Ecological characteristics of co-circulating low pathogenicity avian influenza subtypes at Delaware Bay, USA

Background/Question/Methods

While much attention has been given to high pathogenicity avian influenza (HPAI), low pathogenicity avian influenza (LPAI), which is important for endemic avian influenza (AIV) transmission, is more common. To date, 104 LPAI subtype combinations have been detected in North America, and are maintained in wild waterfowl with little or no mortality. Recent studies indicate that some geographic areas serve as AIV transmission hotspots, harboring several different subtypes; one such hotspot is Delaware Bay.

In an effort to understand the mechanisms of coexistence, we conducted statistical studies of LPAI prevalence in shorebird host species at Delaware Bay. Data comprised 9754 shorebird samples of 431 subtypes collected between 2000 and 2008. The Chao 1 and ACE species richness estimators were used to estimate the absolute number of co-circulating subtypes. Diversity was measured using Simpson's index and relative dominance. To determine whether dynamics are controlled by a single host species, we compared results for prevalence in the dominant host species (Ruddy Turnstone) with community-wide prevalence.

Results/Conclusions

In total, 52 subtype combinations were found to co-circulate in Delaware Bay within a 9 year period. Determination of subtype diversity by species revealed that Ruddy Turnstones harbor a majority of the co-circulating subtypes at Delaware Bay. Estimation of overall subtype diversity showed considerable variation in the amount of subtype diversity by year. Prevalence by host species revealed that Ruddy Turnstones had the highest prevalence of LPAI at Delaware Bay. An examination of subtype dominance revealed that the dominant subtype changed each year, and often the new dominant subtype for a year was not detected in previous and subsequent years.

This is the first study to examine the ecological characteristics of co-circulating LPAI subtypes. Our results suggest that half of the North American subtypes co-circulate in Delaware Bay, 38 of which are present in Ruddy Turnstones. This pattern, combined with the non-detection of the dominant subtype in previous or subsequent years, suggests that Ruddy Turnstones are both an indicator species and probable dominant host driving subtype dynamics at Delaware Bay. This study serves to elucidate some of the basic ecological characteristics of co-circulating LPAI subtypes, which will aid in future studies of AIV dynamics.