Intestinal protozoan infections (parasitic diarrhoea)

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    The most important intestinal protozoan infections are amoebiasis (Entamoeba histolytica), giardiasis (Giardia lamblia), cryptosporidiosis (Cryptosporidium sp), cyclosporiasis (Cyclospora cayetanensis) and isosporiasis (Isospora belli).


    Intestinal protozoa are transmitted by the faecal-oral route (soiled hands, ingestion of food or water contaminated with faeces) and may cause both individual cases of diarrhoea and epidemic diarrhoea outbreaks.

    Clinical features

    • Amoebiasis gives rise to bloody diarrhoea (see Amoebiasis, Chapter 3).


    • Clinical presentation of giardiasis, cryptosporidiosis, cyclosporiasis and isosporiasis is very similar:
      • Diarrhoea is usually mild and self-limiting, except in children and patients with advanced HIV disease (CD4 < 200). These patients are likely to develop severe, intermittent or chronic diarrhoea that may be complicated by malabsorption with significant wasting (or failure to gain weight in children) or severe dehydration.
      • Stools are usually watery, but steatorrhoea (pale, bulky, fatty stools) may be found in the event of secondary fat malabsorption; stools may contain mucus.
      • Diarrhoea is usually associated with non-specific gastrointestinal symptoms (abdominal distension and cramps, flatulence, nausea, anorexia), but patients have low-grade fever or no fever.


    Definitive diagnosis relies on parasite identification in stool specimens (trophozoites and cysts for giardia; oocysts for cryptosporidium, cyclospora, isospora). Two to three samples, collected 2 to 3 days apart are necessary, as pathogens are shed intermittently.


    • Correct dehydration if present (for clinical features and management, see Dehydration, Chapter 1).


    • If the causal agent has been identified in the stool:


    tinidazole PO single dose
    Children: 50 mg/kg (max. 2 g) 
    Adults: 2 g
    metronidazole PO for 3 days
    Children: 30 mg/kg once daily
    Adults: 2 g once daily


    In immunocompetent patients, no aetiological treatment; spontaneous resolution in 1 to 2 weeks.


    co-trimoxazole PO for 7 days
    Children: 25 mg SMX + 5 mg TMP/kg 2 times daily
    Adults: 800 mg SMX + 160 mg TMP 2 times daily
    In immunocompetent patients, symptoms usually resolve spontaneous in 1 to 3 weeks. Treatment is given in case of severe or prolonged symptoms.


    co-trimoxazole PO for 7 to 10 days
    Adults: 800 mg SMX + 160 mg TMP 2 times daily
    In immunocompetent patients, symptoms usually resolve spontaneous in 2 to 3 weeks. Treatment is given in case of severe or prolonged symptoms.


    • If reliable stool examination cannot be carried out: parasitic diarrhoeas cannot be differentiated on clinical grounds, nor is it possible to distinguish these from non- parasitic diarrhoeas. An empirical treatment (using tinidazole or metronidazole and co-trimoxazole as above, together or in succession) may be tried in the case of prolonged diarrhoea or steatorrhoea. In patients with HIV infection, see empirical treatment (HIV infections and AIDS, Chapter 8).


    • In patients with advanced HIV disease, cryptosporidiosis, cyclosporiasis and isosporiasis are opportunistic infections; the most effective intervention is the treatment of the underlying HIV infection with antiretrovirals. Patients remain at high risk for dehydration/death until immunity is restored.