Parasitic disease in amphibians: control by the regulation of worm burdens.

This review considers three case studies based on macroparasites of anurans: (a) natural infections in the permanently-aquatic Xenopus laevis which represent the worm burdens acquired, and the implications for pathology, when hosts are exposed to continuous, year-round, transmission; (b) the desert toad, Scaphiopus couchii, which experiences invasion very briefly each year and provides a simplified system involving only a single significant infection (Pseudodiplorchis americanus); (c) the mesic Bufo bufo which has been the subject of experimental laboratory studies designed to measure the effects of Rhabdias bufonis infection on host growth, physical performance and survival. Experimental manipulation of both Scaphiopus and Bufo provide quantitative data on disease effects of macroparasites, including precise measurements of parasite-induced host mortality. Field data for Xenopus and Scaphiopus show that, despite high initial worm burdens from efficient transmission, infection levels at parasite maturity are modulated below those leading to significant disease. Experimental data for Scaphiopus and Bufo have documented the time-course and magnitude of this decline in intensities, and there is circumstantial evidence for Scaphiopus that this regulation is host-mediated. Immunological studies on Xenopus show that disease effects of the pathogenic Pseudocapillaroides xenopodis are exacerbated in thymectomised hosts and reversed by implantation of thymuses from MHC-compatible donors. Thus, whilst factorial experiments can demonstrate the potential of helminths to cause significant disease and mortality in anuran host-macroparasite interactions, powerful post-invasion regulation of worm burdens appears to exert a strong control of parasite-induced disease in natural host populations.