Pathobiology of Tennessee 2017 H7N9 low and high pathogenicity avian influenza viruses in commercial broiler breeders and specific pathogen free layer chickens

Kateri Bertran,Miria F Criado,Mary J Pantin-Jackwood,Dong-Hun Lee,David E Swayne,Diane Smith

doi:10.1186/s13567-018-0576-0

Abstract

In March 2017, H7N9 highly pathogenic avian influenza (HPAI) virus was detected in 2 broiler breeder farms in the state of Tennessee, USA. Subsequent surveillance detected the low pathogenicity avian influenza (LPAI) virus precursor in multiple broiler breeder farms and backyard poultry in Tennessee and neighboring states. The pathogenesis of the H7N9 LPAI virus was investigated in commercial broiler breeders, the bird type mostly affected in this outbreak. Infectivity, transmissibility, and pathogenesis of the H7N9 HPAI and LPAI viruses were also studied in 4-week-old specific pathogen free (SPF) leghorn chickens. The mean bird infectious doses (BID50) for the LPAI isolate was 5.6 log10 mean egg infectious dose (EID50) for broiler breeders and 4.3 log10 EID50 for SPF layer chickens, and no transmission to contact-exposed birds was observed. In both bird types, virus shedding was almost exclusively from the oropharyngeal route. These findings suggest sub-optimal adaptation for sustained transmission with the H7N9 LPAI isolate, indicating that factors other than the birds genetic background may explain the epidemiology of the outbreak. The BID50 for the HPAI isolate in SPF layer chickens was more than 2 logs lower (<2 log10 EID50) than the LPAI isolate. Also, the HPAI virus was shed by both the oropharyngeal and cloacal routes and transmitted to contacts. Greater susceptibility and easier transmission of the H7N9 HPAI virus are features of the HP phenotype that could favor the spread of HPAI over LPAI viruses during outbreaks.

Highlights

In March 2017, H7N9 highly pathogenic avian influenza (HPAI) virus was detected in 2 broiler breeder farms in the state of Tennessee, USA
Since this H5N2 HPAI virus outbreak (Midwestern USA, 2015) affected commercial turkey and layer farms but not broiler farms [24], our findings suggested that genetic resistance of broilers to infection may have partially accounted for the lack of affected broiler premises, but other factors such as fewer outside-to-on-farm exposure to contacts, type of production management system, leaving broiler farms unpopulated in the control zone, or enhanced biosecurity, could have resulted in the lack of broiler farms being affected [21]
In order to compare the infectivity, transmissibility, and pathogenicity of the Tennessee 2017 H7N9 low pathogenicity avian influenza (LPAI) and HPAI isolates with AI viruses from similar previous outbreaks, such as H7N8 HPAI virus (Indiana, 2016), we studied the viruses in young specific pathogen free (SPF) layer chickens

Summary

Introduction

In March 2017, H7N9 highly pathogenic avian influenza (HPAI) virus was detected in 2 broiler breeder farms in the state of Tennessee, USA. The mean bird infectious doses (BID50) for the LPAI isolate was 5.6 log mean egg infectious dose (EID50) for broiler breeders and 4.3 log10 EID50 for SPF layer chickens, and no transmission to contact-exposed birds was observed In both bird types, virus shedding was almost exclusively from the oropharyn‐ geal route. After circulating in and HPAI virus were confirmed in broiler breeder gallinaceous poultry, some H5 and H7 LPAI viruses can flocks in the state of Tennessee, USA, with additional mutate to the highly pathogenic (HP) phenotype, causing commercial broiler breeder flocks and mixed species in severe systemic disease and high mortality in domestic backyard flocks within the states of Alabama, Kentucky, birds [5]. The outbreak was effectively controlled in a timely fashion, with no further premises affected and no human infections identified

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