Induction of folds or sutures on the walls of Cryptosporidium parvum oocysts and their importance as a diagnostic feature.

The proportion of oocysts of Cryptosporidium parvum showing a fold on oocyst walls when incubated with either fluorescent monoclonal antibody or a surface-reactive fluorescent dye was increased by incubating suspensions of oocysts with dimethyl sulfoxide, sucrose, or Hanks' balanced salt solution. Further incubation of sucrose-incubated oocysts with water showed this to be a reversible phenomenon. Oocysts demonstrating this fold after incubation in dimethyl sulfoxide were of the same viability as control oocysts and followed the same excystation dynamics. Despite this fold having been previously described as a suture, we were unable to find any evidence that this pattern of fluorescence highlighted the same suture that has been described in ultrastructural studies. Furthermore, oocysts were observed in which this fold was not always continuous with the gape in the oocyst wall through which the sporozoites had emerged. We propose that this fluorescently highlighted region or fold should no longer be described as a suture and question its validity as a diagnostic feature. When environmental and other samples are being examined for the presence of C. parvum oocysts, objects of appropriate size, shape, and fluorescence which do not demonstrate a surface fold should not necessarily be excluded.