Pesticides as cofactors in ranavirus emergence

Emerging infectious diseases are a leading challenge for human, wildlife and plant health.  Emerging diseases and environmental contamination are two of the leading hypotheses for global amphibian declines, yet few studies have examined the influence of contaminants on disease susceptibility.  We present results from three factorial studies that investigated the interaction of contaminants and the emerging pathogen, Ambystoma tigrinum virus (ATV), on fitness correlates and disease susceptibility in tiger salamander larvae.  In the three studies we examined the combined effects of ecologically-relevant doses of: 1) atrazine and nitrates; 2) Roundup and malathion; and, 3) atrazine and chlorpyrifos.  Salamanders were laboratory-bred, originally from Arizona populations where a coevolutionary history with ATV is supported, thus cofactors may contribute to disease emergence.  In the first experiment, moderate concentrations of atrazine (16 ppb) significantly decreased host leukocyte levels and increased infection rate by ATV.  Nitrates decreased leukocyte counts at intermediate and high levels (6.8, 68 ppm), but decreased infection rates, perhaps by killing or inhibiting growth of ATV.  In the second experiment, neither Roundup, malathion or their interaction affected disease susceptibility.  However, both pesticides, when considered separately, significantly decreased salamander growth rates.  In the third experiment, chlorpyrifos had a significant effect on survival and preliminary results suggest that there is an interaction between chlorpyrifos and virus susceptibility.  Preliminary results also suggest there is a three-way interaction between chlorpyrifos, atrazine and virus; at intermediate atrazine levels, disease-induced mortality increases with increasing chlorpyrifos concentration.  These results suggest that ecologically relevant concentrations of pesticides can have immunosuppressive effects and may contribute to emergence of ATV, raising concerns about the influence of pesticides on disease emergence in general.