There is a fairly good review of the topic by Khan and Thulin:
Khan,R.A. and Thulin,J. 1991. Influence of pollution on parasites of
aquatic animals. Advances in Parasitology 30:201-238
There is some evidence that many pollutants may increase the
susceptibility of fish to parasite infections. Copper-exposed fish are
more susceptible to outbreaks of things like IHNV, Yersinia ruckeri and
cestode larvae (Eubothrium, I think). Organochlorines tend to suppress
humoral immune responses, and fish from OC-polluted sites have more
cancer and parasites than conrol fish. Croaker intoxicated by oil spills
contain more monogenean parasites, and sheepshead minnows grow tumours
induced by Myxobolus (protozoan). BKME are also reliably connected to
increased parasitism in fish.
It is true that the parasite cycle is sometimes disrupted by pollution,
and this can occasionally pose problems in these studies; the above
paper discusses this as well. Other possibles:
Bucke, D. 1993. Aquatic pollution: effects on the health of fish and
shellfish. Parasitology 106 supp.: 25-37
Couillard,C.M. et al. 1995. Lesions and parasites in white suckers in
bleached-kraft pulp mill-contaminated and reference rivers.
Environmental Toxicology and Chemistry 14(6):1051-1060
email me if you would like to exchange some more references!
blanlev at cam.org