Avian influenza H7N9/13 and H7N7/13: a comparative virulence study in chickens, pigeons, and ferrets

Abstract

Human influenza cases caused by a novel avian H7N9 virus in China emphasize the zoonotic potential of that subtype. We compared the infectivity and pathogenicity of the novel H7N9 virus with those of a recent European avian H7N7 strain in chickens, pigeons, and ferrets. Neither virus induced signs of disease despite substantial replication in inoculated chickens and rapid transmission to contact chickens. Evidence of the replication of both viruses in pigeons, albeit at lower levels of RNA excretion, was also detected. No clear-cut differences between the two H7 isolates emerged regarding replication and antibody development in avian hosts. In ferrets, in contrast, greater replication of the avian H7N9 virus than of the H7N7 strain was observed with significant differences in viral presence, e.g., in nasal wash, lung, and cerebellum samples. Importantly, both viruses showed the potential to spread to the mammal brain. We conclude that efficient asymptomatic viral replication and shedding, as shown in chickens, facilitate the spread of H7 viruses that may harbor zoonotic potential. Biosafety measures are required for the handling of poultry infected with avian influenza viruses of the H7 subtype, independently of their pathogenicity for gallinaceous poultry.

Importance: This study is important to the field since it provides data about the behavior of the novel H7N9 avian influenza virus in chickens, pigeons, and ferrets in comparison with that of a recent low-pathogenicity H7N7 strain isolated from poultry. We clearly show that chickens, but not pigeons, are highly permissive hosts of both H7 viruses, allowing high-titer replication and virus shedding without any relevant clinical signs. In the ferret model, the potential of both viruses to infect mammals could be demonstrated, including infection of the brain. However, the replication efficiency of the H7N9 virus in ferrets was higher than that of the H7N7 strain. In conclusion, valuable data for the risk analysis of low-pathogenicity avian influenza viruses of the H7 subtype are provided that could also be used for the risk assessment of zoonotic potentials and necessary biosafety measures.

FIG 1

Body temperatures (A) and weights (B) of ferrets infected with avian H7N9 (squares) or H7N7 (triangles) virus and uninfected controls (gray line). Measurements were made at the time points indicated while the ferrets were under anesthesia. Body temperature is given as the mean value and standard deviation of the difference from the temperature prior to infection. Body weight is given as the mean value and standard deviation compared to the weight prior to infection.

FIG 2

Viral loads in oropharyngeal (A, C) or cloacal (B, D) swabs obtained from chickens (A, B) or pigeons (C, D) and in nasal wash samples from ferrets (E) infected (solid lines) with avian H7N9 (squares) or H7N7 (triangles) virus and in contact chickens (dotted lines). Swab samples or nasal wash samples taken at the time points indicated were analyzed for the presence of H7 gene-specific RNA by RT-qPCR. Shown are the mean values and standard deviations of the calculated TCID₅₀-eqs. Significant differences (P value = 0.002165; exact Wilcoxon rank sum test) between H7N9- and H7N7-infected ferrets are indicated (*).

FIG 3

Viral loads in organs of chickens (A), pigeons (B), or ferrets (C) infected with avian H7N9 (black bars) or H7N7 virus (shaded bars) and an uninfected ferret (gray bars). Organ samples taken at the time points indicated were analyzed for the presence of H7 gene-specific RNA by RT-qPCRs. Shown are the mean values and standard deviations of the calculated TCID₅₀-eqs. The number of positive samples/total number of samples is shown above each bar.