#19,148
We've a new report in today's MMWR that details the probable feline-to-human transmission of the H5N1 virus - during an outbreak of the virus in domestic cats in California in late 2024 - which was linked to contaminated cat food and/or raw milk.
While long considered possible, actual evidence of cat-to-human transmission of avian flu viruses has been pretty sparse. Two notable exceptions being:
- In 2005, the CDC's EID Journal Probable Tiger-to-Tiger Transmission of Avian Influenza H5N1, reported that while no humans tested positive by RT-PCR, 2 close human contacts (3.5%) tested positive via serology (microneutralization) for the virus 4 weeks later.
- In 2018 we saw evidence suggesting several employers had been infected by LPAI H7N2 during an outbreak in 2016 (see J Infect Dis: Serological Evidence Of H7N2 Infection Among Animal Shelter Workers, NYC 2016).
Admittedly, testing has been fragmented over the years, with many exposed individuals either tested too late, or not at all. And that is a challenge we find once again with today's report.
In today's example, of the 139 people exposed to infected cats, 30 reported flu-like symptoms, but of those only 18 (60%) submitted to PCR testing with 12 declined or were unavailable. Another 15 asymptomatic cases were tested (combined n=33), meaning that < 24% of total number of exposed individuals were tested. And the timing of sample collection was not ideal, as the `window' for PCR detection can be short, particularly in mild or asymptomatic infections. The authors wrote:
The median interval between the most recent exposure date and specimen collection was 8 days (range = 1–13 days). Specimens from 19 (58%) persons who received testing were collected >7 days after the last exposure.
As the above chart indicates, all PCR tests were negative for H5N1.
Again timing may be an issue, since they authors report `. . . the average interval between exposure and serum collection was 104 days (range = 35–137 days)', which could have been long enough for some antibody titers to wane.
Serologic Evidence of Highly Pathogenic Avian Influenza A(H5N1) Virus Infection in a Veterinary Professional Exposed to an Infected Domestic Cat — Los Angeles County, California, December 2024–January 2025
Weekly / May 7, 2026 / 75(17);215–220
Aisling Vaughan1; Allison Joyce2; Elizabeth Traub2; Mellissa Jae3; Emily Beeler3; Erick Paiva2; Kristopher Ananian2; Crystal Holiday4; Stacie Jefferson4; Jessica Richardson2; Cortney Munna2; Cynthia Chan3; Tamerin Scott3; Noah Kojima2; Tanya Seneviratne2; Alexandra Mellis4; Sonja J. Olsen4; Nicole Green5; Matt Feaster6; Dawn Terashita2; Sharon Balter2; Min Z. Levine4; Jamie Middleton3,*; Annabelle de St. Maurice2,*
Summary
What is already known about this topic?
Transmission of influenza A(H5N1) viruses from domestic cats to humans has not been documented.
What is added by this report?
During November 2024–January 2025, a total of 139 persons exposed to 19 A(H5N1)-infected domestic cats that consumed raw animal products were identified in Los Angeles County, California. Among 25 exposed persons who received serologic testing, one asymptomatic veterinary professional had serologic evidence of A(H5N1) infection after occupational exposure to an A(H5N1)-infected cat.
What are the implications for public health practice?
These findings provide evidence of zoonotic transmission of influenza A(H5N1) virus from domestic cats to humans. Pet owners are advised not to feed raw animal products to cats. Veterinary professionals should be aware of infection risks, use appropriate personal protective equipment, and adhere to recommended infection control practices to reduce the risk for zoonotic transmission of influenza A(H5N1).
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Abstract
Since 2021, avian influenza A(H5N1) clade 2.3.4.4b viruses have spread widely among wild birds and domesticated poultry in the United States, with sporadic spillover into mammals. During November 2024–January 2025, 19 domestic cats in Los Angeles County, California, became ill after consumption of commercially purchased raw milk, raw meat, or raw pet food; nine cats tested positive for influenza A(H5N1) virus (clade 2.3.4.4b genotype B3.13).
Overall, 139 persons were exposed to the 19 infected cats, and all were monitored for symptoms. Although 30 persons reported influenza-like illness symptoms, none received a positive influenza A(H5) reverse transcription–polymerase chain reaction (RT-PCR) test result. In April 2025, the Los Angeles County Department of Public Health and CDC invited all exposed persons to participate in an influenza A(H5N1) serosurvey to determine whether transmission of influenza A(H5N1) virus occurred, including in those without symptoms.
Sera from 25 (18%) of the 139 exposed persons were tested. Among these, antibodies specific to A(H5N1) clade 2.3.4.4.b (antigenically similar to the clade 2.3.4.4.b influenza A[H5N1] virus isolated from the infected cats) were detected in serum from one veterinary professional, who was asymptomatic. This person did not use respiratory or eye protection during the exposure, did not report influenza-like illness after the exposure, and reported no other known risk factors for A(H5N1) infection.
These findings represent serologic evidence of possible transmission of influenza A(H5N1) clade 2.3.4.4.b virus from a domestic cat to a human, highlighting concerns about potential cat-to-human transmission of influenza A(H5N1) virus and the importance of infection control practices in veterinary settings.
(SNIP)
Limitations
The findings in this report are subject to at least two limitations.
- First, RT-PCR testing and serologic testing were not performed for all persons; therefore, some infections might have been missed.
- Second, serologic testing was performed 4–5 months after exposure, at which time antibody responses might have waned.
Collection of acute and convalescent serology specimens was not feasible in this investigation; however, this step should be considered during future influenza A(H5N1) virus outbreaks in animals.
We've seen previous examples where the public, or agricultural workers, have been reluctant to cooperate with public health officials or researchers (see EID Journal: Avian Influenza A(H5N1) Virus among Dairy Cattle, Texas, USA).
Around the time this feline outbreak was occuring, we were seeing the first evidence of asymptomatic infections among dairy workers (see MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers), once again uncovered by retrospective serological testing.
Although better cooperation and participation by the public might have turned up more instances of feline-to-human H5N1 transmission - thanks to these researchers - we now have pretty good evidence that feline-to-human transmission of clade 2.3.4.4b. can occur.
