INTRODUCTION Isospora belli is a spore forming coccidian protozoan that infects humans and some primates. It has been studied as the causal agent in the intestinal disease Isosporiasis. Isospora belli has been reported to cause travelers diarrhea in visitors to endemic areas.

MORPHOLOGY Oocysts (the stage of I.belli that exists in the external environment) are ovoid in shape and measure 20-33 x 10-19 µm. Immature Oocyst Containing One Sporoblast

CLASSIFICATION Higher order taxa Domain: Eukaryota Phylum: Apicomplexa Class: Coccidia Order: Eucoccidiorida Family: Eimeriidae

EPIDEMIOLOGY It is found worldwide especially in tropical and subtropical areas. This occurs more frequently under poor sanitation conditions. There is a 3-14 day incubation period. There are no known organism vectors or reservoirs of Isosporiasis. Infection occurs in immunodepressed individuals.

TRANSMISSION Transmission occurs by fecal oral route. It is primarily through ingestion of contaminated food or water

LIFE CYCLE As with other coccidian, Isospora belli has both a sexual and an asexual cycle. Oocysts are passed in the feces unsporulated. Sporulation takes approximately 24 hours and the oocyst becomes infective At time of excretion, the immature oocyst contains usually one sporoblast (more rarely two).In further maturation after excretion, the sporoblast divides in two, so the oocyst now contains two sporoblasts.

The sporoblasts secrete a cyst wall, thus becoming sporocysts; and the sporocysts divide twice to produce four sporozoites each. Infection occurs by ingestion of sporocysts- containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells and initiate schizogony.

Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication. Trophozoites develop into schizonts which contain multiple merozoites. After a minimum of one week the sexual stage begins with the development of male and female gametocytes. Fertilization results in the development of oocysts that are excreted in the stool.

THE SUMMARY OF THE LIFE CYCLE 1) Immature, unsporulated oocyst is excreted through feces. Sporoblast divides into two. Each sporoblast develops into a sporocyst with 4 sporozoites, resulting in mature oocysts. 2) Mature oocyst is ingested. 3) Oocyst bursts. Sporozoites are released and lodge into the intestinal lining.

Sporozoites undergo asexual reproduction to form merozoites Sporozoites undergo asexual reproduction to form merozoites. (The time spent in stages 1 through 3 is 2-3 days). 4)The merozoites mature into gametes 5)The gametes undergo fertilization to produce a new oocyst.

CLINICAL FEATURES In healthy individuals I. belli is usually asymptomatic or produces self limiting diarrhoea. If the host is young or immunodepressed the symptoms can include: profuse watery non-bloody diarrhoea with fluid loss of 2-20L/day (may contain mucous). abdominal pain and cramping. Anorexia. Weight loss .

CONTINUATION OF CLINICAL SYMPTOMS general malaise. low fever. Vomiting. steatorrhea with prolonged illness.

PATHOGENESIS The sporozoites invade epithelial cells in the small intestine which eventually destroys these cells. It has been suggested that the cause of Isosporiasis symptoms is toxin related but no toxin has been identified.

LABORATORY DIAGNOSIS I. belli infection can be diagnosed by stool specimen. The I. belli oocysts are easily dyed with modified acid-fast stains

Immature oocyst of I. belli stained with acid-fast, showing a single sporoblast

If stool examinations are negative, examination of duodenal specimens by biopsy or string test (Enterotest®) may be needed. The oocysts can be visualized on wet mounts by microscopy with bright-field, differential interference contrast (DIC), and epifluorescence. They can also be stained by modified acid- fast stain.

TREATMENT Most treatment for Isosporiasis is simply to relieve symptoms as they appear. It may be necessary to treat patients for malabsorption if severe diarrhoea persists. I. belli infections can also be avoided with trimethoprim-sulfamethoxazole and even patients who are already infected usually respond well to this treatment

PREVENTION The key to prevention is proper sanitation to ensure that the environment is free of feces. Once the Isospora are passed in the feces, they can quickly develop into the infective stage, so rapid removal of the feces is very important. Mature oocysts of Isospora are resistant to most cleaning products and they can survive for months to years in the environment. However, the use of strong ammonia-containing compounds may be helpful in disinfection, and steam cleaning also helps kill the infectious oocysts.

PREVENTION Be sure to allow for adequate ventilation while cleaning the infected areas, as fumes from cleaning products can be harmful to animals and people. Dogs and cats should not be permitted to ingest rodents, since rodents may be carriers of the parasites. The treatment of infected canine and feline mothers soon after parturition may help prevent the spread to the young.

REFERENCES Textbook Of Medical Parasitology;5th Edition by CK Jayaram Paniker;MD. Medical microbiology,Thieme 2005. www.wikipedia.org/wiki/isosporiasis www.practicalscience.com/isospora.htm Center for disease control and prevention (CDC)