BioEd Online

Some of the West Nile virus (WNV) circulating in Mexico may be different from that in the United States, a study has found¹. The discovery suggests that WNV's spread in Central and South America and its public-health impact may be more complicated than in North America.

WNV is a bird disease carried by mosquitoes. Animals and humans can also become sick if bitten. Around 20% of people infected develop a severe fever; in a few cases, mainly among the elderly, the brain becomes infected, causing potentially lethal swelling.

This summer, Mexico declared a state of emergency after blood tests in horses suggested that the virulent form of WNV had crossed the US border, possibly in October 2002. A few animals in the southern Mexican state of the Yucatán had also been exposed to the virus.

Now WNV has turned up in five Mexican states, three in the south, report Scott Weaver, of the University of Texas Medical Branch in Galveston, and colleagues. Antibodies were present in horses as early as July 2002, they estimate. "West Nile seems to be circulating widely in Mexico," says Weaver.

No human cases of WNV have yet been reported in Mexico or further south. "It will be very interesting to see what the clinical situation turns out to be," says Barry Beaty, an expert in insect-borne viruses at Colorado State University in Fort Collins.

Route mapping

Crucially, Weaver's team found live virus in a dead crow from the southern state of Tabasco. Genetic sleuthing suggests the strain is descended from the US virus, but may have been evolving elsewhere for several years. "It may have entered Mexico via a different route," says Weaver.

Top of the list is the Caribbean, where WNV surveys are just beginning. How the US virus could have got there without first going to Mexico is a mystery. Experts expected WNV to march through Central and South America stepwise, just as it did in the United States.

"We just don't know where this virus comes from," says Beaty. He has evidence that the US strain is responsible for WNV in Mexican border states at least.

This patchwork situation has probably arisen because animals and people in the tropics are continually exposed to relatives of WNV, such as Venezuelan equine encephalitis and dengue fever.

Experience of these viruses could protect people in the tropical Americas from WNV, or the virus may be present at such low levels that no one has yet contracted the disease.

Conversely, immunity to dengue-like viruses may exacerbate the symptoms of a secondary infection such as WNV, experts warn.

Surveillance for the symptoms of WNV infection, and for the virus itself, is being stepped up in Mexico and the Caribbean. "We should have a clearer picture of the human disease within a year or so," says Weaver.