Effects of exposure to contaminants such as pesticides along with exposure to pathogens have been listed as two major contributors to the global crisis of declining amphibian populations. These two factors have also been linked in explanations of the causes of these population declines. We conducted a combined exposure experiment to test the hypothesis that exposure to two agricultural
herbicides would increase the susceptibility of post-metamorphic northern leopard frogs (Lithobates pipiens) to the amphibian fungal pathogen Batrachochytrium dendrobatidis (Bd). We assessed the independent and interactive effects of these exposures on the health and survival of the frogs. Wild-caught frogs underwent a 21-day exposure to a nominal concentration of either 2.1 μg/L
atrazine (Aatrex(®) Liquid 480) or 100 μg a.e./L
glyphosate (
Roundup(®) Original), followed by Bd, and then were observed until 94 days post-initial exposure to the
herbicides. Actual levels of
atrazine were between 4.28 ± 0.04 μg/L and 1.70 ± 0.26 μg/L while
glyphosate degraded from 100 μg a.e./L to approximately 7 μg a.e./L within 6 days of initial exposure to the
herbicides. Compared to controls, the
glyphosate formulation reduced the snout-vent length of frogs during the
pesticide exposure (at Day 21), and the
atrazine formulation reduced gain in mass up to Day 94. No treatment affected survival, splenosomatic or hepatosomatic indices, the densities and sizes of hepatic and splenic melanomacrophage aggregates, the density and size of hepatic
granulomas, proportions of circulating leucocytes, the ratio of neutrophils to lymphocytes, or the ratio of leucocytes to erythrocytes. Histological assessment of samples collected at Day 94 revealed no evidence of Bd
infection in any Bd-exposed frogs, while real-time PCR detected only one case of light
infection in a single
atrazine- and Bd-exposed frog. Frogs exposed to Bd shed their skin significantly more frequently than Bd-unexposed frogs, which may have helped them resist or clear
infection, and could explain why no interaction between the
herbicides and Bd was detected. The results suggest that these frogs were resistant to Bd
infection and that pre-exposure to the
herbicides did not alter this resistance. The effects seen on the growth following
herbicide exposure is a concern, as reduced growth can lower the reproductive success and survival of the amphibians.