A diarrhoea-causing parasite outbreak has spread in a number of 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 spans more than a dozen states, and is particularly troublesome in Michigan, where nearly 1,000 people have been diagnosed in a two-week period. 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.

Where have cases been reported?

There were 145 reported cases of cyclosporiasis in the US between 1 May and 16 June, the most recent date up to which the US Centers for Disease Control and Prevention (CDC) has shared data.

No deaths have been reported, and 20 people were admitted to hospital, the CDC said.

Outbreaks have been reported across 17 states since early May, including in Illinois, New York and Texas, says the CDC.

The BBC has asked the public health agency for more up-to-date figures.

State public health departments have been reporting increasing numbers of infections.

As of 8 July, Michigan health officials said there were nearly 1,000 cases reported in that state alone within a two-week period.

In Illinois, public health officials reported 141 cases on 7 July, calling it a “higher-than-average” figure.

Some 177 cases of cyclosporiasis have been reported in Ohio, officials said.

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.

What is cyclospora?

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.

What’s causing the outbreak?

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.

How to stay safe

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. (BBC, excluding headline)