Current trends in research into the waterborne parasite Giardia.

The waterborne flagellated parasite Giardia intestinalis continues to be the most frequent protozoan agent of intestinal disease world-wide, causing an estimated 2.8 x 10(8) cases per annum. Severe symptoms of diarrhea and sickness can be persistent and even life threatening in the immunocompromised, in infants, and in the aged, although self-limiting in the majority of patients. Despite a growing awareness and intensified research many uncertainties remain, especially with respect to the risk of potential zoonotic transmission. Water supplies can be monitored for cysts using automated cytofluorimetric immunoassays, but this does not measure infectivity. Filtration provides the best protection, because cysts are highly resistant to chlorine and ozone. Other incompletely elucidated aspects include mechanisms of pathogenicity, host reaction to infection, immunity and parasite control using vaccines or antigiardial compounds; the 5-nitroimidazole metronidazole is the most effective of these. Molecular typing of various isolates indicates that most animal parasites are not infective to humans, but those that are can be genotypically classified as assemblage A or B. The phylogeny of the organism remains uncertain, but there is a growing opinion that Giardia is not an ancient primitive eukaryote, but that it is derived from a more complex mitochondria-containing protozoon.