Assessment of the conventional detection of fecal Cryptosporidium serpentis oocysts in subclinically infected captive snakes.

Fecal specimens of seven captive snakes naturally subclinically infected with Cryptosporidium serpentis were monitored for 131 days for the presence and concentration of oocysts. Thirty-three of 81 (41%) of the monoclonal antibody positive stools were read as negative (sensitivity 59%) by the acid-fast-stained (AFS) fecal smears. Oocyst concentrations in the false-negative stools ranged between 6.0 x 10(3)/g to 2.5 x 10(4)/g. The experimentally determined concentration-threshold of oocyst detection by AFS fecal smear was 3.0 x 10(4)/g. The stool weights did not conform to a normal distribution; the oocyst concentration was significantly correlated with the stool weight (P < 0.01). Due to this correlation, stools which constituted less than 0.41% of the snake weight were classified as negative by the AFS fecal smears. The AFS fecal smear technique should be used exclusively for the determination of Cryptosporidium-positive snakes, but not for diagnosis of snake negativity for Cryptosporidium; it should be applied only for larger fecal specimens, such as those which constitute more than 0.41% of snake weight. At least five to seven stool samples should be examined by fecal smear in order to determine snake positivity for Cryptosporidium in subclinical infection.