Highly Pathogenic Avian Influenza A(H7N9) Virus, Tennessee, USA, March 2017

Abstract

In March 2017, highly pathogenic avian influenza A(H7N9) was detected at 2 poultry farms in Tennessee, USA. Surveillance data and genetic analyses indicated multiple introductions of low pathogenicity avian influenza virus before mutation to high pathogenicity and interfarm transmission. Poultry surveillance should continue because low pathogenicity viruses circulate and spill over into commercial poultry.

Keywords: Alabama; H7N9; HPAI virus; Iowa; Tennessee; United States; Wyoming; backyard farm; chicken; commercial farm; farm; highly pathogenic avian influenza virus; influenza; mutation; phylogenetic analysis; poultry; respiratory infections; transmission; viruses; wild bird; zoonoses.

Figure 1

Genome constellation of influenza A(H7N9) viruses. Viruses are represented by ovals containing horizontal bars that represent the 8 influenza gene segments (from top to bottom: polymerase basic 2, polymerase basic 1, polymerase acidic, hemagglutinin, nucleoprotein, neuraminidase, matrix, and nonstructural). A genome reassortment event between the H7N9 virus from Saskatchewan, Canada (blue segments) and viruses from wild birds of the US Central and Mississippi Migratory Flyways (yellow segments) led to the genome assortment present in the Wyoming LPAI virus (BWT/WY/2016). The genome constellation of BWT/WY/2016 is the same as those of the Tennessee H7N9 viruses. A red bar in the hemagglutinin gene of the HPAI virus indicates the insertion at the hemagglutinin cleavage site. HPAI, highly pathogenic avian influenza; LPAI, low pathogenicity avian influenza.

Figure 2

Median-joining phylogenetic network of influenza A(H7N9) viruses, United States, 2017. The median-joining network was constructed from concatenated H7N9 virus genomes containing all 8 segments. This network includes all the most parsimonious trees linking the sequences. Each unique sequence is represented by a circle sized relative to its frequency in the dataset. Isolates are colored according to the sample: red inner circle represents HPAI in poultry, yellow inner circle represents LPAI in poultry, green inner circle represents LPAI in a wild bird, purple outer circle represents isolates from Wyoming, black outer circle represents isolates from Alabama, and sky-blue outer circle represents isolates from Tennessee. Bold lines indicate farm-to-farm transmission verified by epidemiologic investigations. Red numbers indicate number of nucleotide changes between isolates. Black numbers are abbreviated isolate names. HPAI, highly pathogenic avian influenza; LPAI, low pathogenicity avian influenza.