Potential Zoonotic Risk Of Giardia Duodenalis Infections From Livestock And Companion Animals
Transmission of Giardia duodenalis infections from animals to humans has been a subject of recent research interest. Several reports have shown animals harboring strains of G. duodenalis that have also been identified from human samples. Much of the work assessing the organism's zoonotic potential has been done on livestock and companion animals, given high prevalence rates, high intensity of shedding, and their relationships with humans as domesticated species. The research presented in this dissertation is a compilation of four complementary studies aimed at assessing the zoonotic potential of Giardia duodenalis infections in dairy cattle and dogs. The first study was a longitudinal cohort study in which the incidence of Giardia infections was evaluated in dairy cattle in the New York City Watershed encompassing Delaware, Greene, Schoharie, Ulster, and Sullivan counties in New York. Three cohorts were analyzed based on their prior infection status with Giardia. 10,672 fecal samples were collected from calves and adult cattle from 40 dairy herds. The cumulative incidence was 25% over the course of the two-year study. Risk factors such as age, prior infection status, and season of sampling were shown to be significantly associated with shedding Giardia cysts. The second study was a repeated prevalence study done on the same target population described in the first study. The major goal was to determine the prevalence of assemblages of G. duodenalis in dairy cattle in order to genetically characterize infections. 2,109 samples were collected of which 504 were positive for Giardia based on fecal flotation for a cumulative incidence of 23.9%. Polymerase chain reaction (PCR) of the beta-giardin and triosephosphate isomerase genes with subsequent DNA sequencing revealed livestock-specific and potentially zoonotic genotypes in dairy cattle in the New York City Watershed. The third study was a cross-sectional study with aims to 1) determine the prevalence of G. duodenalis in dairy cattle in Trinidad and Tobago, 2) conduct multi-locus characterization of G. duodenalis infections using PCR, and 3) identify risk factors associated with G. duodenalis infections in this population. One hundred and ninety-six samples were analyzed by PCR of the small-subunit ribosomal RNA (ssu-rRNA) gene as the only diagnostic test. Twenty-five samples were positive for a prevalence of 12.8%. Of the ssu-rRNA positive samples, ten were positive by PCR amplification of the beta-giardin gene. Three samples showed heterogeneity of assemblage typing between the two loci, likely indicating mixed infections of assemblages A and E. The final study was a cross-sectional study done on owned, shelter, and free-roaming dogs in Trinidad and Tobago. The major goals were to 1) determine the prevalence of G. duodenalis infections in dogs, 2) assess the risk of G. duodenalis infections to humans through genetic characterization of isolates, 3) compare test agreement between a commercially-available ELISA test kit for Giardia and PCR, and 4) identify risk factors associated with G. duodenalis infections in dogs. 104 samples were analyzed by PCR of the ssu-rRNA gene of which 26 were positive for a prevalence of 25%. No zoonotic strains were present in the study population. Dogspecific assemblages C and D were found as well as one sample identified as assemblage E. The kappa statistic for agreement between PCR and ELISA was 0.67 indicating good agreement between the tests. Risk factors for infection in dogs included age, group housing, and ownership status. These findings contribute to greater understanding of the zoonotic potential of G. duodenalis infections in dairy cattle and dogs. The New York Watershed studies highlight growing public health concerns about zoonotic pathogens in the public water supply. The work done in Trinidad and Tobago represents pilot epidemiologic research on G. duodenalis infections from livestock and companion animals and contributes to our knowledge about the global distribution of G. duodenalis. While these studies do not directly link infections in dairy cattle and dogs to humans, they provide information on risk factors that may perpetuate G. duodenalis infections in these populations, other animal populations and humans. Taking significant risk factors into consideration may reduce the occurrence of G. duodenalis infections in dairy cattle and dogs, thereby indirectly reducing the potential for infections in humans.
Giardia duodenalis; zoonotic; epidemiology
Mohammed, Hussni Omar
Chang, Yung-Fu; Wade, Susan Edith; Nydam, Daryl Van
Ph. D., Veterinary Medicine
Doctor of Philosophy
dissertation or thesis