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?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
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
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?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
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 would
need to be undertaken by Spring 2008.
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