Seven per cent of dairy workers tested had already had bird flu

Infections with highly pathogenic avian influenza A(H5) have been detected
sporadically in dairy farm workers in the United States since April 2024,
and the response has included **active monitoring of exposed workers for
illness**.

Sporadically. Health officials in Michigan and Colorado went looking for
the cases nobody had noticed.

What the blood showed

**Serologic testing indicated that 7% of participating dairy workers had
evidence of recent infection with HPAI A(H5) virus.**

Serology finds antibodies, which means it finds infections that happened
whether or not anyone was tested, reported ill, or felt particularly unwell at
the time.

One in fourteen — against a national picture of a handful of confirmed
individual cases.

What that changes

The implication the report draws is about very mild illness:

**The findings support the need for active monitoring of exposed workers and
testing to detect and treat HPAI A(H5) infections, including those in
persons with very mild symptoms.**

A surveillance system that tests workers who are ill enough to seek care will
find the severe end of a distribution and miss the rest. If most infections in
this group are mild, the case count is a poor guide to how much virus is
moving between cattle and people.

And that matters beyond the workers themselves: every human infection is an
opportunity for the virus to adapt, and the ones nobody counts are the ones
nobody investigates.

**These efforts should be coupled with farmworker education about infection
risks and prevention measures.**

*Source: Mellis AM, Coyle J, Marshall KE, et al. Serologic Evidence of Recent
Infection with Highly Pathogenic Avian Influenza A(H5) Virus Among Dairy
Workers — Michigan and Colorado, June–August 2024. MMWR, Centers for Disease
Control and Prevention.*