The most prevalent subtype combinations we found, H4N6, H7N7, and H6N2, were also found in the German study.
Those subtypes showing the high reassortment rates in our study, especially H1N1, H3N8 and H4N6 represent the most common subtype combinations that are found in wild ducks during periods of peak prevalence and have a maximum detectable subtype diversity in the late summer and fall [ 32, 50].
The most frequent subtype combination among northern pintail viruses isolated in California was H11N9 (23%), followed by H5N2 (20%).
The influenza A virus, including all its subtypes and most of their subtype combinations, is commonly found in aquatic birds such as ducks, geese, gulls, and shorebirds, while only a limited number of subtypes have been found in nonavian hosts.
The most commonly identified HA and NA subtype combinations during season 1 were H3N8 and H6N1; during season 2, H3N8 remained, but it was not detected during season 3.
Similarly, both the H4N6 and the H6N2 subtype combinations were among the most common in the North American survey (2 ).
H1N1 was the 18th most frequently identified HA-NA subtype of the 91 subtype combinations isolated.
The prevalence of different subtype combinations may vary from season to season.
These subtype combinations differed substantially from virus isolates from Alaska northern pintails analyzed by Koehler et al. (2008), where H3N8 was the most frequent subtype (Fig. 2).
