Mexico slaughtered chickens on farms in Northern Mexico this month and put a million more in quarantine after discovering low-pathogenic cases bird flu, a government official said on Tuesday.

Jose Angel del Valle, director general of animal health for the agriculture ministry, said the sick birds were discovered as part of a permanent program to monitor for bird flu and that all steps taken had been precautionary.

“These were isolated incidents that we have been attending, which is very different from an emergency situation,” he said.

Del Valle said authorities had to quarantine 1 million chickens to ensure the disease did not spread.

A senior industry source said earlier on Tuesday that Mexico slaughtered poultry this month because of an outbreak of bird flu that was brought under control.

“It is under control,” the source said. “I’m not sure how many (birds) were affected, but it was not many.”

The low-pathogenic form of bird flu causes few, if any, symptoms in infected birds. However, farmers must watch the low-pathogenic virus closely because it can mutate into a more deadly, or high pathogenic, form, according to experts.

Del Valle said 13 poultry business owners in Northern Mexico, including local companies and international interests like Tyson Foods Inc., had participated in the Mexican government’s operation to contain the bird flu incidents.

“You cannot play around with bird flu,” said Del Valle.

The U.S. Department of Agriculture does not restrict chicken imports due to low pathogen bird flu outbreaks, according to Jim Rogers, spokesman for the department’s Animal and Plant Health Inspection Service.

The bird flu virus is easily spread by manure, farm equipment, feed, cages, cars, crates, clothing and shoes. Symptoms of the disease include sudden death, decreased egg production and misshapen eggs, purple discoloration on the legs and combs and diarrhea.