Salinity stress increases the severity of ranavirus epidemics in amphibian populations

The stress-induced susceptibility hypothesis, which predicts
chronic stress weakens immune defenses, was proposed to explain
increasing infectious disease-related mass mortality and population
declines. Previous work characterized wetland salinization as a
chronic stressor to larval amphibian populations. Thus, we combined
field observations with experimental exposures quantifying
epidemiological parameters to test the role of salinity stress in the
occurrence of ranavirus-associated mass mortality events. Despite
ubiquitous pathogen presence (94%), populations exposed to salt runoff
had slightly more frequent ranavirus related mass mortality events,
more lethal infections, and 117-times greater pathogen environmental
DNA. Experimental exposure to chronic elevated salinity (0.8-1.6 g/L
Cl-) reduced tolerance to infection, causing greater mortality at
lower doses. We found a strong negative relationship between
splenocyte proliferation and corticosterone in ranavirus-infected
larvae at a moderate elevation of salinity, supporting
glucocorticoid-medicated immunosuppression, but not at high salinity.
Salinity alone reduced proliferation further at similar corticosterone
levels and infection intensities. Finally, larvae raised in elevated
salinity had 10-times more intense infections and shed 5-times as much
virus with similar viral decay rates, suggesting increased
transmission. Our findings illustrate how a small change in habitat
quality leads to more lethal infections and potentially greater
transmission efficiency, increasing the severity of ranavirus
epidemics.