Age-associated occurrence of Giardia duodenalis genotypes and subtypes in a birth-cohort of dairy calves in Guangdong, China.

Giardia duodenalis is a common enteric pathogen in humans and animals, with the disease giardiasis being a zoonosis. Currently, little is known about the occurrence and age patterns of G. duodenalis genotypes and subtypes in calves. To examine the infection dynamics of G. duodenalis in dairy calves, cross-sectional and longitudinal studies were conducted using PCR and DNA sequencing tools. In the cross-sectional study, 467 fecal samples were obtained from dairy farms in Guangdong Province, China, and age-associated differences in the infection rate of G. duodenalis were observed. In the longitudinal cohort study, 47 calves on Farm 5 were followed from birth to nine months of age. The shedding of G. duodenalis cysts began on day four, peaked at five weeks of age, and maintained at high levels until three months of age. Most calves continued to excrete low numbers of cysts intermittently after three months. Based on the bg locus, assemblages E (n = 486), A (n = 13), B (n = 5) and D (n = 2) were identified. Overall, there were two infection peaks of assemblage E at 3-13 weeks and 20-23 weeks of age, leading to a cumulative incidence of 100% (47/47) for this dominant assemblage. The average duration of cyst shedding for assemblage E in the cohort study was 4.0 ± 2.1 weeks for the initial infection and 2.1 ± 0.5 weeks for the subsequent one. The intensity of cyst shedding was markedly high during the initial infection but was subsequently lower in the second infection. Within assemblage E, high genetic diversity was observed, with E3 (234/486) and E5 (113/486) being the dominant subtypes. In addition, zoonotic assemblages A and B were predominantly identified in calves during the second peak of infection. Among the assemblage A-positive samples, subtypes A5, A8 and A1 were found at the bg, gdh, and tpi loci, respectively, all belonging to the AI sub-assemblage. This is the first longitudinal study of the natural history of G. duodenalis in dairy calves using genotyping and subtyping tools, and we established a standardized qPCR curve to assess the intensity of G. duodenalis infection. The results provide new perspectives on the complexity and dynamics of G. duodenalis infection in these animals.