By Faith Karimi and Debra Goldschmidt, CNN

(CNN) -- An outbreak of cyclospora linked to McDonald's salads has sickened at least 61 people in seven states, federal health officials said Friday.

The Food and Drug Administration is working with McDonald's to identify the salad ingredients making people sick and trace them through the supply chain.

As a precaution, McDonald's has stopped selling the salads in 3,000 fast food restaurants in 14 states to try to contain the outbreak, the FDA said.

The states are Illinois, Iowa, Indiana, Wisconsin, Michigan, Ohio, Minnesota, Nebraska, South Dakota, Montana, North Dakota, Kentucky, West Virginia and Missouri.

"It's early in the investigation, but we are taking steps now to help ensure consumers know about the potentially contaminated product so that they can better protect themselves or seek treatment, especially if they have signs or symptoms of a cyclospora infection," FDA commissioner Dr. Scott Gottlieb said in a statement.

Earlier this week, the Illinois and Iowa health departments said they were investigating outbreaks of a parasite that causes intestinal illness and might be linked to McDonald's salads.

Illinois and Iowa among hardest-hit

Illinois reported 90 cases since mid-May while Iowa had 15 cases since late June. It's unclear why their numbers are higher than those provided by federal officials.

The cyclospora parasite causes intestinal illness as a result of consuming contaminated food or water.

The symptoms, which can begin a week or more after consuming the parasite, include diarrhea and frequent, sometimes explosive bowel movements, according to the US Centers for Disease Control and Prevention. Those infected can 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, then get worse again. Patients can be treated with antibiotics.

Other parasite outbreaks

In an email, McDonald's said it's working with officials in the affected states.

"Out of an abundance of caution, we decided to voluntarily stop selling salads at impacted restaurants until we can switch to another lettuce blend supplier," the email said. "We are in the process of removing existing salad blend from identified restaurants and distribution centers -- which includes approximately 3,000 of our US restaurants primarily located in the Midwest."

Cyclospora is also behind an ongoing outbreak linked to recalled Del Monte Fresh vegetable trays; believed to be responsible for more than 200 illnesses in four states.

The parasite is also to blame for an outbreak in Texas, although officials have not yet identified what's making people sick there.