THIS is the year West Nile virus exploded in North America. More than 3200 people fell ill with fever and, for some, encephalitis, and 177 have died. Last year only 66 people fell ill. Even so, the biggest threat may not be to people but to millions of the continent’s birds.
While any new human disease is frightening, media coverage has exaggerated the public health problem. In contrast, few people seem to care about the impending slaughter of millions of birds of many species. Some species may never recover, because of the way the virus spreads.
West Nile virus, which is carried by mosquitoes and lives mainly in birds, arrived in New York in 1999 from Israel. It had never been seen before outside the Old World (Europe, Asia and Africa), but it has now invaded all but six of the mainland states, and five Canadian provinces. Even worse, this is a strain first identified in Israel in 1998 that is far more lethal to humans than any previously known strain of West Nile.
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This year’s outbreak wasn’t entirely unexpected. Two years ago, American ornithologists warned that birds would soon carry West Nile all over the continent (91av, 8 July 2000, p 4). At the time, some virologists called this sensationalist, and sniffed that ornithologists should keep their noses out of the disease business. Now it seems the ornithologists were right.
The good news for the worried well of North America is that the virus rarely causes serious illness. West Nile is not likely to become a major public health problem in temperate North America. Outbreaks in people happen mainly in late summer, after that season’s mosquitoes have spread the virus among birds. But only one person falls ill for every 150 people infected. People are thought to become immune to the virus after infection.
Prospects for a vaccine look good too. The virus is one of the flaviviruses and we already have vaccines for close relatives, such as the yellow fever and Japanese encephalitis viruses. Those vaccines may even offer some protection against West Nile. One company already has a specific West Nile vaccine in animal trials. Moreover, the virus does not seem to be mutating into anything worse. “So far West Nile has been genetically stable in North America,” says Lyle Petersen of the US Centers for Disease Control in Colorado.
Birds have not been nearly so lucky. West Nile has ripped through North America’s wild birds the way smallpox ravaged Native Americans. No one knows how many birds are affected. State authorities are overwhelmed and are now testing no more dead birds than it takes to show whether West Nile is present in any local area. With that, they can start spraying for mosquitoes to protect humans.
But in work soon to be published, Andre Dhondt of Cornell University in Ithaca, New York, estimates that in the worst-affected places, 90 per cent of the crows, the most sensitive species, are gone. The same may be true for crow relatives, such as blue jays. And this year, birds of prey, including species only now recovering from the DDT era, are dying in 10 times their usual numbers in several states.
Most test positive for West Nile. That doesn’t prove the virus killed them, though the timing makes it seem likely. Dhondt and British colleague David Rogers at the University of Oxford want to try a different approach. They plan to pinpoint West Nile hot spots using satellite images to trace moisture, temperature and other factors that favour mosquitoes, and then see if birds are dying in the same places.
Classically, a wildlife virus is supposed to become less lethal after its initial attack – it is not in its interests to kill off all its hosts. But unusually, West Nile infects more than a 100 known species of North American birds. And ominously, at least one – the ubiquitous house sparrow – carries the virus without falling ill. That means the virus can persist where there are sparrows, no matter how many birds of other species it kills off. “It has no reason to evolve into something less harmful,” warns Dhondt. House sparrows are probably immune because they also emigrated to New York in 1851 and, like most Old World birds they have evolved to resist West Nile virus.
In work soon to be published, John Rappole of the Smithsonian Conservation and Research Center in Virginia, one of the ornithologists who first predicted West Nile’s spread across America, concludes the most likely culprit is the sparrow. He says that the virus has gradually radiated outward from New York at a rate consistent with the dispersal patterns of non-migratory birds such as sparrows, and not long-distance migrants as he first thought. Sparrows stay healthy enough to disperse over tens of kilometres while carrying enough virus in their blood to pass it to mosquitoes – which then pass it to susceptible birds, and people.
Endangered native birds, such as the Florida scrub jay, could disappear altogether, as could some endangered species that live only in American zoos, such as the 65 remaining Micronesian kingfishers. “Hawaii is really scared,” says Emi Saito, West Nile coordinator at the National Wildlife Health Center in Wisconsin. It has a history of losing its unique birds to imported plagues.
Meanwhile, threatened species such as the American bald eagle, only just reintroduced to many states, could decline drastically and even disappear in some areas. Ironically, efforts to protect people from West Nile virus may also hit such birds of prey. Local authorities are reportedly using cheap, highly toxic malathion to kill mosquitoes – which poisons birds when it gets into the food chain.
The picture isn’t completely bleak. If European birds evolved resistance to West Nile, perhaps American birds can too. Dhondt wants to see if surviving American crows have genetic changes that suggest selection for West Nile resistance. But the virus won’t go away as long as mosquitoes keep getting it in their blood meals from sparrows and other carriers. And that seems unlikely to change as long as sparrows do not die from the virus – and it kills their enemies and competitors.
It looks as if people will just have to live with West Nile virus. But America’s birds, including the symbolic eagle itself, may be in for a rough ride.
Bye Bye, Blackbird
EUROPE’S birds may soon face their own crisis. In August and September last year, thousands of blackbirds died in Vienna. The culprit turned out to be Usutu virus, a close relative of West Nile until then seen only in Africa. It reached a few Viennese birds in 2000, possibly carried in by migrating swallows. But last year it exploded.
Herbert Weissenbock of the University of Veterinary Medicine in Vienna says blackbirds were dying again this year, this time up to 30 kilometres outside Vienna. In some areas they’ve gone completely. Blue tits and sparrows were also hit. It is exactly the same strain of Usutu as last year, he says, which means that the virus has taken up residence.
Further spread seems likely, although it is unknown which bird species can carry the virus in their blood and spread it to mosquitoes without falling ill themselves. Candidates might be birds, such as swallows, that encounter the virus while wintering in Africa.
But the real unknown is whether these bird-borne viruses pose risks for people. Weissenbock says Europe should start monitoring birds and mosquitoes for this flavivirus family, which contains several human pathogens, such as yellow fever and dengue. Usutu doesn’t make people seriously ill, though they get a fever and rash. But viruses can mutate.
