Transmission and tissue distribution of H7N9 viruses in chickens

Abstract

Exposure to infected poultry or the contaminated environment in live poultry markets are major risk factors for acquiring Influenza A(H7N9) infections in humans, but the predominate mode of transmission that facilitates the spread of H7N9 virus among poultry and to humans are not fully understood. The H7N9 virus is unusual in its ability to cause serious disease among infected people despite being a low pathogenicity virus in chickens. Severe human disease in the recent past has been associated mainly but not only with HPAI viruses. To gain information to support attempts to control and prevent H7N9 infections, we studied the modes of chicken-to-chicken and chicken-to-ferret transmission, and pathogenicity of H7N9 viruses in chickens.

A chicken origin A/Silkie Chicken/Hong Kong/1772-3/2014 (SCk1772) and a human origin A/Hong Kong/3263/2014 (HK3263) H7N9 viruses, which only differed by 4 amino acids, including the mammalian adaptive marker at the PB2 residue 627, were used to evaluate the differences in viral RNA load in multiple organs of 30 infected chickens, as well as the histopathological changes among them. The presence of infective virus was measured by EID 50/TCID50 or virus isolation.

SCk1772 and HK3263 replicated to comparable titres in tissue of chickens soon after infection at 2 and 4 dpi. Mild histological lesions including tracheitis (14/30), lymphocytic hyperplasia of caecal tonsils (12/30), mild pancreatic acinar degeneration (8/30) and rare interstitial nephritis (1/30) were observed in chickens. Viral RNA was found in a wide range of organs. Infective virus was confirmed in the kidney (8/8) and pancreas (10/21) of selected chickens, with viral titre of 4.0-6.2 and 3.7-4.5 log10 (TCID50/EID50)/g among five and six chickens respectively. The pancreatic acinar cells from 16 chickens also stained positive for viral antigen in the IHC test. The findings suggest the H7N9 virus replicates in the epithelial cells of kidney and pancreas of infected chickens. The H7N9 virus is highly respiratory trophic with dominant oropharyngeal over cloacal shedding.

Overall, we observed rapid efficient transmission of both viruses among chickens via direct contact within 24 hours post-exposure, while airborne transmission was inefficient. The Re-6 H5N1 poultry vaccine did not confer protection against infection and transmission of H7N9 virus in chickens, a result that was expected given the limited evidence of heterosubtypic protection from vaccines. Both SCk1772 and HK3263 exhibited limited transmission from infected chickens to naïve ferrets via the airborne route. Our studies demonstrate that infected chickens shed large quantities of virus and transmitted to other chickens in direct contact efficiently. Experiments in ferrets confirm that infecting human through airborne transmission is possible. Overall, the present study identified direct contact as the main mode for H7N9 transmission among chickens and support applying intervention measures to minimize direct and indirect contact between infected and uninfected poultry at poultry markets in areas where the virus is known to be present.