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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."
Temperature-dependent growth of the virus in midges
The time taken between a midge getting infected with bluetongue virus, after a blood-meal, and sufficient virus being produced in the midge for it to be able to successfully spread the virus to a susceptible animal, is highly dependent on temperature. At a constant 30˚C this can be as little as two days, at 15˚C it takes approximately 16 days, whilst at below 15˚C there is virtually no replication of the virus. 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 in northern Europe
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.
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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.
Overwintering of the virus in adult midges
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˚C. Once temperatures significantly exceeded 15˚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.
Looking ahead to 2008
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 would need to be undertaken by Spring 2008.
* Re-emergence of bluetongue in northern Europe in 2007.
Anthony Wilson, Simon Carpenter, John Gloster and Philip Mellor.
Published in The Veterinary Record, Volume 161, pp. 487-489 (6th October, 2007)
The research was supported by the Biotechnology and Biological Sciences Research Council (BBSRC) and the Department of the
Environment, Food and Rural Affairs (Defra).
For more information visit other pages on the website of the Institute for Animal Health, especially the pages devoted to
the outbreak of bluetongue virus type 8 in northern Europe: Bluetongue,
and in the ‘New to this Site’ area on the Institute’s homepage,
and/or contact Dr Dave Cavanagh at the Institute for Animal Health’s press office:
mobile:07789 941568;
office: 01635 577241.