Issues in clinical parasitology: the treatment of giardiasis

Giardiasis is the most common pathogenic parasitic infection in the United States. The choice of a therapeutic agent for treatment of the infection is unclear; of the three drugs commonly used, metronidazole and furazolidone have been shown to be carcinogenic in animal studies, while quinarcrine causes frequent side effects. In order to evaluate the usefulness of these three drugs, eight randomized trials of effectiveness were evaluated. The trials suffered from unclear methods, variability in study populations and total dosage of medication used, inaccuracy in determining outcomes, and inadequate sample sizes. Both metronidazole and quinacrine were more effective than furazolidone (p less than 0.001), but quinacrine caused frequent side effects which may have been responsible for poor compliance. With regard to the carcinogenicity of these compounds, it appears likely that there is no large short-term risk from the use of metronidazole, but long-term follow-up studies have not been done and no clinical studies have been done regarding the risk of furazolidone or quinacrine. The lack of clear confidence in animal studies is evident in the fact that the Food and Drug Administration has approved ongoing studies with drugs related to metronidazole.