H5 Bird Flu Antibodies in 3 of 150 Cattle Veterinarians, United States, 2024

In short

• Highly pathogenic avian influenza (HPAI) A(H5) infections have been found in people exposed to infected dairy cattle.
• In a blood survey of 150 cattle veterinarians, 3 showed evidence of recent H5 infection. Two had not worked with any animals known or suspected to be infected, and one did not practise in any state with known infected cattle.
• The findings point to the possible value of systematic surveillance, to find H5 virus quickly in dairy cattle, milk and the people exposed to cattle, so that hazards can be properly assessed.

Background

The current outbreak of HPAI A(H5) clade 2.3.4.4.b, genotype B3.13, in dairy cattle was first detected in March 2024, and human cases among dairy farm workers were identified from April. Farm workers and veterinarians who work with infected cattle are at higher risk of exposure. In this outbreak most human infections have been mild, and they were found through stepped-up surveillance of people working with affected animals.

The survey

On September 12–13, 2024, CDC carried out a serosurvey — a test for antibodies in the blood — aiming to enrol 150 bovine veterinary practitioners who had worked with cattle in the previous 3 months, and asked about their exposures since January 2024. Practitioners were recruited in person at an annual veterinary conference and by email to people attending it. Taking part was anonymous, and participants received a $50 gift card after giving blood. At the time, H5 had been detected in dairy cattle in 14 states, and there had been four human cases with dairy cattle exposure, in three states. The Ohio Department of Health's institutional review board approved the survey.

All 150 practitioners were tested for antibodies indicating recent H5 infection. Their main practice was in 46 U.S. states (143 people) or Canada (seven). In all, 82 (55%) practised in states with H5-positive dairy herds, and 25 (17%) had worked with dairy cattle known or suspected to be infected.

What was found

Three practitioners (2%; 95% confidence interval 0.7%–5.7%) had antibodies suggesting recent H5 infection, all based in the United States. None reported respiratory or flu-like symptoms, conjunctivitis included, and none had been tested for influenza since January 2024.

• All three cared for several kinds of animals, dairy cattle among them. Two also treated other cattle, one treated poultry and one worked at livestock markets.
• None had worked with dairy cattle known or suspected to be infected, though one had worked with H5-positive poultry.
• Two practised in more than one state, and two practised in states with known H5 infection in cattle. The third treated dairy cattle only in Georgia and other cattle in South Carolina — states that had not reported H5 in dairy cattle.
• All wore gloves or protective clothing when caring for cattle, during work such as pregnancy checks or surgery. None wore respiratory or eye protection.

States where H5 had been detected in dairy cattle as of September 2024, where enrolled practitioners practised, and where the practitioners with positive results practised. CDC.

What it means

One of the three worked only in two states with no known H5 infection in cattle and no reported human cases, and reported no contact with animals known or suspected to be infected. That suggests there may be infected dairy cattle in states where infection has not yet been identified, and underlines the importance of finding infected herds quickly through herd and bulk milk testing, as recently announced by the U.S. Department of Agriculture.

None of the three had symptoms. Finding antibodies in people without symptoms suggests that watching only exposed workers who feel ill may undercount human infections. CDC has recently recommended offering the antiviral oseltamivir, as prevention after exposure or as treatment, along with H5 testing, to workers without symptoms who have had high exposure to infected animals. Getting that guidance to people who work with dairy cattle, veterinarians included, remains important.

None of the three knew they were working with infected cattle, and none wore respiratory or eye protection. Such protection is not recommended for work with uninfected animals where there are no confirmed cases, but goggles and a respirator are recommended for work with uninfected animals where infected or possibly infected animals have been confirmed. Milk from infected cattle carries high concentrations of the virus, a route for infection through the airways, the eyes and the gut. Continued systematic surveillance of livestock and milk could help assess the hazards of the work.

Since the survey, the outbreak has grown to 67 confirmed human cases, 40 of them with dairy cattle exposure. National antibody surveys of practitioners at higher risk could help gauge the risk to workers in states where H5 has not been confirmed in dairy cattle.

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

Sources

• Jerome Leonard, Elizabeth J. Harker, Christine M. Szablewski, Sara F. Margrey, K. Fred Gingrich II, and others (Samantha M. Olson, corresponding author), "Notes from the Field: Seroprevalence of Highly Pathogenic Avian Influenza A(H5) Virus Infections Among Bovine Veterinary Practitioners — United States, September 2024," Morbidity and Mortality Weekly Report. https://www.cdc.gov/mmwr/volumes/74/wr/mm7404a2.htm
• Authors from CDC, the Ohio Department of Health and the American Association of Bovine Practitioners.