Isosporiasis (Isospora belli Infection)

Isosporiasis Definition

Isosporiasis is an infectious disease caused by a single-celled organism (protozoan), Isospora belli, that is marked by diarrhea and other intestinal symptoms. These days the infection is known as cystoisosporiasis and the protozoan is Cystoisospora belli, however, the former name isosporiasis is still widely used. Isosporiasis is a self-limiting infection and while treatment can hasten recovery, it is not always necessary and the infection does not usually lead to complications in most people. However, immunocompromised patients are at risk of the infection spreading beyond the intestines. AIDS patients are at the greatest risk in this regard. Isosporiasis is an uncommon infection that is spread through contaminated food or water.

Isosporiasis Incidence

Overall isosporiasis is a rare infection especially among people with healthy immune systems. As it causes a mild diarrheal illness, it is often missed and the exact incidence in the general population is difficult to estimate. Even among immuncompromised individuals like AIDS patients, isosporiasis is an uncommon AIDS-defining illness. Isosporiasis is rare in the United States and other developed nations but it has a worldwide distribution. It is more commonly seen in tropical and subtropical climates.

Isosporiasis Pathophysiology

Life cycle

Isospora belli, the parasite that causes isosporiasis, enters the human body through contaminated food and water. The parasites exist in the immature form in the environment (oocytes) and has to mature within the gut to infect it. The maturation occurs within the small intestine and the oocytes become sporozoites. It infects the cells of the small intestine (enterocytes) causing diarrhea and hampering absorption of nutrients (malabsorption).

Once inside the enterocytes, the sporozoites become trophozoites and then merozoites through asexual reproduction. Uninfected enterocytes become infected with these merozoites. New oocytes are then produced through sexual reproduction and passed out of the gut and into the environment through the stool. Within 2 to 3 days these oocytes mature and can then be transmitted to another person who may make contact with contaminated food or water in the environment.

Isospora belli in the body

The parasite causes inflammation of the small intestine but is limited to the gut by the immune system. The symptoms resemble gastroenteritis and more common infectious agents, like viruses and bacteria, are usually suspected. Overall isosporiasis is a mild diarrheal illness but it is protracted and can take 2 to 3 weeks to resolve without treatment. People with a healthy immune system are not at risk of major complications provided that they maintain a good diet and proper hydration throughout the illness. However, immunocompromised patients are a risk of severe complications and even death.

Isosporiasis in HIV and AIDS

Without proper immune functioning, specifically cell-mediated immunity which is compromised in conditions like HIV/AIDS, the protozoa can cause a very severe infection. The symptoms are intense and prolonged. The large volume watery diarrhea that persists indefinitely in immunocompromised patients closely resembles cryptosporidiasis – another AIDS defining illness. Other causes of HIV diarrhea also need to be considered. The patient is at a major risk of dehydration and the infection may become disseminated, spreading to other organs outside of the gut (extra-intestinal). The gallbladder is commonly affected and patients develop acalculous cholecystitis – gallbladder inflammation without gallstones.

Isosporiasis Symptoms

The symptoms of isosporiasis arise about 1 week after contracting the infection but the incubation period can be as short as 3 days or as long as 2 weeks. It is largely similar to other causes of infectious gastroenteritis or enterocolitis and may even resemble inflammatory bowel disease (IBD). Eosinophilia is one of the features that may be seen in isosporiasis which differs from the clinical presentation of other protozoal diarrheal illnesses.

Isosporiasis Diarrhea

Diarrhea is the defining feature of isosporiasis. It is profuse and watery. The diarrhea is usually non-bloody and mucus may sometimes be present in the stool. It has an extremely offensive odor as is the case with the foul-smelling flatus. In prolonged cases, the patient may experience steatorrhea – fatty stool that tends to float due to the malabsorption of fats in the gut.

Isosporiasis Other Symptoms

The other prominent symptoms apart from diarrhea includes :

  • Abdominal pain that is cramping in nature.
  • Vomiting that is sometimes accompanied by nausea.
  • Loss of appetite.
  • Fever which is usually low-grade.
  • Malaise is more severe in immunocompromised patients.

Less common symptoms include :

  • Headaches
  • Muscle pain
  • Abdominal tenderness once pain and cramping subsides.
  • Signs of nutritional deficiencies.

Isosporiasis Causes

Isosporiasis is caused by the Isospora belli protozoan, a parasite that is transmitted through food and water. It is an uncommon infectious agent compared to other viruses, bacteria and protozoa that cause waterborne and foodborne illness. It is closely related to other protozoa that cause a similar illness, such as Toxoplasma, Cryptosporidium and Cyclospora. The immature form of Isospora belli known as oocytes are ingested with water and food and then need to mature in the human gut before it becomes infective. After completing its life cycle in the human body, newly formed oocytes are then eliminated in the stool.

The parasite is transmitted via the feco-oral route. Oocytes that are passed out in the stool of infected humans or animals may contaminate water that is then ingested. Outbreaks may occur when a common water supply is contaminated. However, these infections are uncommon in most developed nations. People with weakened immune systems, patients who are immunocompromised and those living in areas with poor sanitation are at the greatest risk of infection. Spread of the infection from one person to another is uncommon but may occur with oro-anal sexual contact.

Isosporiasis Diagnosis

Isosporiasis is not easily diagnosed without specific diagnostic investigations. Collection of a stool sample and microscopic examination of the stool is the preferred method for diagnosing the infection. There are other procedures that may be considered to collect samples of intestinal fluids which may be examined for the presence of the protozoa. However, these tests are not often conducted. There is no blood test that can conclusively identify an infection with Isospora belli. Since isosporiasis is an AIDS-defining illness, the patient should also be screened for HIV infection.

Isosporiasis Treatment

Isosporiasis is a self-limiting disease and is mild in patients with healthy immune systems. It should be treated for a more rapid recovery as symptoms may abate within 2 to 3 days in patients undergoing the appropriate treatment as compared to 2 to 3 weeks without treatment. In patients with a weakened immune system, the symptoms may be very severe and can continue indefinitely. AIDS patients are at the greatest risk of complications and death.


Isosporiasis is treated with antibiotics. The preferred antibiotic is trimethoprim-sulfamethoxazole (TMP-SMZ) which may be used from 7 to 10 days to as long as 2 to 4 weeks. Patients need to complete the course although symptoms improve within 2 to 3 days in immunocompetent patients or in about 5 days in immunocompromised patients. In cases where patients cannot tolerate sulfonamides (antibiotics like TMP-SMZ), pyrimethamine with folinic acid may be prescribed. Nutritional supplements may be necessary to correct deficiencies that arise from the use of certain medication or with prolonged illness associated with malabsorption.

Home Care

Supportive measures are advisable during the illness for a quicker recover. These measures also prevent dehydration and some of the other complications associated with isosporiasis. Bed rest along with fluid replenishment is important. Oral rehydrating solutions (ORS) containing the appropriate electrolytes needs to be administered on a continuous basis. In cases of severe dehydration, patients may need to be hospitalized and intravenous fluids administered. There is no specific dietary plan that is advised for isosporiasis. Patients should continue to eat solid foods once vomiting subsides and low-protein/lactose-free diets may be better tolerated for the diarrhea.

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