Impact of environmental stressors on the dynamics of disease transmission.

Infectious disease within outmigrant juvenile salmon in the Columbia River Basin is modulated, in part, by abiotic stressors that influence host-susceptibility. Through the application of a dose-structured population dynamic model, we show that chemical (both in the river and in the estuary) and in-river (e.g., dams and/or predation) stressors influence host-susceptibility, increasing the mean force of infection (defined as the per capita acquisition rate of infection) by a factor of 2.2 and 1.6, respectively. Using Listonella anguillarum as a model pathogen, nonchemical in-river and chemical stressors contribute equally to a cumulative incidence of delayed disease-induced mortalities in Chinook salmon that range from 3% to 18% for estuary residence times of 30-120 days, respectively. Mitigation of environmental stressors that increase host-susceptibility could represent a significant component in future management strategies to recover listed stocks.