ARS | Publication request: A longitudinal study of Giardia duodenalis genotypes in dairy cows from birth to two years of age

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 4, 2009
Publication Date: N/A

Interpretive Summary: Giardia is an intestinal parasite that can infect animals and humans; some types of Giardia infect only animals, whereas other types can infect both animals and humans. These different types are called genotypes. The only way to determine which genotype is present in a fecal sample is to isolate DNA from the Giardia and analyze the DNA sequence. Diary cattle can be infected with 3 genotypes of Giardia, two of these A and B are also infectious for humans; the third genotype, E, is only infectious for cattle and other hoofed-livestock. The current research project was conducted to follow a group of 30 dairy cows, and collect fecal samples from the time they were born until they were 2 years old, so that the prevalence of different Giardia genotypes could be determined. Fecal samples were collected weekly from calves up to 8 weeks of age, biweekly from calves 3 to 5 months of age, and monthly from animals 6 to 24 months of age. The parasites were isolated from the fecal samples and analyzed to determine the genotype. The results showed that over time 100% of the animals became infected with genotype E, which is not infectious to humans, but 70% of the animals became infected with genotype A, which can be transmitted to humans. About 2/3 of these animals were infected with both genotypes at the same time. This high level of genotype A, was not reported in other studies when cattle were only sampled one time. This is likely because genotype A is present in lower numbers and so its presence is masked by high levels of genotype E. This data indicates that cattle could be a larger source of Giardia that is infectious for humans than previous studies have reported.

Technical Abstract: Fecal specimens were collected from 30 calves from birth to 24 months of age at a dairy farm in Maryland to determine the prevalence and age distribution of Giardia duodenalis genotypes. Fecal specimens were collected weekly from calves up to 8 weeks of age, biweekly from calves 3 to 5 months of age, and monthly from animals 6 to 24 months of age. Fecal samples were subjected to density gradient centrifugation to remove debris and concentrate cysts. Specimens were analyzed by immunofluorescence microscopy and polymerase chain reaction (PCR). All PCR positive specimens were sequenced using the 16S rRNA gene of Giardia. All 30 calves shed Giardia spp. cysts at some time during the study. Of 990 specimens, 312 were positive for G. duodenalis (31.5%). The highest prevalence of infection occurred at weeks 4 and 5 of age with 25 out of 30 calves shedding cysts at those sampling times. Overall, pre-weaned calves (less than 8 weeks of age) exhibited the highest prevalence (60.8%), followed by post-weaned calves (3-12 months of age) (32.1%) and heifers (12-24 months of age) (11.4%). Sequence analysis of the 312 PCR-positive samples revealed the presence of both Assemblage A and E, G. duodenalis, with cumulative prevalences of 70 and 100%, respectively. Assemblage A was not detected in pre-weaned calves, but was detected in 6.9% and 4.7% of post-weaned calves and heifers, respectively. These data indicate not only that calves are infected with both Assemblage A and E simultaneously, but also that infections with zoonotic Assemblage A, G. duodenalis are more common than previously reported. Thus, calves appear to be a more significant reservoir of human infectious G. duodenalis than previous data has suggested.