Understanding Schmallenberg virus will help experts prepare

Schmallenberg virus has continued to spread across Europe since I last wrote about it two years ago.

Cases have been confirmed in 27 countries, including the United Kingdom and Turkey. More than 5,000 farms have been affected.

The infectious agent was named for the German town where the virus was first discovered.

Schmallenberg virus infects ruminants, including cattle, sheep and goats. It has also been detected in deer, dogs, wild boar, alpacas and 19 zoo species. There have been no human infections.

It causes mild, non-fatal illness in adult sheep and cattle. Clinical signs include fever and loss of appetite and weight, which lasts for days to weeks. Milk production can be substantially reduced in affected dairy cattle.

More serious disease occurs in ewes, nannies and cows that are infected for the first time while pregnant. Outcomes include severe birth defects, abortions and stillbirths.

New studies have found that only a small proportion of infected pregnant cows have these severe outcomes. Most infected cows deliver healthy calves.

Researchers have found Schmallenberg virus in biting midges from several European countries, which supports the theory that these tiny insects play an important role in virus spread.

Transmission continued last winter but it is unknown whether midges were present within barns to spread the virus or if other insects were capable of transmission.

Schmallenberg virus is not spread by contact. Infected dams can pass the virus to offspring, but this is not a major infection route.

The virus is present in blood, milk and semen from affected animals.

Antibodies in infected cattle can last at least two years after infection. Recovered animals are resistant to re-infection for at least a year.

Several tests have been developed over the last two years that use blood or tissues.

Vaccines have been developed but their widespread efficacy, use and cost-benefit have yet to be established.

The source of the virus is still unknown. Researchers found no trace of it in cattle and sheep blood from the years immediately before the first cases occurred, which suggests it was either circulating at low levels or came from elsewhere.

The European Food Safety Authority says the most significant economic impact is from trade restrictions imposed by countries where the disease is not endemic.

The impact at the herd level seems to vary.

One study of dairy cows found only small differences in milk production and slightly reduced fertility during the season when Schmallenberg virus emerged compared to the previous year.

In the United Kingdom, some farms experienced greater than 40 percent lamb mortality.

The disease also took its toll on producers, with one in four farmers reporting a high impact on emotional well-being.

An explosion of research has occurred since 2012, with more than 100 scientific papers published.

One of the studies, which tested blood from cattle, sheep and goats in Mozambique, found that 100 percent of cattle and more than 40 percent of sheep and goats had antibodies against Schmallenberg virus.

It’s possible that the test used in the study could have cross-reacted with other viruses. Researchers need to do further testing to confirm that Schmallenberg is indeed present in Africa.

European scientists, laboratories and research institutions collaborated in response to this outbreak with remarkable speed and openness, which bodes well for future disease emergence scenarios.

The Canadian Food Inspection Agency has modified importation requirements to prevent Schmallenberg virus from entering Canada.

Animals from which sperm or embryos are collected must test negative for the virus before entry into Canada.

The virus hasn’t jumped the ocean to North America yet, but understanding emerging disease issues across the world is important, given the speed and volume of global travel and trade.

Dr. Jamie Rothenburger is a veterinary pathology resident at the Western College of Veterinary Medicine, University of Saskatchewan. Twitter: @DrJamieR_Vet


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