July 29 (UPI) — The number of suspected cyclosporiasis cases in the United States have risen to more than 18,000, with more than 6,700 lab-confirmed cases since May 1, the U.S. Centers for Disease Control and Prevention said Wednesday.

Cases of the gastrointestinal infection, which causes explosive diarrhea, have now been recorded in 45 states as part of this outbreak, the CDC said. The number of confirmed cases marks an increase of more than 2,500 since its last report July 21.

It is now considered the largest known outbreak of cyclosporiasis in U.S. history, The Hill reported. There have been 423 hospitalizations and no deaths, the CDC said.

Michigan has the most cases, with more than 10,000 reported in that state alone. It is followed by Ohio, which has more than 2,600 reported cases. Health officials in other states, including Kentucky, Illinois and Indiana, have also said that cases are on the rise.

Federal authorities were investigating the causes of the outbreak. The largest portion of the outbreak — in Michigan and eight other states — has been linked to iceberg lettuce grown in central Mexico.

“Cyclosporiasis cases may not be limited to these states with known cases,” the CDC update said Wednesday. “The true number of people sick with cyclosporiasis was likely higher than the number reported. This is because some people recover without medical and are not tested for Cyclospora.”

Hawaii, Maine, Montana, New Mexico and South Carolina are the only states without confirmed cases related to the domestic outbreak.

The illness is caused by the cyclospora cayetanensis parasite and spreads through food or water contaminated by feces from an infected person.

In addition to the most well-known symptom of sudden, watery diarrhea, symptoms of cyclosporiasis include vomiting, nausea, severe stomach cramps, loss of appetite and fatigue. It can take up to two weeks after eating contaminated food for symptoms to show themselves.