New York: A flock of 150 dairy ewes were bred in the fall of 2020. Over the course of a 6-week period in January and February of 2021, there were 20 abortions, 5 stillbirths, and 3 small singletons, with no udder development in the ewes. This well-managed dairy herd has not had any past reproductive or production issues, and the normal pregnancy rate across all ages is greater than 98%.

Eight fetuses with placentas were submitted to the Cornell AHDC Pathology Service for necropsy and ancillary abortion panel testing. Overall, gross and histologic findings were unremarkable, however, one had evidence of placentitis. Fetal and placental tissues, as well as fetal fluid were tested for a variety of pathogens. Two fetal fluid sample (heart blood and pleural fluid) were positive for Cache Valley virus antibodies, with titers of 16 and 32. Unfortunately, adequate samples of fetal fluid could not be obtained from the other 6 fetuses. These findings, along with negative test results for other pathogens, indicated that an outbreak of Cache Valley Fever was a likely cause of abortion.

Serum from 13 other ewes on the farm, some of which had aborted and some who had not, were tested for Cache Valley virus antibodies. Eight out of 13 ewes showed titers to the virus, indicating previous exposure concurrent with the clinical picture.

Cache Valley virus is a mosquito-borne bunyavirus that results in devastating reproductive losses in naïve small ruminant herds. When the host is infected, the virus can cross the placenta and result in embryonic loss, or abortion with or without musculoskeletal and central nervous system deformities.1,2  It is important to realize that the virus itself is promptly cleared from both the dam and the fetus, and not present at the time of abortion. Diagnostic approaches must rely entirely on fetal fluid antibody tests, rather than viral detection. Fetal fluid that can be used for fetal serology testing includes heart blood or pericardial, pleural or peritoneal effusion.

References:

1. Rodrigues Hoffmann A, et al. Ovine fetal immune response to Cache Valley virus infection. Journal of Virology, 2013;87:5586-5592.
2. Menzies PI. Abortion in Sheep: Diagnosis and Control. In: Youngquist RA, Threlfall WR, eds. Current Therapy in Large Animal Theriogenology. 2nd ed. St Louis, MO: Saunders Elsevier Inc, 2007:667-680.