Unlucky victims of the parasite that survives despite huge odds

You have to be seriously unlucky to catch malaria in Britain today - but it does happen. The 14 cases of airport malaria identified in the country in the last 30 years were each the result of a chain of events occurring against huge odds.

You have to be seriously unlucky to catch malaria in Britain today - but it does happen. The 14 cases of airport malaria identified in the country in the last 30 years were each the result of a chain of events occurring against huge odds.

First, the mosquito responsible had to be carrying the malaria parasite, which is comparatively rare. Only a small proportion of anopheline mosquitoes, the breed that spread the disease, are infected with the parasite. Then the infected mosquito had to find its way onto the aircraft that brought it from Africa or the Far East - and survive the journey.

On arrival, it had to escape from the aircraft, bite a victim and deliver in that single encounter a sufficient dose of parasite to cause malaria.

Normally, being bitten by one malaria-carrying mosquito would not be enough to cause the disease. In countries where malaria is endemic it is repeated bites by malaria-carrying mosquitoes that eventually overwhelms the immune defence system of the victim and causes them to succumb.

Dr Charles Easmon, specialist adviser in travel health at the London School of Hygiene and Tropical Medicine, said: "Airport malaria is incredibly rare. You have to be pretty unlucky to meet a mosquito which has got it and then the mosquito has to deliver a sufficient dose of the parasite. There is, howeveran issue about who is mostat risk."

Some people, such as pregnant women, are more likely to get the disease, which is why they are advised not to travel to malarial areas. "Studies have shown that pregnant women are more prone to be bitten and there is a suggestion they may be more prone to get the worst type of the disease, plasmodium falciparum," Dr Easmon said.

There are fears that malaria could become re-established in Britain, aided by global warming, in the way that West Nile virus, also spread by mosquitoes, has found its way back into New York, apparently carried by ships bringing exotic birds for American collectors. Seven people died last summer and one has died so far this summer from the disease which led the mayor, Rudy Giuliani, to order the closure of Central Park last month and the mass spraying of streets and open spaces with insecticide.

Malaria was widespread in Britain until improved public health measures eliminated it in the early part of the last century. In Rome, it was common until Mussolini drained the Pontine marsh in the Thirties - the clearest example of how a public-health measure succeeded in eliminating the disease.

Global warming could increase the likelihood of the spread of several diseases, including malaria, northwards. Viral encephalitis, a swelling of the brain spread by ticks; Lyme disease, also spread by ticks and causing arthritis and skin rashes; and leishmaniasis, a disease affecting the liver, are all expected to grow as temperatures rise.

Dr Easmon said: "No one understands why malaria disappeared in England. But we do know that it would only take a slight change in global temperatures to alter the mosquito breeding habits and the disease could re-emerge."