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Giardiasis, balantidiasis, isosporiasis, and microsporidiosis 

Giardiasis, balantidiasis, isosporiasis, and microsporidiosis

Giardiasis, balantidiasis, isosporiasis, and microsporidiosis

Martin F. Heyworth



Giardiasis – report that infection might predispose to irritable bowel syndrome and chronic fatigue; possible anti-giardial effect of proton pump inhibitors.

Updated on 30 Jul 2015. The previous version of this content can be found here.
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date: 25 April 2017


Infection with Giardia intestinalis, a flagellate protozoan that colonizes the lumen of the small intestine, is acquired by ingesting environmentally resistant cysts of the parasite, typically in water or food. Strains of the parasite that can potentially infect humans are harboured by various mammals, including dogs and cattle.

Clinical features—manifestations include watery diarrhoea, abdominal discomfort and distension, weight loss, and malabsorption, with the infection typically being persistent and severe in individuals with genetic impairment of antibody production. G. intestinalis infection can lead to impairment of growth, and possibly of intellectual development, in children.

Diagnosis and treatment—diagnosis is by faecal examination for evidence of G. intestinalis infection, including (1) parasite antigen—by enzyme-linked immunosorbent assay (ELISA) or immunochromatography; (2) cysts—by microscopy (a historic approach that lacks sensitivity); or (3) parasite DNA—by nucleic acid amplification techniques, such as the polymerase chain reaction (PCR). Aside from supportive care, treatment is with metronidazole (although the parasite is becoming increasingly resistant), tinidazole and nitazoxanide.

Prevention—cysts of G. intestinalis in water can be killed by boiling or removed by filtration.


Balantidium coli is a ciliate protozoan that invades the colonic mucosa. Infection—which may or may not be acquired from pigs or other animals—may be asymptomatic or cause diarrhoea that can be watery or contain blood and mucus. Perforation of the colon can occur, leading to peritonitis, and the parasite can also spread to the liver, lungs, and spine. Diagnosis is by recognition of the parasite on microscopic examination of diarrhoeal stools, colonic mucus, or rectal biopsies. Aside from supportive care, treatment with metronidazole or tetracycline has reportedly eradicated infection in some instances. Prevention is by filtration or boiling of drinking-water, hand washing before handling food, and careful cleaning and cooking of food.


Cystoisospora belli is a coccidian protozoan that colonizes epithelial cells of the small intestine. Infection is presumed to occur by ingestion of parasite oocysts in water or food, but vehicles for transmission to humans are unknown, although the organism has been found on cockroaches. Clinical features include watery diarrhoea, dehydration, fever, and weight loss, with isosporiasis being an opportunistic infection associated with HIV infection. Diagnosis is by microscopic examination of faecal specimens for oocysts of C. belli, which show blue autofluorescence under ultraviolet light. Aside from supportive care, trimethoprim–sulphamethoxazole is partially effective.


Microsporidia are minute intracellular parasites, now regarded as fungi, which infect various animals and birds. About a dozen species can cause human infection (some only rarely). In at least some cases, microsporidiosis appears to be acquired by ingestion of spores of the causative organism(s) in water.

Clinical manifestations are most frequently reported in HIV-infected patients, and include diarrhoea ascribed to colonization of the small intestinal mucosa by Enterocytozoon bieneusi or Encephalitozoon intestinalis. Other manifestations of microsporidial infection include acalculous cholecystitis, sinusitis, cough/dyspnoea, urethritis, and keratitis.

Diagnosis, treatment, and prevention—intestinal microsporidiosis is diagnosed by microscopic examination of faecal specimens (after appropriate staining) for microsporidian spores, or by detection of microsporidian DNA in faecal specimens. Aside from supportive care, albendazole is an effective drug for treating Encephalitozoon infections, although Ent. bieneusi does not respond. In HIV-infected patients, remission of Ent. bieneusi infection can be achieved by anti-retroviral drug treatment that reduces the HIV load. Prevention can be achieved by killing spores in water by boiling or exposure to ultraviolet light.

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