I have been following the news about H5N1 infections in cats with interest (and other emotions!) due to the repeated mentioning of “neurologic” signs. I had to do some digging to find any specifics, but I finally found some details and wanted to pass them along. On February 20, 2025, the CDC’s Morbidity and Mortality Weekly Report (MMWR) included information from two household cats in Michigan that were confirmed positive for HPAI-H5N1 on necropsy in May of 2024. Clinical signs for both cats were described. Although these reports are from 1 year ago, I think the information is still relevant today. A link to the CDC report can be found here (https://www.cdc.gov/mmwr/volumes/74/wr/pdfs/mm7405a2-H.pdf) or I’m happy to provide a copy of what I have if you send me an email. The second report was from 2023 out of Nebraska (Journal of Comparative Pathology 205 (2023) 17e23). There were 3 cats in the Nebraska report diagnosed with H5N1
Neurologic Signs
The signs reported come from the two cats in Michigan and 3 cats in Nebraska, so given the small sample size additional neurologic signs may be present and not reported in these cases. Initial signs appear to involve loss of appetite, grooming and change in behavior. In the Nebraska cats, case 1 is described as having anisocoria, injected sclera, nystagmus, loss of proprioception, tremors, hyperesthesia and pyrexia. Signs progressed to seizures which resulted in euthanasia. A second cat from the same house developed compulsive circling and inappropriate mentation 24 hours after the first cat’s signs. This cat was stable with supportive care but acutely progressed on day 10 to lateral recumbence and tremors which resulted in subsequent euthanasia. A third Nebraska cat (unrelated household) developed difficulty ambulating, lethargy and anorexia and was presented to a veterinarian. The Michigan cats were reported to have nasal discharge, decreased mentation, ataxia, swollen right jaw and anorexia. One other household cat in Michigan developed signs of URI but did not test positive for H5N1 and did not die. Most cats have been reported to die within 7 days (reports are 1-7 days) of experimental infection so survival to 10 days is unusual for the one naturally occurring infection. The household cat that developed an URI did not test positive and therefore must be assumed to have an unrelated illness due to their lack of fatality, however this is an assumption.
Clinical Pathology
No significant abnormalities were noted on CBC or serum biochemistry.
Necropsy
Not surprisingly, lesions were present in lungs and brain but also some other visceral organs were affected as well. The brain had wedged shaped laminar necrosis and vasculitis. Mild to moderate spongiosis and other signs of inflammation were present in all cases. The third Nebraska cat had more profound inflammation than the other two, but it was similar in characteristic. Areas of malacia were present in the cats.
Recommended Diagnostic Testing
Based on the data collected, if you have a cat with signs of respiratory and neurologic disease collecting nasal swabs for PCR testing might be useful antemortem test. If the cat is euthanized the authors of the Nebraska paper suggest random brain samples for viral testing. I would add that PPE should be strongly considered given the rapidly adapting capabilities of this virus. According to their website The WVDL can test for Influenza A virus (https://www.wvdl.wisc.edu/testsandfees/) but I’m not a virologist so I’m not sure if this is inclusive of the H5N1 variant. If you have a patient who needs testing, I recommend calling the WVDL to ask (608-262-5432).
Treatment
To date, supportive care has been unsuccessful for managing H5N1 variant infection in cats and no specific treatment is available.
I hope this very quick summary has been useful for your practice. As I said above, I am not a virologist so please reach out to WVDL where the very smart people can give you better information about the virus itself. These reported cases help me see that this is an intracranial disease for cats, and a terribly inflammatory one with lots of thrombi and vasculitis so I am better able to identify cats with possible infection. On that sad note, I hope you are enjoying the pleasant shift of the weather, and I look forward to working with you soon!