A new strain of bird flu has affected cats in the United States.

Cats living on farms may come into contact with birds, consume their carcasses, or be exposed to contaminated feed: in the spring of 2024, 10 cats died in a short period from the highly pathogenic H5N1 avian influenza strain in a rural household in South Dakota, USA.

In addition to respiratory issues, the animals exhibited seizures and loss of coordination. Similar symptoms were noted in wild mammals such as foxes and minks. This suggests that the virus continues to adapt to mammals. The results of a new study were published in the journal Taylor and Francis.

Pathological analysis of the brain and lung tissues of two cats from South Dakota revealed a high concentration of the virus in the brain and minimal levels in the lungs; however, the pathogen primarily affected the central nervous system. According to the authors of the scientific paper, this indicates its pronounced neurotropism—the ability to infect and destroy brain cells.

An international research team led by Suresh V. Kuchipudi from the University of Pittsburgh (USA) analyzed the genome of the virus isolated from the two cats that died on the poultry farm and compared it with a variant previously found on a nearby dairy farm in cattle, as well as with strains isolated from domestic cats in other regions.

The results revealed a number of unique mutations. In particular, the changes found in the hemagglutinin (HA) protein—T143A—may make the pathogen more contagious and better adapted to other hosts, as well as allow it to "evade" the immune response. Two additional mutations—F314L and L342Q—were found in the PA (polymerase) protein, which is involved in assembling the viral genetic material (potentially affecting the strain's virulence).

The mutations apparently occurred within the bodies of the infected cats rather than being transmitted directly from birds or other animals. This is significant because cats have two receptors that avian influenza and seasonal flu viruses easily bind to. Such "dual" infections could lead to mutations and the subsequent transmission of avian influenza virus from pets to humans.

Virologists Angela Rasmussen from the University of Saskatchewan (Canada) and Thijs Kuiken from Erasmus University Rotterdam (Netherlands), who were not involved in the study, also noted that infected cats develop systemic infections, and the virus is transmitted both through the respiratory tract and the gastrointestinal tract, creating multiple routes of infection.

While there is currently no evidence of transmission of the infection from cats to humans, household contact with infected pets, especially on poultry farms, could potentially lead to the spread of the pathogen in the human population.

Outbreaks of avian influenza are affecting numerous countries, involving an increasingly wide range of species—from marine mammals to terrestrial predators and domestic animals. The accumulation of the virus in nature increases the chances of random genetic combinations that could lead to a new pandemic. Similar jumps in human history have already occurred—the exchange of genome segments between avian and seasonal influenza resulted in the pandemics of 1918 and 2009.

Scientists have warned that continuous monitoring of genetic mutations of the H5N1 virus and surveillance of the health of domestic and wild animals are essential for keeping the situation under control. Moreover, the results of a previously published study indicated that over time, the avian influenza virus adapts more quickly and successfully crosses interspecies barriers.