EXPLAINER- What is cyclospora, the diarrhea-causing parasite, and why are infections rising in the US?

• Record outbreak in Michigan has pushed cases above 1,000, while health officials investigate hundreds more in several states

A record outbreak of cyclospora infections in the US state of Michigan has sickened more than 1,000 people, making it the state's largest recorded outbreak of the parasitic illness and one of the biggest reported in the US in recent years.

Health officials have yet to identify the source of the outbreak, while similar infections are under investigation in several other states, including neighboring Ohio.

What is Cyclospora?

Cyclospora (Cyclospora cayetanensis) is a microscopic intestinal parasite that causes an illness known as cyclosporiasis.

It spreads when people consume food or water contaminated with infected human feces.

Unlike many foodborne illnesses, cyclospora is not usually transmitted directly from one person to another because the parasite requires time in the environment before it becomes infectious.

Although the parasite is most common in tropical and subtropical regions, outbreaks in the US have frequently been linked to imported fresh produce.

Why are cases increasing?

Michigan has reported more than 1,000 confirmed infections since late June, far exceeding the roughly 50 cases it typically records in an average year. About 40 people have been hospitalized, but no deaths have been reported.

Ohio has also recorded hundreds of cases, while the US Centers for Disease Control and Prevention (CDC) is investigating additional illnesses reported across dozens of states.

Although investigators believe many of the illnesses are connected, they have not yet identified the contaminated food responsible for the Michigan outbreak. The CDC has also said there is currently no evidence that all reported cases are part of a single nationwide outbreak.

What are the symptoms?

Symptoms usually develop about one week after infection. The most common symptom is watery diarrhea, which can sometimes be severe or even explosive. People may also experience stomach cramps, bloating, fatigue, nausea, increased gas and a loss of appetite. Less common symptoms include vomiting, body aches and a low-grade fever.

Without treatment, the illness can last from several days to more than a month, and symptoms may disappear and return. Some infected people never develop symptoms.

Cyclosporiasis is generally not considered life-threatening for healthy people, but it can cause more severe or prolonged illness in older adults, young children and people with weakened immune systems.

How can it be prevented?

Doctors diagnose the infection by examining stool samples, although more than one sample may be needed because the parasite can be difficult to detect. Cyclosporiasis is commonly treated with antibiotics, although many otherwise healthy people recover without medication. Treatment depends on the severity of symptoms and the patient's overall health.

Because cyclospora is resistant to many routine disinfectants, preventing infection can be challenging once food has become contaminated.

Health authorities recommend washing hands thoroughly before handling food, rinsing fruits, vegetables and herbs under running water, scrubbing firm produce such as melons and cucumbers, removing damaged portions of produce and refrigerating peeled or cut fruits and vegetables promptly. While these measures cannot eliminate all risk, they can significantly reduce the chance of infection.

Health officials continue to investigate the source of the outbreak as cases remain under review in Michigan and several other US states.