The number of people sick from an outbreak of a gastrointestinal illness, called cyclosporiasis, linked to McDonald's salads is now 507, federal health officials said Thursday .
All of the illnesses have been confirmed with lab tests. Twenty-four of those individuals have been hospitalized.
The illnesses began between May 20 and July 21, according to the Centers for Disease Control and Prevention .
Illnesses have been reported in 16 states. The states are Connecticut, Florida, Iowa, Illinois, Indiana, Kentucky, Michigan, Minnesota, Missouri, Nebraska, Ohio, South Dakota, Tennessee, Virginia, Wisconsin and New York.
The patients in Connecticut, New York, Tennessee and Virginia all purchased their salads while they were visiting Illinois and the Florida patient purchased the suspect salad while in Kentucky, according to the CDC.
"Epidemiologic evidence indicates that salads purchased from McDonald's restaurants are one likely source of these infections. The investigation is ongoing, and FDA is working to determine the sources of the ingredients that were in common to the salads served at McDonald's," the CDC said.
The cyclospora parasite causes intestinal illness in people who consume contaminated food or water. Symptoms can begin a week or more after consuming the parasite. They include diarrhea and frequent, sometimes explosive, bowel movements, according to the CDC. Those who are infected might also experience loss of appetite, weight loss, stomach cramps or pain, nausea, gas and fatigue. Vomiting, headache, fever, body aches and flu-like symptoms can also occur.
The illness can last from a few days to a few months, and patients might feel better but then get worse again. Patients can be treated with antibiotics.
In July, McDonald's pulled the suspected salad mix from approximately 3,000 locations and switched its supplier.
Subsequently, the outbreak investigation found the parasite in a sample of Fresh Express salad mix of romaine lettuce and carrots that had been distributed by the restaurant chain.
On July 27, Fresh Express issued a recall for some lettuce that had been supplied to wholesale and institutional customers.