The West Nile virus (WNV) epidemic is growing as it continues its march westwards across the US, public-health experts announced this week.

Next year's situation could be worse still, they warned. The virus is set to reach densely populated California, where mosquitoes are more efficient carriers of the virus.

"We have a potentially very bad problem," says Lyle Petersen, who heads WNV research at the US centres for disease control in Fort Collins, Colorado.

WNV arrived in the US in 1999. It spent its first summer in New York before migrating west and south during subsequent summers, when the mosquitoes that transmit it multiply and birds that carry it migrate.

Since 80% of infections have no symptoms, up to 150,000 people may have been infected. But about 20% of those infected develop a bad fever - and in a few cases, mainly among the elderly, the brain becomes infected, causing potentially lethal swelling.

Last year an outbreak across the southern and midwestern US caused 4,156 confirmed cases and 284 deaths. So far this year, 3,659 cases and 67 deaths have been reported, mainly in the western states of Colorado, South Dakota and Nebraska.

The rise is partly down to doctors' better knowledge of the symptoms. "But we can't rule out an actual increase in cases," says Petersen. This summer the virus has killed more birds than ever before, he points out.

There is good news however. Blood tests in use since June could cut disease spread by transfusions. At least 23 cases of WNV infection were due to donated blood last year, delegates heard at last week's Interscience Conference on Antimicrobial Agents and Chemotherapy in Chicago.