Of feathers and flu: Identifying data gaps in bird flu host dynamics to help conserve vulnerable species

Johanna Harvey, an assistant professor of wildlife disease wildlife ecology at the University of Rhode Island, has described bird flu in public presentations as a quiet virus with loud consequences. 

Now she’s published a new paper in Wildlife Monographs, describing how circulating avian influenza viruses (HPAIV) show an expanded set of susceptible hosts, including many migratory wild birds, and higher transmission rates. In the paper, Harvey examines data gaps in avian influenza host dynamics to prioritize wildlife conservation — and protect human health. 

The flagship wildlife science journal publishes only a small number of papers annually, presenting a thorough, book-length analysis on a particular wildlife topic. The journal is now offering a foundational, go-to reference on avian flu with input from Harvey, an evolutionary ecologist, in collaboration with Jennifer Mullinax at the University of Maryland. 

They are examining how risk factors relate to biological traits to begin disentangling their interactions. 

The magnitude and spread of HPAIV animal transmission have introduced a novel stressor to bird species already facing severe population declines due to the cumulative effects of climate change, habitat loss and degradation, food stress, contaminant exposure, and other pathogens. This represents a conservation crisis for wild birds, particularly the waterbirds and raptors most impacted by HPAIV mortality.

Harvey and Mullinax have compiled an updated primer describing host-virus interactions in wild birds, showing how seasonal patterns, migration, and species interactions are shaping transmission dynamics across diverse landscapes. 

Migration matters

Prior to the early 2000’s, HPAIV outbreaks in wild birds were rare — largely contained to domestic birds. After wild birds began getting infected and dying in Eurasia in 2005, HPAIV transmission and detection in wild birds started to increase globally, due to birds’ migratory flyways. However, long-term virus spread among wild birds was still rare until the clade H5N1 caused more problematic spread via migration.

The H5N1 virus was first identified in wild hosts during outbreaks in 2002, affecting wild species such as waterfowl, egrets, herons, and gulls, and even flamingos. The detection of HPAI H5 in wild bird species marked a pivotal moment, drawing attention to the potential for global spread through seasonal migration. 

Bird flu has continued to evolve, spreading to other continents and arriving in North America in 2014. In the U.S., the H5 virus caused an outbreak primarily impacting poultry and also some wild birds. This virus was seemingly eradicated via culling of infected poultry and was not detected again in wild birds. During its first half decade, HPAI transmission in the U.S. increased by 297%, infecting an increasingly broad range of host species. 

A highly pathogenic virus was not seen again in the U.S. until 2021 with the incursion of the H5N1 into North America via Newfoundland, Canada. Wild birds began to be affected by HPAIV beginning with a highly pathogenic H5N1 virus lineage A/Goose/Guangdong/1/96 (Gs/GD), first isolated in a domestic goose in China in 1996. Descendents of this virus strain became more adapted to wild birds and this current global H5 is a descendant from that Gs/GD virus.

This current wave of transmission has already caused close to 10,000 occurrences in wild birds, impacting 255 total avian species. Harvey’s research shows that the circulating dominant strain possesses an elevated ability to infect hosts, casting another shadow over an already grim situation.

In the paper, Harvey describes how the newest circulating virus’ spread and persistence in the U.S. is closely linked to its expanding host range, host demographics, and birds’ seasonal behaviors. 

Peaks in virus activity also coincide with periods of immunological naivety as young birds enter the population. Nestlings have partially developed immune systems, and receive protection from maternal antibodies. Their immunity develops for several weeks post-hatching, creating a window of heightened susceptibility coinciding with pre-migration staging. 

It appears that transmission is strongly biased toward introduction into domestic birds from wild birds, with relatively few spillover events from domestic poultry into wild populations. 

Vulnerable wild species

Harvey’s paper identifies a few well-known wild bird species, such as house sparrows, mallard ducks, and trumpeter swans, as particularly susceptible to viral infection. The study also indicates that Canada geese and some swans have the potential to be supermover / superspreader hosts. 

Early HPAIV surveillance work also established waterbirds as a primary source of HPAIV, capable of spreading the virus to others.

Some seagulls have recently been identified as key players in HPAIV transmission, with the initial 2021 incursion of HPAIV into North America resulting from a migratory gull. These species may act as biological bridges between continents, with the virus spreading rapidly through waterfowl and seabird colonies after the initial incursion. Transmission generally occurs when species become infected through shared water habitats.

Many marine and coastal birds breed in dense colonies that can facilitate rapid transmission. Raptors and scavenging birds may experience increased exposure risks. Infection exposure and susceptibility is also influenced by flock foraging and the bird trait of gregariousness — generally a positive except when it comes to bird flu transmission. Gulls have been shown to contribute to rapid transoceanic spread of HPAIV H5, while wild ducks facilitate swift regional dispersal and geographic expansion. 

Luckily, Harvey says that the beachgoing public can still enjoy their time by the shore as the summer season commences. “As beach traffic picks up in the heat of summer, virus prevalence and transmission seems to be reduced,” she said. 

In cases where humans are regularly interacting with superspreaders or supershedders — either for research, work activities, or randomly occupying the same space — it may be necessary to adjust, limit, or cease human activities which contribute to HPAIV spread. For example, food piles and feeders may artificially increase the density of animals at a location and facilitate transmission for certain high abundance species such as geese (home bird feeders are not likely to cause HPAIV spread, Harvey said). 

By reviewing and analyzing this considerable amount of data, Harvey hopes to provide direction for future research to inform efforts on mitigating HPAIV impacts on wild birds by providing practical insights for policymakers, public health officials, and wildlife management authorities. 

She also says that communicating these bird flu findings with the public can help guide management and decision-making. 

Support for this study was provided by the U.S. Geological Survey and National Science Foundation.