Explained | What is bird flu, and why is Brazil's first case on a commercial farm concerning?

Sao Paulo: Brazil, the world's largest chicken exporter, on Friday reported its first case of highly pathogenic avian influenza - also known as bird flu - on a commercial farm, triggering fears over global poultry supply chains.

The disease has spread around the globe in the past years, leading to hundreds of millions of poultry being culled. It has also been reported in dairy cows, cats, and humans.

What is bird flu?

Bird flu is a viral disease spread mainly by birds which can also affect mammals, including humans.

The disease, of which there are many strains, is highly contagious between birds.

Why is the case in Brazil important?

Friday's case is the first time an outbreak has been reported on a commercial farm in Brazil.

The South American country accounts for 35% of the global chicken trade, shipping $10 billion worth of the poultry in 2024.

Brazil sends chicken to some 150 countries around the world, with China and Japan among its top customers. China immediately banned imports from Brazil following the news, and other countries could follow.

In 2023, Japan banned chicken imports from Brazil's Espirito Santo state after bird flu was found on a non-commercial farm there.

Eggs could also be affected. In March, Reuters reported that the US had almost doubled imports of eggs from Brazil amid sky-high egg prices there following a spike in US bird flu cases.

What's happening with the number of cases?

As of April 30, just under 1,200 outbreaks of highly pathogenic bird flu had been reported in poultry around the world in the current season that runs from October 2024 to September 2025, according to the World Organisation for Animal Health.

That compares with 786 outbreaks in the whole of the 2023-2024 season, and 1,971 cases in the season before that.

At the same time, during the 2024-2025 season more than 1,400 cases have been reported in wild birds, compared to 1,062 cases and 3,975 cases in the whole of the 2023-2024 and 2022-2023 seasons respectively.

What other species can get bird flu?

Bird flu has also been reported in dozens of mammal species, including dairy cows, sheep, cats - including some house cats - and humans.

The virus is typically transferred direct from birds to humans. The U.S. Centers for Disease Control (CDC) says limited human-to-human transfer has occasionally been reported around the world, though not in the U.S.

Is there vaccine?

There is no vaccine for cases of bird flu in humans.

Vaccines are available for birds. In March this year, France's government said its poultry flocks will be allowed outdoors again after a lull in bird flu infections, which it attributed to a vaccination program.

In January, the US Department of Agriculture said it would rebuild a stockpile of bird flu vaccines to combat the current outbreak, which began in 2022.