TAYLOR, C.N.; WASSERSUG, R.J.; Dalhousie University; Dalhousie University: Mechanical and Synergistic Factors Influencing Echinostomatoid Trematode Infections in Anurans

We used high-speed videography of staged encounters between tadpoles of either Bufo americanus or Rana sylvatica and Echinostoma cercariae to understand why echinostomatoids, such as Echinostoma and Ribeiroia (which has been implicated in epidemics of limb deformities in anurans), attack specific anatomical regions of tadpoles. Both Bufo and Rana tadpoles can shed cercariae on their skin from some parts of their body more easily than others. In particular, because of the “dead water zone” at the junction of a tadpole’s body and tail, cercariae that make their way into this recess are more difficult for tadpoles to brush off than elsewhere on the tadpole’s body. Cercariae that reach this recess can easily enter the inguinal region of tadpoles, as do Ribeiroia, or ascend the tadpole’s cloaca, as do Echinostoma. Tadpoles often sense cercariae contacting their skin and make explosive movements to shed those parasites before they enter the tadpole’s body. Factors that reduce tadpoles’ activity, such as the threat of predation or exposure to certain pesticides, will necessarily increase a tadpole’s susceptibility to echinostomatoid infection. Because Bufo tadpoles are unpalatable to many predators, they can afford to make more conspicuous evasive maneuvers than Rana tadpoles, and do so in the laboratory. Bufo tadpoles also have lower rates of Ribeiroia infection than Rana tadpoles in the field. Our observations help explain how factors, such as the introduction of predator fishes or certain pesticides in tadpole habitats, may act synergistically to increase parasite loads and subsequent deformities in anurans.