Cryptosporidium parvum sporozoite staining by propidium iodide.

Modified Ziehl-Neelsen (ZN) acid-fast stain is the usual method for detection of Cryptosporidium oocysts in feces. Propidium iodide permitted us to stain free or intra-oocyst sporozoites. With the ZN method only 3-5% of the oocysts purified from three human and one experimentally infected lamb dichromate-preserved feces were stained by carbol fuchsin. These fuchsin-stained oocysts were free of intact sporozoites as identified by propidium iodide staining. Treatment with 10% formalin or 0.5% sodium hypochlorite increased the percentage of acid-fast stained oocysts and thus the sensitivity of acid-fast staining. Treatment with sodium hypochlorite induced intra-oocyst sporozoite alterations as demonstrated by flow cytometric analysis of the oocysts' DNA content. Propidium iodide staining of fixed oocysts is a simple and rapid method to visualize sporozoites and to assess oocyst preservation after different treatments.