#18,634
The CDC has published the MMWR report on H5N1 in household cats that was telegraphed a couple of weeks ago, which provides some tantalizing information about the potential indirect transmission of the virus, while at the same time illustrating some of the barriers to investigating these types of cases.
Two different households with dairy farm employees saw fatal infections of indoor household cats in Michigan last May. Cats that had no direct contact with the farm, and were never fed raw milk.
In both cases, the farm workers declined testing for the virus, and several household members described illnesses concurrent to or following the cat's onset of illness. The few PCR tests that were conducted were taken 10 to 11 days after likely exposure; well beyond the optimal time for such tests.
As a result, no humans in either of these two households tested positive for the virus.
This has been an all too familiar story over the past year, with many farm workers reluctant to report illnesses, or be tested for the virus, over fears of losing their jobs (see EID Journal: Avian Influenza A(H5N1) Virus among Dairy Cattle, Texas, USA).
Anecdotal reports suggest there may have been scores of symptomatic farm workers who were never identified as infected, numbers which are further supported by a serological study which found 7% (of 115 farm workers tested) showed antibodies for the virus, but only half reported `mild symptoms', and none were ever identified as having been infected.
Today's report provides a good deal of detail into the investigation, but in many instances tests were either delayed or declined, leaving many more questions than answers.
I've only posted some excerpts from a much longer report. Follow the link to read it in its entirety. I'll have a bit more after the break.
Highly Pathogenic Avian Influenza A(H5N1) Virus Infection of Indoor Domestic Cats Within Dairy Industry Worker Households — Michigan, May 2024
Weekly / February 20, 2025 / 74(5);61–65
Ramya Naraharisetti, PhD1,2; Meghan Weinberg, PhD2; Becky Stoddard3; Mary Grace Stobierski, DVM2; Kimberly A. Dodd, DVM4; Nora Wineland, DVM5; Mathew Beal, DVM4; Jennifer Morse, MD3; Samantha Hatter, DVM4; Dodd Sledge, DVM4; Katelynn Youatt, DVM4; Joseph Coyle, MPH2; Jevon McFadden, MD2,6; Timothy M. Uyeki, MD7; Lizette O. Durand, VMD, PhD7 (VIEW AUTHOR AFFILIATIONS)View suggested citation
Summary
What is already known about this topic?
Outdoor cats on U.S. dairy farms have been infected with highly pathogenic avian influenza (HPAI) A(H5N1) virus; infection has not been reported in indoor cats.
What is added by this report?
HPAI A(H5N1) virus was detected in two indoor domestic cats with respiratory and neurologic illness that lived in homes of dairy workers but had no known direct exposure to HPAI A(H5N1)–affected farms. Both dairy workers declined testing; other household members received negative test results for influenza A.
What are the implications for public health practice?
Veterinarians in states with confirmed HPAI A(H5N1) in livestock should consider obtaining household occupational information, testing for influenza A viruses, and wearing personal protective equipment when evaluating companion cats with respiratory or neurologic illness. Suspected cases should be reported to public and animal health officials.
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Full Issue PDF
Abstract
Highly pathogenic avian influenza (HPAI) A(H5N1) virus, clade 2.3.4.4b, genotype B3.13 infection has been documented in cats on U.S. dairy cattle farms. In May 2024, the detection of HPAI A(H5N1) virus infection in two cats that were reported to be exclusively indoor, and that had respiratory and neurologic illness in different households, prompted an investigation by the Michigan Department of Health and Human Services and Mid-Michigan District Health Department (MDHHS/MMDHD).
The cats’ owners and household members were interviewed and offered testing for influenza A(H5) virus. The owner of one cat worked on a dairy farm but declined A(H5) testing; three other household members received negative A(H5) test results. The owner of the other cat lived alone and worked on multiple dairy farms transporting unpasteurized milk; this worker also reported getting splashed in the face and eyes by unpasteurized milk but declined A(H5) testing.
Both workers were employed in a county known by MDHHS/MMDHD to have HPAI A(H5N1) virus, clade 2.3.4.4b, genotype B3.13–positive dairy cattle. In states with confirmed HPAI A(H5N1) in livestock, veterinary care can be aided if veterinarians obtain household members’ occupational information, especially when evaluating cats with signs of respiratory or neurologic illness. If occupational exposure to HPAI A(H5N1)-infected livestock is identified among cat owners, and their companion cats are suspected to have HPAI A(H5N1) virus infection, it is important that veterinarians contact state and federal public health and animal health officials to collaborate on joint One Health investigations and testing to protect human and animal health.
(SNIP)
Discussion
HPAI A(H5N1) virus, clade 2.3.4.4b, has been detected in wild birds, poultry, and wildlife in the United States since 2022, and in commercial U.S. dairy cattle since 2024 (2–4). In the ongoing U.S. outbreak of HPAI A(H5N1) in dairy cattle, serious illness, including neurologic signs, and death from HPAI A(H5N1) virus infection in cats that are frequent inhabitants of farms have been attributed to consumption of unpasteurized milk from infected dairy cattle, wild birds, or raw poultry products¶¶ (4–6). Continued epizootic circulation of HPAI A(H5N1) virus increases the potential for emergence of mutations that might increase risk for mammalian adaption and transmission to and among humans, and this finding has been documented in the case of domestic cats (7). Isolated, sporadic instances of cow-to-human transmission of HPAI A(H5N1) virus, clade 2.3. 4.4b, genotype B3.13 have occurred in California, Colorado, Michigan, and Texas (1,8). Presumed cat-to-human transmission of low pathogenic avian influenza A(H7N2) virus in an animal shelter in 2016 suggests that exposure to cats infected with HPAI A(H5N1) virus might also pose a transmission risk to humans (9).
Although reported cases of infection of indoor cats with HPAI A(H5N1) viruses are rare, such cats might pose a risk for human infection. The source of HPAI A(H5N1) virus infection in these two cats is unknown; however, the cats’ owners worked on dairy farms and potentially had occupational exposures to HPAI A(H5N1)–positive dairy cattle or contaminated products or environments. Further research is necessary to evaluate the risk of fomite transmission and other types of transmission routes of HPAI A(H5N1) virus to cats. The two dairy workers described in this report did not use recommended PPE before their illnesses and could have been exposed to HPAI A(H5N1) virus. However, because neither dairy worker received testing for A(H5), whether cat 1A’s owner’s gastrointestinal symptoms or cat 2A’s owner’s ocular symptoms were because of HPAI A(H5N1) virus infection or a different etiology is unknown.
Implications for Public Health Practice
Given the potential for fomite contamination, farmworkers are encouraged to consider removing clothing and footwear and to rinse off any animal byproduct residue (including milk and feces) before entering households.*** Veterinarians evaluating companion cats with signs of respiratory or neurologic illness in areas with HPAI A(H5N1) virus circulating in cattle or poultry or other animals are recommended to wear PPE when examining these animals or collecting specimens for influenza testing and to obtain occupational information from household members to help prevent unprotected exposures and guide coordinated One Health††† (i.e., human, animal, and environmental) public health investigations of potential animal-to-human spread of HPAI A(H5N1) virus. Implementation of standard precautions for zoonotic disease prevention and CDC guidance for veterinarians at veterinary clinics can help limit the number of staff members exposed to sick animals potentially infected with pathogens, including HPAI A(H5N1) virus. Further, given the widespread outbreak in animals, including poultry and wild birds, throughout the United States, anyone who has occupational or recreational exposure should wear the recommended PPE when interacting with any potentially infected animals.§§§
Acknowledgments
James Barber, Smeralda Bushi, Ebone Colbert, Seth Eckel, Justin Henderson, Tiffany Henderson, Shannon Johnson, Sue Kim, Fatema Mamou, Mat Myers, Sarah Pruett, Briana Putrus, Diana Riner, Michigan Department of Health and Human Services Bureau of Infectious Disease Prevention; Wai-Yi Leung, Ingham County Health Department.
It is sobering to realize that we are nearly a year into this HPAI H5 crisis in dairy cows, and we still don't have a decent handle on how it is being transmitted, or how widespread it really is. For far too long HPAI has been viewed primarily as an `economic' problem for agriculture, rather than a genuine public health concern.
Whether that perception will change in 2025, remains to be seen.