By Michael Kahn

LONDON, July 4 (Reuters) - An increase in Britons travelling to malaria-infested countries has steadily increased the number of imported cases of the disease over the past 20 years, researchers said on Friday.

These imported cases stem mainly from people travelling to West Africa who often fail to take malaria pills or other preventative measures during their visit, they reported in the British Medical Journal.

"As severe acute respiratory syndrome (SARS) showed, 21st century threats to global public health and travel are inextricably interlinked, and they present ready opportunities for the rapid spread of infectious disease," World Health Organisation Researcher Jane Zuckerman wrote in a commentary.

Malaria infects between 300 million and 500 million people each year, mainly in Africa. The disease kills about 1 million people each year, including a child every 30 seconds.

Malaria is difficult to fight because its complex life cycle allows the parasite to evade drugs. The tiny parasites live and reproduce inside mosquitoes, which spread them when they bite.

The disease has become resistant to some drugs and work on a vaccine has been slow. One effective treatment is artesimin-based therapies like Novartis AG's <NOVN.VX> Coartem.

The British study found the number of reported cases of Plasmodium falciparum -- the deadliest malaria parasite -- jumped from 5,120 between 1987 to 1991 to 6,753 in 2000 to 2006.

Travellers to Nigeria and Ghana accounted for about half of all the imported falciparum cases, Adrian Smith and colleagues from Britain's Health Protection Agency reported.

More importantly only 42 percent of all British residents travelling abroad reported taking pills during their trip, added Peter Chiodini, who worked on the study.

The team analysed the nearly 40,000 reported malaria cases and found that while the overall number each year has remained steady those related to the deadliest form now account for about 64 percent of the total, Chiodini said.

The findings also highlight the need for health officials to relate the dangers of malaria to at risk ethnic minority groups and to consider making anti-malarial drugs available through the National Health Service, he added.

"The overall numbers of malaria cases have remained about the same but the proportion of the dangerous sort has gone up significantly," he said in a telephone interview. "That is the real concern." (Editing by Matthew Jones)

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