How annual bird migration could spread avian flu

Scientists are still trying to understand exactly how a virulent bird flu is spreading through farms in the United States, but one pattern is clear: poultry and cows risk exposure to sick wild birds migrating across the Americas.

One way to understand the relationship between migrating birds and agricultural livestock is to look at the path of the mallard duck which has the most recorded avian flu infections.

January

Mallards spend winters across the United States. As they migrate north for the breeding season, the mallards pass over chicken farms big and small.

April

They may use farms as a water and food source throughout their journey.

"Outbreaks among poultry are more likely to happen where migratory waterfowl are present," said Claire Teitelbaum, a researcher with Bay Area Environmental Research Institute.

July

In the breeding season, birds return to cooler landscapes that may still contain viruses from a previous season.

A virus can persist for weeks or months in colder environments, meaning birds could contract multiple virus variants. Those variants can undergo genetic changes that enable the virus to infect new species.

September

Viruses travel along bird migration routes or "flyways," transferring from one wild bird to another.

While only the mallard's path is shown here, migratory bird flyways cover the entire United States.

December

Winter comes back around and the ducks make their way south.

They once again fly over farms across North America seeking warmer weather and possibly carrying new viral variants.

The concern is not just the beginning and end of the birds' journey. They stop to take breaks along the way, sometimes for months at a time, in what scientists call "stopover locations."

"They're going to wetlands," said Mike Casazza, a wildlife biologist for the US Geological Survey. "But now a major food source for the waterfowl is the surrounding agricultural landscape that provides grains and invertebrates and other opportunities for them to feed because that's what replaced the wetlands that they used to have."

Vast swaths of wetlands were converted for agricultural use in the early 1900s, reducing places migratory waterfowl can congregate. Drawn to the remaining wetlands, waterfowl can venture to nearby farms where they risk spreading avian flu via their contaminated saliva and faeces.

Most bird flu transmission to livestock happens indirectly, according to the US Department of Agriculture (USDA), usually from workers' boots or equipment that have been contaminated with wild bird faeces or saliva carrying the virus. But hospitable farmland increases the likelihood of direct contact between livestock and wild birds, furthering the need for "biosecurity" or efforts to prevent viral spread. Most industrial farms have strict biosecurity measures.

USDA posts guidance for farmers to keep their livestock safe from infection. The key is to limit water and food that birds are attracted to.

Almost 200 wild bird species carrying bird flu have been recorded by USDA. Mallard ducks top the list and are a focus in the agency's surveillance programme. These ducks and other waterfowl are particularly resistant to the virus, allowing them to carry it far distances.

The WHO has expressed concern that migration could lead to spread of the deadly strain circulating in North America to other countries.

Some migratory birds like Snow Geese travel as far north as Russia. Blue-winged Teal can migrate all the way to South America.

The US government in late March reported cases of the disease in dairy cows in Texas and said one person who had contact with cows had been infected and suffered conjunctivitis, commonly known as pink eye. In a preliminary report, government scientists indicated a wild bird was likely responsible for the initial introduction of the virus to a cow. Nine states have since detected bird flu among dairy cattle.

Scientists at USDA are still trying to understand which variants are circulating through cows to determine how avian flu is spreading. It has long been on the list of viruses with pandemic potential, and any expansion to a new mammal species is concerning.

As mallards and other wild birds travel north into Canada for the summer, Canadian officials are urging precaution among dairy farms to prevent the spread already racing through the United States.

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