An outbreak of a diarrhoea-causing parasite is hitting more than half US states, with thousands of cases reported in recent weeks.

Cyclosporiasis is an infection caused by a microscopic parasite, with the main symptom being frequent, watery and explosive diarrhoea. People can become infected by consuming food or water that contains the parasite.

The outbreak now spread to 31 states. Michigan and Ohio have been hit particularly hard, with the two states reporting more than 3,000 cases on Friday. More than 1,000 of those cases were diagnosed over the previous two weeks.

No deaths from the recent infections have been reported in the US. While the source has not yet been identified, past outbreaks have been linked to foodborne illness in raw produce.

Between 1 May and 10 July there were 843 confirmed cases of cyclosporiasis in the US, according to data released by the US Centers for Disease Control and Prevention (CDC).

There has been a lag in reporting from the federal government that accounts for the state reported numbers being higher than the US-wide numbers.

There are possibly hundreds more, with the public health agency saying on Friday it was aware of more than 1,500 cases that require further analysis to confirm the illness.

No deaths have been reported, and 86 people were admitted to hospital, the CDC said.

In Michigan, no particular product or supplier has been identified as the source, the state health department said.

“Although we do not have a definite product identified as the source of the outbreak, we want to let Michiganders know what we have learned so far so they can take steps to protect their families,” Natasha Bagdasarian, Michigan’s chief medical executive, said in a statement. “Early information has shown lettuce as a common product that regularly comes up during the investigation.”

After Michigan, New York has seen the highest number of cases so far. Some 470 cases were reported there, state health officials said.

In Illinois, public health officials reported 194 probable cases on 13 July, calling it a “higher-than-average” figure, and more than 400 cases of cyclosporiasis have been reported in Ohio in the past month, according to the health department.

The number of people sick with cyclosporiasis is likely higher than the number reported because some people recover without medical care and are not tested for the illness.

The CDC said it anticipates the case counts will continue to rise as new data comes in.

Cyclosporiasis is an intestinal illness caused by a microscopic parasite, according to the CDC.

People can become infected by consuming food or water that contains the parasite.

The illness is not usually life threatening, according to the CDC, and is less common than other foodborne illnesses such as salmonella and E. coli.

People who are infected with cyclospora may or may not experience symptoms. The illness usually causes diarrhoea “with frequent and sometimes explosive bowel movements”, says the public health agency.

If untreated, the illness may last from a few days to over a month, and symptoms may return after appearing to dissipate.

It usually takes about one week after infection to become sick.

Person-to-person transmission does not occur, according to Dr Caitlin Rivers, of the Johns Hopkins Center for Health Security.

The epidemiologist wrote in an 8 July newsletter that transmission was exclusively faecal-oral via ingestion of contaminated food or water.

Those who were infected “became sick after eating food in the United States”, the CDC said, and did not report any travel during the 14 days before they got sick.

No specific type of produce or any grower or supplier has been identified as the source.

“Contamination typically occurs at the farm or irrigation level, making traceback investigations difficult,” Rivers wrote.

Previous cyclospora outbreaks in the US and Canada have been linked to bagged salad mixes and kits, fresh cilantro, fresh basil, raspberries, snow peas and green onions.

Given the large and increasing number of cases, Michigan’s health department has recommended restaurants and kitchens preparing or serving raw produce reduce risk by thoroughly washing greens, cooking raspberries and leafy greens when possible, and removing outer layers of lettuce and green onions.

While the US Food and Drug Administration says rinsing produce is unlikely to be effective, the CDC still recommends it.

People who are experiencing diarrhoea are urged to contact a health provider and ask about possible infection, officials said.