Influenza virus in a natural host, the mallard: experimental infection data

Abstract

Wild waterfowl, particularly dabbling ducks such as mallards (Anas platyrhynchos), are considered the main reservoir of low-pathogenic avian influenza viruses (LPAIVs). They carry viruses that may evolve and become highly pathogenic for poultry or zoonotic. Understanding the ecology of LPAIVs in these natural hosts is therefore essential. We assessed the clinical response, viral shedding and antibody production of juvenile mallards after intra-esophageal inoculation of two LPAIV subtypes previously isolated from wild congeners. Six ducks, equipped with data loggers that continually monitored body temperature, heart rate and activity, were successively inoculated with an H7N7 LPAI isolate (day 0), the same H7N7 isolate again (day 21) and an H5N2 LPAI isolate (day 35). After the first H7N7 inoculation, the ducks remained alert with no modification of heart rate or activity. However, body temperature transiently increased in four individuals, suggesting that LPAIV strains may have minor clinical effects on their natural hosts. The excretion patterns observed after both re-inoculations differed strongly from those observed after the primary H7N7 inoculation, suggesting that not only homosubtypic but also heterosubtypic immunity exist. Our study suggests that LPAI infection has minor clinically measurable effects on mallards and that mallard ducks are able to mount immunological responses protective against heterologous infections. Because the transmission dynamics of LPAIVs in wild populations is greatly influenced by individual susceptibility and herd immunity, these findings are of high importance. Our study also shows the relevance of using telemetry to monitor disease in animals.

Figure 1. Experimental design.

Figure 2. Significant (p<0.05) day to day changes in mean daily body temperature, heart rate and activity recorded for each duck throughout the experiment using General Linear Models and Tukey's post hoc test.

For body temperature, when both DSI and iButton data loggers were functional, only the changes detected by both data loggers are indicated.

Figure 3. Body mass for each mallard throughout the study.

Figure 4. Matrix gene RTT-PCR results for the six implanted mallards.

Dash line: H7N7 inoculation, dot line: H5N2 inoculation. Results for oral swabs are presented as supportive material online (Figure S1).

Figure 5. Matrix gene RTT-PCR results for the two control mallards.

Dash line: H7N7 infection, dot line: H5N2 infection; euthanasia occurred on day 7 post-inoculation. Results for oral swabs are presented as supportive material online (Figure S2).

Figure 6. ELISA and matrix gene RTT-PCR results for the six implanted mallards.

Primary y-axis: ELISA inhibition values (blue line: NP-ELISA, pink line H7-ELISA and red line: H5-ELISA); secondary y-axis (black line): minimum ct-value considering oral, cloacal, fecal and water samples; the vertical lines illustrate the successive inoculations (dash line: H7N7 inoculation, dot line: H5N2 inoculation).