Schmallenberg Virus in Sheep: Pregnancy Risks and Congenital Defects

Schmallenberg virus, often shortened to SBV, is a viral disease of ruminants including sheep, goats, and cattle. It belongs to the Simbu serogroup of orthobunyaviruses and is spread mainly by biting midges. In adult sheep, illness is often mild or even unnoticed. The more important problem is what can happen when a ewe is infected during a vulnerable stage of pregnancy.

In sheep, SBV is best known for causing fetal infection and congenital defects. A ewe may look normal, yet the developing lamb can be severely affected. Outcomes may include abortion, stillbirth, mummified fetuses, weak newborn lambs, or lambs born with fixed joints, spinal curvature, or neurologic defects. Merck Veterinary Manual notes that Schmallenberg-related abnormalities in sheep and goats most often develop after infection between days 28 and 56 of gestation.

This virus first emerged in Europe in 2011 and has remained a concern there because outbreaks can reappear when enough susceptible animals and insect vectors are present. WOAH describes SBV as a disease found in malformed lambs, kids, and calves, while adult ruminants may have only mild signs such as fever or reduced production. In the U.S., USDA import-health materials have stated that no cases caused by Schmallenberg virus have been detected or reported in New Zealand, reflecting its importance in international animal health screening rather than routine U.S. flock disease. Your vet can help sort out whether SBV is even a realistic concern in your region. (woah.org)

Schmallenberg virus is caused by an orthobunyavirus transmitted primarily by Culicoides biting midges. Sheep become infected when these insects feed during vector season. The virus then circulates briefly in the ewe. Experimental work in sheep found that the period when viral RNA can be detected in blood is short, roughly 3 to 5 days, which helps explain why adult infection can be easy to miss unless testing happens quickly. (pubmed.ncbi.nlm.nih.gov)

The main pregnancy risk comes from transplacental infection. If a ewe is infected during a susceptible window in gestation, the virus can disrupt development of the fetal brain, spinal cord, and musculoskeletal system. Merck Veterinary Manual states that in sheep and goats, most Schmallenberg-related abnormalities develop after infection between days 28 and 56 of gestation. Because sheep have a shorter pregnancy than cattle, the vulnerable period is narrower but still very important for breeding management. (msdvetmanual.com)

Risk is shaped by timing. Flocks bred so that early pregnancy overlaps with heavy midge activity may face greater reproductive losses if SBV is circulating. WOAH technical guidance notes that rescheduling breeding outside vector season may reduce fetal malformations. Local climate, humidity, and warm late summers can also favor midge activity and increase exposure risk. (woah.org)

Diagnosis starts with the history and pattern of losses. Your vet will ask about breeding dates, when ewes were likely in early gestation, whether abortions or malformed lambs appeared after midge season, and whether several animals are affected. Because adult sheep often show little illness, the most useful clues may come from the fetuses or newborn lambs, not the ewe.

Laboratory confirmation usually involves PCR testing and sometimes antibody testing. PCR may be performed on fetal tissues such as brain or spinal cord, placenta, or other submitted samples. Serology can help show prior exposure in dams or pre-colostral newborns. A recent surveillance update from APHA noted that PCR on brain tissue can be negative depending on the stage of gestation when infection occurred, so a negative result does not always rule out SBV. That is one reason your vet may recommend a broader abortion panel and careful sample selection. (gov.uk)

Necropsy findings can be very helpful. Typical lesions may include arthrogryposis, scoliosis, torticollis, and central nervous system malformations. Because several diseases can look similar, your vet may also test for other causes of congenital defects and abortion, especially Cache Valley virus in North America. Cornell notes that Cache Valley fever can also cause abortions and malformed lambs with musculoskeletal and CNS lesions, making differential diagnosis especially important in U.S. flocks. (vet.cornell.edu)

Prevention focuses on reducing exposure during the most vulnerable stage of pregnancy. Because SBV is spread by biting midges, flock planning matters. WOAH technical guidance notes that breeding outside vector season may reduce fetal malformations. In practical terms, your vet can help you look at local insect pressure, weather patterns, and breeding dates so early gestation does not overlap as much with peak midge activity. (woah.org)

Vector control can help, although it is not perfect. Measures may include reducing standing water where feasible, improving airflow in housing, bringing sheep indoors during peak midge activity when practical, and discussing insecticide or repellent strategies with your vet. These steps are usually part of a broader flock plan rather than a guarantee.

Vaccination is complicated. A vaccine has existed in parts of Europe, but current availability has been inconsistent, and some regions have reported that commercial production is not ongoing. That means many flocks rely mainly on surveillance, breeding management, and prompt diagnostic workups instead of vaccination alone. If you are concerned about import risk, regional spread, or whether any vaccine option is relevant where you live, ask your vet and local animal health authorities for current guidance. (gov.scot)

Finally, treat every abortion storm or cluster of malformed lambs as a diagnostic event, not something to guess at. Quick testing helps protect the rest of the flock, supports future breeding decisions, and rules out other important infectious causes. In the U.S., where SBV is not a routine field diagnosis, that step is especially important.