Effects of environmental change on helminth infections in amphibians: exploring the emergence of Ribeiroia and Echinostoma infections in North America.

Abstract

Amphibians have long served as model organisms for studying animal physiology, vertebrate anatomy, and host–parasite interactions. Recently, however, the occurrence of precipitous declines in many amphibian populations and of severe limb malformations in others has catalyzed renewed efforts to understand the effects of parasites on amphibians. In this brief review, we examine the importance of two groups of trematodes that utilize amphibians as intermediate hosts: species in the genus Ribeiroia and the broader “echinostome” group which collectively includes the genera Echinostoma and Echinoparyphium. For each, we specifically explore the pathology resulting from infection, whether the parasite has recently increased in abundance or geographic range, and the biotic and abiotic factors likely to influence infection. Both groups of parasites can induce significant pathology in amphibian hosts. Exposure to Ribeiroia cercariae causes substantial increases in mortality and limb malformations in larval amphibians. These malformations, which include missing, malformed and extra limbs, may further reduce survival in amphibians; malformations are extremely rare in adult frogs, even following years in which they are abundant (>50%) among juvenile frogs. Similarly, the echinostomes, which colonize the kidneys of amphibians, can reduce the survival and increase the incidence of edema and renal failure, particularly in laboratory experiments. Recent surveys of National Wildlife Refuges across the USA suggest that both groups of parasites are widespread and sometimes extremely abundant (~1,000 metacercariae per frog). Infections appear to be most common along major rivers and bird flyways in the northern half of the country. While limited evidence suggests a recent increase in amphibian malformations and Ribeiroia infection, the paucity of available historical data precludes a definitive assessment of whether either parasite group has recently emerged. We discuss future approaches to this question and explore contemporary ecological changes known or hypothesized to influence patterns of infection, including changes in land use, increases in nutrient and pesticide runoff, decreases in community diversity and shifts in climate. Considering the documented pathologies of each parasite group, their widespread and often abundant infection patterns, and the ongoing declines observed in amphibian populations, we emphasize the urgent need for further study of Ribeiroia and echinostome infections in amphibians.