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

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 log(10) 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.