Deadly bird flu reappears in US commercial flocks

Highly pathogenic bird flu has made its first appearances in US commercial poultry flocks this season, affecting one turkey farm in South Dakota and one in Utah, raising concerns that more outbreaks could follow.

The US Department of Agriculture reported avian influenza, which is deadly to commercial poultry, was confirmed in a flock of 47,300 turkeys in Jerauld County, South Dakota, on October 4 and at a farm with 141,800 birds in Utah's Sanpete County last Friday.

The outbreaks are the first reported among commercial flocks in the US since the disease struck two turkey farms in the Dakotas in April. Infected flocks are normally destroyed to prevent the flu's spread, and then the farms are decontaminated.

Before last week, the only reports of bird flu in recent months in the US in recent months were sporadic appearances in backyard flocks or among wild birds such as ducks, geese and eagles. While wild birds often show no symptoms of avian influenza, infections in them are a concern to the poultry industry because migrating birds can spread the disease to vulnerable commercial flocks.

"I don't doubt that we will have more cases," South Dakota State Veterinarian Beth Thompson said in an interview on Tuesday.

"I would be very pleasantly surprised if we're done because migration is just starting."

Bird flu last year cost US poultry producers nearly 59 million birds across 47 states, including egg-laying chickens and turkeys and chickens raised for meat, making it the country's deadliest outbreak ever, according to USDA figures. The outbreak caused spikes in egg and turkey prices for consumers and cost the government over $US660 million ($A1 billion).

The toll from a 2015 outbreak, deemed the most expensive animal health disaster in US history with over $US1 billion ($A1.6 billion) in costs to the government, was nearly 51 million birds in 15 states.

Bird flu infections in humans are relatively rare and aren't considered a food safety risk. But as it hits other species, including some mammals, scientists fear the virus could evolve to spread more easily among people. Cambodia this week reported its third human death from bird flu this year.