Prevalence of Giardia duodenalis assemblages among dairy herds in the New York City Watershed.

A longitudinal herd-level study was carried out to determine the cumulative incidence of Giardia duodenalis infections in dairy cattle in the New York City Watershed. We also sought to assess the changes in infection pattern of animals diagnosed as shedding Giardia over time, determine risk factors that may be associated with G. duodenalis infections, and identify potentially zoonotic infections. A total of 2109 fecal samples were randomly collected from dairy cattle at 34 farms in the New York City Watershed on a seasonal basis. A total of 504 Giardia-positive samples were identified by zinc sulfate flotation. The overall cumulative incidence of G. duodenalis based on flotation results was 23.9% with 73.8% of all infections occurring in animals under 180 days of age (372/504). The intensity of infection ranged from 2 to 563,200 cysts/gram of feces. Cattle shedding Cryptosporidium spp. oocysts were twice as likely to shed G. duodenalis cysts in comparison to the animals that did not shed oocysts (1.81 95% CI 1.26-2.60 p=0.0012). In the multivariate analysis, only the age of the animal and the presence of dogs on the farm were significantly associated with the likelihood of shedding G. duodenalis. DNA was extracted from positive samples and analyzed by polymerase chain reaction (PCR) of the beta-giardin and triosephosphate isomerase genes of Giardia spp. 304 samples were analyzed by PCR of which 131 were sequenced. 22.1% of sequenced samples were identified as assemblage A and 77.9% were identified as assemblage E. Interestingly, 100% of specimens identified as assemblage A were from calves under 84 days of age indicating that younger cattle are important reservoirs for potentially zoonotic assemblages of G. duodenalis.