Updated 21 April 2017


Isosporiasis is an uncommon intestinal infection caused by a parasite, isospora belli.


An uncommon intestinal infection caused by a parasite, isospora belli, which is especially common in people with immune deficiency.

Alternative names


What is this?

A rather rare human infection, now most often seen in association with HIV/Aids, due to infection of the cells lining the small intestines with the parasite isospora belli. Humans are the only hosts for this parasite, which isn't seen in animals. The parasite has a complex life cycle.

What causes this?

The parasite occurs widely, and infections are seen mainly in people with Aids, though outbreaks have also been seen among inhabitants of institutions such as psychiatric hospitals and day-care centres. It can cause severe diarrhoea in infants. It affects men and women equally. The infection is common around the world, including Africa and South America. The incubation period for the infection is about a week.

What are its symptoms?

An acute, but not blood-stained, and profoundly smelly diarrhoea, accompanied by cramping abdominal pains and a low fever. There can be malaise, loss of appetite and weight, malabsorption, and a low fever. The stools may be fatty/greasy, but nausea and vomiting are uncommon. It can last for weeks, and is especially severe in people with a weakened immune system, as well as in infants and children. The use of trimethoprim-sullfamethoxazole to prevent pneumocystis carninii infections in patients with Aids can also reduce the risk of this infection occurring.

How is it diagnosed?

Stool samples need to be concentrated and examined microscopically, so that the parasite may be seen and identified, to confirm the diagnosis. Raised levels of eosinophils in the blood in someone with diarrhoea suggest this diagnosis, as this isn't seen in other protozoal infections.

How is it treated?

It doesn't respond well to most antibiotics used to treat other diarrhoeal infections. Usually it is managed with trimethoprim-sullfamethoxazole, and by the replacement of fluids lost., as dehydration can be marked In patients with Aids, indefinitely long treatment with these drugs may be needed, to attempt to suppress the infection and limit relapses. Alternatively pyrimethamine and sulphadoxine may be used. Although immune-deficient people tend to respond rather rapidly to treatment, there is a high rate of relapse.

What is the prognosis?

It is usually a minor and self-limited illness in immunologically healthy people, though it can cause chronic and debilitating diarrhoea. In people with Aids. it can be an intermittent, chronic problem, or may be severe and life-threatening. With careful therapy and prophylaxis, the outlook is reasonably good, however.

When to call your doctor

When diarrhoea arises in someone with immune-deficiency.

How can it be prevented?

It is spread through faecal contamination of food or water, so maintaining careful hygiene can help reduce the risk of infection or spread of it to others.


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