Giardia lamblia (syn.: G. intestinalis, G. duodenalis) is distributed worldwide and has been identified in humans and domestic livestock, particularly in young animals such as calves, lambs, piglets, and foals and birds. The flagellate (with four pairs of flagella) living in the small intestine may produce acute or chronic enteritis with profuse and heavy diarrhea and growth rate reduction in mammals and birds. Giardia spp. infections in animals may pose a serious zoonotic threat to humans. Humans may acquire infection either through waterborne transmission by Giardia cysts occurring in the drinking water contaminated with infectious feces or by direct contact with contaminated feces. Although quinacrine and furazolidone resistance has been induced in G. duodenalistrophozoites, the substituted acridine dye derivative quinacrine can be of value in humans suffering from giardiasis, mainly in patients showing reduced response to 5-nitroimidazoles. However,...