Cache Valley Fever in Sheep: Abortions and Congenital Defects

Cache Valley fever is a disease caused by Cache Valley virus, a mosquito-borne orthobunyavirus found across much of the United States. In sheep, the biggest concern is not usually illness in the ewe herself. Instead, the virus can cross the placenta and damage the developing fetus, leading to infertility, early pregnancy loss, abortion, stillbirth, or lambs born with serious congenital defects.

Many adult sheep infected outside of pregnancy have no obvious signs or only a mild fever. That makes this disease easy to miss until lambing season. A flock may first notice a rise in open ewes, abortions, stillborn lambs, or newborns with twisted limbs, curved spines, or severe brain abnormalities.

Timing during pregnancy matters. Infection very early in gestation is more likely to cause embryonic death or abortion. Infection during a narrow early fetal development window can lead to central nervous system and musculoskeletal defects, while infection later in gestation may cause little to no visible fetal damage. Because of that pattern, winter lambing flocks exposed to mosquitoes in late summer or early fall can be at particular risk.

Cache Valley fever is caused by Cache Valley virus, which is spread mainly by mosquitoes. Sheep do not usually pass the disease directly to one another in the way pet parents might think of with a contagious barn outbreak. Instead, the usual pattern is exposure of susceptible ewes to infected mosquitoes during a critical stage of pregnancy.

The ewe's stage of gestation strongly affects what happens next. Very early infection can lead to embryonic death or abortion. Infection during early fetal development can damage the fetal brain, spinal cord, and muscles, causing defects such as arthrogryposis, scoliosis, torticollis, hydrocephalus, hydranencephaly, microcephaly, and muscle hypoplasia. Later infection is less likely to cause visible congenital disease because the fetus may clear the virus.

Risk tends to be higher when naïve breeding animals are exposed during late summer and fall, when mosquito activity is still present. In many regions, that means flocks bred for winter lambing may show the consequences months later. Prior exposure in a flock may reduce risk in some animals, but it does not remove the need for prevention planning with your vet.

Diagnosis usually starts with the flock history. Your vet will look at breeding dates, the timing of mosquito exposure, the number of abortions or malformed lambs, and whether the defects fit the pattern seen with Cache Valley virus. Because adult ewes often have few signs, the fetus, placenta, and pre-colostral samples from affected lambs are often the most useful pieces of the puzzle.

Testing can be tricky because the virus may be gone by the time abortion or birth occurs. For that reason, diagnosis often relies on fetal antibody testing, especially serum neutralization on fetal fluids such as heart blood, pleural fluid, pericardial fluid, or abdominal fluid. Pre-colostral serum from a live newborn can also help. Some diagnostic laboratories may also use PCR, histopathology, and necropsy findings to support the diagnosis.

Your vet will also want to rule out other important causes of abortion and congenital defects in sheep, including campylobacteriosis, chlamydial abortion, listeriosis, toxoplasmosis, border disease, bluetongue, and nutritional or toxic causes. A complete abortion workup often gives the most useful answer for flock decisions, even when Cache Valley fever is strongly suspected.

Prevention focuses on reducing mosquito exposure during the earliest part of pregnancy. Work with your vet to review when your flock is bred and when mosquitoes are most active in your area. In some operations, adjusting breeding so the most vulnerable gestation window does not overlap with peak late-summer or early-fall mosquito activity may lower risk.

Environmental mosquito control can also help. That may include draining standing water where practical, improving manure and water management, using fans or housing strategies that reduce insect pressure, and discussing safe insect-control products or premises treatments with your vet. No single step is perfect, but layered prevention is often more realistic than relying on one measure.

There is no widely used commercial vaccine or specific antiviral treatment for Cache Valley virus in sheep in the United States. Because of that, early recognition matters. If abortions or malformed lambs occur, involve your vet quickly, submit diagnostic samples, and use the results to shape next season's breeding and vector-control plan.