Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Jeffrey R. Laursen


Miracidia exhibit observable host-finding behaviors, and their sympatric snails have attributes that either permit or prohibit infection. This study was designed to assess the factors involved in intermediate host finding and host-parasite compatibility in the deer liver fluke (Fascioloides magna). The main aim was to determine the extent to which the parasite and/or the intermediate host are involved in host-parasite compatibility. A secondary goal was to determine what factors may lead to miracidial transformation. The study used a panel of four sympatric snails (Lymnaea caperata, Lymnaea palustris, Lymnaea exilis, and Physa sp.) that display a range of susceptibility to the trematode from L. caperata which is the natural intermediate host, to experimentally susceptible L. palustris, to resistant L. exilis and Physa sp. Miracidial host finding behaviors were examined to determine the extent of the role of the miracidium in initiating infection. This was tested by observing single miracidium infections for 30 minute time periods to record number of contacts, attachment time, infection success, and whether the miracidium was harmed. Miracidia attached to susceptible L. caperata more often (χ 2 = 6.6561, p = 0.0359) and for longer periods of time (χ 2 = 8.5290, p = 0.0141) than to resistant L. exilis or Physa sp. Miracidia exposed to a physid snail were harmed more often than those exposed to the lymnaeids (χ 2 = 5.4000, p = 0.0251). To assess the role of primary barriers of snail immunity in host-parasite compatibility, miracidia were exposed to snail mucus in vitro to assess snail toxicity. Following the pattern seen with intact snails, mucus from Physa sp. was 100% cytotoxic to miracidia at 1:3 and 1:30 dilutions. Mucus from L. caperata, L. palustris, and L. exilis showed no difference from control for up to 4 hours. This demonstrated an overall species effect (F3,32 = 23.59; p<0.0001). Mucus did not significantly stimulate transformation in any species at any dilution. To assess the role of internal snail products on compatibility and miracidial transformation, miracidia were exposed to tissue homogenate from L. caperata, L. palustris, L. exilis, and Physa sp. in protein dilutions of 0.5, 0.05, and 0.005 mg/mL. Tissue homogenate from the natural host, L. caperata stimulated transformation more frequently than any other species (F3,77 = 4.43; p = 0.0063) particularly at the 0.5 mg/mL dilution (F2,78 = 25.19; p<0.0001). To examine in vivo snail immune response, single snails of L. caperata, L. palustris, and L. exilis were subjected to mass exposures of F. magna miracidia then fixed at 1 h, 2 h, 3 h, 4 h, and 7 h. Specimens were serial sectioned and examined for in vivo snail immune response. Infection progressed farthest in L. caperata, but no significant hemocyte accumulation was observed in any species. This study showed that miracidia play an active role in locating and attacking a preferred host. Snails may avoid infection due to toxins in mucus or tissue homogenate.

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Parasitology Commons