Cross-sectional study of faecal shedding of Giardia duodenalis and Cryptosporidium parvum among packstock in the Sierra Nevada Range

Faecal specimens from 305 horses and mules used as packstock at one of 17 c ommercial or governmental (National Park Service, US Forest Service) operat ions were examined for Giardia duodenalis and Cryptosporidium parvum using immunofluorescent microscopy. Fourteen packstock (4.6%) were shedding G. du odenalis cysts, with herd-level prevalences ranging 0-22%. Number of packst ock in the corral, size of corral and density of packstock in the corral we re associated with the odds of shedding G. duodenalis cysts. None of the ho rses had detectable C. parvum oocysts. Assuming a sensitivity of at least 4 3% and a specificity of 100% for our assay, the estimated maximum true prev alence of shedding of C. parvum for packstock would be less than or equal t o 2.3% of the population. These data suggest that faecal dispersal of C. pa rvum on back country watersheds is unlikely with packstock.