Isosporosis

Isospora belli was first described by Virchow in 1860 but not named until 1923. It is a coccidian parasite of the duodenum and proximal small intestine (jejunum) in humans. It is cosmopolitan, but more frequent in a tropical environment. No reservoir hosts other than man are known. The oocysts are very resistant to environmental conditions and may remain viable for months if kept cool and moist. The sexual and asexual cycles occur in the same host. The parasites are located intracytoplasmic, unlike Cryptosporidium. There is a prepatent period of about 9-10 days. Infection may be latent or lead to diarrhoea for one to two weeks, occasionally with mild fever, headache, malaise and abdominal pain. The stools tend to be soft, watery or foamy, with an offensive smell, suggesting malabsorption. In immunosuppressed people, the infection can become chronic. In such cases, oocyst shedding can continue for years. In AIDS patients, the parasites can occasionally be found in lymph nodes and walls of large and small intestine, mesenterium and even liver and spleen. Diagnosis is difficult and is based on coprological examination, duodenal tubage and biopsy of the duodeno-jejunal mucosa, in which the parasites are not very numerous. The oocysts are rather large and measure 20-33 µm by 10-19 µm. The oocysts are very pale, transparent and are easily overlooked, especially in a concentrated sediment of a polyvinyl alcohol-preserved stool sample. For this reason, it is best to diminish the light intensity of the microscope and additional contrast should be obtained for optimal examination conditions. Wet preparations are generally preferred. Charcot-Leydig crystals (derived from eosinophils) are occasionally found in isosporiasis cases. The oocysts are acid-fast and can also be detected with auramine-rhodamine staining. Usually the oocyst contains only one immature sporont, but two may be present. Continued development occurs outside the human host with the development of two mature sporocysts, each containing four sporozoites. Normally this takes about 48 hours. The sporulated oocyst is the infective stage which will excyst in the duodenum. The condition can be treated with cotrimoxazole (e.g. Bactrim forte® 4 x 1 tablets/day for 10 days). If there is diminished sensitivity or resistance, either pyrimethamine (Daraprim®) or the combination ornidazole/albendazole is used. Ciprofloxacin is also moderately effective (70% cure rate). Prevention is based on improved personal hygiene measures and improvement of the sanitary conditions.

Category: Medicine Notes