In this image provided by the Centers for Disease Control and Prevention, a photomicrograph of a fresh stool sample, which had been prepared using a 10% formalin solution, and stained with modified acid-fast stain, reveals the presence of four Cyclospora cayetanensis oocysts in the field of view.
The Food and Drug Administration says an outbreak of stomach illnesses in Iowa and Nebraska is linked to salad mix served at Olive Garden and Red Lobster restaurants in those states and supplied by a Mexican farm.
The outbreak has sickened more than 400 people in 16 states, including more than 100 in North Texas. The FDA said it is working to determine whether the salad mix is the source of illnesses in the other 14 states.
"It is not yet clear whether the cases reported from other states are all part of the same outbreak," the agency said in a statement. "The investigation of increased cases of cyclosporiasis in other states continues."
Both Olive Garden and Red Lobster are owned by Orlando-based Darden Restaurants. In a statement, Darden spokesman Mike Bernstein said the FDA's announcement is "new information."
"Nothing we have seen prior to this announcement gave us any reason to be concerned about the products we've received from this supplier," Bernstein said.
The FDA said it traced illnesses from the restaurants in Nebraska and Iowa to Taylor Farms de Mexico, the Mexican branch of Salinas, Calif.-based Taylor Farms. The company, which provides produce to the food service industry, said its facility located about 180 miles north of Mexico City in San Miguel de Allende is the only one of its 12 sites to be connected to the cases.
In a statement on the company's website, Taylor Farms says the Mexican facility is "state of the art and has an exceptional food safety record." The statement said the company is working with FDA investigators who are looking at the facility and that the product is out of the food supply.
The FDA said it had audited the Mexican processing facility in 2001 and found "no notable issues." The agency said it would increase surveillance efforts for green leafy products imported from Mexico.
The most recent known illness in the two states linked to the infected salad was in Nebraska a month ago. The typical shelf life for a salad mix is up to 14 days.
There have been more recent illnesses in other states. According to the Centers for Disease Control and Prevention, the most recent illness was July 23 but centers did not specify a location.
The agency said its investigation has not implicated any packaged salad sold in grocery stores.
Tarrant County is investigating 34 cases of cyclospora infections. Dallas County has 27 cases, Colling County has 25, and Denton County has 18 cases.
North Texas doctors have been unable to pinpoint the source of the outbreak in Texas. Epidemiologists have been interviewing people who fell ill to find out where they purchased fruits and vegetables and what restaurants they visited.
The CDC is looking at Texas cases that occurred in June and July. Investigators are also checking to make sure infected people did not travel outside of the United States and Canada within two weeks being sickened.
In some previous outbreaks of cyclospora, the cause was never discovered, federal officials say.
The Associated Press' Mary Clare Jalonick contributed to this report.