Environmental contaminants can have numerous effects on infectious diseases. Antibiotics are a contaminant of increasing concern as they enter freshwater systems through runoff from agriculture, hospitals, and municipalities. Antibiotics can have numerous effects on aquatic organisms including alteration of the microbiome or direct toxic effects, which can influence susceptibility to infectious diseases. We performed a series of experiments to measure the impact of two common antibiotics (tetracycline and sulfadimethoxine) on fungal infection in a species of freshwater zooplankton, Daphnia dentifera. First, we performed individual-level studies to measure the effects of antibiotics on growth, feeding rates, and susceptibility to infection. Then, we conducted a population-level experiment to test the joint impacts of infection and antibiotics on population densities and infection prevalence.
Results/Conclusions
In our individual-level experiments, exposure to antibiotics decreased infection prevalence and reduced body length, but did not alter feeding rates. In our population-level experiment, both exposure to the parasite and antibiotics reduced population densities. However, the presence of the parasite caused a greater reduction in population densities in the control (no antibiotic) treatments compared to the antibiotic-exposed treatments. This occurred even though infection prevalence was higher in antibiotic-exposed treatments compared to control treatments. Thus, we found different patterns of infection depending on the scale of our experiment. More broadly, our results suggest that exposure to antibiotics can have both direct and indirect effects on freshwater organisms.