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Watch for Cache Valley virus during lambing

Mosquito-borne virus can cause neurological and muscular abnormalities in lambs.

South Dakota State University (SDSU) extension specialist Jeff Held is encouraging sheep producers in South Dakota and surrounding areas to be aware of the potential for Cache Valley virus (CVV) affecting their lamb crop this lambing season.

"The winter lambing season is well underway, and in addition to dealing with cold temperatures, many flock owners have reported an unusual number of lambs born with skeletal and other developmental deformities," Held said. "We've collaborated with the South Dakota Animal Disease Research & Diagnostic Laboratory and with other animal diagnostic labs in the region. They all have confirmed cases of lamb abnormalities caused by CVV."

CVV is a virus that is transmitted by mosquitoes to sheep. If a ewe is pregnant when infected, the virus can cause dramatic neurological and muscular damage to the lambs she carries. CVV cannot be spread from an infected animal to a human or another animal, explained Larry Holler, professor and SDSU extension veterinarian/pathologist at the South Dakota Animal Disease Research & Diagnostic Laboratory.

"The most dramatic effects of the CVV virus leads to birth defects in lambs, mostly affecting the brain and nervous system. The virus also affects the skeleton and muscle," Holler said. "These defects show up as fused joints, curved or twisted spines, unusually thin and underdeveloped muscles and enlarged skulls."

Holler is working with Held to monitor the CVV outbreak this lambing season. They believe that CVV has affected South Dakota flocks this lambing season, because high populations of mosquitoes remained in the region during the early breeding season (from August into September).

"That's why we'll start to see effects now in the early lambing season," Holler said. "They are a result of mosquitoes that carried the virus biting ewes last summer and fall during critical stages of gestation. CVV has no apparent effects on non-pregnant ewes or other classes of sheep."

Infection of a ewe early in gestation, up to day 28 generally, results in fetal reabsorption, but Held said another equally critical period is between days 28 and 45 of gestation. "Infection at this stage of pregnancy has the highest risk of CVV-caused neonatal developmental abnormalities," he said. "After day 45 of gestation, a CVV infection is not expected to cause abnormalities in lambs."

Held said although CVV is found throughout the U.S., the reported cases affecting sheep in South Dakota and the Upper Midwest region generally are minimal. The 2011 winter lambing season was the most recent with widespread reports of high incidences of CVV-affected lambs in the state.

In flocks with clinical CVV, the percentage of the total lamb crop affected was generally less than 5%. "With CVV infections, sheep flock owners often report a higher incidence of open ewes, decreased lambing rates and subsequently lower overall ewe reproductive efficiency," Held said.

"Lambs born with severe defects are stillborn, yet in other CVV cases, the result is the birth of live lambs that are compromised due to skeletal and nervous system abnormalities," Held said. "They can be drowsy, weak or unsteady, and reports indicate that normal and abnormal lambs are possible in the same litter."

Even with excellent management care practices, the mortality rate is high for lambs born with CVV. Holler added that the virus infects pregnant ewes, and mosquitoes are the sole carrier.

Held said ewes bred later last fall -- in late-September or later, when mosquito activity was lower -- will have a decreased chance of having abnormal lambs. Therefore, lamb crops arriving in mid-February or later are expected to have lower probabilities for CVV-affected newborns.

"We want to remind producers that the virus is not contagious or spread from ewe to ewe, even during the lambing season," said Holler. "Unfortunately, there is no vaccine for CVV. Since the cause is a virus, there are no treatments available, either."

Holler and Held both noted that CVV is endemic -- or constantly present -- in sheep populations in the U.S. Clinical manifestations of the disease tend to occur in cycles, because the sheep population seems to gain some natural immunity after infection. As this immunity wanes over a period of years, the clinical effects become more prevalent.

"Sheep producers suspecting CVV should contact a veterinarian in order to rule out other causes of birth defects, miscarriages or infertility," Holler said. "Diagnosis of CVV is sometimes difficult but can be made in the laboratory by detecting specific antibodies against CVV in the lambs."

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