The unholy alliance between viruses and insect vectors like midges has become a signature of emerging diseases around the world. Culicoides, among the most abundant of bloodsucking insects, occur throughout most of the inhabited world. The tiny fly known as the biting midge, or no-see-um because of its diminutive size (one to four mm in length), are far more than just a human and animal irritant. Culicoides transmit a great number of assorted pathogens affecting both wild and domestic animals, and humans. Of prime importance are the 50-some viruses known as arboviruses that midges carry and transmit; a constant source of emerging and re-emerging disease in humans and domestic livestock as the tiny fly is blown around the world in search of a blood meal.
Diseases borne by Culicoides have become central figures in the waves of major disease outbreaks in the Mediterranean Basin and Europe that are having a serious impact on livestock industries. Closer to home, Culicoides play a role in the dynamics of bluetongue virus, and a secondary role in the spread of West Nile virus across North America, are directly linked to epidemics of hemorrhagic disease in deer and will unquestionably be a factor in the introduction of other viruses lurking at our borders.
North American livestock populations are vulnerable on many fronts. Risks increase in the face of environmental change that enhances adaptation of new Culicoides species to North American environs.
Diseases from sub-Saharan Africa and the Middle East to Europe have been associated with the distribution of Culicoides as they ride the wind.
Several Northern European countries reported blue-tongue outbreaks for the first time in August 2006. The disease was rapidly and widely disseminated with outbreaks in France and in the U.K. through 2007 and 2008. The virus involved was shown to be the BTV-serotype 8, a form not previously reported in Europe and which prior to the present epidemic has only occurred in Africa, Central America, Malaysia and India. Probabilities are high that infected midges carried by wind were associated with the European incursions of bluetongue. The same could be said about the introduction of bluetongue in domestic livestock, and epizootic hemorrhagic disease in deer throughout the Okanagan Valley of southern British Columbia from the U.S. in the 1980s.
Midges harvested at altitudes of over two kilometres (km) carry animal pathogens. The temperatures and relative humidity typically found at altitudes of one km are most agreeable to the survival and spread of Culicoides over long distances. Environmental change that triggers the shift in climatic conditions around the world only enhances the chance of midge survival and, ultimately, the international hopscotching of viral pathogens.
And what might the future hold?
We need only look as far as the Schmallenberg virus outbreak that took Northern Europe by surprise; a disease directly linked to emergence of a new virus and its adaptation to Culicoides as a vector in the virus’s march across Europe.
In September 2011, an unidentified disease in cattle was reported in Germany and the Netherlands. Initially, clinical signs included fever, decreased milk production, and diarrhea. In the months that followed, animals that recovered from initial symptoms showed higher rates of abortions and birth defects in calves. The virus then jumped species with the result that the majority of cases now appear in sheep and goats.
Disease caused by the Schmallenberg virus has touched about 3,000 European farms. Among the eight countries known to be infected — Netherlands, Germany, Belgium, France, U.K., Italy, Luxembourg and Spain — the most severely affected countries are Germany and France with about 1,100 infected farms each. The virus breached the English Channel and is now established in the U.K.
Genetic material recovered directly from environmental samples identified a novel virus previously undetected in Europe. The Schmallenberg virus is closely related to similar viruses circulating and widely distributed in Asia, Africa, and Oceania. Transmission occurs predominantly through biting midges. Because of the origin of the first positive samples, the virus was provisionally named Schmallenberg virus (SBV).
Considering the total number of cattle and sheep establishments in affected countries, the scientific regulatory authorities consider SBV a low impact disease. Despite being classified as a minor disease, symptoms like fever and diarrhea in dairy cattle has turned SBV infection into an overblown trade nightmare for the European Union as countries imposed trade restrictions in response to a misunderstood disease that quickly spread well beyond its original infection zone. The lack of information on control and eradication is causing serious concern among trade partners. In the business of livestock export, the real enemy is an unpredictable response by customers to the unknown. Even though OIE says SBV can’t be transmitted through meat and milk, and the likelihood of transmission through embryos and semen is low, bans have included beef, live cattle, goats and sheep and semen. Russia extended the livestock ban to pigs from Eastern Europe effectively quashing a market representing 188 million euros in 2011. A study conducted by the Robert Koch Institute has shown that the new virus hasn’t led to infection in people in contact with large amounts of the virus.
The EU is bracing itself for another round of the disease through the summer and fall of 2012 following the peak of midge activity, uncertain of where it might spread.
— Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).