The arrival of bluetongue in eastern England in the late summer of last year was not a surprise. There were large outbreaks of the virus among farm animals in Belgium and the Netherlands, close enough to Norfolk, Suffolk and Essex for these counties already to be designated at risk because it was known that the infection could be carried by wind over the sea for hundreds of kilometres. An early warning service for Britain had been in operation from 1 April in anticipation. It used meteorological models originally devised to track the dispersal of radioactivity following a nuclear power station accident. Wind conditions were optimal on the night of 4 August for spread from the Low Countries to England. Beyond doubt this is when the virus arrived, carried by infected midges blown across the North Sea.

Bluetongue infects ruminants, but not humans. It is feared by farmers because it is an efficient killer of sheep and because of the import/export controls and bans on moving animals that governments impose to impede its spread. The virus targets small blood vessels. The damage this causes can shut off the circulation, so in some cases the tongue goes blue not long before death. Bluetongue is not contagious in that its spread depends on an intermediate host, the bloodsucking midge, Culicoides: the virus must grow in the insect, which must then bite a second susceptible animal. The virus grows faster and the midge bites more often when it is warm. Too hot, and the lifespan of the midge is significantly shortened; too cold, and the virus doesn’t grow and the midges stop biting.

Like most infections spread by insect bites, bluetongue has traditionally been seen as a disease of warmer countries. So is climate change aiding its spread? Since 1998 it has swept across the Mediterranean Basin into Southern Europe and the Balkans, affecting many countries for the first time. An Afro-Asiatic midge, Culicoides imicola, has been the most important transmitter by far. It was first detected in Southern Europe in 1982, and the prediction is that as the climate gets hotter, there will be a slow but steady extension of the distribution of the disease caused by the spread of the midge.

Maybe this will come about, eventually. But, as so often with microbes, making predictions is difficult. The virus surprised the experts by appearing, without warning, in the Netherlands. Sheep in Kerkrade, a village in Limburg, were the first to fall ill, in August 2006. Over the next four months the disease spread to Belgium, Germany, France and Luxembourg. It was being transmitted by local, indigenous Culicoides, not C. imicola. And the virus had not jumped from Southern Europe; its closest relative was a bluetongue strain of serotype 8 (a type new to Europe) isolated in West Africa in 1982. Scientists agree that the virus must have come from afar, but nobody knows from where, or how. But we do know that in 2006 the summer in Limburg was the warmest on record, producing for a while the climate of regions where bluetongue has its natural home. That will have helped the virus to establish itself in its new one. So the climate change pessimists were right – but for the wrong reason. It was expected that the winter would see the end of the virus: cold weather would stop its transmission and it would die out. Wrong again. In June 2007 it reappeared in North Rhine-Westphalia and spread with renewed vigour.

Britain’s midge season is long over and virus transmission has stopped. But 75 farms were infected when the midges were busy (and surveillance is still picking up old cases that were missed last year). Will it start again in the summer? No one knows. Bluetongue is new to Britain. It is more than a hundred years since such a serious infection was regularly spread in this country by insect bite. That was typhus fever, transmitted by the human body louse. Its incidence diminished rapidly in the last decades of the 19th century. The last natural homes of the disease in Britain were Scotland’s dirtiest dwellings, the picturesque thick-walled blackhouses inhabited by the cattle and people of the Highlands and Islands, and Glasgow’s sour-smelling single-ends. Its survival was even more temperature-dependent than bluetongue. The louse lays its eggs on clothes close to the body. If they are taken off at night egg mortality increases. People who don’t sleep in their underclothes and who change their garments more than once a month don’t get lousy, and don’t get typhus. Such simple control measures will not work for bluetongue because the midges breed in abundance on farms in dung heaps and piles of compost. If the virus overwinters the best hope for prevention will be the serotype 8 vaccine that will soon be available.

Maybe the British outbreak is over and the virus will have died out before the weather warms and the midges start biting again. If it does, it may join the other imported insect-borne diseases that have failed to establish themselves in the UK. The copper ore-carrying sailing ship Hecla brought Aedes aegypti mosquitoes infected with yellow fever virus from Cuba to Swansea in 1865. It docked on 9 September. The first case in the town fell ill six days later: 22 people were affected; 15 died. There was a heatwave in Wales during the outbreak. The temperature dropped sharply in the autumn and the last case manifested symptoms on 5 October. Aedes aegypti has never been found in Wales since. The climate isn’t warm enough for it to breed. Yellow fever hasn’t reappeared either. Mosquitoes that can transmit malaria live naturally in Britain and every year about two thousand travellers are infected abroad and bring the disease back with them. A handful of cases are classed as ‘cryptic’, because they claim never to have gone to a malarious area or even abroad. But the last time investigators were convinced that the disease had been transmitted by British mosquitoes was when two cases occurred in Stockwell in South London in 1953. The parasite cannot establish itself because even in southern England the temperature is often too cool for it to grow well in the mosquito, of which there are too few anyway.

There is no treatment for bluetongue, and the temperature is getting warmer. But many malariologists make a living by controlling mosquitoes. Perhaps midges could be attacked as successfully. British Culicoides experts are world-class and have been for generations; the review of British species by J.A. Campbell and E.C. Pelham-Clinton published in the Proceedings of the Royal Society of Edinburgh in 1960 is the standard work. (The Society still has a few unsold copies and so was able to respond positively to recent orders from Japan and Denmark). There is much pessimism, however. Insecticide-treated bed-nets work for malaria, but will not work for cows. And swamping farms with DDT went out with Rachel Carson’s Silent Spring. Midges flourish when it is warm and wet. We haven’t (so far) had a hard winter to kill them off; so the best that can be hoped for is a cool spring and summer, and a drought. But whatever the weather, it is certain that Britain will continue to suffer aerial invasion.

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