Individual variation in influenza A virus infection histories and long-term immune responses in Mallards

Abstract

Wild dabbling ducks (genus Anas) are the main reservoir for influenza A virus (IAV) in the Northern Hemisphere. Current understanding of disease dynamics and epidemiology in this virus-host system has primarily been based on population-level surveillance studies and infection experiments conducted in laboratory settings. Using a combined experimental-natural approach with wild-strain captive mallards (Anas platyrhynchos), we monitored individual IAV infection histories and immunological responses of 10 birds over the course of 15 months. This is the first detailed study to track natural IAV infection histories over several seasons amongst the same individuals growing from juvenile to adults. The general trends in the infection histories of the monitored birds reflected seasonal variation in prevalence at the population level. However, within the study group there were significant differences between individuals in infection frequency as well as in short and long term anti-IAV antibody response. Further observations included individual variation in the number of infecting virus subtypes, and a strong tendency for long-lasting hemagglutinin-related homosubtypic immunity. Specifically, all infections in the second autumn, except one, were of different subtypes compared to the first autumn. The variation among birds concerning these epidemiologically important traits illustrates the necessity for IAV studies to move from the level of populations to examine individuals in order to further our understanding of IAV disease and epidemiology.

Figure 1. Number of influenza A virus infection (RRT-PCR positive) days per month in 2009 and 2010 given for 10 individual mallards kept under close to natural conditions in close proximity to wild mallards.

Figure 2. Mean number of influenza A virus (RRT-PCR) positive samples per day over the autumn months in 2009 and 2010 (corrected for sample effort).

Means and 95% confidence intervals are provided, based on samples taken every day of 10 mallards kept under close to natural conditions in close proximity to wild mallards.

Figure 3. Changes in influenza A virus shedding (measured as C_T-value in RRT-PCR positive faecal samples) with progress of autumn in 2009 and 2010.

Figure 4. Temporal change of anti-NP-antibodies in 10 mallards kept under close to natural conditions in close proximity to wild mallards.

Data are presented as the inversed monthly mean of the sample to negative control ratio for each individual. The cut-off for positivity is shown by a hatched line. Colours correspond to Figure 1.