#18,399
As we've discussed many times before, HPAI H5N1 infection can produce a wide range of symptoms in humans; ranging from mild or even asymptomatic presentation, to severe and life threatening illness.
As an example, Cambodia's 16 H5N1 cases over the past 2 years have demonstrated a 40% fatality rate, while all of North America's reported cases have been mild.
Some of these differences in virulence are likely due to different strains (clades, or genotypes) of the virus, while in other cases it could have more to do with the route of exposure, viral load, or even in differences in immunity and/or health of the patients (see Differences In Virulence Between Closely Related H5N1 Strains).
We've seen evidence that - even with the more severe strains - that some mild and/or asymptomatic infections have occurred. A small sampling of past studies include:
- In 2012, in H5N1 Seroprevalence Among Jiangsu Province Poultry Workers, we saw a study that found across three locations tested (Gaochun, Jianhu and Gaoyou counties) the percentage of workers testing positive ranged from zero (Gaochun) to 5.38% (95%CI, 2.19%–10.78%) in Gaoyou.
- In May of 2009 (see Cambodian Study Finds Rare Asymptomatic H5N1 Infections) we saw a study published in the Journal of Infectious Diseases on more than 600 members of a Cambodian village where 2 human H5N1 cases were detected in 2006. Antibody titers showed that only 1% (7 of 674) of the villagers tested had contracted, and fought off, the H5N1 virus. A figure much lower than many had expected.
Today, the CDC has published an update to that seroprevalence study in the MMWR, which expanded the number of farm workers tested to 115 across 2 states (Michigan & Colorado), and which returned a significantly different result.
Eight of those tested (7%) tested were positive for antibodies for H5N1, yet half of them could recall no symptoms. Four others reported `very mild' symptoms.
Some excerpts from today's report, after which I'll have a postscript:
Weekly / November 7, 2024 / 73(44);1004–1009
Alexandra M. Mellis1; Joseph Coyle2; Kristen E. Marshall3,4; Aaron M. Frutos1,5; Jordan Singleton5,6; Cara Drehoff3,5; Angiezel Merced-Morales1; H. Pamela Pagano1; Rachel O. Alade5,7; Elizabeth B. White1; Emma K. Noble1; Crystal Holiday1; Feng Liu1; Stacie Jefferson1; Zhu-Nan Li1; F. Liani Gross1; Sonja J. Olsen1; Vivien G. Dugan1; Carrie Reed1; Sascha Ellington1; Sophia Montoya3; Allison Kohnen3; Ginger Stringer3; Nisha Alden3; Peter Blank2; Derick Chia2; Natasha Bagdasarian2; Rachel Herlihy3; Sarah Lyon-Callo2; Min Z. Levine1 (VIEW AUTHOR AFFILIATIONS)View suggested citation
Summary
What is already known about this topic?
Infections with highly pathogenic avian influenza (HPAI) A(H5) viruses have been detected sporadically in dairy farm workers in the United States since April 2024. Public health response efforts include active monitoring of workers exposed to HPAI A(H5) virus for illness.
What is added by this report?
Health officials conducted surveys and serologic testing to identify recent HPAI A(H5) infections among dairy workers in two states. Serologic testing indicated that 7% of participating dairy workers had evidence of recent infection with HPAI A(H5) virus.
What are the implications for public health practice?
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. These efforts should be coupled with farmworker education about infection risks and prevention measures.
(SNIP)
Discussion(Continue . . . )
In this analysis, 7% of exposed dairy farm workers in Michigan and Colorado had serologic evidence of infection with HPAI A(H5). These data reaffirm the importance of identifying and implementing interventions to prevent dairy cattle infections to reduce worker exposure and using infection prevention measures among farm workers when HPAI A(H5) virus infection is confirmed or suspected in a herd.¶¶¶¶ Before the emergence of clade 2.3.4.4.b viruses, estimates of anti-HPAI A(H5) seroprevalence among workers exposed to infected poultry were approximately 0%–0.6% globally (4) and approximately 4.6% in Egypt after the emergence of clade 2.3.4.4.b viruses in poultry (5). Preliminary data available from a single dairy in the United States showed that two of 14 exposed workers had elevated neutralizing antibodies against HPAI A(H5) (6). These data from Michigan and Colorado provide the largest sample to date, estimating the risk to dairy farm workers associated with the ongoing cattle epizootic.
Among workers who had antibodies to HPAI A(H5) virus, all (100%) reported cleaning the milking parlor, compared with 38% of workers without HPAI A(H5) virus antibodies. Cleaning the milking parlor might be a higher-risk workplace activity given the high HPAI A(H5) viral load in the milk of infected cows (7). None of the workers with HPAI A(H5) virus antibodies reported using the PPE recommended for working with HPAI A(H5)–infected animals, and use of recommended PPE was low among all workers (8). These findings support the need for improved outreach to employers and workers about the risk for infection when working with dairy cattle infected with HPAI A(H5) viruses, and for the use of infection prevention measures such as PPE (8). Only one of the persons whose test results indicated antibodies to HPAI A(H5) virus reported working with known HPAI A(H5) virus–infected cows, supporting the need for additional education and outreach to employers and farm workers once HPAI A(H5) is identified in herds. Because most workers (and all those with positive serology results) spoke Spanish, this outreach should be culturally appropriate (9) and delivered in the workers’ spoken languages. Approximately 80% of the dairy workers from this investigation population might also benefit from outreach offering seasonal influenza vaccination.
One half of the persons with antibodies to HPAI A(H5) virus did not report illness; asymptomatic infection has been observed in past HPAI A(H5) serologic investigations (4). Some of the persons who did not report being ill might have experienced only very mild symptoms. This finding highlights the need to actively monitor exposed workers by assessing the presence of any mild symptoms and provide a safe environment that encourages reporting of even mild illness and allows for rapid treatment with antivirals to prevent progression to severe disease, without risk for repercussions in terms of job security and pay (8). Some of the persons with antibodies to HPAI A(H5) virus reported illnesses before herds were identified, underscoring the need for early outreach to dairy workers and rapid identification of herds as through expanded herd testing***** and bulk milk testing programs.†††††
The CDC is expected to release updated guidance on the testing of exposed, but asymptomatic farm workers, more aggressive use of prophylactic antivirals, and new recommendations for PPEs today or tomorrow.
