North American wintering mallards infected with highly pathogenic avian influenza show few signs of altered local or migratory movements

Abstract

Avian influenza viruses pose a threat to wildlife and livestock health. The emergence of highly pathogenic avian influenza (HPAI) in wild birds and poultry in North America in late 2021 was the first such outbreak since 2015 and the largest outbreak in North America to date. Despite its prominence and economic impacts, we know relatively little about how HPAI spreads in wild bird populations. In January 2022, we captured 43 mallards (Anas platyrhynchos) in Tennessee, USA, 11 of which were actively infected with HPAI. These were the first confirmed detections of HPAI H5N1 clade 2.3.4.4b in the Mississippi Flyway. We compared movement patterns of infected and uninfected birds and found no clear differences; infected birds moved just as much during winter, migrated slightly earlier, and migrated similar distances as uninfected birds. Infected mallards also contacted and shared space with uninfected birds while on their wintering grounds, suggesting ongoing transmission of the virus. We found no differences in body condition or survival rates between infected and uninfected birds. Together, these results show that HPAI H5N1 clade 2.3.4.4b infection was unrelated to body condition or movement behavior in mallards infected at this location during winter; if these results are confirmed in other seasons and as HPAI H5N1 continues to evolve, they suggest that these birds could contribute to the maintenance and dispersal of HPAI in North America. Further research on more species across larger geographic areas and multiple seasons would help clarify potential impacts of HPAI on waterfowl and how this emerging disease spreads at continental scales, across species, and potentially between wildlife and domestic animals.

Figure 1

Local movement patterns are unrelated to infection with HPAI H5N1 clade 2.3.4.4b in 43 mallards (Anas platyrhynchos) sampled in Tennessee, USA during winter 2022. In each plot, points show raw data, lines show estimated means from a linear mixed-effects model, and shaded areas show 95% confidence intervals of the mean. Models also included terms for age, sex, and a temporal autoregressive term for each individual; plots show marginal values averaged across age and sex. For plots that show predictions conditional on random effects, see Fig. S1. (A) Area of a 100% minimum convex polygon (MCP), a measurement of space use. (B) Mean hourly step lengths, a measurement of overall movement. (C) Net displacement, i.e., mean daily distance from the first GPS fix, a measurement of dispersal from the capture site.

Figure 2

Time spent in the potentially HPAI-contaminated area declines prior to initiation of migration. The contaminated area was defined as the total area of all 95% utilization distributions of HPAI-infected mallards (Anas platyrhynchos) in the first four days following sampling. The proportion of time was calculated as the proportion of daily fixes for each mallard within the contaminated area. Points show raw data and are jittered to increase visibility. The line and shaded area show marginal means from a generalized linear mixed-effects model. The model also included terms for HPAI infection status, age, and sex; only sex was related to time spent in the contaminated area (Table S5, Fig. S3).

Figure 3

Relationships between HPAI infection status, sex, and migration patterns in mallards (Anas platyrhynchos). Each panel shows the estimated mean and 95% confidence interval of the mean from a linear model. Partially transparent points show raw data. Models also included a term for age; plots show values for juveniles. (A) HPAI-infected birds departed on spring migration slightly earlier than uninfected birds and males migrated earlier than females. The y-axis shows the day of year of spring migration initiation (day 80 = March 21). (B) The duration of migration was unrelated to infection status. (C) Migration distance was unrelated to infection status, but males migrated farther than females. (D) Migration speed was unrelated to infection status or sex.