Among all 115 dairy workers, 46 (40%) reported feeling ill shortly before or during the period that A(H5) virus infection was confirmed in cows on the farms where they worked (Table 3). Four of these illnesses were among the eight workers with serologic evidence of infection; among these persons, signs and symptoms most frequently reported were red, draining, or itching eyes (three). These signs and symptoms were also frequently reported among workers who were ill but who had negative HPAI A(H5) serology (26 of 42; 62%). Among the four workers with positive test results, feverishness, sore throat, runny or stuffy nose, sneezing, diarrhea, and headache were each reported by one worker; these signs and symptoms were also reported by persons with negative serology results. Among persons with serologic evidence of infection, illness onset occurred a median of 5 days before the date of detection of HPAI A(H5) virus among cows within the dairy where they worked.
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 findings in this report are subject to at least five limitations. First, enrolled persons volunteered to participate; therefore, this sample might not be representative of all farmworkers. Second, no demographic or medical history data were collected to examine host factors associated with infection. Third, the fraction of HPAI A(H5) infections that are completely asymptomatic might be lower than the frequency of persons with positive serologic results who did not report illness in this report, because of perceptions of mild or subclinical illness and inability to recall. Fourth, PPE questions were not cross-referenced with specific job duties, limiting inferences that can made about PPE effectiveness. Finally, some persons with negative serologic results might have been infected but failed to mount detectable antibody responses for a variety of reasons.
Primary prevention of HPAI A(H5) virus infections in animals, including dairy cows, is critical to reducing the risk for human infection and mitigating changes in the virus that could lead to a potential HPAI A(H5) pandemic. During the period cattle are infected, employers can reduce the risk for worker infection by following CDC recommendations for engineering controls, worker education on the proper use of PPE, other administrative controls (e.g., testing animals for HPAI A(H5) and developing plans to monitor workers for illness), and providing appropriate PPE to workers (8). This investigation identified low PPE adherence among dairy workers, which has been an ongoing challenge in hot, tight spaces where visibility around large animals is important and the use of eye protection can be challenging (10). Increased use of PPE might be achieved through adapting current recommendations to meet the needs of dairy farm workers such as simplifying messaging and focusing on highest risk activities (10). Employers should prioritize implementation of controls in hot work environments (e.g., worker training acclimatizing protocols, and work/rest schedules) to minimize heat exposures and heat injuries while wearing PPE. Another challenge in these environments with significant sources of particulate matter and bioaerosols (e.g., dirt, feces, and milk), is that mild irritation of eyes or the respiratory tract can occur frequently; a low threshold for reporting mild symptoms and seeking testing should be encouraged to identify whether these mild symptoms are caused by HPAI A(H5) virus. Public health practitioners should modify messaging to address the unique setting of exposed dairy workers to identify and treat all HPAI A(H5) virus infections , including mild infections. Finally, data from additional serosurveys could identify additional risk factors for infection and continue refinement of best practices for prevention.