The U.S. Centers for Disease Control and Prevention has confirmed a multistate outbreak of cyclosporiasis. File Photo by Erik S. Lesser/EPA

July 15 (UPI) -- The U.S. Centers for Disease Control and Prevention has confirmed the multistate outbreak of cyclosporiasis, the parasite-based illness, in at least four states.

The four states are Michigan, which has seen the highest rate of illness; Ohio; Kentucky; and West Virginia. But cases have been identified in 34 states, the CDC said.

The CDC had said that there was "no evidence of a single, multistate" outbreak, The Hill reported, but on Tuesday said that the hundreds of cases in the region appeared to link to a common source. The source hasn't been confirmed, said Dr. Gwen Biggerstaff, deputy director of the CDC's Foodbourne, Waterbourne and Environmental Diseases division. Biggerstaff hosted a call with reporters on Tuesday.

Michigan health officials have reported more than 3,700 cases of the illness and believe lettuce or salad greens could be the culprit. Dr. Donald Prater, FDA acting deputy commissioner for food, said the agency is continuing to investigate "multiple produce items, including lettuce."