"We're monitoring wild birds around the world and organizing global avian-influenza surveillance programs in wild birdsboth in the pet trade and free ranging," he said.
"We're trying to bring together government groups and [nonprofits] working with wild birds to generate what's almost an early warning system."
But in some places migratory animals don't appear to play a role.
"If you look at what's going on in Africa, it looks like almost all of that spread is due to [smuggling] birds," Karesh said.
Nigerian authorities believe that H5N1 entered their country with a shipment of illegally imported day-old chicks and has spread to at least seven states through the trade in domestic birds.
The virus appeared in Nigeria's northern Kano state (map of Nigeria) back in February and has proliferated despite its apparent absence in the area's wild birds.
Smuggling is a serious concern in the developing world, where regulators lack the funding to enforce controls.
"[Many such] governments say that they are going to ban the movement of birds, but they don't really have the funding or support to do that," Karesh explained.
How Might Bird Flu Reach the U.S.?
Still, no country is immune to the illegal bird trade, not even the U.S., experts warn.
"It is a possible risk in the U.S.," said Brett Sandercock, an avian ecologist at Kansas State University in Manhattan.
"There have been outbreaks of Exotic Newcastle disease carried by poultry, and it's thought that those birds came [illegally] from Mexico."
The 2002-03 California outbreak of Exotic Newcastle diseasea fatal and highly contagious viral disease in birdskilled more than 3 million birds and cost poultry producers upwards of 150 million U.S. dollars.
Some birds are smuggled into the U.S. for illegal cockfighting, while others supply specialty markets.
Recently officials at the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) uncovered shipments of frozen, boneless Thai chicken feet in Connecticut and Massachusetts.
Thousands of pounds of the illegal chicken feet had been imported since 2004, labeled as jellyfish in frozen containers holding various other seafood products.
Jim Rogers, APHIS spokesperson in Riverdale, Maryland, says that, while USDA personnel no longer patrol the nation's borders, they do run a compliance team that looks to uncover illegal trade in potentially harmful agricultural products.
"They go to places where they'd be likely to find prohibited productsethnic markets and restaurants. That's one of the ways we look for smuggling," he explained.
"We'll trace it back along the supply chain. For us it's more important to shut down the supply line than to just find the person that received it at the end."
Wild Birds May Increase Exposure
Still, some experts say such measures, along with tighter border security by the Department of Homeland Security, may not be enough to stop birds from getting into the country illegally.
The prospect concerns APHIS head Ron Dehaven.
Dehaven told the Associated Press late last month of his worries regarding "millions of international passengers coming into the United Statesany one of which could be bringing poultry or poultry products that could be infected."
James Gilardi, an ecologist and director of the World Parrot Trust in Davis, California, agrees.
"When people bring the birds onto passenger airlines, that's contained air space," he said.
"For many hours those animals are breathing the same air as the passengers and you have very, very direct exposure. The risk is great."
Yet Gilardi is even more worried about the legal trade in wild birds, such as parrots and songbirds.
His organization opposes the trade in wild-caught birds because of conservation and cruelty concerns, but he warns of its health hazards as well.
"We've grown comfortable with the idea that quarantines protect us, when in fact there are many cases where they don't protect us," he said.
"[Think of] everybody upstream of that quarantine: the trapper, the trader, the exporter, the importer, the baggage handlers. None of those people are protected from this disease, because [quarantine] happens after all of those people have been exposed," he said.
Gilardi also explained that human exposure to the disease is likely to be at higher levels from pet birds than from poultry animals in a country like the United States.
"What's unique about the wild-bird trade is that they are going directly into people's homes, sitting in the kitchen and singing away and pooping away," he said. "The exposure to the general public is really direct."
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