Pathogenicity of Highly Pathogenic Avian Influenza Virus (H5N1) in Different Duck Breeds

Avian influenza viruses (AIVs) pose a global threat, with wild waterfowl serving as key reservoirs for transmission to poultry. The present study investigated the pathogenicity, viral shedding patterns, tissue distribution, and pathological effects of a highly pathogenic avian influenza virus (HPAIV) in two duck breeds, including Muscovy and Sudani ducks. An Egyptian H5N1 strain (A/ibis/Egypt/RLQP-229S/2022), originally isolated from a wild ibis, was used. Forty ducks (20 Muscovy and 20 Sudani) were divided into infected and control groups (10 per group per breed). At four weeks of age (Average weight of 1.2 ± 0.1 kg), each infected duck received a single intranasal dose of 10⁶ EID₅₀. Cloacal and oropharyngeal swabs were collected at 3, 5, 7, and 10 days post-infection (DPI) to monitor viral shedding, while clinical signs were recorded daily. Mortality was higher in Muscovy ducks, which exhibited higher mortality (70%) than Sudani ducks (50%), with both breeds showing neurological signs and lethargy. Viral load analysis of cloacal swabs via RT-PCR (Targeting the AIV M gene), exceeded oropharyngeal shedding, peaking by five DPI and persisting longer in Muscovy ducks (Seven DPI compared to five DPI in Sudani ducks), suggesting that fecal-oral transmission is the primary route of spread and that viral replication is more active in the intestinal tract. Tissue distribution analysis revealed broader viral dissemination in Muscovy ducks, particularly in the brain, lung, kidney, and spleen. These findings demonstrated differential susceptibility between breeds, with Muscovy ducks posing a higher transmission risk due to prolonged viral shedding and tissue tropism. The virus used in the present study carried pathogenicity markers across several proteins, including hemagglutinin (HA), neuraminidase (NA), polymerase basic 1 (PB1), polymerase basic 2 (PB2), nucleoprotein (NP), non-structural protein 1 (NS1), and polymerase acidic (PA) protein. Overall, while both duck breeds are vulnerable to the circulating H5N1 HPAI strain, their susceptibility and clinical outcomes differ. These findings demonstrated that both Muscovy and Sudani ducks are susceptible to H5N1 HPAIV infection, Muscovy ducks showing higher mortality and more extensive viral shedding and histopathological alterations. However, both duck breeds are variable in their susceptibility to H5N1 infection.