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Isosporiasis is a chronic intestinal infection caused by an opportunistic parasite that mainly infects immunocompromised individuals (e.g. HIV/AIDS patients). It is a diarrheal disease that can also occur in dogs. Immunocompetent individuals can also be infected by the causative agent of Isosporiasis by chance. Isosporiasis is caused by Isospora belli, a coccidian protozoan found in the genus Isospora and parasite group known as sporozoa. I. belli is related to Cryptosporidium species and they are found in the phylum Apicomplexa. I. belli is a widespread protozoan, and it is normally found in the tropics and subtropical regions of the world. It is the most important human pathogen in the genus Isospora. I. hominis is another Isospora species that is of less clinical significance in humans. Cryptosporidium parvum and Cyclospora cayetanensis are other clinically important coccidia (singular: coccidian) parasites that infect humans.   

Type and morphology of Isospora belli

I. belli mainly exists in oocysts forms. Oocysts of I. belli are oval in shape. I. belli undergo both asexual and sexual reproduction in the intestinal epithelial cells of their human host. Individuals infected with I. belli usually excrete unsporulated oocysts that contain two sporoblasts. Each of the two sporoblasts contains four sporozoites. Excreted oocysts of I. belli are usually not infectious but remain in the environment until sporulation occurs. Only sporulated oocysts of I. belli are infectious to humans. Isosporiasis is not a zoonotic infection; and thus, the protozoan that causes the infection has no insect vector or animal reservoir. Humans are the only known source of infection with the parasite. I. belli can also be carried in the intestinal tract of dogs.  

Clinical signs and symptoms of Isospora belli infection

The clinical signs and symptoms produced by an infection with I. belli include fever, weight loss, diarrhea and malaise. I. belli causes enteritis or intestinal infections in its human host. Usually, it produces a non-bloody and non-specific type of watery diarrhea. Abdominal cramp, anorexia and vomiting usually accompany diarrhea caused by I. belli infection.  

Pathogenesis of Isospora belli infection

I. belli is an opportunistic protozoan that frequently causes chronic intestinal watery diarrheal disease in immunocompromised individuals. Human infection with I. belli is through the feacal-oral route; and this normally occurs through the consumption of contaminated water or food (Figure 1). After ingestion, infective oocysts of I. belli migrate to the epithelial walls of the intestines where it attaches and cause an episode of watery diarrheal disease. Unsporulated oocysts of I. belli are released in the feaces of infected individuals into the environment from where they mature or sporulate to cause human infections. Acute isosporiasis can also occur in immunocompetent people but with lesser clinical episodes. However, a prolonged type of infection can be experienced in these individuals but it is usually transitory. In immunocompromised people, a chronic watery diarrheal disease is commonly experienced. Severe forms of Isosporiasis usually results in dehydration due to massive fluid loss from the body; and it only occurs in immunocompromised individuals e.g. HIV/AIDS patients. Children and malnourished individuals can also become infected with I. belli especially in cases where there is a history of poor personal and environmental hygiene. I. belli produces both covert and overt diseases depending on whether the infected human hosts are immunocompetent or immunocompromised.  

Figure 1. Life cycle of Isospora belli. 1. Immature oocysts of I. belli are excreted in the feaces of infected human host. Oocysts usually contain one sporoblast (more rarely two). 2. The sporoblast divides in two in further maturation after excretion (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. 3. Human infection occurs by ingestion of sporocysts-containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells of the intestine and initiate schizogony. 4. Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication. 5. Trophozoites develop into schizonts which contain multiple merozoites. This marks the end of the asexual stage of replication. 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. CDC

Laboratory diagnosis and treatment of Isospora belli infection

The laboratory diagnosis of I. belli infection is usually based on the microscopical detection of the parasites oocysts (Figure 2) in wet stool preparations of infected individuals. Oocysts of I. belli can also be detected microscopically in safranin-stained stool samples (Figure 3). The shedding of oocysts of I. belli in the stool of infected patients occurs intermittently, and is not frequent. Thus the collection of stool specimens in suspected cases of isosporiasis should be done repeatedly. Fresh stool samples should be collected; and concentration techniques can also be employed in the detection of I. belli oocysts. Isosporiasis is usually treated with antibiotic therapy. Sulphamethoxazole-trimethoprim, ciprofloxacin and pyrimethamine are the antibiotics of choice for the clinical management of the disease. Oral rehydration should also be accompanied with antibiotic therapy to take care of fluid loss from the body, a condition that is characteristic of I. belli infection in humans.

Figure 2. Unstained oocysts of Isospora belli in wet preparation. CDC
Figure 3. Oocysts of Isospora belli stained with safranin. CDC

Prevention and control of Isospora belli infection

Since I. belli produces a chronic diarrheal disease in its human host, it is important to maintain proper re-hydration of the affected individuals through   to maintain the body’s osmotic/fluid balance. I. belli is an opportunistic protozoan that can be acquired by humans via the consumption of contaminated food or water. Thus, proper personal and environmental hygiene is critical to the prevention of Isosporiasis in human population.


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