Pyrosequencing of cDNA from brains of parrots with proventricular dilatation disease (PDD), an unexplained fatal inflammatory central, autonomic, and peripheral nervous system disease, showed 2 strains of a novel Borna virus. Real-time PCR confirmed virus presence in brain, proventriculus, and adrenal gland of 3 birds with PDD but not in 4 unaffected birds.

Avian influenza (AI) viruses have been detected in more than 105 wild bird species from 12 different orders but species-related differences in susceptibility to AI viruses exist. Expression of α2,3-linked (avian-type) and α2,6linked (human type) sialic acid (SA) influenza virus receptors in tissues is considered to be one of the determinants of the host range and tissue tropism of influenza viruses. We investigated the expression of these SA receptors in 37 wild bird species from 11 different orders by lectin histochemistry. Two isoforms of Maackia amurensis (MAA) lectin, MAA1 and MAA2, were used to detect α2,3-linked SA and Sambucus nigra (SNA) lectin was used to detect α2,6-linked SA. All species evaluated expressed α2,3-linked and α2,6-linked SA receptors in endothelial cells and renal tubular epithelial cells. Both α2,3-linked and α-2,6-linked SA receptors were expressed in respiratory and intestinal tract tissues of aquatic and terrestrial wild bird species from different taxa, but differences in SA expression and in the predominant isoform of MAA lectin bound were observed. With a few possible exceptions, these observed differences were not generally predictive of reported species susceptibility to AI viruses based on published experimental and field data.

Mallards are important natural hosts involved in the epidemiology of low pathogenic avian influenza viruses (LPAIVs). LPAIVs are mainly transmitted by a fecal-oral route and are excreted in high concentrations in the feces. We investigated the pathology, viral antigen distribution, and the expression of alpha2,3 sialic acid (SA) influenza virus receptors in mallards after intranasal inoculation with A/Mallard/MN/199106/99 (H3N8) or A/Mallard/MN/355779/00 (H5N2). Gross lesions were not observed. Avian influenza virus (AIV) nucleoprotein (NP) antigen was detected in rare epithelial cells of the larynx and trachea only at 1-day postinoculation (dpi) in the birds infected with H3N8 LPAIV, but infection with either virus was associated with lymphocytic tracheitis and laryngitis on 1 and 2 dpi. AIV NP antigen was detected in enterocytes of the lower intestine from 1 to 4 dpi and in epithelial cells of the bursa of Fabricius from 2 to 3 dpi in birds infected with either virus. Oropharyngeal and cloacal viral shedding was detected from 1 dpi, with higher cloacal viral shedding detected at 2 and 3 dpi with both viruses. Mallards abundantly expressed alpha2,3 sialic acid receptors in epithelial cells of the respiratory tract, lower intestine, and bursa of Fabricius. Some infected birds had decreased alpha2,3 sialic acid expression in epithelial cells of the bursa of Fabricius and in enterocytes of the ceca and colon. In conclusion, the main sites of LPAIV replication in mallards are the enterocytes of the lower intestinal tract and epithelial cells of the bursa of Fabricius in the first days after infection, when these birds are shedding AIV in high titers in the feces.

Wild birds in the orders Anseriformes and Charadriiformes are the natural and asymptomatic reservoirs of influenza A viruses representing all of the avian hemagglutinin (HA) and neuraminidase (NA) subtypes. Transmission of avian influenza (AI) viruses from wild birds to gallinaceous poultry species occurs regularly and outcomes vary, ranging from asymptomatic infections to mortality. Circulation of H5 and H7 low pathogenic AI (LPAI) viruses in gallinaceous poultry may result in mutations in the HA protein cleavage site and the emergence of highly pathogenic AI (HPAI) viruses, which in poultry can cause severe disease with high economic losses. Since 2002, various wild bird species also have succumbed to infection with the Eurasian H5N1 HPAI viruses. The pathogenesis of AI is complex and the ability of these viruses to produce disease and death in avian species is dependent on various host, viral and environmental factors, which are not completely understood.

Our overall hypothesis is that host population immunity directed at multiple antigens will influence the prevalence, diversity and evolution of influenza A virus (IAV) in avian populations where the vast subtype diversity is maintained. To investigate how initial infection influences the outcome of later infections with homologous or heterologous IAV subtypes and how viruses interact through host immune responses, we carried out experimental infections in mallard ducks (Anas platyrhynchos). Mallards were pre-challenged with an H3N8 low-pathogenic IAV and were divided into six groups. At five weeks post H3N8 inoculation, each group was challenged with a different IAV subtype (H4N5, H10N7, H6N2, H12N5) or the same H3N8. Two additional pre-challenged groups were inoculated with the homologous H3N8 virus at weeks 11 and 15 after pre-challenge to evaluate the duration of protection. The results showed that mallards were still resistant to re-infection after 15 weeks. There was a significant reduction in shedding for all pre-challenged groups compared to controls and the outcome of the heterologous challenges varied according to hemagglutinin (HA) phylogenetic relatedness between the viruses used. There was a boost in the H3 antibody titer after re-infection with H4N5, which is consistent with original antigenic sin or antigenic seniority and suggest a putative strategy of virus evasion. These results imply competition between related subtypes that could regulate IAV subtype population dynamics in nature. Collectively, we provide new insights into within-host IAV complex interactions as drivers of IAV antigenic diversity that could allow the circulation of multiple subtypes in wild ducks.