ARS | Publication request: Prevalence of Giardia duodenalis genotypes in adult dairy cows

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 16, 2007
Publication Date: June 12, 2007
Citation: Trout, J.M., Santin, M., Fayer, R. 2007. Prevalence of Giardia duodenalis genotypes in adult dairy cows. Veterinary Parasitology. 147:205-209.

Interpretive Summary: Giardia duodenalis is a parasite of the intestine that causes disease in humans and many animals. Within the G. duodenalis species, there are different types (genotypes or assemblages) that seem to be specific for different animal hosts. For example, Assemblages A and B are zoonotic – they infect humans and many other mammals. Assemblages C-G are generally restricted to a specific animal host and have not been found in human infections. All of the organisms in these assemblages look identical under the microscope; therefore they can be differentiated only by the use of molecular analysis techniques, such as PCR and DNA sequence analysis. Although Giardia has been historically reported from many animal sources, data collected before the use of molecular methods is useless in determining if Giardia from a particular animal host represents a risk of human infection. The current study examined adult dairy cows in 7 states along the east coast of the U.S. in order to determine if cows can harbor Giardia that is infectious for humans. Fecal samples were collected from 541 adult cows and analyzed with molecular techniques to determine the genotypes of Giardia that were present. Overall, 27% of the cows were infected with Giardia, however, only 2% of the animals had a Giardia that can infect humans (Assemblage A); the remaining 25% were infected with Assemblage E, Giardia, which currently appears to infect only hoofed-livestock. Although only a small percentage of these cows harbored a type of Giardia that can infect humans, according to the National Agricultural Statistics Service, there were approximately 9,000,000 dairy cows in the U.S. in 2005. Thus, there are potentially 180,000 adult cows that are excreting Giardia with the potential to infect humans. Therefore, even adult animals should be considered potential sources of human infectious organisms.

Technical Abstract: The prevalence of G. duodenalis genotypes was determined in adult dairy cows. Fecal specimens were collected from two farms each in Vermont, New York, Pennsylvania, Maryland, Virginia, North Carolina, and Florida. Specimens, cleaned of fecal debris and concentrated using CsCl density gradient centrifugation, were subjected to PCR and DNA sequence analysis. The prevalence of G. duodenalis infection, ranged from 3% to 64%, with an average prevalence of 27% (144 positive cows out of 541 examined). DNA sequence analysis of the 16S rRNA gene revealed the presence of both Assemblage A and Assemblage E, G. duodenalis. Overall, Assemblage E was present in 25% of all animals tested and Assemblage A was present in 1% of the animals. As a percentage of G. duodenalis isolates, Assemblage E represented 94% and Assemblage A represented 6%. However, Assemblage A was present on only 5 of the 14 farms sampled. Although, most of the cows were infected with a genotype that is not known to be infectious for humans, adult cows on 5 farms did harbor varying levels of zoonotic Assemblage A, G. duodenalis. Therefore, although adult cows do not appear to be a significant source of human infectious cysts in the environment, the risk from this age group should not entirely be discounted.