Submitted to: Veterinary Parasitology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/1/2005
Publication Date: 6/30/2005
Citation: Trout, J.M., Santin, M., Greiner, E., Fayer, R. 2005. Prevalence and genotypes of giardia duodenalis in post-weaned dairy calves. Veterinary Parasitology. pp. 177-183. Interpretive Summary: Giardia infections are common in livestock, thus animals have long been considered a major source of infections in humans. Recently, application of molecular analysis techniques such as DNA sequencing, has indicated that Giardia from some animals may be host specific, and therefore not represent a risk of infection for humans. To better understand these different genotypes of Giardia that might be present in dairy cattle, we collected fecal samples from 14 farms in seven states: VT, NY, PA, MD, VA, NC, and FL. A total of 456 calves, aged 3-11 months were sampled (number of samples per farm ranged from 21 to 49). Samples were tested for the presence of Giardia using a PCR reaction and all samples that were positive by PCR were subjected to DNA sequence analysis to determine the genotype of Giardia that was present. Fifty-two percent (237/456) of the calves were infected with Giardia. On average, 45% harbored the Assemblage E genotype, which is currently believed to infect only hoofed-livestock. However, 7% harbored the Assemblage A genotype, which has been reported in human infections. The overall averages are somewhat misleading, however, as the levels of the two genotypes varied considerably between farms; some farms had no Assemblage A while other farms had greater than 15% of the calves infected with Assemblage A Giardia. Thus, while on average it may appear that calves represent a low level risk of human Giardia infection, the farm to farm variation observed in this study indicates that use of data averaged over a large number of farms does not provide an accurate assessment of the risk of human infection.
Technical Abstract: To determine the prevalence of Giardia genotypes in post-weaned dairy calves, fecal specimens were collected from three to eleven-month-old dairy calves per farm on two farms in Vermont, New York, Pennsylvania, Maryland, Virginia, North Carolina, and Florida. Specimens cleaned of fecal debris and concentrated using CsCl density gradient centrifugation were stained and examined by immunofluorescence microscopy and also subjected to PCR and gene sequence analysis. Overall, PCR provided more sensitive detection than IFA. Prevalence of Giardia infection, as detected by PCR ranged from 20% on NC-2 to 81% on VT-2, with an overall prevalence of 52% (237 positive samples out of 456 total). DNA sequence analysis of the 16S rRNA gene revealed 87% of the 237 Giardia isolates were Assemblage E, and 13% were Assemblage A although the prevalence of these genotypes varied greatly from farm to farm, with five farms having no Assemblage A Giardia. Therefore, Assemblage E was present in 45% of all animals tested and Assemblage A was present in 7% of the animals. Thus, while many of the calves were infected with a genotype that is not known to be infectious for humans, post-weaned calves on 9 of 14 farms did harbor Assemblage A Giardia. Therefore calves should be considered as a potential source of human infectious cysts in the environment, with some farms representing a much higher risk than others.