A new CDC report seems to indicate that there is more transmission of bird flu from animals to humans that are undetected because the cases are asymptomatic. The CDC had been suspicious that there may be more bird flu cases than had been detected was backed by a voluntary sero survey among veterinary practitioners who work with cattle.

As I have written previously, the problem isn’t that these cases are dangerous, it’s that the virus could undergo reassortment, when an individual is infected by two viruses at the same time, the viruses can trade genetic material. If the asymptomatic bird flu cases are a larger problem than we thought, then the bird flu virus might be jumping between species and may become a public health concern.

This post will review this new CDC data (and it’s good that we’re getting new CDC data) and describe what it may mean.

Asymptomatic bird flu study

In a paper published on 13 February 2025 in the CDC’s Morbidity and Mortality Weekly Report, Samantha M Olson, MPH, Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, and colleagues tested 150 veterinary practitioners who worked with cattle for antibodies indicating recent infection with influenza Type A subtype H5N1 bird flu based on blood samples collected in September 2024.

The researchers found that three out of the 150 had antibodies to H5N1. Out of those three, none reported exposure to cattle with known or suspected bird flu infection. However, one of the individuals did work with bird flu-positive poultry. Interestingly, one had been practicing in Georgia and South Carolina, two states with no known H5N1-infected cattle.

The authors concluded:

These findings suggest that there might be HPAI [highly pathogenic avian influenza] A(H5)-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.

Summary

Since the first cattle-to-human transmission of H5N1 in April 2024, there have been 69 confirmed total reported human cases in the USA. These cases have been mild although one person in Louisiana died from H5N1, although they had several comorbidities. Most of the cases have been in California and people with exposure to commercial cattle or poultry agriculture.

Although they only found three cases of asymptomatic H5N1 bird flu in these veterinary practitioners, it is concerning that they showed no symptoms and they may have passed the disease to others. This could mean that there are many more H5N1 infections than we thought.

I know I’ve been writing a lot about H5N1 lately, it’s mostly to keep me and the readers up-to-date on the latest news because we don’t want to relive the misinformation of the last pandemic.

The CDC continues to state that the current risk of bird flu to the general public remains low. I don’t think we’re on the brink of a pandemic, but then again, I thought the same about COVID. I’ll keep writing about this until I think the risk has completely gone.

Citations

• Leonard J, Harker EJ, Szablewski CM, Margrey SF, Gingrich KF 2nd, Crossley K, Fletcher E, McCreavy CJ, Weis-Torres S, Wang D, Noble EK, Levine MZ, Pagano HP, Holiday C, Liu F, Jefferson S, Li ZN, Gross FL, Reed C, Ellington S, Mellis AM, Olson SM. Notes from the Field: Seroprevalence of Highly Pathogenic Avian Influenza A(H5) Virus Infections Among Bovine Veterinary Practitioners – United States, September 2024. MMWR Morb Mortal Wkly Rep. 2025 Feb 13;74(4):50-52. doi: 10.15585/mmwr.mm7404a2. PMID: 39946278; PMCID: PMC11824947.