Signalment:  

One of twin male and female lambs stillborn at term from a white, mixed breed ewe (Ovis aries).Twin stillborn lambs were received from a property in New York state housing 65 adult sheep purchased in November 2011. They began lambing in December 2012 and of the 22 lambs dropped by 14 ewes between December and early January, 12 were stillborn, some of which had deformities of the spine and limbs. Ten were born healthy and one was born deformed but alive. There were no clinical signs reported in the ewes.

Gross Description:  

In one lamb, there was contraction of the elbow, stifle, carpal and tarsal joints (arthrogryposis). There was moderate lateral and dorsal deviation of the thoracic spine (scoliosis and kyphosis, respectively) and the sternum was curved dorsally. In the skull, the right orbit was located more rostroventally compared to the left orbit and there was deviation of the mandible to the right of the maxilla. The calvarium contained a thin membrane of cerebral cortex surrounding abundant clear, lightly red, thin fluid (hydranencephaly). The brainstem and hippocampi were small and the cerebellum absent. In the other lamb, there was arthrogryposis with contraction of the elbow and carpal joints and extension of the stifle and tarsal joints. There was severe scoliosis and kyphosis of the thoracic spine and the sternum was curved ventrally. The brain was fluctuant with marked dilation of the lateral ventricles (hydrocephalus) and cerebellar hypoplasia. 

Histopathologic Description:

Throughout the section, myocytes are small, thin, individualized and rounded. There is an absence of cross-striations and increased cytoplasmic basophilia. Diffusely, there is moderate expansion of the endomysium and perimysium with loosely arranged fibromyxomatous extracellular matrix material and edema. Scattered bundles of less affected muscle fibers composed of smaller myofibers with decreased sarcoplasm and peripheral and occasional central nuclei. Multifocally, there is parenchymal replacement by adipocytes and few small areas of hemorrhage. 

Morphologic Diagnosis:  

Skeletal muscle: Severe, diffuse myofiber hypoplasia and atrophy with fatty replacement. 

Lab Results:  

Virus neutralization testing (NVSL, Iowa) was performed in 7 ewes and 1 live affected lamb: 6 ewes, including the ewe of the fetuses examined, and the affected lamb were positive for Cache Valley virus and all were negative for Schmallenberg virus. Serum neutralization testing for Bovine Virus Diarrhea virus was negative. Polymerase chain reaction (PCR) testing for Bunyavirus and Schmallenberg virus and virus isolation in the two lambs were negative. 

Condition:  

Cache valley virus

Contributor Comment:  

Cache Valley Virus (CVV) belongs to the family Bunyaviridae, genus Orthobunyavirus, serogroup Bunyamwera, and is transmitted by arthropod vectors (arbovirus) to mammalian hosts. CVV is endemic to North America with a wide geographic distribution and was first isolated from Culiseta inornata mosquitoes collected in Cache Valley, Utah, in 1956. Other vectors for CVV include Aedes spp., Anopheles spp. and Aedeomyia spp. mosquitoes and Culicoides spp. biting midges. CVV is known to infect a wide range of mammals, including sheep, cattle, horses, white-tailed deer, cottontail rabbits and, rarely, humans. The lifecycle of CVV is poorly studied and the primary mammalian amplifying host is unknown. A potential primary host is the white-tailed deer, when experimentally infected develops a transient viremia lasting 1-3 days in na+�-�ve animals or less in previously exposed animals.(1) However, experimental infection of cottontail rabbits revealed a similar duration of viremia which was insufficient to infect mosquito vectors.(2) CVV is known to be present in the area of this farm(6) but malformed lambs are rare as local farmers breed at times which coincide with low numbers of flying insects. 

The clinical signs of CVV infection in adults are generally subclinical causing only a transient febrile response. In sheep, CVV is a well-recognized cause of congenital malformations and early fetal loss. Infection of ewes between 27 to 50 days of gestation results in congenital abnormalities including arthrogryposis, torticollis, hydranencephaly, hydrocephalus, porencephaly, microencephaly, cerebral and cerebellar hypoplasia, micromelia, anasarca and oligohydroamnios; mummification, reabsorption and dead embryos without deformities are also seen.(4) Infection at 2836 days gives rise to central nervous system and musculoskeletal defects while infection at 3742 days gives rise to musculoskeletal deformities only. Histological changes in addition to the muscular changes presented in this case include areas of necrosis and loss of paraventricular neuropil in the brain together with a reduction in the number of motor neurons.

CVV can be diagnosed on the basis of suggestive fetal malformations, histopathological changes, the demonstration maternal and neonatal antibodies and the presence of virus in pools of resident mosquitoes and viremic adults. At birth, CVV generally cannot be isolated aborted fetuses. Experimentally in sheep, CVV could be isolated from the allantoic fluid at less than 70 days of gestation but was not recovered from the allantoic fluid of fetuses after 76 days gestation.(4) Virus neutralization and ELISA testing are available for serological testing while PCR and virus isolation are available for virus detection. 

JPC Diagnosis:  

Skeletal muscle and adipose tissue: Myofiber hypoplasia, diffuse, severe, with fatty infiltration.

Conference Comment:  

Histochemical staining with PTAH demonstrates the shrunken, irregular nature of fetal myocytes and highlights the multifocal loss of cross striations. The edema in the submitted tissue sections incited some debate among conference participants; several considered this a normal finding in fetal tissue and speculated that ongoing vasculogenesis may play a role, while others attributed the edema in the perimuscular connective tissue to a diminished intensity of skeletal muscle contraction secondary to myocyte hypoplasia. Since muscle contraction normally helps transport fluid from the interstitium into local lymphatics, skeletal muscle hypoplasia could theoretically contribute to widespread edema. Ultimately, participants concluded that both of these factors likely contributed to the edema.

Plant toxins and teratogenic viruses, including those belonging to the genera Orthobunyavirus, Pestivirus and Orbivirus, are often associated with congenital fetal malformations of the musculoskeletal and central nervous systems in ruminants. The most common cause of arthrogryposis, whether due to teratogenic plants or viruses, is denervation.(7) Ingestion of wild lupins (Lupinus caudatus, sericeus, or formosus) by pregnant cows may result in crooked calf disease, which is characterized by fetal musculoskeletal malformations such as arthrogryposis, torticollis, scoliosis, kyphosis, brachygnathia superior or palatoschisis. Similarly, maternal ingestion of Veratrum californicum can cause arthrogryposis or cyclopia in neonatal ruminants, and Nicotiana spp. (tobacco), jimsonweed and wild black cherry have also been associated with arthrogryposis; however, these plant toxins are not generally linked with hydranencephaly or cerebellar hypoplasia.(7)

Cache Valley, Akbane, Schmallenberg and Aino viruses belong to the genus Orthobunyavirus and are known for causing outbreaks of arthrogryposis, hydranencephaly and occasionally cerebellar hypoplasia in calves and lambs (see WSC 2012-2013, conference 22, case 3, table 1). Cache Valley virus is more common in sheep, while Akbane and Aino viruses primarily affect cattle. Fetal infection of calves and lambs with bovine virus diarrhea virus or border disease virus (pestiviruses) often results in cerebellar hypoplasia, or, less commonly, hydranencephaly, porencephaly, hydrocephalus or ocular abnormalities. Likewise, classical swine fever virus (also a pestivirus) infection can cause mummification, arthrogryposis and cerebellar hypoplasia in piglets. Infection with bluetongue virus (orbivirus) causes hydranencephaly and porencephaly in lambs and occasionally calves; however, arthrogryposis is not a characteristic lesion. Wesselsbron virus (flavivirus) is associated with mummification, hydranencephaly, arthrogryposis, porencephaly and cerebellar hypoplasia in lambs and calves.(3,7)

Virus-induced teratogenic effects are far less common in small domestic animals and laboratory species than in livestock. One exception is feline parvovirus which causes cerebellar hypoplasia in kittens due to selective necrosis of the external granular cell layer. Less commonly, canine parvovirus and Kilham rat virus (also a parvovirus) can also result in cerebellar hypoplasia in neonatal dogs and rats, respectively.(3,5)

References:

1. Blackmore CG, Grimstad PR. Evaluation of the eastern cottontail Sylvilagus floridanus as an amplifying vertebrate host for Cache Valley virus (Bunyaviridae) in Indiana. J Wildl Dis. 2008;44(1):188-192.

2. Blackmore CGM, Grimstad PR. Cache Valley and Potosi viruses (Bunyaviridae) in white-tailed deer (Odocoileus virginianus): experimental infections and antibody prevalence in natural populations. Am J Trop Med Hyg. 1998;59(5):704-709.

3. Maxie MG, Youssef S. Nervous system. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. 5th ed. Vol. 1. St. Louis, MO: Elsevier Limited; 2007:304-322.

4. OIE. 2008. Chapter 2.9.1 - Bunyaviral diseases of animals (excluding Rift Valley fever). Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2012. Accessed: 25/02/2013. URL: http://www.oie.int/fileadmin/Home/eng/Health_standards/tahm/2.09.01_BUNYAVIRAL_DISEASES.pdf

5. Percy DH, Barthold SW. Pathology of Laboratory Rodents and Rabbits. 3rd ed. Ames, IA: Blackwell Publishing; 2007;127-129.

6. Sahu, SP, Pedersen, DD, Ridpath, HD, Ostlund, EN, Schmitt, BJ, Alstad, DA. Serological survey of cattle in the northeastern and north central United States, Virginia, Alaska, and Hawaii for antibodies to Cache Valley and antigenically related viruses (Bunyamwera serogroup viruses). Am J Trop Med Hyg. 2002;67(1):119-122.

7. Thompson KG. Bones and joints. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. Vol. 1. 5th ed. St. Louis, MO: Elsevier Limited; 2007:60-62, 204-206.

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4-4. Stillborn lamb, skeletal muscle

4-5. Stillborn lamb, skeletal muscle

4-6. Stillborn lamb, skeletal muscle

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