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Isosporosis: causes, symptoms, diagnosis, treatment
Last reviewed: 04.07.2025
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Isosporiasis is an anthroponotic disease that affects only humans and is characterized by acute enteritis or enterocolitis and spontaneous recovery. In immunosuppressed individuals, the disease becomes chronic (chronic diarrhea) and can be fatal.
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Epidemiology of isosporiasis
Isosporiasis is an anthroponotic disease, the only host of the pathogen of which is a person. The source of invasion is only a person sick with acute or chronic forms of isosporiasis, or a carrier. Oocysts in the feces of the patient appear only on the 10-12th day from the onset of the disease. Direct transmission of the pathogen from person to person does not occur, since oocysts mature in aerobic soil conditions for 2-3 days. Thus, the minimum time of infection turnover is 2 weeks (2-3 days in the environment and 10-12 days in the human body). The most active release of pathogens occurs after 16-30 days from the onset of the disease, therefore, patients with isosporiasis are most dangerous during the period of subsiding clinical symptoms.
The mechanism of infection is feco-oral.
Foci of isosporiasis are confined to tropical and subtropical regions. Sporadic cases occur everywhere.
What causes isosporiasis?
Isosporosis is caused by representatives of the protozoan genus Isospora. In humans, the disease is caused by I. belli and I. natalensis. The life cycle of isospores consists of two phases - exogenous (development in the external environment) and endogenous (development in the human body). Characteristic is the alternation of asexual (in the external environment and in the human body) and sexual (only in the human body) reproduction. After the maturation of the oocyst that has entered the human intestine, sporozoites emerge from it, which penetrate the epithelial cells of the duodenum and jejunum, where they are located under the nucleus of enterocytes. Sporozoites turn into trophozoites, which grow and increase in size, then, after maturation, the nucleus divides repeatedly, and as a result, a schizont is formed. Cytoplasm is isolated around each daughter nucleus. Merozoites formed from the schizont "fall out" of the affected epithelial cells into the intestinal lumen and affect more and more new enterocytes. Later, some of the merozoites are transformed into male (microgametocytes) and female (macrogametocytes). Mature macrogamets are formed from macrogametocytes, occupying the entire enterocyte. In the microgametocyte, the nucleus undergoes multiple divisions. The newly formed nuclei have an elongated shape, they are separated from each other by a small amount of cytoplasm and turn into small microgametes with 2 flagella, and leave the enterocyte. Then the microgametes actively penetrate the macrogamete, which turns into a zygote, and then into an immature oocyst. Immature oocysts are released into the environment with feces, where at a temperature of +25 °C they transform into sporocysts within 2-3 days, each of which forms 4 sporozoites after 18-36 hours. Mature isospore oocysts are very stable in the environment: low temperatures of 0-5 °C slow down sporulation, but when favorable conditions occur, it resumes; at a temperature of -21 °C, sporulated isospores survive up to 1 month.
Pathogenesis of isosporiasis
Endogenous forms of isospores destroy the epithelium of the villi of the jejunum and ileum, where merogony is completed. With extensive lesions, leukocyte exudate is formed, the structure of the cylindrical epithelium changes, atrophy of the villi, metaplasia of enterocytes, and hyperplasia of the crypts occur. All this leads to impaired absorption and the development of malabsorption syndrome.
Symptoms of isosporiasis
The incubation period of isosporiasis is on average 7 days. In immunocompetent individuals, isosporiasis occurs as acute enteritis or enterocolitis, accompanied by loose stools with mucus, sometimes blood; at the same time, general symptoms of isosporiasis appear: nausea, vomiting, anorexia). Pain throughout the entire surface of the abdomen, both constant and cramping, is a concern. Isosporiasis is self-healing within 18-31 days.
In immunosuppressed individuals, including HIV-infected individuals, a chronic form of the disease develops, accompanied by steatorrhea, protein loss, which leads to a rapid decrease in body weight of up to 25% or more. The absorption of D-xylose and vitamin B 12 is impaired. The outcome can be fatal. In AIDS patients, isosporiasis is quite common and is one of the causes of death in these patients.
Diagnosis of isosporosis
A characteristic sign of isosporiasis is increasing eosinophilia. Isospores can be detected in human feces using the enrichment method followed by smear microscopy, since there are few parasites in the feces. The study is carried out repeatedly, and is considered most informative in the period from the 16th to the 31st day of the onset of symptoms.
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Treatment of isosporiasis
Treatment of isosporiasis is mainly symptomatic (see Cryptosporidiosis ).Etiotropic treatment for isosporiasis has not been developed.