Washington

Public-health authorities in North America are gearing up for the return and probable spread of West Nile virus, which last year claimed the lives of more than 200 people.

But despite a national effort involving several federal agencies, including a $40-million investment in research and surveillance by the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, officials still have no reliable estimate of the likely scale of this year's outbreak.

West Nile virus is carried by birds and is mainly spread to humans by mosquitoes. It is a member of the flavivirus family, which includes yellow fever and St Louis encephalitis, but in humans it is not usually life-threatening, although in about 1% of cases it causes severe meningitis and encephalitis.

Since it was detected in New York state in 1999, the virus has surged westwards. Last year it infected at least 4,000 people in the United States, killing 284, and public-health officials are predicting that the virus will spread to cover the whole country this year.

But virologists also expect both humans and animal host populations in the United States to begin developing immunity to the virus, as has happened in other parts of the world where the virus is already endemic. However, there is so far no reliable estimate of how long this immunity will take to build up, leaving researchers unsure of the likely death toll for this year.

“We should be prepared for an outbreak just as large, if not larger than last year's,” says Lyle Petersen, of the CDC's division for vector-borne diseases. “There is nothing to prevent it from happening again.”

US researchers have identified 36 species of mosquito that can carry and transmit the virus to humans, although three species of Culex mosquito seem to be responsible for most US cases.

Last year, 21 people were infected through blood transfusions, and four became infected after receiving organ transplants from infected donors. Two companies are developing screening tests in an effort to prevent further human-to-human transmission of the virus. The tests are expected to be ready by 1 July.

But experts are criticizing the US response to the virus, describing current efforts as patchy. Because of funding cuts, some state health departments have dispensed with bird surveillance programmes, which track and identify the incidence of the virus in birds, on the premise that the virus has already become endemic.

In California, where the virus has yet to arrive and where the relatively high number of outdoor workers, such as farm labourers, could heighten health risks, officials are preparing for the worst. “We fully expect that West Nile virus will be introduced and become established this summer,” says Vicki Kramer, chief of the vector-borne disease section of the state's health department. California has established a network of 200 chicken flocks which will be monitored by health officials for the arrival of the virus.

University researchers and biotechnology companies, meanwhile, are trying to develop a vaccine. Drug firm Acambis in Cambridge, Massachusetts, for example, this summer plans to begin human safety trials of a live vaccine, based on an established vaccine against yellow fever. A DNA vaccine — derived from the virus's RNA — and one derived from its protein coat are also being developed.

Other researchers are looking at how to improve control of the mosquitoes. Researchers at the US Agricultural Research Service in Gainesville, Florida, have identified a baculovirus that selectively infects and kills the Culex mosquito. But the team has yet to find a commercial partner to develop the idea into an insecticide that could be approved and used.