Influenza-A viruses in ducks in northwestern Minnesota: fine scale spatial and temporal variation in prevalence and subtype diversity

Abstract

Waterfowl from northwestern Minnesota were sampled by cloacal swabbing for Avian Influenza Virus (AIV) from July-October in 2007 and 2008. AIV was detected in 222 (9.1%) of 2,441 ducks in 2007 and in 438 (17.9%) of 2,452 ducks in 2008. Prevalence of AIV peaked in late summer. We detected 27 AIV subtypes during 2007 and 31 during 2008. Ten hemagglutinin (HA) subtypes were detected each year (i.e., H1, 3-8, and 10-12 during 2007; H1-8, 10 and 11 during 2008). All neuraminidase (NA) subtypes were detected during each year of the study. Subtype diversity varied between years and increased with prevalence into September. Predominant subtypes during 2007 (comprising ≥5% of subtype diversity) included H1N1, H3N6, H3N8, H4N6, H7N3, H10N7, and H11N9. Predominant subtypes during 2008 included H3N6, H3N8, H4N6, H4N8, H6N1, and H10N7. Additionally, within each HA subtype, the same predominant HA/NA subtype combinations were detected each year and included H1N1, H3N8, H4N6, H5N2, H6N1, H7N3, H8N4, H10N7, and H11N9. The H2N3 and H12N5 viruses also predominated within the H2 and H12 subtypes, respectively, but only were detected during a single year (H2 and H12 viruses were not detected during 2007 and 2008, respectively). Mallards were the predominant species sampled (63.7% of the total), and 531 AIV were isolated from this species (80.5% of the total isolates). Mallard data collected during both years adequately described the observed temporal and spatial prevalence from the total sample and also adequately represented subtype diversity. Juvenile mallards also were adequate in describing the temporal and spatial prevalence of AIV as well as subtype diversity.

Figure 1. The location of sampled areas for avian influenza virus in northwestern Minnesota, USA, 2007 and 2008.

Figure 2. Epidemic curves showing the within-year temporal change in percent of ducks in northwestern Minnesota infected with avian influenza virus, 2007 (red lines) and 2008 (black lines; A =  all species; B =  Mallards; C =  Adult Mallards; and D =  Juvenile Mallards).

Trend lines were calculated using SigmaPlot software (Systat Software Inc., Richmond California, USA) using a peak, log normal, 3 parameter function (A.B, and D) and exponential decay, single, 2 parameter functions (C).

Figure 3. The temporal variation of subtype diversity of avian influenza viruses from ducks captured in northwestern Minnesota, USA (A = 2007; B = 2008).

This graph demonstrates that the peak collection period to gather subtypes is early September.

Figure 4. The number of subtype combinations of avian influenza virus that were detected in ducks captured at multiple sites in northwestern Minnesota, USA, 2007 and 2008.