A comparison of enumeration techniques for Cryptosporidium parvum oocysts

A variety of methods have been used to enumerate Cryptosporidium parvum ooc ysts from source or drinking waters. The reliability of these counting meth ods varies, in part, with suspension density, sample purity, and other fact ors. Frequently, the method of determination of suspension density is not r eported by authors. To confound the problem, each method of counting has la rge inherent variation. There is a relationship between suspension density, overall number of organisms counted, and counting mechanism accuracy that should be accounted for when selecting a counting mechanism. This study sel ected a maximum acceptable coefficient of variation (CV) to be 10%. A metho d was considered unreliable if this standard was not achieved. Flow cytomet ry achieved this standard at 486 oocysts/ml. Counting with a Coulter counte r achieved this level of reliability at about 1,230 oocysts/ml. Neither cha mber slides nor fluorescent antibody-stained well slides ever demonstrated less than 10% CV. However, estimates of the minimum required concentrations were 5,100 oocysts/ml and approximately 6,500 oocysts/ml, respectively. Th e hemacytometer provided counts accurate to a 10% CV at a concentration of at least 60,000 organisms/ml. Of the methods tested, flow cytometry provide d the least amount of variability at low suspension densities.