o carry the virus.
But the virus had a second route into Europe. Bluetongue suddenly appeared in the Netherlands in August 2006, and rapidly spread to Belgium, Germany, France and Luxembourg, carried by local midges. The strain was completely different from the viruses in southern Europe - instead matching viruses from South Africa.
The outbreak was 6° of latitude farther north than bluetongue had ever been seen before, and there were hundreds of kilometres of uninfected territory between it and the affected southern regions. No one knows how it arrived. Midges like to breed in damp manure, and one notion is that manure with infected flies accompanied a horse shipped in by air.
Then the virus did something really new: it survived the winter, and broke out again in August 2007. It invaded eastern England, as scientists had predicted, probably in midges blown across the Channel from Belgium. So far, more than 1.8 million sheep have died across Europe. Unusually, the northern outbreak is also killing cattle. (See a map of the current outbreak.)
Q Why is this happening now?
A The virus's spread from the Mediterranean is a clear case of a tropical insect moving north as Europe warms. The midge that carries bluetongue, Culicoides imicola, likes higher average winter and summer night-time temperatures than Europe used to have. The northward migration of bluetongue has precisely tracked the areas where winter and night-time temperatures have increased.
In northern Europe, warming had quietly created conditions that allowed a stray introduction to persist. Some local midges could always carry bluetongue. But for the virus to persist, many of the insects had to be infected and then live long enough to bite lots of animals. These then developed large amounts of virus in their blood, which were passed on to other midges, and so on.
Warming made this more likely in several ways. Warmer, wetter weather means more midges being more active for more of the year. Warmer virus replicates faster. Then, at warmer temperatures, midge species that normally can't carry bluetongue become able to carry it, while in species that can carry it, more individual midges acquire the ability.
This is because the midge genes that block or limit infection are temperature-sensitive. One effect is that warmer flies grow faster and are smaller, with thinner gut walls that allow virus swallowed with a blood meal to break through into the body fluids, move into the saliva, then infect the next bitten animal.
The real mystery is how the northern outbreak over-wintered. The last case in 2006 was in December; it reappeared in July. The virus doesn't persist in larvae and adults don't live that long.
Either some infected midges did last out, or a cow carried the virus through. One theory is that certain T cells, a type of immune cell in the blood, pick up the virus when they flock to inflamed areas around a midge bite. The virus then becomes quiescent. When the midges bite again, the T cells again move in, and the virus reactivates and escapes, just where it is likely to be picked up by a biting midge. Much of this has been observed in cows.
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