Cache Valley virus (CVV) is a mosquito‐borne virus belonging to the Bunyamwera serogroup of the Orthobunyavirus genus. The virus was first isolated from Culiseta inornata mosquitoes in Cache Valley, Utah, in 1956. CVV circulates in mosquitoes and mammals throughout North America, Central America and parts of South America. Recently, the Canadian Sheep Federation from the Saskatchewan Sheep Development Board shared a report regarding surveillance on Cache Valley Virus in Saskatchewan. This was driven by the fact that during November and December 2024, sheep producers across Saskatchewan noticed an unusual number of stillbirths, abortions, and lambs born with skeletal deformities. In response, the Disease Investigation Unit at the Western College of Veterinary Medicine (WCVM) launched a project to encourage producers to send affected lambs for examination at Prairie Diagnostic Services (PDS). A total of 29 lambs from 12 different flocks were sent to PDS between December and mid-February. Each lamb underwent a post-mortem by a veterinary pathologist. Tissue samples were sent to the Animal Health Laboratory in Guelph for PCR testing to detect CVV. Some samples were also tested at the Texas Veterinary Medical Diagnostic Laboratory for antibodies. Additionally, most lambs had their tissues examined for other potential causes of abortion.
Out of the 29 lambs, 20 (69%) were suspected to have CVV, affecting 8 out of the 12 flocks (67%). Of the 20 suspected cases, 70% (14/20) were confirmed positive to CVV by PCR testing. It is likely that the virus was no longer present at birth for those lambs with typical CVV symptoms that tested negative. Interestingly, joint stiffness and fusion (arthrogryposis) was found in 100% of confirmed CVV cases but also appeared in 22% of negative cases. Almost all CVV-positive lambs (95%) had curved, bent, or fused spines, including conditions like kyphosis (humpbacked), lordosis (inward spine curve), or scoliosis (sideways spine curve). In addition, 80% had hydrocephalus or hydranencephaly, where parts of the brain were missing and replaced with fluid. Approximately 50% of cases also had a shortened lower jaw. Of note, one of the CVV-positive lambs also tested positive for Q fever (Coxiella).
In the 4 flocks where CVV was not confirmed, additional diagnostics demonstrated Ureaplasma infection, Campylobacter fetus infection, and one flock in which all submitted lambs had goiter.
These results highlight why post-mortems are important—similar symptoms can have different causes, and proper diagnoses help prevent future losses.
Like many mosquito-borne diseases, CVV may follow cycles based on mosquito infection rates and flock immunity levels. Although the duration of immunity is unknown, a similar virus in Europe, Schmallenberg virus, causes the same problems in sheep and goats and tends to reappear every 3-5 years. The good news is that sheep that become infected with CVV will be unlikely to abort in subsequent pregnancies due to CVV. Unfortunately, there is no vaccine for CVV so the only way to reduce risk is to avoid exposing ewes to mosquitoes during the first 48 days of pregnancy. Some strategies might include adjusting the breeding schedule to avoid peak mosquito season or keeping early-pregnancy ewes away from mosquito-heavy areas.