An Investigation of the Prevalence of Giardia agilis in Anuran Amphibians Reveals Special Parasitic Adaptations of this Giardia Species

Abstract

Background: Giardia agilis is a Giardia species with a very narrow and elongated body distinct from others. It was first reported in 1882, and was detected in several species of anuran amphibians. Although there were some studies about its morphology, no investigations about its prevalence have ever been reported to date.Methods: We detected G. agilis in frogs and tadpoles from some areas of China based on its distinct morphology. Statistical analysis was performed using mid-P exact probability tests and differences were considered significant when p-values ≤0.05 were obtained.Results: We investigated the prevalence of G. agilis in 25 anuran amphibian species and found that 195 of the 463 (42.1%) samples were detected to be positive. Our molecular phylogenetic analysis indicated all the detected G. agilis were from the same species. The 195 positive frog samples were from 9 frog species, which are distributed scatteredly in four families that are not closely related rather than just restricted to a narrow lineage. The statistical prevalence among adults of different frog species showed no significant difference, and so did among tadpoles, but the prevalence in the tadpoles is significantly higher than in their adults. More interestingly, although the prevalence in Kaloula verrucosa from the same area showed no significant differences between its tadpoles without legs and the ones with two legs, but is significantly higher in these two developmental stages than in the four-legged stage, which is still much higher than in the adults. Moreover, all the positive samples were found to be from the areas with relatively high altitude (more than 870 meters).Conclusions: G. agilis is probably able to infect all anuran amphibians without species-bias. The turning point of prevalence in the period of front leg development might be related with the development of immune system of the hosts. That G. agilis tends to infect easily the frogs living in high altitude areas means that it has adapted to the dramatic temperature change in the poikilothermal animal hosts. Therefore, G. agilis has evolved some special successful parasitism strategies for parastizing the poikilothermal hosts with metamorphosis development -- anuran amphibians.