H7N9 confirmed in Mississippi broiler flock

March 27, 2025

An outbreak of highly pathogenic avian influenza (HPAI) at a commercial broiler facility in Noxubee County, Mississippi, has been confirmed as the H7N9 strain of the virus, a type not seen in U.S. poultry in eight years.

The infected flock of roughly 46,000 broiler chickens was depopulated and did not enter the food system, the Mississippi Board of Animal Health (MBAH) said in a statement. This is the third HPAI outbreak among commercial poultry in Mississippi since the spring of 2023, according to the MBAH, adding that the virus has been detected in migratory waterfowl in areas of Mississippi since November 2024.

The U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) confirmed the presence of H7N9 on March 12. The agency has documented low pathogenic avian influenza (LPAI) H7 in wild birds this year and in previous years, but this is the first HPAI H7 case seen in U.S. poultry since March 2017, APHIS said.

Indoors chicken farm, chicken feeding, farm for growing broiler chickens — The flock of roughly 46,000 broiler chickens was showing signs of illness, including multiple deaths, when highly pathogenic avian influenza H7N9 was confirmed. The birds were depopulated, according to Mississippi animal health officials, who insist none of the birds entered the food supply.

“This H7N9 virus is a fully North American (AM) virus of wild bird-origin and is unrelated to the Eurasian H5N1 clade 2.3.4.4b virus currently circulating in the United States,” APHIS explained.

Some North American H7 LPAI viruses detected as part of APHIS’s wild bird surveillance are closely related to this HPAI virus, indicating a recent spillover of wild bird-origin LPAI virus, according to the agency. APHIS closely monitors these subtypes because H5 and H7 LPAI viruses in poultry species, such as chickens and turkeys, can mutate into HPAI.

The outbreak began on March 8 when clinical signs in the Mississippi flock were noticed, including increased deaths. Bird samples analyzed at the Mississippi Veterinary Research & Diagnostic Laboratory, a member of the National Animal Health Laboratory Network, were presumptive positive for HPAI then confirmed as H7N9 by USDA’s National Veterinary Services Laboratories in Ames, Iowa.

A notification of the outbreak submitted to the World Organisation for Animal Health (WOAH, founded as the OIE) noted the H7N9 subtype. As part of existing avian influenza response plans, federal and state officials are working jointly on additional surveillance and testing in areas around the Mississippi poultry operation.

The 2017 outbreak of HPAI H7N9 in the United States occurred in commercial poultry operations in Tennessee, Alabama, Georgia, and Kentucky, and resulted in the culling of hundreds of thousands of birds. At the time, China was in its fourth and final year of an HPAI H7N9 epidemic that had infected more than 1,200 people, one of whom died. No human cases were reported in the United States during that period.