One Health: Outbreak of HPAI a(H5N1) Among House Cats: A Case Series Involving Oseltamivir Treatment

#18,878

Although the USDA's dashboard of mammalian H5N1 infections reports 145 domestic cats (along with 38 lions, 16 bobcats, 6 tigers, etc.) infected in the U.S. since 2022, there is little doubt this is a considerable undercount.  

• First, only a few states appear to be aggressively looking for, testing, or reporting cases (see map above). And reporting has dropped precipitously over the past year across the nation. 

• Second, Mammals - including domestic cats - often die in remote and difficult to access places, meaning many are never discovered or tested. And of course, some of these animals may recover, and are also never tested.

• And third, we've seen reports (such as today's) describing outbreaks which are not always fully captured by this dashboard.

Regular readers will recall that 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 case report of 10 cats that died in a South Dakota household, but only 2 were ever tested for the virus.

In today's One Health Journal report, we learn of household in Tulare, CA where 4 cats were sickened last October (2 died without testing), while 2 surviving cats (treated with oseltamivir) showed strong antibodies to H5N1.  

But the USDA's dashboard only shows 1 cat in Tulare, CA as infected. 

Obviously, the USDA can only publish what is submitted to them, and untested cats in a cluster - even if likely infected - can't really be counted.  

The point being, the true number of cats (and other mammals) infected by H5N1 across the nation - and the world - is likely far higher than reported. 

Today's report is of interest because it illustrates the difference in survivability of H5N1 infection (in cats) with - and without - antiviral medication; in this case,  oseltamivir (15 mg PO q12h) for 10 days.

This report highly how quickly HPAI can spread through cats, and is also notable for a lack of an obvious source of exposure for these cats, although they discuss some intriguing possibilities.

 

Cat 2 had indoor/outdoor access, allowing potential contact with contaminated environments, wild birds, or fomites, while cats 1, 3, and 4 were described as indoor only.  However, the report also states:

The owner family lives less than 1 mile from the nearest cluster of dairy farms and the husband works within the dairy sector as a traveling hay salesman who frequently visits farms. 

I've only posted some excerpts, so follow the link to read the report in full.  I'll have a brief postscript when you return. 

Outbreak of highly pathogenic avian influenza a(H5N1) among house cats: A case series involving oseltamivir treatment

Jacob F. Gomez a 1, Ian G. Bemis b c 1, Ismaila Shittu d 1, Gregory C. Gray d e f, Kristen K. Coleman b c 
https://doi.org/10.1016/j.onehlt.2025.101211Get rights and content
Open access

Highlights

• We report a highly pathogenic avian influenza A(H5N1) outbreak among four domestic cats from the same household.
• With early supportive care and antiviral treatment, H5N1 is survivable in domestic cats.
• Survivor cats may maintain high titers of neutralizing antibodies against H5N1 at least 3–4 months post recovery.

Abstract

We present a highly pathogenic avian influenza A(H5N1) outbreak among four domestic cats from the same household within close proximity to a dairy farm in Tulare, California – the epicenter of the H5N1 dairy cattle outbreaks in California, USA. We demonstrate that with early supportive care and treatment with oseltamivir, H5N1 is survivable in domestic cats, and that survivor cats may maintain high titers of neutralizing antibodies against H5N1 at least 3–4 months post recovery.

(SNIP) 

This outbreak of H5N1 among house cats demonstrates how quickly H5N1 can spread through multiple animals in a home, posing an unexpected spillover risk to family members and veterinarians caring for them. Most importantly, we demonstrate that with early supportive care and early treatment with oseltamivir, H5N1 is a survivable disease in domestic cats. Our results support the continued prioritization of neuraminidase inhibitors in the treatment of H5N1 in mammals and highlight the importance of early identification and targeted treatment. Furthermore, we demonstrate that survivor cats can regain full health and maintain high titers of neutralizing antibodies against H5N1 at least 3–4 months post recovery. These findings suggest that cats and other mammalian species may sustain strong protection from reinfection of H5N1 after natural infection and continue to thrive.

        (Continue . . . .)

 
This study is limited by its small sample size (n=4), of which real data was only available for the 2 surviving cats, but it does suggest that oseltamivir can improve survivability in infected felines. 

Without environmental testing in the household (obviously far beyond the purview of the veterinary clinic), and/or sequencing data, identifying the source of this outbreak is a matter of speculation. 

What we do know is the amount of HPAI H5 virus in the environment increases during the fall and winter months, and now is a good time to think about how you will protect your companion animals (dogs and cats) during the months ahead. 

The CDC offers the following advice to pet owners on how they may keep their animals safe from avian flu, and what to do if they suspect infection.