The U.S. Department of Agriculture announced Thursday that they will permit Foster Farms to continue operating the two poultry plants that have been tied to an outbreak of salmonella due to changes Foster Farms has made in both plants, the Associated Press reported.

The outbreak, which began in March, has affected 278 people in 17 different states according to a sample by the USDA taken in September, the Washington Post reported. Most of the infected chicken was sold in California, Oregon and Washington, with 77 percent of the illnesses happening in California, Center for Disease Control told the AP.

The two Foster Farm slaughterhouses are located in Fresno and Livingston, California, and are one of the nation's largest poultry processors and sellers, the AP reported. There were seven strains of salmonella involved in the outbreak, several of which are antibiotic-resistant causing the outbreak to be considered more dangerous, USA Today reported.

"One of the strains that we've tested is resistant to seven antibiotics," Christopher Braden, director of the CDC and prevention division of foodborne diseases, told USA Today.

In a statement Thursday, Foster Farms President Ron Foster said the company has been working for two months to add increased food safety controls, according to the Post.

"This officially validates our progress, but we are not stopping here," Foster told the Post.

According to USA Today, Foster Farms is not obligated to recall chicken processed at the three plants because USDA investigators have not yet been able to tie the outbreak to specific products and lots.

"It should be noted that while no illness is ever acceptable, the time period for this issue was over the course of six months from March to mid-September," company CEO Ron Foster said, according to the AP. "During that time, more than 25 million consumers safely consumed Foster Farms chicken."

Although Salmonella can contaminate meat during slaughter and processing and is especially common in raw chicken, the infections can be avoided by proper handling and cooking of raw poultry.

Foster emphasized that raw chicken is not a ready-to-eat product and that it "must be prepared following safe handling procedures, avoiding cross-contamination, and must be fully cooked to 165 degrees to ensure safety," the AP reported.

Symptoms of salmonella include diarrhea, fever, and abdominal cramps. They develop 12 to 72 hours after infection, and the illness usually lasts 4 to 7 days.