Q I keep hearing about bluetongue. Why? What is it?
A Bluetongue is an animal virus, spread by tiny flies called midges - also called biting gnats, or no-see-ums. Scientists have been predicting for ages that insect-borne diseases would move north as global warming takes hold. They have predicted since at least 2002 that bluetongue could invade northern Europe and Britain. Those predictions have now come true.
Q Where did it come from?
A Bluetongue infects ruminants - cattle, sheep, goats, deer, and the like - throughout Africa, southern Asia, and parts of Latin America. It does not infect humans. In most places that have it, livestock have evolved resistance to the virus, though it still takes a toll.
Sheep get much sicker with bluetongue than cattle; they develop fever, difficulty with breathing and swallowing, and swollen feet and faces. Sheep's tongues can turn blue if the swelling restricts circulation to the mouth, but despite the disease's name, this rarely happens. In non-resistant sheep, up to 80% of those infected can die.
The virus is present in northern Australia, but causes no disease, partly because it has not spread to sheep country, which is too dry for midges. Bluetongue also occurs in the southern US, but kills less than 5% of sheep, which have high levels of resistance. The US and other wealthier infected countries such as South Africa also use vaccines made of live, weakened virus.
Q How did it get to Europe?
A Firstly, the midge species that normally carries bluetongue in Africa and the Middle East has moved north into Europe, taking the virus with it. In 1998 just the southern tips of Spain, Italy and Portugal, and a few Greek islands, had bluetongue. Now the midge has carried five of the 24 known strains of the virus through much of Spain, Italy and Portugal, southern France and Greece, and across the Balkans.
Scientists warned that the disease might continue spreading past the most northerly point reached by the invading southern midge, because some midge species in northern Europe can also 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 