TREMATODE INDUCED MALFORMATIONS IN AMPHIBIANS

Cercariae of digenetic trematodes have a complex life cycle involving sequential transmission from a freshwater snail as the first intermediate host and invertebrate or a vertebrate second intermediate host to a definitive vertebrate host. Free swimming cercariae emerge from infective snails and actively penetrate tadpoles and encyst within frog tissues as metacercariae. Encysted metacercariae induce malformations, primarily affecting the limbs, including extra limbs or limb elements, missing limbs and abnormal skin webbing. These malformations increase the likelihood an infected frog is consumed by a suitable definitive vertebrate host. Risk of pathology depends on the intensity of infection and the influenced by both host resistance and host tolerance.

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We recorded the occurrence of malformations in local amphibian populations in protected areas and nature reserve and subsequently  conducted laboratory studies to investigate whether the cercarial stage of a digenetic trematode species, Acanthostomum burminis is responsible for the malformations. Adult Acanthostomum burminis known to infect the freshwater snakes. We exposed two anuran species: the Common hourglass tree frog Polypedates cruciger and the Asian common toad, Duttaphrynus melanostictus to the cercairiae under laboratory conditions.  Our studies reported high frequencies of malformations, (up to 92%in the tree frog) including limb malformations such as amelia, ectromelia, ectrodactyly, and also axial malformations, such as kyphosis and scoliosis. We also observed a reduction in the survival and growth in the exposed tadpoles.  We also found that when the tadpoles were exposed at an earlier pre-limb bud stage (Gosner stage 25 and 26) they experienced higher mortality and fewer malformations than exposed at limb bud stage (Gosner stage 27).

 

Recent studies in our laboratory showed  age dependent tolerance and resistance to parasitism and variation in sequence of exposure of parasites leading to additive, antagonistic, or synergistic effects, reinforcing the notion of the existence of cryptic interactions between parasite and host communities in nature. Resistance is predicted to reduce parasite prevalence, whereas tolerance could be neutral towards or increase parasite prevalence in a population and therefore these two strategies have epidemiological significance.

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