Impact of body condition on influenza A virus infection dynamics in mallards following a secondary exposure

Abstract

Migratory waterfowl are often viewed as vehicles for the global spread of influenza A viruses (IAVs), with mallards (Anas platyrhynchos) implicated as particularly important reservoir hosts. The physical demands and energetic costs of migration have been shown to influence birds' body condition; poorer body condition may suppress immune function and affect the course of IAV infection. Our study evaluated the impact of body condition on immune function and viral shedding dynamics in mallards naturally exposed to an H9 IAV, and then secondarily exposed to an H4N6 IAV. Mallards were divided into three treatment groups of 10 birds per group, with each bird's body condition manipulated as a function of body weight by restricting food availability to achieve either a -10%, -20%, or control body weight class. We found that mallards exhibit moderate heterosubtypic immunity against an H4N6 IAV infection after an infection from an H9 IAV, and that body condition did not have an impact on shedding dynamics in response to a secondary exposure. Furthermore, body condition did not affect aspects of the innate and adaptive immune system, including the acute phase protein haptoglobin, heterophil/lymphocyte ratios, and antibody production. Contrary to recently proposed hypotheses and some experimental evidence, our data do not support relationships between body condition, infection and immunocompetence following a second exposure to IAV in mallards. Consequently, while annual migration may be a driver in the maintenance and spread of IAVs, the energetic demands of migration may not affect susceptibility in mallards.

Fig 1. Median Treatment Weights.

The median weights of each treatment group every week over the duration of the study. Error bars show +/- one standard error for each respective day. Line A denotes the beginning of diet manipulation and Line B denotes when the birds were experimentally inoculated.

Fig 2. Heterosubtypic Immunity.

Box plots summarize the distribution of total viral shedding in oral/cloacal swabs taken from the H9-exposed control group and naïve ducks from another study [22]. Boxes show the interquartile range (IQR, middle 50% of values) for each group, horizontal lines represent medians, and vertical lines are values within 1.5*IQR. Outliers are plotted as individual points.

Fig 3. PCR Results.

Line plots of median viral RNA concentrations. The areas under the curves are indexes for the total amount of virus detected across the infection. (A) oral swabs, (B) cloacal swabs, and (C) fecal swabs.

Fig 4. Immune Function.

(A) Boxplots of the interpolated S/N ratios over the duration of the study. Values above the threshold S/N ratio of 0.7 (the red line) are considered positive for antibodies to IAV. (B) Boxplots of haptoglobin concentrations over the duration of the study. (C) Boxplots of heterophil/lymphocyte ratios over the duration of the study.