Belgian scientists have found out how the infamous Schmallenberg virus is transmitted from animal to animal. The culprits are biting midges, the same that transmit bluetongue in Europe. This was proven in a joint effort by researchers from the Antwerp institute of Tropical Medicine (ITG) and the Belgian Veterinary and Agrochemical Research Centre (VAR).
The Schmallenberg virus showed up for the first time in November last year, in the idyllic German ski resort Schmallenberg. It causes grave congenital malformations and stillbirths in cattle, sheep and goats. Shortly thereafter, the new disease emerged in the Netherlands. In the mean time it has reached Belgium, Luxemburg, France, the UK, Italy and recently Spain. In Germany more than eight hundred livestock farms are hit.
Midges are really small. There is a reason why they are popularly called no-see-ums. Here one is compared to a common mosquito. © ITG
Nobody knew how the virus was transmitted. But it was known that a related virus, Shamonda, was transmitted by midges. It emerged in the sixties in Nigeria, disappeared for thirty years, reappeared in the nineties in Japan to disappear again. The bluetongue virus, also hitting ruminants, is transmitted by midges as well.
So midges, Culicoides in scientific Latin, were logical suspects. Midges are minuscule insects, living in damp areas, where they can be a real nuisance to humans, attacking and biting in large swarms. There are many species of midges, and it takes a trained specialist to discriminate them under a microscope. That’s why the ITG scientists, who monitor for the Belgian authorities the distribution of midges as vectors of bluetongue, have developed a ‘microarray’, a molecular technology that can be used by non-specialists to simply and accurately recognize midge species.
To map the distribution of midges, the scientists trapped them at several locations. This collection they now revisited to find out if it also contained the Schmallenberg virus. But it is not sufficient to demonstrate that a midge contains the virus. The virus might be accidentally present in ingested blood, without the midge being able to transmit it at the next bite. To achieve transmission, the virus has to leave the gut, travel to the salivary glands and wriggle its way into them.
Therefore the ITG scientists determined the species of each midge, decapitated them and gave only the heads to their colleagues at VAR, who had the molecular analytic technology to detect the virus. If they detected something, it should have come from the salivary glands.
In a few midges they had caught in September and October (so even before the disease emerged in Schmallenberg) they indeed found the Schmallenberg virus. The virus was demonstrated in Culicoides obsoletus, C. dewulfi and C. pulicaris, three of the five species that have been shown to transmit bluetongue and that are common in Belgium.
The scientists now work through the rest of their midge archive, in order to publish a well-documented article in a scientific journal. Of course they want to examine the new catch, but their commission to monitor midges ends in April, because in the meantime Belgium is bluetongue-free. They hope for an extension.
Institute of Tropical Medicine, Antwerp