FINGERUT, J.T.; ZIMMER, C.A.; ZIMMER, R.K.: From host to host: interaction of behavior and the environment during parasite transmission

Parasites that transmit infection between hosts through free-swimming larval stages must interact both with the host’s internal environment and external habitat. The trematode Himasthla rhigedana moves from its 1st to 2nd intermediary hosts as a free-swimming cercarial larva. In both laboratory and field studies we quantified the interaction between cercaria and the environment throughout the entire transmission process, including movement through, and emergence from, the host, transport in the water column, and subsequent host recognition and infection. A hierarchy of abiotic cues, including light, temperature, water availability and time of day, controls emergence from the 1st intermediary host (a snail). Once emerged, transport to the next benthic host is determined by the relative strength of downward cercarial swimming (increasing proximity to their benthic hosts) versus re-suspension due to turbulent mixing. Finally, the settlement of cercaria on their 2nd intermediary host (snail or crab) is mediated by larval ability to distinguish both visual and tactile cues. Through the application of recent technologies (such as flumes, IR laser illumination and Computer Assisted Video Motion Analysis) in the lab and real-time studies of all three stages of transmission in the field, we have defined the environmental conditions under which parasites can maximize the transmission of infection. The results of this study are relevant to topics beyond the parasite system studied here, from broad questions regarding larval transport and settlement to specific issues such as the control of medically and agriculturally important trematode species.