Blow to blood suckers

Global warming may slowly reduce the incidence of tick-borne disease in Europe over the next 50 years.

Tick-borne encephalitis, a disease of the central nervous system, is transmitted by tick bites or by drinking milk from infected animals. The infection has recently been on the increase in Central Europe as the economic situation forces people into more subsistence lifestyles.

However, new findings that will be published in Proceedings of the Royal Society next week, suggest this worrying trend may be reversed as the climate changes.

Sarah Randolph and David Rogers at the University of Oxford found the range of the disease is likely to contract markedly as hotter, drier summers disrupt the specialised feeding behaviour of ticks that allows the disease to be transmitted.

"Ticks can feed only three times during their whole life, once at each of their three stages of development after hatching - larva, nymph and adult. This means there are relatively few occasions they can feed on an infected animal," says Professor Rogers.

To make things more difficult, the disease is very transient in its mammal host, lasting just four to five days. The virus therefore gets transmitted by the process of co-feeding transmission.

"While one tick at one stage of its development takes its blood meal, it can transmit the disease to other ticks, at other stages of development, that are feeding on the same animal at the same time," Professor Rogers says. Ticks will feed on an animal for up to ten days, so there is ample time for the transfer of the virus.

However, successful co-feeding transmission requires the different stages of ticks to be active at the same time. This occurs only under very particular weather patterns.

"The ticks have a life cycle that takes more than one year to complete. Development and, therefore, further feeding, can be delayed from the end of one year - when life stages would not be synchronised - to the next year - when they are. Rapid chilling in autumn sends ticks into early winter hibernation and is typical of affected areas in Europe. But these conditions are likely to occur less and less in future," Professor Rogers says.

Ticks are second only to mosquitoes in their medical importance as carriers of disease on a worldwide scale. The effects of climatic change on cerebral malaria, which still affects more than 2.5 billion people, are shortly to be published by the same team in the journal Science.

Emma Cunningham