Epizootiology of avian influenza: effect of season on incidence in sentinel ducks and domestic turkeys in Minnesota.

Sentinel ducks and domestic turkey flocks were monitored for influenza infection during a 4-year period. The onset of infection among ducks was similar each year, occurring in late July or early August. Influenza in turkeys was also shown to be seasonal, but the usual onset was 6 to 8 weeks after the detection of influenza in sentinel ducks. Possible explanations for the delayed infection in turkeys are (i) increased waterfowl activity associated with fledging and congregating in late summer and early fall; (ii) vectors transmitting virus from the waterfowl habitat to poultry farms; (iii) cooler environmental temperature, allowing prolonged virus viability; (iv) cooler surface water temperature, allowing prolonged virus viability; (v) groundwater contamination from contaminated surface water; and (vi) virus adaptation in domestic turkeys before infection is detected. We conclude that ducks are not only a natural reservoir of influenza but also have a seasonal infection that appears to be related to seasonal influenza outbreaks in domestic turkeys in Minnesota. However, only some influenza A virus isolates circulating among waterfowl at any given time appear capable of causing detectable infection in turkeys. It is speculated that the seasonal infection in migratory waterfowl may also be related to seasonal influenza infections in other species including humans.