Abstract
Batrachochytrium dendrobatidis (Bd) is a fungus that causes the disease chytridiomycosis and is associated with widespread amphibian declines. Populations vary in their susceptibility to Bd infections, and the virulence of the infecting lineage can also vary. Both of these factors may manifest as a differential physiological stress response. In addition, variation in disease susceptibility across amphibian populations may be influenced by immunosuppression caused by chronic stress imposed by environmental factors. Here, we use a non-invasive water-borne hormone technique to assess stress levels (corticosterone) of free-living tadpole populations that are infected by Bd. We found that corticosterone release rates were higher in infected populations of two species of tadpoles (Alytes obstetricans and A. muletensis) than in an uninfected population for both species. The relationship between corticosterone and the intensity of infection differed between species, with only the infected A. obstetricans population showing a significant positive correlation. The higher corticosterone release rates found in A. obstetricans may be an outcome of infection by a highly virulent lineage of Bd (BdGPL), whereas A. muletensis is infected with a less virulent lineage (BdCAPE). These results suggest that different lineages of Bd impose different levels of stress on the infected animals, and that this may influence survival. The next step is to determine whether higher corticosterone levels make individuals more susceptible to Bd or if Bd infections drive the higher corticosterone levels.
Highlights
Batrachochytrium dendrobatidis (Bd) is an aquatic chytrid fungus that causes the globally emerging disease chytridiomycosis in amphibians [1,2,3]
Uninfected populations of A. obstetricans and A. muletensis were confirmed negative for infection by quantitative real-time polymerase chain reaction (qPCR)
We found that the infected population of each species had higher CORT release rates than the uninfected population (Unpaired t-test: A. obstetricans: t = 8.29, d.f. = 37, P,0.0001; A. muletensis: t = 2.44, d.f. = 36, P = 0.019; Figure 1)
Summary
Batrachochytrium dendrobatidis (Bd) is an aquatic chytrid fungus that causes the globally emerging disease chytridiomycosis in amphibians [1,2,3]. It is known that environmental stressors impair host-produced defenses against Bd [5,6], leading to higher burdens of infection. The amphibian immune response to infection is modulated by the hypothalamus-pituitary-interrenal (HPI) axis [7]. Glucocorticoids, the main vertebrate stress hormones, are released within 10 min of stress-induced activation of the HPI but generally actions are not exerted for about an hour after the onset of the stressor [8]. Initial release of corticosterone (CORT), the main amphibian glucocorticoid, may initially activate the immune system [9]. Over longer periods of time (hours or days) with chronic exposure to a stressor, as CORT levels rise, immune cell function becomes inhibited and acts as a negative feedback on the HPI resulting in decreased growth, development and immunosuppression. These factors can result in increased susceptibility to disease [6]
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