemia. Fluid therapy included I .3% sodium bicarbonate (200 m l k ) and 0.45% sodium chloride-5% dextrose (200 m l k ) with potassium chloride supplementation (20 mequivn). Pulmonary oedema was suspected when orangecoloured froth appeared in the nasotracheal tube lumen during suctioning. Furosemide ( I .5 m a g bwt) and dexamethasone (0.I mgkg bwt) were administered intravenously. Controlled ventilation was initiated using 100% inspired oxygen and a high frequency, positive pressure ventilator (Servolator Percussionator-Percussionator Corp, Sandpoint, ID 83864, USA). Ventilator settings were adjusted to deliver an oscillatory frequency of 400 cycles/min, a mean airway pressure of 50 cm water and a respiratory rate of 36 breathdmin. Chest excursions remained poor and the filly continued to breathe against the ventilator, resulting in sudden increases in peak airway pressures and reduced tidal volumes. Attempts to increase respiratory rate, without delivering excessive airways pressures. resulted in diminished chest excursions. Improved ventilation was obtained using a mechanical pressure cycled respirator (Bird Mark 9 Servo respirator - Bird Corp, Palm Springs, CA 92262, USA). Serial arterial blood gas analysis (Table 2) revealed persistent hypoxaemia, hypercapnia, and exacerbation of the pre-existing respiratory acidosis. A slow infusion of phenobarbital ( I 5 mg/kg) was administered to sedate the filly and permit continued assisted ventilation. Intravenous dimethyl sulphoxide ( 1 mgkg) was given as a 20%

solution to reduce inflammation. Despite aggressive ventilatory support the filly's condition deteriorated. The foal was killed on the evening of Day 4 and a complete necropsy performed.

Necropsy findings The most severe pathology was present in the respiratory and alimentary systems. The lungs did not collapse when the thorax was opened and the surface of the lungs revealed rib impressions (Fig I). There was a moderate excess of clear serosanguinous fluid in both the pleural and pericardial cavities. All lung lobes were diffusely congested, dark red, and oedematous (Fig 2). When placed in formalin, lung tissue samples sank to the bottom of the jar. The lesions in the alimentary tract were confined to the small intestines (predominantly jejunum), where multifocal serosal and mucosal petechiae and ecchymoses were seen. The wall of affected intestine was oedematous with dark red-black fluid within the lumen. Microscopic examination revealed lesions in the lung, intestines and lymphoid organs. In the lungs, there was diffuse capillary congestion, marked interlobular oedema, moderate mononuclear cell infiltration, and randomly distributed areas of acute haemorrhage in the parenchyma (Fig 3). The air spaces were filled with pink proteinaceous fluid, many foamy macrophages and lesser numbers of neutrophils. In many areas the proteinaceous fluid was coating the alveolar walls. On special stained preparations (periodic acid-Shiff, PAS), the eosinophilic layer over the alveolar surface was PASpositive. indicating the presence of hyaline membrane (Fig 3). The jejunum revealed locally extensive areas of mucosal haemorrhage and necrosis (Fig 4). There was cellular debris over the epithelial surface and numerous microthrombi in the Raminae propria (Fig 4). There was marked, diffuse oedema and multifocal haemorrhages in submucosal and subserosal areas (Fig 5). At both of these sites, capillary and arterial walls displayed areas of segmental fibrinoid necrosis (Fig 6). The thymus had multifocal acute haemorrhages and moderate lymphoid cell depletion. The latter change was also seen in the spleen and mesenteric lymph nodes.

Virus isolation and serology Specimens of neonatal lung tissue were positive for EAV in cell culture and negative for equine adenovirus using a fluorescent antibody screen. Pre-suckle foal serum was weakly positive for neutralising antibody against EAV at a titre of 1.4. Arteritis virus

TABLE 2: Respiratory and blood characteristics of the filly infected congenitally with EAV

Hospital Day: Time (h):

4 08:30

09:45

4 14:40

4 22:20

PH PO2 (mmHg) PC02 (mmHg) HCO3 (mequiv/l) BE (mequiv 1/1)

7.156 83.1 56.2 18.7 -9.7

7.081 43.8 75.3 22.1 -7.6

7.323 57.1 53.8 27.6 +1.6

7.192 34.3 60.5 22.8 -5.0

Ventilatory management

Demand

HFPPV

Mark IX

Mark IX

F102

1.o 52 (40 psi)

1.o 36 50 cm H 2 0

1 .o

1 .O

44 32mHg

48 30mmHg

RR (breaths/min)

PAP

4

value

BE = base excess; Flop = fraction of inspired 0 2 ; RR = respiratory rate; PAP = peak airway pressure: HFPPV = high frequency

Fig I ; N ( V t - ~ ' O / / U / J . w/ltrr,qs d of rhc irifec'rcvl filly Mmirh ,YI~&P

positive pressure ventilation

of the ribs

inlpt~r,~,~io,r,s

EQUINE VETERINARY JOURNAL

I57

Fig 5 : P hornmicrograph of jejunum. There is extensive diffuse oedema and multifi~calhaemorrhanes in the suhmucosa. H & E stain. X36

Fig 3: Phoroniii~rographnfthe lung nVth diffuse interlohulur oedemo and

c,upillary and oedema and haemorrhage in the surrounding urea. H & E stuin, X36O

Discussion

Fig 4 : Phoromicrograph qf mucosal area of jejunitni showing multifocul haeniorrhages. necrotic, cellulur debris and manv mic,rothrnnihi (arrous). H & E stain. XYO.

was isolated from amniotic fluid collected at parturition. Maternal serum samples collected at Days 15 and 28 post partum were positive for EAV-neutralising antibodies at dilutions of 1:256 and 21:8, respectively. Culture of the mare's nasopharyngeal and uterine swabs obtained on Day 15 post partum were negative for EAV.

Relatively few EVA epizootics have been reported in the veterinary literature (Burki 1965; McCollum and Bryans 1973; McCollum and Swerczek 1978: Golnik and Michalak 1979; Timoney 1985; Traub-Dargatz, Ralston, Bennett and Collins 1985). Serological surveys indicate that EAV has a world-wide distribution (Matumoti, Shimizu and Ishizaki 1965; McCollum and Bryans 1973; Moraillon and Moraillon 1978; Nosetto et a/ 1984), although there is considerable disparity in the prevalence of EAV infection among different horse breeds. In the USA, the serological incidence of infection amongst Standardbreds is between 70 and 90% while