CDC report: Avian influenza found in two cats linked to dairy workers

Two indoor cats living in separate homes with dairy farmworkers experienced severe neurological signs and died from infection with highly pathogenic avian influenza type A H5N1, according to recent reports.

The report appeared in the February 20 issue of the Centers for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report (MMWR).

Samples from both cats were genetically sequenced at Michigan State University’s (MSU) Veterinary Diagnostic Laboratory, which identified the H5N1 virus as clade 2.3.4.4b, genotype B3.13, the same strain of bird flu circulating among dairy cattle in Michigan and 16 other states. The U.S. Department of Agriculture’s National Veterinary Services Laboratories confirmed the results.

The cases prompted an investigation by staff members at the Michigan Department of Health and Human Services and Mid-Michigan District Health Department.

“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 researchers wrote. “Further research is necessary to evaluate the risk of fomite transmission and other types of transmission routes of HPAI A(H5N1) virus to cats.”

Case reports

The two cases occurred in May 2024 within days of each other and involved a 5-year-old female domestic shorthair cat and a 6-month-old male Maine Coon cat.

According to the MMWR report, the domestic shorthair cat lived exclusively indoors with two other cats in a household of two adults—including the farm worker—and two teenagers. The worker reportedly removed work clothes and boots outside the home, placing them in an area not accessible to the cats. The cats and household members did not drink unpasteurized milk.

The cat initially stopped eating as much, had decreased energy, and appeared disoriented. Worsening neurologic signs became evident the next day, including ataxia, and the cat was taken to a local veterinary clinic. By the fourth day, the cat’s condition rapidly deteriorated and a referral was made to Michigan State’s Veterinary Medical Center (VMC). Clinical signs included cranial nerve dysfunction, motor impairment in all four limbs, a swollen right jaw, and an unsteady gait. The cat was euthanized when it could no longer hold up its head.

Of the other two cats in the household, one was reported to have signs of watery eye discharge, increased respirations, and decreased appetite, which resolved 11 days after the first cat's illness onset. The third cat had no signs of illness and tested negative for influenza A viruses.

Six days after the domestic shorthair cat was sent to the VMC, the Maine Coon cat was brought to the center by its owner, another dairy farmworker, with a one-day history of progressive neurological deterioration, anorexia, lethargy, and facial swelling. Upon examination, the cat was losing consciousness, with abnormalities of cranial nerve function, abnormal motor function, puffiness of the eyes and nose, and minimal movement. The cat died within 24 hours. Another cat lived with the Maine Coon and dairy worker but did not show any signs of illness, and nasal swabs tested negative for influenza A viruses.

The owner of the Maine Coon cat worked on multiple dairy farms transporting unpasteurized milk. The worker reported having been splashed in the face and eyes by unpasteurized milk, but declined influenza A testing. The worker also reported eye irritation two days before the cat’s illness.

Exposure and protective measures

Though the source of H5N1 infection in these two cats remains unknown, the cats’ owners had occupational exposures to H5N1-positive dairy cattle or contaminated products or environments.

Neither dairy farmworker wore personal protective equipment (PPE) on the job, and both declined testing for influenza A virus after their cats tested positive for H5N1. Other household members in the home with the domestic shorthair cat tested negative for influenza A.

Given the potential for fomite contamination, the report’s authors suggest that farmworkers should consider removing clothing and footwear and rinsing off any animal byproduct residue prior to entering households.

Twenty-four veterinary staff members were also exposed to the two sick cats. Eighteen were successfully contacted and enrolled by public health authorities for 10 days of symptom monitoring after their last exposure to the cats. “Among seven people who reported signs or symptoms after exposure to the ill cats, including four who reported nasal congestion and three who reported headache, five agreed to testing,” according to the report. All tested negative for influenza A.

The report’s authors cautioned that veterinarians evaluating pet cats with signs of respiratory or neurologic illness in areas where H5N1 virus is circulating should wear PPE when examining these animals or collecting specimens for influenza testing. They also recommended obtaining occupational information from household members to help prevent unprotected exposures and guide coordinated public health investigations of potential animal-to-human spread of 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,” the report’s authors wrote. “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.”