An unusual number of cats are dying across Poland, and authorities found more than half of those tested harbored the worrisome strain of bird flu known as H5N1, the World Health Organization said.

Of 47 samples tested — including one wild feline in captivity — 29 were positive for the H5N1 flu, marking the first report of “high numbers of infected cats over a wide geographical area within a country,” the WHO said in a statement.

The United Nations agency was notified by local health authorities amid concern that the flu strain found in birds could combine with one that is able to circulate in humans, resulting in a new version that transmits easily enough to spark a pandemic.

Some cats developed severe symptoms, including breathing difficulty, bloody diarrhea and neurological problems, the WHO said. The general population is at low risk of contamination, while the group assesses the threat for cat owners and vets as “low to moderate.”

As of July 12, no human contacts of the infected cats had reported any symptoms, and the surveillance period for those individuals is now complete, according to the WHO.

The felines could have had contact with infected birds or their environments, the WHO said. Some of them were outdoor animals while others lived indoors, either with or without access to a balcony or backyard.