Treatment Options for the Eradication of Intestinal Protozoa

Michael JG Farthing


Nat Clin Pract Gastroenterol Hepatol. 2006;3(8) 

In This Article

Summary and Introduction

Pathogenic intestinal protozoa are responsible for clinically important infections in both the developed and the developing world. These organisms are responsible for both acute and chronic diarrhea, and Entamoeba histolytica, which affects the colon, can spread to involve the liver. Many of these pathogens, particularly the intracellular protozoa that predominantly affect the small intestine, produce their most devastating effects in patients with HIV/AIDS and other forms of immune deficiency. There are also various intestinal protozoa that do not seem to have any adverse effects on humans and can, therefore, be regarded as harmless commensal organisms. Although treatment has been available for several decades for giardiasis, isosporiasis and amoebiasis, until recently there have been no effective remedies for infection with intestinal coccidia—Cryptosporidium, Microsporidium and Cyclospora species. Cyclospora respond well to co-trimoxazole, microsporidia respond variably to albendazole, and cryptosporidia can often be eradicated by nitazoxanide. In chronically infected HIV-positive patients, treatment with multidrug regimens usually results in rapid resolution of the diarrhea and, in many instances, eradication of the parasite.

Pathogenic intestinal protozoa produce diarrhea and other intestinal symptoms by colonizing the human small and/or large intestine. Infections are found worldwide both in developing countries and the industrialized world. Intestinal protozoa are most prevalent in the developing world, where they are responsible for substantial morbidity and mortality. The small intestinal protozoa Giardia intestinalis and Cryptosporidium parvum have their major impact in children, whereas the large-bowel pathogen Entamoeba histolytica infects all age-groups but has its most profound effects in adults. Intestinal protozoan infections took center stage with the rapid spread of HIV infection and AIDS. Interestingly, some of the protozoa, particularly C. parvum and Isospora belli, are associated with profoundly increased morbidity in immunocompromised patients, whereas the severity of giardiasis and amoebiasis is little affected.

For many of the protozoa the host immune response remains poorly defined, as are the mechanisms involved in their eradication and the development of protective immunity. As yet, there is no candidate vaccine for any of the intestinal protozoan infections. It is likely that the next decade will produce great advances in our understanding of many of these areas because of the research currently being undertaken on pathogenetic mechanisms and immune responses, with commensurate progress in treatment and prevention. For the present, however, we must depend on antimicrobial chemotherapy and a healthy immune system to resolve acute and chronic infections, and thereby minimize their clinical impact. The major protozoan pathogens of clinical relevance to humans are summarized in Table 1 . There are other protozoa that can be isolated from human feces, for which there is no clear evidence of pathogenicity. There are others whose clinical impact remains controversial. The discussion in this review is limited to those organisms that are definitely known to be pathogenic to humans.


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