JPC SYSTEMIC PATHOLOGY
Signalment (JPC #2469833): Merino lamb
HISTORY: This animal was from a flock where a few abortions and neonatal deaths had occurred. There were no floodwater pools on the farm, but there were increased numbers of mosquitoes when compared to the previous 4-5 years.
HISTOPATHOLOGIC DESCRIPTION: Liver: Diffusely there is massive lytic necrosis of all regions of the hepatic lobule characterized by dissociation of hepatic cord architecture with loss of hepatocytes and replacement by small amounts of cellular and karyorrhectic debris, a moderate number of viable and degenerate neutrophils often in small aggregates, fibrin, and hemorrhage. Remaining hepatocytes are either degenerate, characterized by swollen, pale, vacuolated cytoplasm with faded nuclei (karyolysis) or individualized and shrunken with hypereosinophilic cytoplasm and pyknotic or karyorrhectic nuclei (necrotic). Few degenerate and necrotic hepatocytes contain 5 to 10 um eosinophilic, intranuclear viral inclusion bodies and/or variably sized, typically round eosinophilic intracytoplasmic bodies (cytosegresomes). Expanding periportal connective tissue are low numbers of macrophages, lymphocytes, and plasma cells.
MORPHOLOGIC DIAGNOSIS: Liver: Necrosis, massive, subacute, diffuse, moderate, with hepatocellular eosinophilic intranuclear viral inclusion bodies and eosinophilic intracytoplasmic inclusion bodies, Merino sheep, ovine.
ETIOLOGIC DIAGNOSIS: Flaviviral hepatitis
CAUSE: Wesselsbron virus (flavivirus)
CONDITION: Wesselsbron’s disease
- Wesselsbron’s disease is an acute zoonotic arthropod-borne flavivirus infection of sheep, goats, cattle and birds in southern Africa
- Causes abortion outbreaks and perinatal death in sheep
- High mortality in new-born lambs and kids and subclinical infection in adults
- Enveloped RNA virus transmitted by Aedes mosquitoes
- Outbreaks occur during and after heavy rains and end abruptly with first frost; lower prevalence in dryer regions
- 2-5 day incubation period
- Hepatotropic (newborn lambs) and neurotropic (embryonic and fetal tissue in pregnant ewes)
- Mosquito bite à blood vessel penetration à virus enters monocytes, or deposited in vascularized extracellular matrix and gains access to Langerhans cells and trafficking tissue macrophagesà lymphatic vesselsà lymph nodesà other lymphoid tissues (spleen, liver) à targets cells are hepatocytes and Kupffer cells (monocyte-macrophage system)
- During epizootics, infection via other biting insects may also occur
- Direct transmission between animals has not been reported
TYPICAL CLINICAL FINDINGS:
- Newborn lambs:
- Fever, anorexia, listlessness, weakness, tachypnea
- 18-27% mortality rate in newborns: Death within 72 hours
- Adult sheep, goats, cattle and calves:
- Generally non-fatal; biphasic fever; clinically unapparent infection; jaundice
- Abortion or fetal mummification in ruminant species
TYPICAL GROSS FINDINGS:
- Acute: Widespread petechiae and gastrointestinal hemorrhage in the abomasal mucosa; digested blood in abomasum
- Yellow-orange liver with hepatomegaly, patchy congestion, friability and moderate to severe jaundice
- Congested, edematous lymph nodes; lymphadenomegaly
TYPICAL MICROSCOPIC FINDINGS:
- Random foci of hepatocellular necrosis of individual or small groups of hepatocytes (less extensive than Rift Valley fever)
- Irregular eosinophilic intranuclear viral inclusion bodies occasionally within degenerative hepatocytes; chromatin is not marginated
- Scattered pigmented macrophages and mononuclear inflammation in portal regions and sinusoids
- Hydropic degeneration of less affected hepatocytes
- Kupffer cell proliferation
- Bile duct proliferation
- Canalicular cholestasis
- Spleen and lymph nodes: Lymphocyte necrosis and lymphoblast stimulation
- Virus isolation from a variety of tissues
- Cell culture with immunofluorescence
- ELISA most sensitive and least cross-reactive test
- Hemagglutinin inhibition most specific test, cross reacts with other flaviviruses
- Rift Valley Fever (family Bunyaviridae, genus Phlebovirus): More extensive periacinar to midzonal or massive necrosis and hemorrhage; high morbidity and mortality; also affects adult sheep; canalicular cholestasis is less prominent
- Akabane virus (family Bunyaviridae, genus Arbovirus): Affects mostly cattle in Japan, Australia, Israel and the Middle East
- Humans: Severe flu-like symptoms, fever, headache, blurred vision, myalgia, and arthralgia
- A reported cause of high mortality in a flock of juvenile ostriches
- Subclinical infection in horses, pigs, a camel and a Namaqua gerbil
- Other flaviviruses present in southern Africa but causing little morbidity in livestock: Yellow fever, West Nile, Banzi, Spondweni, Bagaza, Uganda-S, Israel turkey meningoencephalitis, Usutu, Ntaya
- Brown DL, Van Wettere AJ, Cullen JM. Hepatobiliary system and exocrine pancreas. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017: 457-458.
- Systemic and multi-organ diseases. In: Constable PD, Hinchcliff KW, Done SH, Grunberg W, eds. Veterinary Medicine, A Textbook of the Diseases of Cattle, Horses, Sheep, Pigs, and Goats. 11th ed. St. Louis, MO: Elsevier; 2017:2084-2086.
- Uzal FA, Plattner BL, Hostetter JM. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier; 2016:312.
- Zachary JF. Mechanisms of microbial infections. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:206-207.