Submitted to: Risk Analysis
Publication Type: Abstract Only
Publication Acceptance Date: June 30, 2004
Publication Date: December 7, 2004
Citation: Fayer, R., Trout, J.M., Santin, M. 2004. On farm protozoan pathogens: The prevalence of zoonotic cryptosporidium and Giardia. [Abstract]. Risk Analysis. p.T4.3
Dairy farms in 7 states in the eastern U.S. were visited on 2 consecutive years to determine the prevalence of Cryptosporidium and Giardia species in pre-weaned (5 days to 2 months) and post-weaned calves (3 to 11 months). A total of 971 fecal specimens were collected, sieved, and subjected to density gradient centrifugation to remove debris and concentrate parasites. Specimens were examined by immunofluorescence microscopy and polymerase chain reaction (PCR). For all PCR positive specimens the following genes were sequenced: for Cryptosporidium, the 18S rRNA gene; for Giardia, the 16s rRNA, TPI, or beta-giardin genes. Both parasites were identified from all farms. Housing type appeared to have no effect on prevalence. Overall, 36% were infected with Cryptosporidium, but more pre-weaned calves (50%) than post-weaned calves (20%) were infected. DNA sequence analysis revealed C. parvum, C. andersoni, and two unnamed Cryptosporidium genotypes: Bovine B and deer-like genotype. The prevalence of Cryptosporidium species and genotypes appeared to be age related. Cryptosporidium parvum, the zoonotic species/genotype, constituted 85% of the Cryptosporidium infections in pre-weaned calves but only 1% of the infections in post-weaned calves. This finding indicates that persons handling or otherwise exposed to calves under 2 months of age are at greater risk of infection from Cryptosporidium than persons exposed to older calves. For Giardia, 46% of the calves were infected. Gene sequence analysis revealed two genotypes of Giardia duodenalis, Assemblage E, which infects hoofed-livestock, and Assemblage A, which infects various mammals, including humans. Assemblage A represented 14% of the infections while Assemblage E represented 86% of the infections; these percentages did not vary significantly with age. The results indicate that although a significant percentage of dairy calves are infected with Giardia, only a small number of those represent a source of organisms infectious for humans. These findings clearly demonstrate that in order to conduct appropriate risk analysis, earlier reports on the prevalence of C. parvum and Giardia in cattle that were based solely on oocyst and cyst morphology must be reassessed using molecular methods to determine the presence of species and genotypes infectious for humans.