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

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.Electronic supplementary materialThe online version of this article (10.1186/s13567-018-0576-0) contains supplementary material, which is available to authorized users.

Loss of Fitness of Mexican H7N3 Highly Pathogenic Avian Influenza Virus in Mallards after Circulating in Chickens

“…Therefore, this discrepancy is likely caused by differences in susceptibility in individual birds. The transmission results obtained in this (47)(48)(49)(50). However, LPAI viruses that are known to be well adapted to chickens, including the Asian H9N2 and H7N9 viruses, readily transmitted in chickens in our model (51).…”

Virus Shedding of Avian Influenza in Poultry: A Systematic Review and Meta-Analysis

“…In contrast to HPAIV, no significant association between the inoculation dose and LPAIV shedding was found. It might be expected that for LPAIV the inoculation dose is more critical compared to HPAIV as it is less virulent and pathogenic, a relatively low dose of HPAIV might be sufficient for infection, whilst infection with LPAIV requires a higher dose to achieve virus shedding in chickens [36,37]. At lower doses not all experimental units might be infected, which cannot be seen from the data as only poultry that shed viruses were included in the meta-analysis.…”

“…Given the low virulence and pathogenicity of LPAIV, it could be that once chickens become successfully infected with LPAIV, regardless of the inoculation dose, they would end up shedding similar levels of virus. Considering the need of ensuring that most (or all) poultry become infected during performing experiments and the observation that there is no effect of the inoculation dose on LPAIV shedding, high doses (around 10 6 EID 50 /mL [36,37]) may be advisable for LPAIV experiments.…”

Association of wild bird densities around poultry farms with the risk of highly pathogenic avian influenza virus subtype H5N8 outbreaks in the Netherlands, 2016

“…Broiler farms were excluded, as HPAI (H5N8) outbreaks are rare in broilers (EFSA AHAW Panel, 2017;Napp et al, 2018). This may be related to genetically reduced susceptibility of broilers compared to layers, ducks or turkeys, in combination with factors related to the production system, such as fewer outside-to-on-farm exposure to contacts, or enhanced biosecurity (Bertran et al, 2016(Bertran et al, , 2018. The final selection of 21 R farms included layer, broiler (grand)parent, layer parent, rearing hen, broiler breeder rearing, fattening duck and meat turkey farms (Table S1).…”