#18,752
While the USDA's reporting of HPAI H5 infections in peridomestic animals in the United States has slowed markedly (130 Jan-April 2025, only 6 since May 1st, 2025), we continue to see reports in wild birds, poultry, and livestock, indicating the virus is still active in the environment.
The USDA can only report on what is submitted to them, and as we've seen over the past few years, some states appear to be looking harder for cases than others (see map below). Many states have yet to report their first detection of the virus in mammalian wildlife.
Despite this lack of reporting, 139 HPAI infected domestic cats have been added to the USDA's list over the past year, and it is likely that many others have gone unreported (see California: San Mateo County Warns Residents After Stray Cat Found With H5N1).
Last December, in Emerg. Microbes & Inf.: Marked Neurotropism and Potential Adaptation of H5N1 Clade 2.3.4.4.b Virus in Naturally Infected Domestic Cats, we looked at a report on the HPAI H5 infection of a house full of domestic cats (n=8) in South Dakota 15 months ago.
Isolates from the two cats that were tested showed signs of viral adaptation to a mammalian host. The authors wrote:
Cat H5N1 genomes had unique mutations, including T143A in haemagglutinin, known to affect infectivity and immune evasion, and two novel mutations in PA protein (F314L, L342Q) that may affect polymerase activity and virulence, suggesting potential virus adaptation.Dead cats showed systemic infection with lesions and viral antigens in multiple organs. Higher viral RNA and antigen in the brain indicated pronounced neurotropism.
Although the risks of human infection from cats is thought to be low, we've seen instances where animal shelter workers have been infected with (H7N2) avian flu from cats (see J Infect Dis: Serological Evidence Of H7N2 Infection Among Animal Shelter Workers, NYC 2016).
The CDC issued new HPAI guidance for veterinarians in the spring of 2024 (see CDC Guidance for Veterinarians: Evaluating & Handling Cats Potentially Exposed to HPAI H5N1), including the use of PPEs.
Earlier this year, however, the CDC reported serological evidence of HPAI exposure in 3 veterinarians (see MMWR: Seroprevalence of Highly Pathogenic Avian Influenza A(H5) Infections among Bovine Veterinary Practitioners – United States, September 2024), from (n=150) samples collected last August.
None of the positive cases suspected they had contracted the virus, and two denied working with infected cattle (including one who worked in two states that have never reported bovine H5). A finding the authors suggested may indicate `. . . . there could be U.S. states with A(H5)-positive people and animals that have not yet been identified.'
Given the ongoing risks of exposure, yesterday Ontario Health published a 7-page updated guidance document for veterinarians which they summarize as:
Veterinarians should be aware of the potential for exposure of backyard poultry, as well as pets such as dogs and cats, to HPAI in Ontario and should be aware of the potential clinical signs associated with this disease, in the event that an animal presents with a compatible illness. Additionally, pet owners should be aware of actions that can be taken to minimize the risk of themselves or their pets being exposed to the virus.
Follow the link below to download and read the guidance in its entirety.
