Quantitative exposure assessment of waterfowl hunters to avian influenza viruses

Abstract

The potential for direct transmission of type A influenza viruses from wild waterfowl to humans is undefined. This study estimated exposure of hunters to avian influenza virus (AIV) resulting from direct contact with potentially infected waterfowl in Georgia (GA), Louisiana (LA) and Minnesota (MN), and demonstrated variation in the risk of exposure to AIV by hunting location and time. Hunting begins earlier in MN, starting in October, and later in GA and LA, usually starting in November. In addition, the numbers of hunters and birds harvested varies considerably in each state, with LA hosting the largest harvest in the USA Temporal effects resulted in variation of the exposure risk per hunter-day, with a higher risk associated with the earlier months of the hunting season. Exposure risk in locations varied due to AIV prevalence during each hunting season, average bird harvest per hunter-day, and ratio of juveniles/adult birds harvested (higher risk associated with higher ratios). Population risk is discussed based on the exposure risk and number of active hunters in each state per month. The risk of human exposure to AIV was also shown to be temporally distinct from the time of greatest risk of human influenza A infection during circulation of seasonal human influenza viruses, making recombination events due to co-infection unlikely.

Fig. 1.

Summary measures for the random variable Eviral-load_ist, which represents the estimated viral load a hunter is exposed to when a hunter-day results in exposure. The right axis indicates the percentage of iterations which resulted in non-zero exposure (Enon-zero_ist) (). GA, Georgia; LA, Louisiana; MN, Minnesota.

Fig. 2.

Distribution of estimated values for the random variable Eviral-load_ist, for Louisiana, in December, for iterations with non-zero exposure (2·1%).

Fig. 3.

Expected size of the hunter population in each state and month, according to the number of weekends of open hunting per month (left axis) and probability of hunter exposure to any load of avian influenza viruses (Enon-zero_ist) (right axis). GA, Georgia; LA, Louisiana; MN, Minnesota.

Fig. 4.

Comparison between the relative population risk of avian influenza infection in hunters (combining exposure risk and active hunter population per month) and the prevalence of human influenza A (lines, right axis) per month, from September to January. Different lines represent different influenza seasons. GA, Georgia; LA, Louisiana; MN, Minnesota.