Highly pathogenic avian influenza (HPAI) in a recent outbreak may have spread from one farm to a second, but low pathogenic AI (LPAI) was more likely introduced through the environment, according to an epidemiological report on the outbreaks.

The report, “Epidemiologic and Other Analyses of HPAI/LPAI Affected Poultry Flocks,” was issued by USDA’s Animal and Plant Health Inspection Service (APHIS) to share findings on the most recent U.S. outbreaks. The spring 2017 outbreaks involved HPAI in two broiler-breeder flocks in the same Tennessee county, as well as LPAI outbreaks in broiler-breeder and backyard flocks in Tennessee, Alabama, Kentucky and Georgia. Results of genetic analyses determined that all H7N9 viruses detected from this event are of North American wild bird lineage, not the Asian H7N9 virus. Comparing the HPAI and LPAI H7N9 viruses showed they are highly similar, and it is therefore likely the LPAI virus was introduced first into commercial poultry and mutated to HPAI.

APHIS wrote: “While lateral spread was implicated between two HPAI sites based on genetic homology between the isolates, close geographic proximity and company ties between the two farms, direct farm-to-farm transfer routes among the AM H7NO LPAI commercial broiler breeder sites were not apparent.” APHIS added that the occurrences in backyard flocks also point to environmental introduction of the virus, rather than farm-to-farm transmission.