The pathogenesis of a 2022 North American highly pathogenic clade 2.3.4.4b H5N1 avian influenza in mallards (Anas platyrhynchos)

Authors: Spackman, Erica; Pantin Jackwood, Mary; Lee, Scott; PROSSER, DIANN - Us Geological Survey (USGS)

Submitted to: Avian Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2023
Publication Date: 5/9/2023
Citation: Spackman, E., Pantin Jackwood, M.J., Lee, S.A., Prosser, D. 2023. The pathogenesis of a 2022 North American highly pathogenic clade 2.3.4.4b H5N1 avian influenza in mallards (Anas platyrhynchos). Avian Pathology. 52(3):219-228. https://doi.org/10.1080/03079457.2023.2196258.
DOI: https://doi.org/10.1080/03079457.2023.2196258

Interpretive Summary: Dabbling ducks, such as mallards, are known carriers of avian influenza virus. Most strains don't cause any disease, but rare strains of the virulent form of the virus, i.e., bird flu, sometimes do make them sick. Unfortunately, ducks can also infect chickens and turkeys with bird flu when they carry it. A new strain of bird flu, called "H5N1 clade 2.3.4.4", was introduced by wild bird migrations into North America in late 2021. Because this virus seems to be spread by ducks, this new strain was tested for it's ability to infect mallards and to see how much virus they excreted. It was found that mallards are highly susceptible to infection with this virus and excreted high quantities for at least 2 weeks. Most of the ducks did not get sick. This suggests that mallards can serve as a major source of virus for other species by moving it during migration or within their territories and excreting high levels into the environment.

Technical Abstract: Highly pathogenic (HP) avian influenza viruses (AIVs) of the clade 2.3.4.4 goose/Guangdong/1996 H5 lineage continue to be a problem in poultry and wild birds in much of the world. The recent incursion of a H5N1 clade 2.3.4.4b HP AIV from this lineage into North America has resulted in widespread outbreaks in poultry and consistent detections of the virus across diverse families of birds and occasionally mammals. To characterize the pathobiology of this virus in mallards (Anas platyrhynchos), which are a primary reservoir of AIV, a challenge study was conducted with 2-week-old birds. The 50% bird infectious dose was determined to be < 2 log10 50% egg infectious doses (EID50) and all exposed ducks, including ducks co-housed with inoculated ducks, were infected. Infection appeared to be subclinical for 58.8% (20/34) of the ducks, one duck was lethargic, about 20% developed neurological signs and were euthanized, and 18% developed corneal opacity. The mallards shed virus by both the oral and cloacal routes within 24–48 h post-infection. Oral shedding substantially decreased by 6–7 days post-infection, but 65% of the ducks continued to shed virus cloacally through 14 days post-exposure (DPE) for the direct inoculates and 13 DPE for contact-exposed ducks. Based on the high transmissibility, high virus shed titres, and mild-to-moderate disease, mallards could serve as efficient reservoirs to amplify and disseminate recent North American clade 2.3.4.4b viruses.