Bluetongue virus-infected adult midges surviving winter may have been responsible for the re-emergence of bluetongue this year in northern Europe.

In a paper just published in the Veterinary Record*, Anthony Wilson and colleagues of the Institute for Animal Health's (IAH) Pirbright Laboratory have presented evidence to support the view that the reappearance of bluetongue in northern Europe this year could have been
caused by adult midges surviving from the previous year.

The question as to whether this second season of bluetongue was due to re-introduction of the BTV-8, survival of the virus in some animals that had been infected in 2006 or in larval forms of the midge, or in adult midges that survived the winter, is not known, and is addressed in this
Veterinary Record paper.

Anthony Wilson said: "The ability of bluetongue virus to re-emerge in temperate areas after absences of several months has been a source of discussion for years. Other researchers have proposed complex and unusual mechanisms. Our work suggests that simpler mechanisms may also have a major role to play."

Below 15 degrees C there is virtually no replication of the virus in a midge. However, at low temperatures the virus remains dormant in the midges and the midges themselves can survive for much longer than at summer temperatures.

Bluetongue virus came to northern Europe for the first time in summer 2006. The last clinical case of that first season of bluetongue was reported in mid-Jan 2007. After a break of several months the disease re-emerged, and the first BTV infection in the UK (with BTV type 8, the
same as in Belgium and surrounding countries) was confirmed by IAH on 22 September this year.

Taking into account air temperatures at the centre of the 2006 outbreak, and the growth of the virus at various temperatures, Wilson and colleagues have calculated that mid-April 2007 was the earliest time by when the virus would have grown to sufficient levels for the overwintering midges to be able to successfully infect animals when taking a blood meal. This agrees with the estimate made in Germany based on blood tests detecting antibody to the virus in the first cow observed to have developed disease.

The findings of Wilson and colleagues suggest that overwintering of the disease could simply be a combination of the long term survival of adult midges at low temperatures, retention of the virus in the midges, with short periods of virus growth during periods when the temperature
reached 15 degrees C. Once temperatures significantly exceeded 15 degrees C, which they
did in early to mid April, the virus more rapidly grew to the point at which the midges became fully infectious; the second season of bluetongue had begun.

It is quite possible that this phenomenon will be repeated in 2008, unless steps are taken to prevent it. Vaccination is considered to be the best defence against bluetongue. Vaccines against BTV-8 are expected to become available during early 2008. The progression of the disease on
the continent during 2007, combined with the analysis of Wilson and colleagues, suggests that for vaccination to be most successful it wouldneed to be undertaken by Spring 2008.