Climate change fears for virus outbreaks in livestock

Science Daily: Global warming could have chilling consequences for European livestock, warned Professor Peter Mertens from the Institute for Animal Health, at the meeting of the Society for General Microbiology in Harrogate on March 30.

Since 1998, rising temperatures have led to outbreaks of bluetongue (BT) across most of Europe, which have killed over 2 million ruminants (mainly sheep). The outbreak (the largest on record) caused by Bluetongue virus serotype 8 (BTV-8), which started in the Netherlands and Belgium during 2006, has since spread to most European countries, including the UK in August and September 2007. This outbreak, the first ever recorded in northern Europe, was not an isolated event. There are also fears that related viruses, such as African horse sickness virus, which can have a fatality rate of more than 95% and shares the same insect vectors as bluetongue, could also be introduced.

Bluetongue is spread by the biting midge, Culicoides imicola, which has recently colonised the northern Mediterranean coast, leading to outbreaks in affected regions. However, BT outbreaks have also been spread by other novel vector species of midge (C.pulicaris and C obsoletus groups), which are abundant across the whole of central and northern Europe. In experiments, a single bite from a fully infected midge can transmit the virus and as midges are blown across Europe "like aerial plankton" it is almost impossible to prevent them getting to the United Kingdom.

Warmer weather increases the rate of infection and virus replication in the midge itself, and increases their activity in more northern areas. Indeed, the 2006 outbreak started in the Netherlands when temperatures were six degrees higher than previously recorded. Mild winters may also play a significant part in the problem, as the midges that are not killed by the cold (in the absence of frosts) may survive in sufficient numbers to maintain a reservoir of the disease. It is clear that BTV-8 can also be transmitted directly between cattle, providing an overwintering mechanism for the virus to survive from one midge season to the next.

Negatively stained bluetongue virus–like particle that caused a cytopathic effect in BHK-21 cells. Scale bar = 50 nm