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Midges spread bluetongue upwind as well as down

Wed, 02/15/2012 - 1:23am
Planet Earth Online

You might imagine midges are so small and weak that they're blown about at the mercy of the elements.

Sheep

But an innovative study has found that the tiny insects that spread bluetongue disease during the 2006 European epidemic were almost as likely to fly upwind as downwind in search of animals to bite. The findings may help control future outbreaks more effectively.

An analysis of the relationship between bluetongue infections and wind conditions during the last outbreak shows that 38 per cent of infections came from midges that had moved upwind (using a conservative estimate of midge flight speed), almost equal to the 39 per cent caused by downwind movement. Scientists think midges are lured upwind by the chemicals given off by potential victims, including carbon dioxide and sweat.

'For the first time we can say that midges under their own power travel upwind as well as downwind during this kind of epidemic,' said Dr Luigi Sedda of the University of Oxford's Department of Zoology, who led the research with Professor David Rogers. 'This has very important implications for the control of future epidemics because previous efforts had concentrated only on downwind infection.'

The bluetongue virus infects animals like cows, sheep and deer. It causes drooling and swelling of the head and neck; it severely harms the productivity of dairy and beef farms, and can kill sheep. It can't spread directly from animal to animal, though, but has to be carried by bloodsucking midges.

Another of the study's findings was that the disease usually moves in short hops. The outbreak itself moved across Europe at just 2km a day, and individual midges probably travelled even shorter distances.

54 per cent of cases where one farm passed on infection to another took place over distances of 5km or less, and 92 per cent over no more than 31km. In less than one per cent of cases did the disease spread by more than 50km in a single jump, and there were no jumps of more than 100Km.

'Previous studies tended to focus on long-range, high-altitude journeys over the sea. For example midges are thought to have been carried by the wind from Turkey to the Greek islands, triggering a bluetongue outbreak there,' says Dr Bethan Purse, an ecological modeller and entomologist at the Centre for Ecology & Hydrology, and another of the report's authors, which is published in Proceedings of the Royal Society B. 'Our study looks at what happens once the disease has already arrived on a landmass, and this has implications for the best way to control an outbreak.'

'Early detection is very important, so that midge control measures and vaccination can be used at a very local level,' she continues, arguing that official protection zones, within which surveillance and vaccination are deployed as soon as infection is detected, may need rethinking. A zone of 100km radius could still be advisable if wild animals are involved in spreading disease, but it may be better to concentrate resources in a smaller area to try to crush an outbreak before it becomes serious. 'We cannot ignore long-distance journeys, as they can lead to new disease clusters in previously uninfected areas. But they are very rare, so we may need to reconsider our priorities.'

The study suggests that a surprising proportion of infected farms - around 70 per cent - were dead-ends that didn't cause any further infections. In many cases this was probably because they were infected in late summer or autumn, when midge activity was already falling ahead of winter. These farms still need attention eventually as they could potentially trigger new outbreaks the following spring, but Purse argues that if we can learn how to predict which farms won't pass the infection on, authorities will be able to focus their early efforts on the others.

There hasn't been much detailed research into the flight capabilities of midges, and the authors suggest more is needed to clarify our understanding of how diseases like bluetongue are spread.

The researchers say their method explains 94 per cent of all bluetongue farm infections recorded in 2006. Purse says similar methods of analysis could help improve our understanding of how other diseases spread - and not only those passed on by insects. For instance, it could help disentangle the relationship between movements of livestock and those of the insects that spread a disease between them.

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