Isosporiasis is a human intestinal disease caused by the parasite Isospora belli

Isospora belli

Isospora belli is a species of internal parasites classified under Coccidia. It is responsible for the condition isosporiasis. Autofluorescence aids detection....

. It is found worldwide, especially in tropical and subtropical areas. Infection often occurs in immuno-compromised

Immune system

An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 individuals, notably AIDS

AIDS

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

 patients, and outbreaks have been reported in institutionalized groups in the United States. The first documented case was in 1915.

Causal Agent

The coccidia

Coccidia

Coccidia is a subclass of microscopic, spore-forming, single-celled obligate parasites belonging to the apicomplexan class Conoidasida. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa....

n parasite Isospora belli infects the epithelial

Epithelium

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 cells of the small intestine, and is the least common of the three intestinal coccidia that infect humans (Toxoplasma, Cryptosporidium

Cryptosporidium

Cryptosporidium is a protozoan that can cause gastro-intestinal illness with diarrhea in humans.Cryptosporidium is the organism most commonly isolated in HIV positive patients presenting with diarrhea...

, and Isospora).

Life cycle

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 sporocyst-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. Isospora belli infects both humans and animals.

Clinical Features

Infection causes acute, non-bloody diarrhea

Diarrhea

Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...

 with crampy abdominal pain, which can last for weeks and result in malabsorption

Malabsorption

Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal tract.Impairment can be of single or multiple nutrients depending on the abnormality...

 and weight loss. In immunodepressed patients, and in infants and children, the diarrhea can be severe. Eosinophilia

Eosinophilia

Eosinophilia is a condition in which the eosinophil count in the peripheral blood exceeds 0.45×109/L . A marked increase in non-blood tissue eosinophil count noticed upon histopathologic examination is diagnostic for tissue eosinophilia. Several causes are known, with the most common being...

 may be present (differently from other protozoan infections).

Laboratory 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. 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.

Typical laboratory analyses include:

• Microscopy
• Morphologic comparison with other intestinal parasites
• Bench aids for Isospora

Treatment

Trimethoprim-sulfamethoxazole

Co-trimoxazole

Trimethoprim/sulfamethoxazole or co-trimoxazole is a sulfonamide antibiotic combination of trimethoprim and sulfamethoxazole, in the ratio of 1 to 5, used in the treatment of a variety of bacterial infections.The name co-trimoxazole is the British Approved Name, and has been marketed worldwide...

is the usual treatment choice. See recommendations in The Medical Letter (Drugs for Parasitic Infections) for complete information.