Abstract
The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused mass mortality leading to population declines and extinctions in many frog species worldwide. The lack of host resistance may be due to fungal immunosuppressive effects that have been observed when Bd is incubated with cultured lymphocytes, but whether in vivo host immunosuppression occurs is unknown. We used a broad range of hematologic and protein electrophoresis biomarkers, along with various functional tests, to assess immune competence in common green (Litoria caerulea) and white-lipped (L. infrafrenata) tree frogs experimentally infected with Bd. Compared with uninfected frogs, Bd infection in L. caerulea caused a reduction in immunoglobulin and splenic lymphocyte responses to antigenic stimulation with sheep red blood cells, along with decreased white blood cell and serum protein concentrations, indicating possible impaired immune response capability of Bd-infected frogs. This is the first in vivo study suggesting that infection with Bd causes multiple defects in systemic host immune function, and this may contribute to disease development in susceptible host species. Although L. infrafrenata failed to maintain Bd infection after exposure, white blood cell and serum globulin concentrations were lower in recovered frogs compared with unexposed frogs, but antigen-specific serum and splenic antibody, and splenic cellular, responses were similar in both recovered and unexposed frogs. This may indicate potential systemic costs associated with infection clearance and/or redirection of host resources towards more effective mechanisms to overcome infection. No clear mechanism for resistance was identified in L. infrafrenata, suggesting that localized and/or innate immune defense mechanisms may be important factors involved in disease resistance in this species.
Highlights
The recent global spread of the emerging infectious disease chytridiomycosis has caused declines and extinctions of many amphibian species [1,2,3]
Mean splenic total lymphocyte and Rosette-forming cells (RFC) counts, splenic cell viability and liver-body weight ratio post-immune stimulation were lower in Batrachochytrium dendrobatidis (Bd)-infected L. caerulea (n = 10) compared with the uninfected frogs (n = 10) (Table 1)
Our results provide the first direct evidence suggesting that Bd infection causes multiple in vivo systemic immune function defects in a susceptible amphibian host, which likely enables disease development
Summary
The recent global spread of the emerging infectious disease chytridiomycosis has caused declines and extinctions of many amphibian species [1,2,3]. The causative fungal skin pathogen, Batrachochytrium dendrobatidis (Bd), has had the most devastating impact in remote and protected mountainous regions, where abundant populations crashed within months of its arrival [1,2,4]. Environmental changes such as pollution and climate are generally not considered primary factors in its emergence – Bd can clearly cause high mortality rates in healthy, immune competent populations [3]. Fatal pathophysiological changes include epidermal degeneration, inhibited epidermal electrolyte transport, systemic electrolyte disturbances (hyponatremia and hypokalemia) [11,12,13], severe hypovolemia secondary to dehydration [13] and asystolic cardiac arrest [12]
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