Cow Flu Caution

Highly pathogenic avian influenza (HPAI) A(H5) viruses have circulated among animals globally since 1997, but their emergence in US dairy cattle in March 2024 marked a new animal reservoir. Now, a study published by the US Centers for Disease Control and Prevention (CDC) suggests that human infections with the H5N1 avian influenza virus, also referred to as "cow flu," may be significantly underdiagnosed among US dairy farm workers.

Of 115 dairy workers tested in Michigan and Colorado from June to August 2024, seven percent showed evidence of recent H5N1 infections. Though no signs of human-to-human transmission were detected, these findings raise concerns about the extent of exposure in workers who handle infected cattle.

The CDC study focused on workers at farms with lab-confirmed HPAI A(H5) infections. Among the eight seropositive individuals, four reported mild illness – with symptoms including conjunctivitis, sore throat, and nasal congestion – around the time cows were sick. Notably, the remaining four seropositive individuals were asymptomatic.

A key observation was that all the seropositive workers cleaned milking parlors, whereas only 38 percent of noninfected individuals performed this task. Despite CDC recommendations, adherence to personal protective equipment (PPE) use was low; none of the seropositive workers used respiratory protection, and only three reported using eye protection.

The study highlights the importance of early herd testing, enhanced monitoring, and preventive measures to reduce human exposure. CDC guidelines recommend increased outreach efforts to educate dairy workers, particularly in culturally and linguistically appropriate formats, about infection risks and proper PPE usage. The findings also emphasize the need for broader surveillance to detect asymptomatic or mild cases, which can go unnoticed without systematic testing.

Although there is no evidence of sustained human-to-human transmission, the study underscores the importance of mitigating risks associated with zoonotic spillover events, particularly in occupational settings. Future efforts will focus on expanding testing and refining prevention strategies to protect agricultural workers and the broader community from potential influenza outbreaks.