A strain of bird flu has been detected in one of major food producer Tyson’s chicken breeder’s flocks in Tennessee.

According to CNBC, the U.S. Department of Agriculture said this represented the first confirmed case of highly pathogenic H7 avian influenza (HPAI) in commercial poultry in the United States this year. It is also the first time HPAI has been found in Tennessee, the state government said.

Company officials from Tyson, the largest chicken meat producer in the United States, stated on Sunday that roughly 70,000 birds will be euthanized in compliance with Tennessee and federal officials to prevent the spread of the disease to other birds as well as to the food processing plants.

While it may seem inhumane to euthanize thousands of birds, scientists learned after the 2015 outbreak that it’s crucial to euthanize entire infected flocks immediately so as to prevent further spread of the disease.

Dr. Carol Cardona, a poultry disease expert at the University of Minnesota, told The Washington Times, “You want a very rapid response and a very rapid stamping out. … The faster the birds die, the faster the outbreak stops.”

This is not the first time that a strain of avian flu has impacted American chicken breeders and food producers alike. According to Reuters, in 2014 and 2015, during a widespread outbreak of HPAI, the United States killed nearly 50 million birds, mostly egg-laying hens. The losses pushed U.S. egg prices to record highs and prompted trading partners to ban imports of American poultry, even though there was little infection then in the broiler industry.

The USDA stresses that the current illness is not the same H7N9 virus of Chinese lineage that has sickened poultry and people in Asia, nor is it related to the virus that caused the 2015 U.S. outbreak. This is good and bad news: while the pathogen currently affecting poultry is not to same as in previous years, it is still somewhat of a mystery to researchers in terms of what exactly caused the outbreak.

Though experts are not completely certain on the origin of the disease, they have a good idea. According to The Washington Times, wild waterfowl are considered the main reservoirs of bird flu, so farmers and scientists must be on high alert when birds are migrating. Droppings from infected birds flying north in the spring, or south in the fall, can get tracked into barns or carried in on contaminated equipment. This is a possible source of infection for the affected birds.

The risk of human infection during bird flu outbreak is generally low, although in China people have died this winter amid an outbreak of the H7N9 virus in birds.