Pathobiology of Asian highly pathogenic avian influenza H5N1 virus infection in ducks

Authors: Pantin Jackwood, Mary; Swayne, David

Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2006
Publication Date: 3/1/2007
Citation: Pantin Jackwood, M.J., Swayne, D.E. 2007. Pathobiology of Asian highly pathogenic avian influenza H5N1 virus infection in ducks. Avian Diseases. 51:250-259.

Interpretive Summary: Ducks are natural reservoirs of influenza type A viruses which normally don’t cause disease in these birds. However, the Asian H5N1 avian influenza (AI) viruses have changed from producing no disease or mild respiratory infections in ducks, to some strains producing severe disease and mortality. Lesions and virus distribution in tissues were studied in 2 and 5-week-old white Pekin ducks infected with different Asian-origin H5N1 AI viruses. Seven out of eight 2-week-old ducks inoculated with A/Egret/HK/757.2/02 developed severe disease, including neurological signs and death. However, this virus only killed two out of eight 5-week-old ducks. Other two viruses, A/Vietnam/1203/04 and A/Crow/Thailand/04, also produced high mortality in 2-week-old ducks. Another virus, A/Thailand PB/6231/04, killed three out of eight 2-week-old ducks, but did not induce neurological signs. In addition, older ducks infected with this virus did not present clinical signs and their tissues showed very few microscopic lesions. In conclusion, viral replication in tissues correlated with the severity of the clinical signs; however, the differences observed between ducks infected at different ages is not clear and may be associated with several factors including the virus strain, the ducks immune response, and the capacity of the ducks tissues of supporting viral replication.

Technical Abstract: Ducks and other wild aquatic birds are the natural reservoir of influenza type A viruses which normally are nonpathogenic in these birds. However, the Asian H5N1 avian influenza (AI) viruses have evolved from producing no disease or mild respiratory infections in ducks, to some strains producing severe systemic disease and mortality. In order to further understand the pathogenicity of these strains in ducks, we studied the gross and microscopic lesions and tissue distribution of viral antigen in 2 and 5-week-old white Pekin ducks infected with different Asian-origin H5N1 AI viruses. Seven out of eight 2-week-old ducks inoculated with A/Egret/HK/757.2/02 developed acute disease, including severe neurological dysfunction and death. However, this virus only killed two out of eight 5-week-old ducks. Other two viruses, A/Vietnam/1203/04 and A/Crow/Thailand/04, also produced high mortality in 2-week-old ducks. Microscopic lesions and AI viral antigen were observed most frequently in the nasal cavity, brain, heart, adrenal glands and pancreas. Another virus, A/Thailand PB/6231/04, killed three out of eight 2-week-old ducks, but did not induce neurological signs. Furthermore, older ducks infected with this virus did not present clinical signs or gross lesions, and their tissues showed very few microscopic lesions. All the viruses studied established systemic infections in both younger and older ducks, with viral replication in tissues correlating with the severity of the clinical signs. The differences in mortality induced by H5N1 AI viruses in ducks are reflected in the pathological findings and in the viral tissue distribution patterns. However, the differences in pathology existent between ducks infected at different ages is unclear and may be associated with a variety of factors including the virus strain, the host immune response, the host cell maturation and the capacity to support viral replication.