Influenza/A viruses naturally infect aquatic wildfowl like ducks and there are a lot of influenza/A subtypes besides the H5N1 that has been in the news. Some people are heavily exposed to wild ducks, namely serious duck hunters and game handlers. Why don’t they get infected with some of the other influenza virus subtypes? It turns out nobody has looked to see if they do until now. In a paper just published in CDC’s journal, Emerging Infectious Diseases scientists from the University of Iowa and St. Jude’s Children’s Hospital report that on occasion it is possible to see evidence of infection with other influenza/A subtypes.
The study looked at blood samples from 39 Iowa duck hunters and 68 Iowa Department of Natural Resources game handlers, many of whom worked banding (marking) wild ducks. Three of these subjects had antibodies against one influenza subtype, H11N9, the first time we have seen evidence of human infection from this virus and the first report of transmission to humans of influenza virus from wild birds. All of the positive subjects had long time and substantial histories of contacts with wild ducks, one as a hunter with a history of one to two months duck hunting in the marshes per year harvesting 100 ducks and handling another 300 per season. This even exceeded the two DNR employees who also had in excess of 30 year histories. None of the three were smokers (a risk factor for other zoonotic diseases, presumably from the hand to mouth contamination entailed by smoking) and none used any personal protective gear while working or hunting. No health histories were obtained so it isn’t known if the subjects experienced any symptoms. For other avian viruses (H5N1 excepted) when illnesses occur they are usually mild or asymptomatic. Infections with both H7 and H9 subtypes have previously been reported, but not H11.
How do we interpret these findings? Previous studies infecting human volunteers with H4N8, H6N1 and H10N7 were successful resulting in some viral shedding and mild symptoms but not detectable antibody response (Beare and Webster, “Replication of avian influenza viruses in humans,” Arch Virol. 1991;119(1-2):37-42). Thus we know that humans can be infected but not show serological signs. The evidence of H11 infection in three subjects may be because the H11 subtype is more immunogenic, produces a heavier infection or because the current assays were more sensitive than those used in the previous 1991 study. The absence of serological evidence of H4 and H6 antibodies despite the fact that these subtypes are quite prevalent in wild ducks ducking the hunting and banding seasons suggests the sensitivity of the assay is not the explanation, however.
Taken together, the authors suggest that infection with avian infuenza/A subtypes may be more common than serological evidence indicates. Whether this also holds true for the H5N1 subtypes we don’t know, but it does raise the question as to whether still scant negative seroprevalence data for H5N1 accurately reflects the rate of infection in human populations in contact with infected poultry, sick patients and possibly other animal reservoirs.