Bird Flu Antibodies in a Veterinary Worker Exposed to an Infected Cat — Los Angeles County

Summary

• Known before: transmission of influenza A(H5N1) from domestic cats to people had not been documented.
• New here: from November 2024 to January 2025, 139 people were exposed to 19 H5N1-infected cats in Los Angeles County, California, that had eaten raw animal products. Of 25 exposed people who gave blood for testing, one — a veterinary professional with no symptoms — had antibodies showing H5N1 infection after exposure to an infected cat at work.
• What it means: this is evidence of zoonotic spread of H5N1 from a cat to a person. Pet owners are advised not to feed cats raw animal products. Veterinary staff should know the risk, use personal protective equipment (PPE) and follow infection control practices.

Background

Since 2021, highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b viruses have spread widely among wild birds and poultry in the United States, spilling over more and more into mammals — dairy cattle, domestic cats and people. Most U.S. human cases have been mild and linked to known exposure to sick or infected animals. In cats, H5N1 often causes severe disease, linked to raw milk, raw meat, raw pet food or dairy farm environments. Close contact between cats and people — especially owners and veterinary staff — could create chances for exposure.

The sick cats

• 19 cats from five households fell ill with severe respiratory, liver or neurologic signs; 14 died or were euthanized.
• Nine were tested, and all were positive for H5N1 clade 2.3.4.4b genotype B3.13.
• Every owner had fed their cats commercially bought raw milk, raw poultry or raw pet food in the weeks before illness; some of those products had tested positive for H5N1.
• 14 cats had been seen at 10 veterinary practices. Cases were found through veterinarians' and commercial laboratory reports, routine flu testing of brain tissue from euthanized cats sent for rabies testing, and links to confirmed cases.

Following the exposed people

The Los Angeles County Department of Public Health (LACDPH) interviewed owners and reviewed staff lists from the 10 clinics and an animal control agency that carried cat carcasses to its laboratory. It identified 139 exposed people — anyone who had handled, or taken part within 6 feet in the care of, a suspected or confirmed infected cat, with or without PPE:

• All were monitored for symptoms for 10 days after exposure through daily text-message questionnaires, and offered testing; oseltamivir was offered case by case to people with compatible symptoms within 5 days of exposure.
• 30 (22%) reported flu-like symptoms — runny nose, cough, sore throat, fatigue, muscle aches, headache, sneezing — an average of 7 days after exposure.
• 33 people were swabbed (18 symptomatic veterinary staff, 14 without symptoms, and one pet owner), a median of 8 days after exposure. None tested positive for H5N1. Of 12 positive respiratory panels, nine were seasonal flu A(H3N2), two rhinovirus, and one rhinovirus plus a common coronavirus.

Testing results for people exposed to cats with suspected or confirmed H5N1 infection. Chart: CDC.

The blood survey

In April 2025, all exposed people were invited to give a blood sample and answer questions about exposures, PPE use and other risks. CDC tested the samples in enhanced biosafety level 3 laboratories by microneutralization and hemagglutination inhibition, against two H5N1 viruses (genotypes B3.13 and D1.1), after removing cross-reactions with seasonal flu. A titer of 40 or more in both tests counted as positive.

• 25 (18%) took part, an average of 104 days after exposure.
• One (4%) was positive: a veterinary professional without symptoms, sampled 120 days after exposure to an ill cat, with antibodies to both viruses — which are antigenically similar to the virus from the infected cats.

The cat and the worker

The cat: an indoor cat fell ill after eating commercial raw pet food (a poultry blend) and was seen at four practices over 11 days for respiratory signs, lung lesions, transient ataxia and hind-leg weakness, and progressive uveitis with retinal hemorrhage, detachment and blindness. It survived, with permanent vision loss. 31 people were exposed to it. A swab taken at the second visit came back 7 days later positive for feline calicivirus and influenza A; federal sequencing identified H5N1 B3.13 2 weeks after that. Because the cat moved between clinics before results arrived, and the result's public health meaning was not passed on, staff did not know they had been exposed.

The worker stayed symptom-free, and a flu test 7 days after exposure was negative. Their duties included restraining animals, assisting in surgery, giving inhaled anesthesia, cardiopulmonary resuscitation, intubation, collecting many kinds of samples, and cleaning exam rooms. Staff at the clinic routinely wore gloves but no other PPE, and the worker used no respiratory or eye protection. They had no other known H5N1 risk — no other infected cats, poultry, wild birds, dairy cattle or raw animal products — no underlying conditions, and had received neither the 2024–25 flu vaccine (which is not meant to prevent H5N1) nor preventive antivirals.

What it means

These findings are serologic documentation of possible transmission of H5N1 from a domestic cat to a person, an infection that would otherwise have gone undetected. Animal-to-human flu involving cats and big cats has been seen before: a veterinarian infected with low-pathogenicity H7N2 by shelter cats in New York City, and zoo workers who developed antibodies after an H5N1 outbreak among captive tigers in Thailand. The exposure also happened during a seasonal H3N2 outbreak, raising the risk of co-infection and reassortment — which could produce a new virus able to spread between people.

Limitations: not everyone was tested, so some infections may have been missed, and blood was drawn 4–5 months after exposure, when antibodies may have waned. Paired early and later samples should be considered in future outbreaks.

What to do

• Pet owners: don't feed cats raw milk or other raw animal products, which have been documented to infect pets with H5N1.
• Veterinarians: consider H5N1 in cats with sudden respiratory or neurologic illness, and use PPE and infection prevention practices.
• Public health: a prompt One Health response to infected cats — confirming cases, finding the source, and quickly evaluating and testing exposed people — is essential.

Sources

Based on Vaughan A, Joyce A, Traub E, et al., "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," MMWR Morbidity and Mortality Weekly Report, volume 75, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report writes the clade both "2.3.4.4b" and "2.3.4.4.b"; the first is used.