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Giardia intestinalis (syn. duodenalis or lamblia) is one of the most common intestinal parasites in the world, with an estimated 2.8×106 infections per year in humans, and it contributes to diarrhea and nutritional deficiencies in children in developing regions. The wide prevalence of Giardia and its unique place in evolutionary biology have led to ongoing research.Research into the basic biology of Giardia has highlighted some of its unique properties as an ‘early-branching’ eukaryote. Although Giardia do not contain mitochondria, they have developed pathways to perform some mitochondrial functions. Investigations into encystation and excystation have identified new gene products that are important in cyst wall formation, and signal transduction events that occur during excystation. The ability to transfect Giardia stably will lead to an improved understanding of its development and metabolism. Molecular typing of G. intestinalis isolates indicates that most animal parasites are not associated with human infection. Insights into immunology have helped define the role of IL-6 in the early control of murine giardiasis, and the contributions of IgA in controlling infection. Further studies of giardiasis in poorly nourished children in developing regions supports an important contributing role of Giardia in stunting and cognitive impairment. Finally, new diagnostic assays using antigen detection are being evaluated and a new agent, nitazoxanide, has been approved in the USA for the treatment of giardiasis and cryptosporidiosis in children.Research into the biology of Giardia should increase knowledge about protist differentiation and will complement studies in other biological systems. Continued study of the role of Giardia in chronic diarrhea and malnutrition in developing regions will help focus strategies to improve childhood growth and nutrition.