Emerging disease and zoonoses #20–subclinical infections with avian influenza

I’ve mentioned previously the potential role that mild or asymptomatic infections with influenza may play in transmission and evolution of the virus. Right now, most of our reports of H5N1 have been due to serious infections that caused a patient to seek medical care. These cases are the tip of the iceberg in a normal influenza outbreak, when most of us don’t have physician-diagnosed influenza. Instead, we recognize the signs and symptoms, and stay at home to ride it out. Is this happening in Indonesia, Thailand, and elsewhere? For every person who shows symptoms of H5N1 infection, are there 2 more that have been infected but remained well, or only suffered mild illness? We don’t know yet. However, a new study by some folks here at CEID suggests that even in the US, people in close contact with waterfowl can be infected with avian influenza viruses.

This was just a small pilot study, enrolling 39 duck hunters and 68 Department of Natural Resources (DNR) workers, including many who’d been in close contact with ducks and geese either due to their job or hobby hunting. Their sera were then tested against a panel of avian influenza isolates, and three samples were found to be positive. As noted in the paper, these individuals had roughly 3 decades apiece of duck-hunting experience, handling hundreds (even thousands) of ducks over the years. All were positive for a H11 serotype virus, which is not known to infect humans. This was somewhat surprising, as all the samples were negative for other serotypes that have previously been implicated in human infection. They say:

In our study, a less common hemagglutinin subtype (H11) has apparently caused serologically detectable infections in high-exposure groups, whereas the more common hemagglutinin subtypes H4 and H6 in wild ducks have not. The reason for this finding is unknown but may include the following: 1) H11 may have increased ability to infect humans, 2) H11 may provoke a relatively strong and detectable immune response, and 3) our serologic assays may be more sensitive in detecting H11 infection than other H subtypes.

This is always a problem when testing for something completely novel. You don’t have positive control sera from a human who’s been infected with the virus, so it’s difficult to really validate your test. In this case, they used 3 different methods of testing: a microneutralization assay, and two different types of hemagglutinin inhibition (HI) assays. All tests gave similar results, thereby strengthening the confidence of their findings (since, if they were due simply to chance, it would be unlikely that the same 3 serum samples would react with all 3 tests, with the others remaining negative).

So, this research suggests that human infection with avian influenza viruses may be occurring more often than we’re aware of. However, the effect–if any–that this has on the generation of a human-adapted avian virus with pandemic potential remains unknown, and will stay that way until larger surveillance studies are carried out to determine how often these trans-species infections actually occur.

[Edited to add: link to Reuters story on the article]


Gill et al. 2006. Avian influenza among waterfowl hunters and wildlife professionals. EID. 12:1284-6. Link.

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