Identification of the Intermediate Host for Learedius learedi: Good news for Sea Turtles

              Spirorchiidiasis is one of the most common infectious diseases of sea turtles, which is, therefore why it is important to expand on what is already known about spirorchiidiasis. Many sea turtles are an endangered species and in order to help prevent the spread of a parasite that is life threatening to them, the life cycle of spirorchiid trematodes needs to be studied. Stacy et al. (2010) expand on what is already known about Learedius learedi, a common parasite in sea turtles. More specifically, the authors found the first evidence that the intermediate hosts for L. learedi is an intertidal keyhole limpet Fissurella nodosa.

            Stacy et al. (2010) used a DNA extraction technique and PCR to detect the presence of L. learedi  and  species of Hepalotrema (a family of spirorchiid trematode that infect sea turtles). The authors used ITS2 sequenced by voucher specimens of both genus’ for the PCR. Multiple methods of tissue lysis and DNA extraction were tested to discover the best way to extract DNA from gastropod tissue (Stacy et al., 2010). The gastropods that were tested were collected from a coastal habitat adjoining the Cayman Turtles Farm, Cayman Island where infected turtles were found.

            The preliminary studies revealed that there was a high prevalence of infection (greater than 98%) of L. learedi when turtles from the farm were necropsied (Stacy et al., 2010).  It was hard to determine the intermediate host because there were no snail colonies within the holding tanks at the farm, which caused the authors to deduce that the intermediate hosts were likely in the nearby costal habitat and metacercariae were entering via the water-flow system. Stacy et al. (2010) gathered several thousand gastropods from the outside environment and screened them using PCR and spirorchiid cercariae development was found in 2.3% of F. nodosa samples.

            The detection of L. learedi in F. nodosa is the first evidence of an intermediate host for a marine spirorchiid (Stacy et al., 2010). The prevalence among limpets was less than 1%, which is very small but is consistant with low prevalence observed in other primary intermediate host populations. The abundant production of cercariae and the prolonged survival of an infective intermediate hosts results in a biological advantage (Stacey et al., 2010). Those intermediate hosts that live longer will produce more cercariae over time and result in a higher prevalence in definitive hosts. The prevalence of L. learedi is very high in sea turtles at the observational site, but only 10% of those infected turtles had eggs in their feces (Stacy et al., 2010). Even when eggs were present, they were few in number and hard to find. The authors mention that there may be a correlation between the low numbers of eggs and environmental factors (i.e. seasons and current flow) which has been proven to influence egg production in freshwater spirorchiid trematodes.

            Despite the research that still needs to be done on the life cycle of L. learedi, Stacy et al. (2010) produced positive PCR results that support F. nadosa as the intermediate host of L. learedi. Future investigation may want to look at other parts of the life cycle, but it would be very interesting to examine the prevalence of eggs production in variant environmental conditions. Studies that have already been done with freshwater spirorchiid Trematode could be used as guidelines.

Works Cited:

Stacy, B.A., T. Frankovich, E. Greiner, A.R. Alleman, L.H. Herbst, P. Klein, A. Bolton, A. McIntosh and E.R. Jacobson. 2010. Detection of spirorchiid trematodes in gastropod tissue by polymerase chain reaction: preliminary identification of an intermediate host of Learedius learedi. The Journal of Parasitology 96 (4): 752 – 757.