H5N1 Bird Flu in Two Indoor Cats of Dairy Workers' Households, Michigan, May 2024

In short

• Outdoor cats on U.S. dairy farms had been infected with highly pathogenic avian influenza (HPAI) A(H5N1), but infection had not been reported in indoor cats.
• In May 2024, H5N1 was found in two indoor cats, in different Michigan households, that fell ill with respiratory and neurological signs. Both lived with dairy workers but had no known direct contact with affected farms. Both workers declined testing; the other people in the households tested negative for influenza A.
• Where H5N1 has been confirmed in livestock, veterinarians seeing pet cats with respiratory or neurological illness should consider asking about household members' jobs, testing for influenza A and wearing personal protective equipment (PPE). Suspected cases should be reported to public health and animal health officials.

The virus in both cats was clade 2.3.4.4b, genotype B3.13 — the type circulating in U.S. dairy cattle. The Michigan Department of Health and Human Services and the Mid-Michigan District Health Department investigated.

The first cat

Cat 1A, a five-year-old spayed female domestic shorthair that lived only indoors, lost her appetite, stopped grooming, and became disoriented and lethargic; her nervous system then steadily failed. A local vet saw her on the second day of illness, and on the fourth she was referred to Michigan State University's Veterinary Medical Center. Examination found severe neurological disease — dulled consciousness, abnormal cranial nerves and abnormal movement in all four limbs — along with loss of appetite and lethargy. She had improved a little on day 3, the day after an injection of antibiotics, but on day 4 she could not hold up her head and all the earlier signs returned. Because the disease was advancing so fast, she was euthanised that day.

Her owner worked on a dairy farm, and H5N1 was known to be circulating in Michigan dairy herds, so with the state veterinarian's approval her body was sent to the university's diagnostic laboratory. Brain and nasal swabs were positive for influenza A(H5) by RT-PCR. Sequencing, confirmed by the U.S. Department of Agriculture's National Veterinary Services Laboratories, identified HPAI A(H5N1) of the same kind circulating in Michigan dairy cattle.

Household 1

Cat 1A lived with two adults, two adolescents and two other indoor-only cats.

• Cat 1B developed a watery, pus-like eye discharge, fast breathing and poor appetite 4 days after cat 1A fell ill. The owner was asked to swab her, as she was too ill to bring in, but no sample was submitted. Her signs were reported gone 11 days after cat 1A's illness began.
• Cat 1C showed no signs of illness and tested negative for influenza A.
• There were no other pets. All four people had daily contact with all three cats.

The adult who worked on a dairy farm did not handle the animals but worked on the premises, in a county known to have H5N1-positive cattle. The farm was not known to be affected, but it was near farms that were. The farm's manager said no animals there were ill; the worker said many of the farm's barn cats had recently died. The worker took off work clothes and boots outside before coming home, and they were kept where the cats could not reach them. Neither the cats nor the family drank raw milk or ate raw milk products. The owners were not asked whether their pets ate raw pet food.

The three other household members were swabbed 11 days after cat 1A fell ill and all tested negative for influenza A. They were given a 10-day course of oseltamivir twice a day as prevention; the report notes that 5 days is the recommendation, but a longer course can be considered when exposure continues. The dairy worker declined both testing and the medicine, and reported a day of vomiting and diarrhoea just before cat 1A became ill.

The report counts one person as ill: adolescent 1A, who had no other health conditions, developed a cough, sore throat, headache and muscle aches 6 days after cat 1A fell ill, and tested positive only for rhinovirus/enterovirus. Adolescent 1B reported a dry, croup-like cough on the same day, put down to severe allergies; contact ended before it was known how that resolved.

Illness, test results and follow-up for each person and cat in the two households, by day after cat 1A fell ill. CDC.

Household 2

Six days after cat 1A's referral, the owner of cat 2A, a six-month-old intact male Maine Coon kept only indoors, brought him straight to the university's veterinary centre after a day of worsening neurological signs, loss of appetite, lethargy and a swollen face. He was barely responsive, with abnormal cranial nerves and movement and puffy eyes and nose, and he died within 24 hours of falling ill. Nasal swabs were positive for influenza A and confirmed as the same H5N1 clade and genotype. Cat 2B, the household's other indoor cat, stayed well and tested negative.

The owner lived alone and hauled raw milk from several farms in a Michigan county that included farms with confirmed infected cattle — the same county where cat 1A's owner lived. The owner wore no PPE when handling raw milk, was often splashed on the face, eyes and clothes, and did not change out of work clothes before going indoors. Cat 2A used to roll in those clothes; cat 2B did not. The owner's eyes became irritated 2 days before cat 2A fell ill, with no other symptoms. The owner was not tested, declined oseltamivir and ended contact with health officials, citing fear of losing work for talking to them and implicating the farms that supplied the milk.

Veterinary staff

Twenty-four staff at the local practice and the university centre may have been exposed to the two cats: one veterinarian, five nurses, three technicians, five assistants, two caregivers, three interns and five students. Eighteen (75%) were reached and monitored for 10 days after their last exposure. Their exposure was limited, so they were not offered preventive medicine. The isolation unit where the cats were treated has PPE rules — for H5N1, a Tyvek suit, boot covers, nitrile gloves, a surgical head cover and a face mask — but use varied from gloves alone to the full protocol. Staff probably wore surgical masks when cat 1A first came in, and N95 masks afterwards; laboratory staff wear powered air-purifying or N95 respirators. Seven staff reported symptoms such as nasal congestion or headache; the five who agreed to testing were negative for influenza A.

What it means

H5N1 clade 2.3.4.4b has been found in U.S. wild birds, poultry and wildlife since 2022 and in commercial dairy cattle since 2024. On farms, serious illness and death in cats have been attributed to drinking raw milk from infected cows or eating wild birds or raw poultry products. The longer the virus circulates in animals, the greater the chance of mutations that could help it adapt to mammals and spread to and between people — something documented in domestic cats. Isolated cases of cow-to-human spread have occurred in California, Colorado, Michigan and Texas, and a low pathogenic bird flu, A(H7N2), is presumed to have passed from cats to a person at an animal shelter in 2016, suggesting that infected cats could also be a risk to people.

Infection of indoor cats is rarely reported. How these two were infected is unknown, but their owners worked on dairy farms and may have been exposed to infected cattle or contaminated products or surroundings. More research is needed on whether the virus can reach cats on objects such as clothing, or by other routes. Neither worker used the recommended PPE, and because neither was tested, whether one worker's stomach upset or the other's eye irritation was H5N1 is unknown. This investigation came before cats were found infected, presumably through contaminated commercial raw pet food, in late 2024; the virus in that outbreak was a different genotype.

What to do

• Farmworkers are encouraged to consider taking off work clothes and footwear, and rinsing off milk, manure and other animal residue, before going into the house.
• Veterinarians seeing pet cats with respiratory or neurological illness where H5N1 is circulating in cattle, poultry or other animals should wear PPE when examining them or taking samples, and ask about household members' jobs, so that exposures can be avoided and human, animal and environmental health officials can investigate together.
• Standard precautions against diseases passed from animals, and CDC guidance for veterinary clinics, can limit how many staff are exposed to a sick animal.
• Anyone exposed at work or during recreation to animals that may be infected should wear the recommended PPE.

The report's text puts cat 1C's negative test 11 days after cat 1A fell ill, while its figure shows it on day 9; this page gives neither.

Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.

Sources

• Ramya Naraharisetti, Meghan Weinberg, Becky Stoddard, Mary Grace Stobierski, Kimberly A. Dodd, and others, "Highly Pathogenic Avian Influenza A(H5N1) Virus Infection of Indoor Domestic Cats Within Dairy Industry Worker Households — Michigan, May 2024," Morbidity and Mortality Weekly Report. https://www.cdc.gov/mmwr/volumes/74/wr/mm7405a2.htm
• Authors from CDC, the Michigan Department of Health and Human Services, the Mid-Michigan District Health Department, the Michigan State University College of Veterinary Medicine and the Michigan Department of Agriculture and Rural Development.