Internal movement of a digenetic trematode through its' intermediate snail host

Background/Question/Methods For digenetic trematodes dispersal is critical for completion of their complex lifecycles.  Successful transmission requires these parasites to be temporally and spatially in sync with their next host.  In estuaries, with constantly changing abiotic factors, this can be challenging. Transmission of the digenetic trematode Himasthala rhigedana through the free swimming cercarial larval stage in southern Californian estuaries has been well studied yet their internal movement through the intermediate snail host prior to emergence has received little attention.

Results/Conclusions In this study, we investigated the internal movement of H. rhigedana through its’ secondary host, the California Horn snail Cerithidea californica.  H. rhigedana cercaria are produced in the gonads and emerge from the anus.  We propose two pathways through this host.  The first involves travel through the lumen of the intestinal tract.  By utilizing the snail’s digestive system, the parasites have an energy efficient mode of transportation but might be limited in the timing of their emergence.  Cercariae would be expelled continuously with fecal matter and have the potential of emerging into unfavorable abiotic conditions.  The other proposed route is through the muscle tissue surrounding the intestinal tract.  Despite being presumably more energetically costly for these larvae, this pathway would provide more control over their emergence timing.  Histological techniques and video analysis were utilized to examine infected and non-infected snails. Cross sections of host intestines show that H. rhigedana cercariae are not located in the lumen of the intestine. Additional observations of infected snail intestines indicated that the gut lumens remained intact despite infection and the cercariae were congregated more frequently on the dorsal side of the lumen.  Early indications are that the cercariae move within the peri-intestinal sinus or between the layers of smooth muscle surrounding the lumen. This pathway may be relatively easy for the cercariae to move through. By traveling through the intestinal tract outside of the lumen, the cercariae can control their migration through this host and their entrance into the external environment.  With conditions dramatically changing in an estuary on the order of minutes, the ability to control the timing of expulsion optimizes the chances for successful transmission, Our study indicates that these cercariae may have developed a pathway that maximizes control of emergence with minimal increase in energy costs.