Tritia obsoleta (Say) is a snail inhabiting estuarine mudflats and salt marshes along the Eastern Coast of the United States. It is the intermediate host of Pleurogonius malaclemys whose definitive host is the diamondback terrapin (Malaclemys terrapin terrapin). P. malaclemys forms dome-like cysts on the snail's operculum. The study was aimed at observing differences in the snail's physical parameters (shell height, thickness, and snail weight) under the effect of the environmental quality and the prevalence of P. malaclemys. Snails were collected, along with samples of water and bottom deposits, from two locations in Barnegat Bay, New Jersey. In the area with poorer quality of sediment and water, the prevalence of parasite's cysts was significantly higher, and the shells were thinner and lower. Histological analyses revealed the presence of metacercariae and tissue anomalies. Increasing prevalence of the parasite was correlated with deteriorating ecosystem health, leading to a combined detrimental effect on the host. This study is a model of the repercussions of deteriorating habitat quality on host-parasite relationships, and the health of the organisms in question.