In this report we document serological evidence that Iowans who self-reported hunting birds or recent poultry work had elevated antibodies against low pathogenic avian H5, H6, and H7 viruses representing strains recently circulating in the United States. These data are consistent with previous investigations identifying hunting [9
] and poultry agriculture [1
] as risk factors for avian influenza infections in man.
These data are unique in that we avoided using a cutpoint in titer (binary outcome) approach which we have previously shown severely limits analytical power [19
]. Instead we compared the entire distribution of antibody titers against exposures with the proportional odds model. We conjecture that had a proportional odds modeling method been used in other studies that used binary outcome methodology [6
], their results would have likely been very different. Our study is also unique in that we had a nonexposed control group. Without such a control group it is difficult to evaluate titer activity among the exposed.
One might ask “What do the findings mean?” While these data do not show the magnitude of risk (odds radios) that our study of US veterinarians who work with poultry demonstrated [10
], these study data support the position that US hunters and poultry workers are at increased risk of recreational or occupational avian influenza virus infections. We posit here as we have detailed before [24
], that their increased risk merits special public health attention. They should be educated about their increased risk, encouraged to use personal protective equipment, and to seek medical attention whenever they develop an influenza-like-illness. They should also receive priority access to annual and pandemic influenza vaccines so that they do not facilitate the reassortment of novel strains of influenza virus [24
] and do not accelerate human or avian influenza epidemics [26
Our cross-sectional data are limited in that we cannot discern if the increase in antibody reflects infection or simply antigen exposure. However, other reports do seem to shed light on these questions. Hayden and Croisier [5
] considered similar findings among Italian poultry workers and concluded that the low prevalence of antibody and temporal association with avian influenza epizootics argued for human infection with avian viruses as an explanation. We agree and further argue that as vaccine-generated immunity to influenza viruses wanes over time and as inactivated avian virus immunization may require large doses of unadjuvanted antigen [27
] to cause an increase in antibody, one might point out that positive serological findings are more likely to represent true infection with avian viruses and their replication in tissue. Regarding the question of clinical illness we can only speculate as relatively few prospective studies of humans exposed to ill birds have been conducted. However, the available data suggest that subclinical avian influenza virus infections may be more common than expected [1
]. Comprehensive, prospective studies of large numbers of poultry-exposed individuals and their contacts are required to better understand the spectrum of illnesses associated with clinical avian influenza virus infections.
Our study data have a number of other limitations. A number of potential biases may have influenced results: voluntary participation, self-reporting exposure data, a younger mean age of the university controls, potential mismatching between study and circulating viruses, possible cross-reacting antibodies against avian viruses, and passive surveillance for humans with acute avian influenza virus infections. However, as previously described [13
] adjustments have been made to examine or reduce these limitations, and study findings are biologically plausible and consistent with previous reports.
In summary, these data suggest that US bird hunters and poultry workers are at increased risk of avian H5, H6, and H7 influenza virus infections. Efforts should be made to include these citizens in influenza pandemic preparedness plans.