LONDON (AP) — The World Health Organization said more than two dozen cats have been infected with bird flu across Poland, but no people appeared to have been sickened.

In a statement on Monday, the U.N. health agency said it was the first time so many cats had been reported to have bird flu over such a wide geographical area in a single country, amid an unprecedented global outbreak of the latest version of the H5N1 version of the disease.

WHO said that late last month, Polish authorities informed agency officials of the unusual deaths of more than 45 cats in 13 geographical regions of the country. Testing last week found that 29 had H5N1.

As of June, the most recent variant of H5N1 has been reported in birds and other animal species in Asia, Africa, Europe and the Americas. Since 2020, WHO said a dozen human cases have been reported.

Scientists worry that rising cases of H5N1, particularly in animals that have frequent contact with humans, might lead to a mutated version of the disease that could spread easily between people, triggering another pandemic.

Before the COVID-19 pandemic, many experts had suspected that the next global outbreak would be sparked by H5N1. But while bird flu has killed hundreds of millions of birds globally, it has sickened fewer than 900 people since 2003 and has not been able to spread easily among humans.

WHO said it was unclear how the domestic cats in Poland became infected with bird flu and said officials were still investigating possible sources of exposure, including contact with wild birds that are known to carry H5N1. The agency said the risk of people in Poland being infected with bird flu was “low” and “low to moderate” for people exposed to cats, including cat owners and veterinarians.

Last week, WHO and partners warned that the increasing numbers of mammals infected with H5N1 were unusual. Experts have previously cautioned that pigs, which are susceptible to flu viruses from both humans and birds, might act as a “mixing vessel,” leading to the emergence of mutated viruses that could be lethal to people.

Since last year, authorities in 10 countries have reported bird flu outbreaks in mammals, including farmed mink in Spain, seals in the U.S., and sea lions in Peru and Chile.