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Bird flu is infecting the brains of wild mammals such as foxes and raccoons and may be making them behave in unusual ways, according to a study in the US. Out of 57 live mammals found to be infected, 53 had neurological symptoms, such as seizures, problems with balance, tremors and a lack of fear of people.

The risk to people appears to be low. 鈥淭here isn鈥檛 yet any evidence that red foxes or other wild mammals that have avian influenza could spread that infection to humans,鈥� says at the University of Wisconsin-Madison.

But this doesn鈥檛 mean it cannot happen, Elsmo says. 鈥淗umans should never interact with animals, particularly carnivores, that are behaving abnormally for multiple reasons, rabies of course being one.鈥�

Her team has reported 67 wild mammals in the US confirmed to be infected with H5N1 bird flu between 1 April and 22 July 2022: 50 red foxes (Vulpes vulpes), six striped skunks (Mephitis mephitis), four raccoons (Procyon lotor), two bobcats (Lynx rufus), two Virginia opossums (Didelphis virginiana), one coyote (Canis latrans), one fisher (Pekania pennanti) and one grey fox (Urocyon cinereoargenteus).

Ten of these animals were found dead, but the rest were found ill and either taken to wildlife rehabilitation centres or collected by wildlife professionals.

Two of the foxes recovered, but the rest of the animals died or were euthanised. Post-mortems revealed brain damage in 51 out of 54 brains examined, and the bird flu virus was detected in many samples of brain tissue.

There have been previous reports of bird flu causing neurological symptoms in infected mammals. Elsmo thinks the relatively high number of brain infections found by her team isn鈥檛 something specific to the virus strain that is currently circulating around the world, known as clade 2.3.4.4b, but simply because this virus is so widespread.

鈥淭his new strain of avian influenza is much more widespread in wild birds than previous strains have been,鈥� says Elsmo. 鈥淭here are increased opportunities, then, for wild mammals to interact with dead and dying birds.鈥�

A by the European Centre for Disease Prevention and Control (ECDC) lists numerous recent cases of bird flu spilling over to mammals, including marine mammals such as seals and dolphins. Besides the US, cases have been detected in Canada, Japan, Peru, Chile and many countries in Europe, including the UK.

In other countries, the lack of cases may be due to a lack of testing. Elsmo notes that the mammal infections her team found are probably a very small proportion of actual cases in the US. She expects numbers to go up as more people become aware that odd behaviour in mammals could be a sign of bird flu.

The worry is that as more mammals become infected, the higher the odds of the virus acquiring mutations that allow it to spread from mammal to mammal, and thus potentially among humans.

According to the ECDC report, 鈥渕utations associated with genetic adaptation to mammals鈥� have recently been detected in some bird flu viruses. It also says there may have been mammal-to-mammal transmission in harbour seals in the US in 2022, in farmed mink in Spain in 2022 and in sea lions in Peru earlier this year.

Neurological symptoms were seen in the mink and sea lions, the report says.

Meanwhile, a lab study in ferrets by at Erasmus University Medical Center in Rotterdam, the Netherlands, has shown that H5N1 bird flu , even if there is no respiratory infection.

鈥淭he virus could enter the nose by sniffing, but I think it is more likely that when mammals eat infected birds, aerosols are generated which end up in the nose,鈥� says van Riel.

鈥淚 would advise people to not have contact with sick birds or mammals, especially when they have respiratory or neurological signs including unusual behaviour,鈥� she says. 鈥淭he risk of transmission is low, but I would not take the risk, and would report any sick or dead mammal to the appropriate agencies.鈥�

In the UK, bird flu has in foxes, otters, . The UK鈥檚 Animal and Plant Health Agency (APHA) couldn鈥檛 say whether any had neurological symptoms. 鈥淢ost of the animals were found dead,鈥� said a spokesperson. The public has been asked to , but APHA didn鈥檛 respond to a question about whether people in the UK should report sick or oddly behaving foxes or other mammals.