Study: Three bovine veterinarians unknowingly infected with H5N1

Since its initial detection in March 2024, infection with highly pathogenic avian influenza type A  H5N1 in dairy cows has become widespread in the U.S., affecting at least 968 herds in 16 states.

In mid-September 2024, a serosurvey examined the prevalence of recent H5N1 infections among U.S veterinary practitioners exposed to cattle.

The results of that study, “Seroprevalence of Highly Pathogenic Avian Influenza A(H5) Virus Infections Among Bovine Veterinary Practitioners,” was published in the Centers for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report (MMWR) on February 13.

Among 150 practitioners tested, three had evidence of recent infection with H5N1, including two who hadn’t been exposed to animals confirmed or suspected to have H5N1 infections and one who did not practice in a state with confirmed H5N1-infected cattle.

“These data highlight the possible benefit of national seroprevalence assessments of recent HPAI A(H5) infection among practitioners at increased risk for exposure, which might help assess occupational risk in states without confirmed HPAI A(H5) virus detections in dairy cattle,” the study authors wrote.

The CDC, American Association of Bovine Practitioners (AABP), and the Ohio Department of Health (ODH) jointly conducted the study.

CDC and ODH staff members surveyed attendees of the 2024 AABP conference in September who had worked with cattle during the previous three months. They were asked about exposures to various species of animals; what personal protective equipment they had worn around these animals; whether they were vaccinated for seasonal influenza; and their thoughts regarding a potential H5N1 vaccine, if one were to be authorized.

CDC epidemiologists obtained blood samples so they could test for antibodies against the virus.

Veterinarians who participated in the study practiced in 46 states and Canada. Slightly more than half (55%) reported practicing in states with H5N1-positive dairy herds, and 17% worked with dairy cattle with known or suspected H5N1 infection.

Among the three veterinarians who had H5N1 antibodies detected:

• None reported respiratory or flu-like symptoms, including conjunctivitis, nor had any been tested for influenza since January 2024.
• None had worked with dairy cattle confirmed or suspected to have H5N1 infections; however, one practitioner did work with poultry confirmed to be infected with H5N1.
• Two said they practiced in multiple states, and two practiced in states with known H5N1 infections among cattle. One had only practiced in two states (Georgia and South Carolina) with no known H5N1 infections in cattle and no reported human cases. This practitioner reported no exposures to animals with confirmed or suspected H5N1 infections.
• All three reported wearing gloves or a clothing cover when providing veterinary care to cattle, but none reported wearing respiratory or eye protection.

Since the serosurvey was conducted, the H5N1 outbreak has expanded to include confirmed H5N1 infections in 68 people in the U.S. Twenty-three of these cases were associated with exposure to infected poultry; 41 were associated with exposure to infected dairy cows; three exposure sources remain unknown; and one source of infection remains unknown with potential exposure to other animals such as backyard flocks, wild birds, or other mammals.

Most human H5N1 infections have involved H5N1 clade 2.3.4.4b, genotype B3.13, but recently, the D1.1 genotype was confirmed in dairy cows and a dairy worker in Nevada. Genotype D1.1 represents the main genotype in the North American flyways this past fall and winter, and has been identified in wild birds, mammals, and spillovers into domestic poultry.

The USDA continues to emphasize the importance of practicing rigorous biosecurity and offers financial assistance to herd owners for planning and implementing enhanced biosecurity practices. The USDA’s Animal and Plant Health Inspection Service (APHIS) is also working closely with state partners on surveillance, reporting, and control efforts.