#18,617
Last summer we looked at a (small) seroprevalence study out of Michigan, that tested 35 farm workers exposed to H5N1 infected cattle, which reassuringly found no evidence of neutralizing or HI antibodies specific to avian influenza A(H5N1) virus.
A few months later, however - in November's MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers - we learned that in an extended sampling of 115 farm workers (in Michigan & Colorado) Eight (7%) showed antibodies for H5N1, yet half of them could recall no symptoms, while four others reported `very mild' symptoms.
This report, along with numerous anecdotal reports of symptomatic farm workers going untested for the virus (see EID Journal: Avian Influenza A(H5N1) Virus among Dairy Cattle, Texas, USA), strongly suggested that some (mild or subclinical) cases were being missed by surveillance.
These findings prompted the CDC to release Multiple revised H5N1 Guidance Documents, geared towards farm workers, their employers, clinicians, veterinarians, public health officials, and the general public.
Today we've a new study, published in this week's MMWR which looks at the seroprevalence of H5 antibodies in veterinarians who work with dairy cattle. Out of 150 samples studied, 3 (2%) were positive for H5 antibodies.
None of these veterinarians suspected they had ever contracted the virus, and two deny working with infected cattle (including one who worked in two states that have never reported bovine H5). A finding the authors suggest may indicate `. . . . there could be U.S. states with A(H5)-positive people and animals that have not yet been identified.'It should be noted that these samples were gather at an annual veterinary conference held over two days (12th & 13th) last September in Ohio, at a time when just 14 human H5N1 infections had been reported nationally, and only 4 were linked to cattle exposure.
I've posted much of the report below, but you'll want to follow the link for the full references and related material. I'll have a brief postscript after the break.Since then, the number of identified infected cattle herds has nearly quintupled, as have the number of confirmed human infections in the United States.
Notes from the Field: Seroprevalence of Highly Pathogenic Avian Influenza A(H5) Virus Infections Among Bovine Veterinary Practitioners — United States, September 2024
Weekly / February 13, 2025 / 74(4);50–52
Jerome Leonard1,2; Elizabeth J. Harker1; Christine M. Szablewski1; Sara F. Margrey3; K. Fred Gingrich II4; Keyana Crossley5; Emily Fletcher3; Claire J. McCreavy1; Sabrina Weis-Torres6; Dennis Wang2,7; Emma K. Noble1; Min Z. Levine1; H. Pamela Pagano1; Crystal Holiday1; Feng Liu1; Stacie Jefferson1; Zhu-Nan Li1; F. Liaini Gross1; Carrie Reed1; Sascha Ellington1; Alexandra M. Mellis1; Samantha M. Olson1 (VIEW AUTHOR AFFILIATIONS)View suggested citation
Summary
What is already known about this topic?
Highly pathogenic avian influenza (HPAI) A(H5) virus infections have been detected in humans exposed to infected dairy cattle.
What is added by this report?
Public health officials conducted a serosurvey among 150 bovine veterinary practitioners. Three practitioners had evidence of recent infection with HPAI A(H5) virus, including two without exposures to animals with known or suspected HPAI A(H5) virus infections and one who did not practice in a U.S. state with known HPAI A(H5) virus–infected cattle.
What are the implications for public health practice?
These findings suggest the possible benefit of systematic surveillance for rapid identification of HPAI A(H5) virus in dairy cattle, milk, and humans who are exposed to cattle to ensure appropriate hazard assessments.
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The current outbreak of highly pathogenic avian influenza (HPAI) A(H5) clade 2.3.4.4.b viruses, genotype B3.13, among dairy cattle was first detected in March 2024 (1), with human cases of HPAI A(H5) among dairy farm workers identified beginning in April (2). Farm workers and bovine veterinary practitioners working with HPAI A(H5) virus–infected cattle are at increased risk for HPAI A(H5) exposure; in the current outbreak, most human infections with HPAI A(H5) have been mild and were detected through enhanced surveillance of persons working with affected animals (2).
Investigation and Outcomes
To investigate the prevalence of recent HPAI A(H5) infection among U.S. bovine veterinary practitioners, CDC conducted an HPAI A(H5) serosurvey (September 12–13, 2024) with a target enrollment of 150 bovine veterinary practitioners with cattle exposure in the previous 3 months and assessed their exposures since January 2024. Practitioners were recruited in-person at an annual veterinary conference and through emails to conference attendees. Participation was anonymous, and participants received a $50 USD gift card as compensation after the blood draw. At the time of this serosurvey, HPAI A(H5) in dairy cattle had been detected in 14 U.S. states, with four human cases in persons with dairy cattle exposure in three states.* This serosurvey was reviewed and approved by the Ohio Department of Health institutional review board and was conducted consistent with applicable federal law and CDC policy.†
All 150 surveyed practitioners were serologically tested for antibodies to recent HPAI A(H5) virus infection (3). Participants reported their primary practice in 46 U.S. states (143) and Canada (seven). Among all survey participants, 82 (55%) practiced in states with HPAI A(H5) virus–positive dairy herds, and 25 (17%) worked with dairy cattle with known or suspected HPAI A(H5) infection.§
Three (2%; 95% CI = 0.7%–5.7%) survey participants had antibodies to HPAI A(H5)¶ suggestive of recent HPAI A(H5) infection; all were U.S.-based practitioners. None of the practitioners with positive serology results reported respiratory or influenza-like symptoms, including conjunctivitis** nor had any received testing for influenza since January 2024.
All three practitioners with positive serology results provided care to multiple animals,†† including dairy cattle; two also provided care to nondairy cattle, one provided care to poultry, and one worked at livestock markets. None worked with dairy cattle with known or suspected HPAI A(H5) virus infection; however, one practitioner did work with HPAI A(H5) virus–positive poultry. Two of the participants with a positive serologic test result reported practicing in multiple U.S. states, and two practiced in states with known HPAI A(H5) infection among cattle (Figure). However, one reported providing veterinary care to dairy cattle only in Georgia and to nondairy cattle in South Carolina; these states had not previously reported HPAI A(H5) infection in dairy cattle (1). All reported wearing gloves or a clothing cover when providing veterinary care to cattle (including a variety of clinical activities, such as pregnancy checking or surgery)§§; none reported wearing respiratory or eye protection.
Preliminary Conclusions and Actions
Among 150 bovine veterinary practitioners, three had evidence of recent infection with HPAI A(H5) virus, including one who only practiced in two states (Georgia and South Carolina) with no known HPAI A(H5) virus infection in cattle and no reported human cases (1,2); this practitioner reported no exposures to animals with known or suspected HPAI A(H5) virus infections. These findings suggest that there might be HPAI A(H5) virus–infected dairy cattle in states where infection in dairy cattle has not yet been identified, highlighting the importance of rapid identification of infected dairy cattle through herd and bulk milk testing as recently announced by the U.S. Department of Agriculture.¶¶
No practitioners with positive HPAI A(H5) serology results in this study reported influenza-like symptoms, including conjunctivitis. Detection of HPAI A(H5) antibodies in persons without reported symptoms suggests that surveillance of symptomatic exposed workers might underestimate human infection. CDC recently recommended offering the influenza antiviral oseltamivir as postexposure prophylaxis or treatment and HPAI A(H5) molecular testing to asymptomatic workers with high exposure*** to infected animals (4). Continued efforts to disseminate these guidelines are important for persons with exposure to dairy cattle, including bovine veterinary practitioners.
No seropositive practitioner knew that they were working with dairy cattle with known or suspected HPAI A(H5) infection. None of the seropositive practitioners reported wearing respiratory or eye protection while providing veterinary care to cattle. Neither respiratory or eye protection is recommended when working with uninfected animals in regions without confirmed cases; however, safety goggles and a respirator are recommended when working with uninfected animals in regions where there are confirmed or potentially infected animals (4). HPAI A(H5) virus is known to be present in high concentrations in milk produced by infected cattle, introducing infection risk through respiratory, ocular, and gastrointestinal exposure (5). Continued systematic surveillance of livestock and milk could aid in appropriate occupational hazard assessment.†††
Since the time that this serosurvey was conducted, the HPAI A(H5) outbreak has expanded to include 67 confirmed human cases, including 40 with dairy cattle exposure (1,2). 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.
We now have two (relatively small) seroprevalence studies - conducted more than 6 months ago - which suggest that somewhere between 2% and 7% of those working with farm animals may have antibodies for the H5 virus.
Most of those with positive results had no idea they had been exposed or infected.
On one hand, it is comforting that so many people experienced mild or subclinical infections, but it is sobering that so many spillover events have gone unnoticed. Particularly when they are presumably still occurring now, during the height of flu season, raising the chances of seeing a reassortment.
Kudos are rightfully deserved for the researchers who took the initiative to mount this study, and the volunteers who provided blood samples.
But the lack of nationwide, aggressive testing of livestock (and humans) - now nearly a year into this crisis - is both disappointing and hard to understand.
