Infectivity and transmissibility of H9N2 avian influenza virus in chickens and wild terrestrial birds

Abstract

Genetic changes in avian influenza viruses influence their infectivity, virulence and transmission. Recently we identified a novel genotype of H9N2 viruses in widespread circulation in poultry in Pakistan that contained polymerases (PB2, PB1 and PA) and non-structural (NS) gene segments identical to highly pathogenic H7N3 viruses. Here, we investigated the potential of these viruses to cause disease and assessed the transmission capability of the virus within and between poultry and wild terrestrial avian species. Groups of broilers, layers, jungle fowl, quail, sparrows or crows were infected with a representative strain (A/chicken/UDL-01/08) of this H9N2 virus and then mixed with naïve birds of the same breed or species, or different species to examine transmission. With the exception of crows, all directly inoculated and contact birds showed clinical signs, varying in severity with quail showing the most pronounced clinical signs. Virus shedding was detected in all infected birds, with quail showing the greatest levels of virus secretion, but only very low levels of virus were found in directly infected crow samples. Efficient virus intra-species transmission was observed within each group with the exception of crows in which no evidence of transmission was seen. Interspecies transmission was examined between chickens and sparrows and vice versa and efficient transmission was seen in either direction. These results highlight the ease of spread of this group of H9N2 viruses between domesticated poultry and sparrows and show that sparrows need to be considered as a high risk species for transmitting H9N2 viruses between premises.

Figure 1

Viral titres (EID₅₀ /mL) determined in EHE inoculated with (a) buccal and (b) cloacal swabs taken from the infected groups on 3 dpi and from the contact groups on 4 dpi. Infected broilers and naïve contact sparrows were housed together in cage 1, infected sparrows and naïve contact broilers were housed together in cage 2 and infected quail were housed in cage 3. The square below the axis represents the cages. Data are presented as box-and-whisker plot indicating maximum and minimum titres observed in six birds selected from each group that were positive for virus infection within the infected and contact groups. The horizontal line marks the median and the error bars indicate standard deviation. (*) indicates significant differences (P < 0.05) compared with both of the broiler and sparrow groups and (#) indicates significant differences (P < 0.05) compared with two sparrow groups. Differences were calculated using ANOVA and Tukey’s multiple comparison tests.