Prevalence and multilocus analysis of Giardia duodenalis in racehorses in China.

Giardia duodenalis is a zoonotic intestinal parasite infecting humans and mammals worldwide. In this study, we evaluated the prevalence of G. duodenalis in racehorses in China and genetically characterized it. In total, 621 fecal samples were collected from racehorses at 17 equestrian clubs in 15 cities in China. Forty-eight (7.7%) animals from 11 equestrian clubs were positive for G. duodenalis of assemblages A (n = 10), B (n = 36), and E (n = 2), based on the small subunit ribosomal RNA (SSU rRNA) gene. Statistically significant differences in the prevalence of this parasite were detected among the different equestrian clubs (χ2 = 49.55, df = 16, p < 0.01), whereas no significant differences were detected according to age (χ2 = 0.64, df = 1, p > 0.05) or sex (χ2 = 1.41, df = 2, p > 0.05). The G. duodenalis-positive samples were further subtyped based on three other genes, which identified 5, 4, and 4 genotypes at the triose phosphate isomerase (tpi), glutamate dehydrogenase (gdh), and β-giardin (bg) loci, respectively. Subassemblage BIV was the predominant genotype. A phylogenetic analysis of the concatenated sequences of subassemblage BIV showed that the multilocus genotypes from the horses were genetically different from those of humans and nonhuman primates, indicating the evolution of host separation in G. duodenalis subassemblage BIV. Our study extends our understanding of the transmission of G. duodenalis between animals and humans.