Flow cytometric detection of Cryptosporidium oocysts in human stool samples

Cryptosporidium parvum is an important pathogen that causes diarrhea in virtually all human populations. Improved diagnostic methods are needed to understand the risk factors, modes of transmission, and impact of cryptosporidiosis. In the present study, we fluorescently labeled and counted C. parvum oocysts by flow cytometry (FC) and developed a simple and efficient method of processing human stool samples for FC analysis. Formed stool (suspended in phosphate-buffered saline) from an asymptomatic, healthy individual was seeded with known concentrations of oocysts, and oocysts were labeled with a cell wall-specific monoclonal antibody and detected by FC. The method described herein resulted in a mean oocyst recovery rate of 45% ± 16% (median 42%), which consistently yielded a fourfold increase in sensitivity compared to direct fluorescent-antibody assay of seeded stool samples. However, in many instances, FC detected as few as 10³ oocysts per ml. Thus, FC provides a reproducible and sensitive method for C. parvum oocyst detection.