More than 500 McDonald's customers became ill after a parasite typically found in human waste was found inside the fast food chain's salads.

The Food and Drug Administration (FDA) launched an investigation after 507 people across 16 states reported sickness and diarrhea.

The unwanted ingredient hidden within the lettuce is known to cause intestinal illness.

Symptoms typically kick in after a week and include 'explosive' bowel movements.

Yesterday the FDA confirmed cyclospora was detected within the Fresh Express romaine lettuce and carrot mix that McDonald's use in their meals.

Fresh Express Salad tested positive for cyclospora (
Image:
Getty Images North America)

The gruesome situation first came to light in May 20, according to the Centers for Disease Control (CDC) and Prevention.

The restaurant decided temporarily remove the salads at the 3,000 affected restaurants on July 13.

Connecticut, Florida, Iowa, Illinois, Indiana, Kentucky, Michigan, Minnesota, Missouri, Nebraska, Ohio, South Dakota, Tennessee, Virginia, Wisconsin and New York were all affected by the contaminated salads.

Cyclospora parasite can also cause weight loss, stomach cramps, nausea, vomiting, fever and body aches and in some cases it can last as long as a few months.

McDonald's HQ in Illinois - one of the states affected by the bug (
Image:
Collect Unknown)

The CDC's statement read: "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."

On July 27, Fresh Express issued a recall for some lettuce that had been supplied to wholesale and institutional customers.

They also reported that the carrots used in the mix were only sent to McDonald's locations.

McDonald's said: "The health and safety of our customers and the people who work in McDonald’s restaurants is always our top priority.

"The additional states identified by the FDA and CDC are among the same states where a week ago we proactively decided to remove our lettuce blend in impacted restaurants and replace it through a different supplier.

"McDonald's is committed to the highest standards of food safety and quality and we continue to cooperate and support regulatory and public health officials in their investigations.

"For those seeking additional information about Cyclospora, we encourage them to visit the CDC and FDA websites."