Abstract
An outbreak caused by H7N3 low pathogenicity avian influenza virus (LPAIV) occurred in commercial turkey farms in the states of North Carolina (NC) and South Carolina (SC), United States in March of 2020. Subsequently, H7N3 high pathogenicity avian influenza virus (HPAIV) was detected on a turkey farm in SC. The infectivity, transmissibility, and pathogenicity of the H7N3 HPAIV and two LPAIV isolates, including one with a deletion in the neuraminidase (NA) protein stalk, were studied in turkeys and chickens. High infectivity [<2 log10 50% bird infectious dose (BID50)] and transmission to birds exposed by direct contact were observed with the HPAIV in turkeys. In contrast, the HPAIV dose to infect chickens was higher than for turkeys (3.7 log10 BID50), and no transmission was observed. Similarly, higher infectivity (<2–2.5 log10 BID50) and transmissibility were observed with the H7N3 LPAIVs in turkeys compared to chickens, which required higher virus doses to become infected (5.4–5.7 log10 BID50). The LPAIV with the NA stalk deletion was more infectious in turkeys but did not have enhanced infectivity in chickens. These results show clear differences in the pathobiology of AIVs in turkeys and chickens and corroborate the high susceptibility of turkeys to both LPAIV and HPAIV infections.
Highlights
Avian influenza viruses (AIV) are type A influenza viruses belonging to the Orthomyxoviridae family and are classified into subtypes based on the two viral surface glycoproteins: the hemagglutinin (HA) and the neuraminidase (NA)
None of the turkeys inoculated with the low dose (2 log10 EID50) of low pathogenicity avian influenza virus (LPAIV)-1 became infected; all turkeys were infected in the medium (3 log10 EID50) and high dose (5 log10 EID50) groups, resulting in a BID50 of 2.5 log10 EID50 (Table 1)
We compared the infectivity, transmissibility, and pathogenicity in turkeys and chickens of two LPAIVs, differing by a 66-nucleotide deletion in the NA stalk, and one high pathogenicity avian influenza virus (HPAIV) isolate from the H7N3 outbreak in turkeys in North Carolina and South Carolina, United States (US), in 2020
Summary
Avian influenza viruses (AIV) are type A influenza viruses belonging to the Orthomyxoviridae family and are classified into subtypes based on the two viral surface glycoproteins: the hemagglutinin (HA) and the neuraminidase (NA). Wild aquatic birds are the natural reservoir of AIVs; viruses from these species are usually of the LP phenotype, and infections are asymptomatic [2]. These LPAIVs transmit from wild to domestic birds, resulting in subclinical infections, mild respiratory disease, and/or drops in egg production [3]. After circulating in chickens or turkeys, H5 and H7 LPAIVs have mutated on numerous occasions to HPAIV, causing poultry outbreaks worldwide [2,4]. In 2017, H7N9 HPAIV was identified in two broiler breeder farms in Tennessee, with the precursor LPAIV found in multiple broiler breeder farms and backyard poultry in Tennessee and neighboring states [12]
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