Quantitation of Giardia cysts and Cryptosporidium oocysts in fecal samples by direct immunofluorescence assay.

The lack of quick, simple, and sensitive quantitative tests has impeded studies on infection patterns and treatment of Giardia spp. and Cryptosporidium spp. A quantitative direct immunofluorescence assay (FA) using a commercial FA kit was developed and evaluated. Recovery rates of the FA for Cryptosporidium oocysts in calf feces seeded with 1,000, 10,000, 100,000, and 1,000,000 oocysts per g were 14.8, 40.8, 84.2, and 78.2%, respectively. Interassay coefficients of variation were 10.6 to 47.1%. Recovery rates of the FA for Giardia cysts in feces seeded with 1,000, 10,000, and 100,000 cysts per g were 76.4, 96.9, and 89.6%, respectively. Interassay coefficients of variation were 7.4 to 22.1%. By comparison, recovery rates of Giardia cyst by sucrose gradient flotation were only 20.5, 51.2, and 42.9%, respectively. Counts of cysts-per-gram obtained by sucrose gradient flotation with samples from calves, lambs, and ewes were only 49.1 to 54.8% of those obtained by the FA. Zinc sulfate flotation detected only 36.4% of infections when there were < or = 1,000 cysts per g. The quantitative FA offers a useful technique for epidemiological and control studies of these two parasites.