Cystoisosporiasis [Cystoisospora belli (synonym: Isospora belli)]

It is intestinal coccidian parasite that infects humans It is intestinal coccidian parasite that infects humans. It is Worldwide, especially in tropical and subtropical areas.

Transmission: Humans are the only known hosts for C. belli, which has no known animal reservoir. The mode of transmission of isosporiasis is fecal-oral, i.e. through food or water contaminated with human feces. Oocysts become mature and infective after their release to the environment by 2-3 days therefore, direct person-to-person transmission is unlikely.  

Morphology: The oocysts of Cystoisospora belli are long and oval shaped. They measure between 20 and 33 micrometers in length and between 10 and 19 micrometers wide. A fully mature (sporulated) oocyst of Isospora genus has two sporocysts and each sporocyst contains four sporozoites.

Oocyst of Cystoisospora belli with 2 sporocysts  

Life cycle: C. belli is ingested in contaminated food or water, and its life cycle requires a stage outside the host (becomes mature oocyst out side the host). After mature C. belli oocysts are ingested, they liberate sporozoites (possibly in response to bile in the small intestine), which invade the enterocytes of the proximal small intestine. Here, they become trophozoites, and a sexual multiplication (schizogony) produces merozoites that invade previously uninfected cells.

Sexual multiplication cycle (sporogony) occurs too, generating oocysts that pass into the environment. Outside the host, oocysts become mature and infectious after 2-3 days. The oocysts of C. belli are resistant and remain viable in the environment for months.

Clinical picture: C belli infection is most commonly observed in immunocompromised individuals or in individuals who have recently traveled to tropical areas, in people who are institutionalized, or in persons who live in poor sanitary conditions. The incubation period ranges from 3 to 14 days. Symptoms begin approximately 1 week after ingestion of the oocysts and last 2-3 weeks, with gradual improvement. Infection in people who are immunocompromised may continue indefinitely.

Symptoms of isosporiasis suggest a toxin-mediated mechanism, but no toxin has been identified. In humans, extraintestinal forms of cystoisosporiasis are reported in patients with AIDS. Clinical presentation may mimic those of inflammatory bowel disease and irritable bowel syndrome.

Symptoms and signs may include the following: Profuse, watery, non bloody, offensive-smelling diarrhea, which may contain mucus Foul-smelling flatus Abdominal colic, vomiting Malaise, loss of appetite, weight loss Low-grade fever Steatorrhea (malabsorption and passage of fatty stool) in chronic cases

Complications: Severe dehydration is the most common complication and almost always occurs in patients who are very young or immunocompromised. Acalculous cholecystitis, colitis, reactive arthritis and tissue invasion and dissemination have been reported in patients with AIDS.  

Diagnosis Microscopic demonstration of the large, typically shaped oocysts, is the basis for diagnosis. Because the oocysts may be passed in small amounts and intermittently, repeated stool examinations and concentration procedures are recommended as Zinc sulfate or sugar flotation which is the most sensitive stool concentration technique. Fluorescent stains, modified acid-fast, hematoxylin/eosin, and Giemsa staining are helpful in identifying the translucent oocysts. Duodenal specimens biopsy may be needed if stool examinations are negative.

In the stool examination we can find microscopic crystals called charcot-Leyden crystals. It is found in people who have allergic diseases such as asthma or parasitic infections such as parasitic pneumonia or ascariasis. These crystals are formed by the reaction between Charcot-Leyden crystal protein and eosinophil lysophospholipases. They vary in size and may be as large as 50 µm in length. Charcot–Leyden crystals are slender and pointed at both ends, consisting of a pair of hexagonal pyramids joined at their bases. Normally colorless, they are stained purplish-red by trichrome stain to differentiate them from fruits and other crystals. Blood picture revealed Peripheral eosinophilia.

Microscopic photograph showing acid fast I Microscopic photograph showing acid fast I. belli oocysts in stool smear (×400). Oocysts appear pinkish red oval 20 and 33 micrometers in length and between 10 and 19 micrometers wide

Charcot-Leyden Crystals in feces Charcot-Leyden crystals stained by trichrome stain  

Treatment: Co-trimoxazole: Trimethoprim-sulfamethoxazole (TMP-SMX) is the medication of choice for Cystoisospora infection. The typical treatment regimen for adults is 960 mg, orally, twice a day, for 7 to 10 days. Ciprofloxacin is a second-line alternative. It is less effective than Co-trimoxazole but might have some activity against Cystoisospora. For adults, the treatment regimen is 500 mg, orally, twice a day, for 7 days.  

Prevention Prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMX, 160 mg and 800 mg, respectively) is effective in preventing isosporiasis in adults with HIV infection Do not drink untreated water and always wash fruits and vegetables before eating them. Take care of toilets hygiene. Do not allow patients who have had C.belli to go swimming at least two weeks after being free from diarrhoea.