Jul 28, 2006 (CIDRAP News) – Researchers who studied Iowa duck hunters and wildlife workers have reported the first laboratory evidence of transmission of an avian influenza virus from wild birds to humans, though not the deadly H5N1 strain.
The researchers report that blood tests of a duck hunter and two wildlife workers revealed evidence of past infection with a lesser known-strain of avian flu virus, influenza A/H11N9. The three men had handled many wild birds in their hunting and work activities.
"Although the sample size of our study was relatively small, our results suggest that handling wild waterfowl, especially ducks, is a risk factor for direct transmission of avian influenza virus to humans," says their report in the August issue of Emerging Infectious Diseases.
Most human cases of H5N1 avian flu infection reported over the past 3 years have been associated with exposure to sick domestic birds, although several cases of human-to-human transmission have been recorded, most recently in a family case cluster in Indonesia in May. Wild waterfowl often carry influenza viruses, usually without looking sick.
The researchers conducted what they believe to be the first documented search for avian flu viruses in waterfowl hunters. The team included scientists from the University of Iowa in Iowa City and St. Jude Children's Research Hospital in St. Louis, with James S. Gill of the University of Iowa as first author.
The study group consisted of 39 men who hunted ducks at a southeastern Iowa wildlife refuge in October 2004 and 68 employees of the Iowa Department of Natural Resources (DNR), many of whom were duck hunters or had captured and banded wild ducks and geese in their work. Overall, they had handled wild birds an average of about 20 years.
Flu viruses were widespread among ducks at the time of the study, with 60% of a sample of mallards testing positive, the report says.
The researchers took blood samples from all the volunteers and used microneutralization assay to test the serum for avian influenza A subtypes H1 through H12. In addition, hemagglutination inhibition was used to test the hunter serum samples for subtype H11.
The microneutralization assay showed that one 39-year-old hunter and two male DNR workers, aged 52 and 53, had antibodies to an H11N9 virus. None of the three men had received flu shots in the preceding 3 years.
The hunter tested negative for all the other virus subtypes in the study, but serum from the two DNR workers reacted to an H2N2 avian flu virus, the report says. The latter finding, the authors say, probably means that the two men were naturally infected with the human H2N2 virus, which was derived from birds and circulated from 1957 to 1967.
All three men had more than 25 years of duck-hunting experience, and one of the DNR workers had participated for several years in duck-banding projects. They did not wear gloves, masks, or eye protection when handling birds. The researchers did not try to find out if the men had experienced any illness related to their contact with wild birds.
"In our study, a less common hemagglutinin subtype (H11) has apparently caused serologically detectable infections in high-exposure groups, whereas the more common hemagglutninin subtypes H4 and H6 in wild ducks have not," the authors write. Possible reasons, they say, are that H11 viruses may be more able to infect humans or may induce a relatively strong immune response, or that the available serologic tests are more sensitive to H11 than to other strains.
The researchers say their findings are consistent with a 1991 study in which 40 volunteers were inoculated with the avian virus subtypes H4N8, H6N1, and H10N7. Eleven of the volunteers experienced a mild illness, but they produced no detectable antibodies, probably because the viruses did not multiply enough to generate much of an immune response, according to the abstract of the study.
The authors of the Iowa study write that their findings, in combination with the 1991 study, suggest that people may contract flu viruses from wild birds more often than serologic testing suggests.
"The relative lack of antibody response in our study population, who had substantial exposures to waterfowl with influenza A infections, and in inoculated volunteers from Beare and Webster [the 1991 study] suggests that avian influenza infections in humans exposed to wild waterfowl may occur more commonly than we are able to detect with current methods," they write.
Gill JS, Webby R, Gilchrist MJR, et al. Avian influenza among waterfowl hunters and wildlife professionals. Emerg Infect Dis 2006;12(8) [Full text]
Beare AS, Webster RG. Replication of avian influenza viruses in humans. Arch Virol 1991;119(1-2):37-42 [PubMed abstract]