Limnol. Oceanogr., 44(3, part 2), 1999, 925–931

Parasites occur in nearly every population. They often interact in complex ways with other stressors. In some cases, the interaction may lead to a disproportionately negative effect on the host population. In other cases, the stressor may ameliorate the effects of parasitism. Here we illustrate intersections of four types of environmental stressors with infectious diseases. First, pollutants may increase parasitism by increasing host susceptibility or by increasing the abundance of intermediate hosts and vectors. Pollutants can also decrease parasitism if infected hosts suffer differentially high mortality, parasites are more susceptible to pollution than their hosts, or if pollutants negatively affect intermediate hosts or vectors. These effects vary depending on the particular parasite and pollutant that interact. Second, habitat alterations such as impounding water or development can affect both intermediate host and vector populations such that the abundance of their attendant parasites is either increased or reduced. Third, fisheries can impact populations already stressed by disease. However, they may act to lower the density of a host population below the threshold for sustained transmission to such an extent that the parasite population can no longer persist. Fourth, introduced species may introduce new diseases to susceptible native populations or they may gain an advantage if they invade without the parasites from their native range. The complexity and ubiquity of these interactions are good arguments for considering parasitism when evaluating stressors of aquatic systems. In this review, we examine what may happen when stressors to a population overlap with parasitism—a ubiquitous stressor. There are several possible qualitative outcomes when parasites interact with other stressors. The most obvious is that some stressors may make hosts more susceptible to parasitism. Related to this are stressors that disproportionately increase mortality rates of infected hosts. Less obvious are stressors that decrease parasitism by killing parasites directly, by reducing the parasite’s intermediate hosts or vectors or otherwise degrading their ease of transmission. By the same logic, some stressors may act to increase the abundance of intermediate hosts or vectors. Epidemiological theory points to another interaction: if a stressor reduces the abundance of a host, its parasites may not be able to persist. Finally, if the stressor is an introduced species, it may introduce new parasites to the native species it competes with or preys upon. Alternatively, and probably more commonly, an introduced species might gain a competitive advantage by being less susceptible to the parasites that are already present (Kuris and Gurney 1997; Lafferty et al. in prep.).