Immunological Aspects of Chytridiomycosis

Abstract

Amphibians are currently the most threatened vertebrate class, with the disease chytridiomycosis being a major contributor to their global declines. Chytridiomycosis is a frequently fatal skin disease caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). The severity and extent of the impact of the infection caused by these pathogens across modern Amphibia are unprecedented in the history of vertebrate infectious diseases. The immune system of amphibians is thought to be largely similar to that of other jawed vertebrates, such as mammals. However, amphibian hosts are both ectothermic and water-dependent, which are characteristics favouring fungal proliferation. Although amphibians possess robust constitutive host defences, Bd/Bsal replicate within host cells once these defences have been breached. Intracellular fungal localisation may contribute to evasion of the induced innate immune response. Increasing evidence suggests that once the innate defences are surpassed, fungal virulence factors suppress the targeted adaptive immune responses whilst promoting an ineffectual inflammatory cascade, resulting in immunopathology and systemic metabolic disruption. Thus, although infections are contained within the integument, crucial homeostatic processes become compromised, leading to mortality. In this paper, we present an integrated synthesis of amphibian post-metamorphic immunological responses and the corresponding outcomes of infection with Bd, focusing on recent developments within the field and highlighting future directions.

Keywords: Batrachochytrium dendrobatidis; Batrachochytrium salamandrivorans; adaptive; amphibian; chytridiomycosis; constitutive; disease; immunopathology; immunosuppression; innate.

Figure 1

Schematic of the in vivo life cycle of Batrachochytrium dendrobatidis (Bd) depicting a microscopic cross-section of amphibian skin (cellular epidermis) and the process of Bd invasion, replication and release. Approximate times/stages during the initial infection process are depicted from left to right (A–E). Bd is depicted in violet and host skin cells are depicted in mauve, with increasing cell keratinisation being darker. During the first 2–8 h post exposure (A), free-living infectious zoospores undergo chemotaxis towards the skin surface using their motile flagellum, where they encyst into a thallus, absorb the flagellum and develop rhizoids. From there (B), they produce a germ tube through which they inject their contents into deep cells of the host epidermis. From 1–2 days post exposure (C), the developing sporangium undergoes division of its nucleus, then cytoplasm, yielding individual zoospores which develop inside the sporangial capsule. These zoospores are then released through a discharge tube (D). Note that the infected skin demonstrates cellular hypertrophy (larger cells), epidermal hyperplasia (increased thickness due to an increase in cell number), hyperkeratosis (increased cellular keratin shown in darker mauve), and skin sloughing becomes dysregulated. Continued infection (E) disrupts the integrity of the epithelial barrier, permitting secondary bacterial infection.

Figure 2

Effects of temperature on Batrachochytrium dendrobatidis (Bd) growth rates and amphibian survival. The Bd growth rate (black line) from Piotrowski et al. [65], using a curve fitted by Rohr et al. [81], is shown together with a frequency distribution (yellow) of upper critical temperature limits for 77 anurans based on data in the supplementary materials of Sunday et al. [82]. Vertical lines show the temperatures at which Andre et al. [78] found 95% mortality (red) and 50% mortality (cyan) in Rana muscosa.

Figure 3

Schematic depicting the layered nature of the immune responses to infection in simplified terms. Robust constitutive defences are the most efficient and effective way to combat a pathogen because they can inhibit infection. Induced innate defences are the second line of defence against pathogens. They can rapidly arrest infection processes or reduce pathogen growth rates, buying survival time. Adaptive immune defences are initially slower to develop in a naively infected host but can be highly effective in providing a targeted defence against pathogens if the host survives the initial infection long enough. If the combined effect of these layers of defence is inadequate, then the pathogen will continue to replicate and the host will eventually die due to overwhelming pathogen burdens, if not before this, due to immunopathology.

Figure 4

Schematic depicting the expected infection load trajectories against time post exposure and infection outcomes associated with different types of immune responses. Type A individuals (purple) possess fully effective constitutive defences, are completely resistant and do not develop infection. Type B individuals (cyan) have partially effective constitutive defences and induced innate defences, reducing their overall pathogen growth rate throughout the course of the infection (demonstrating greater resistance than type C individuals (red)). Type B individuals with sufficiently effective early innate immune responses (corresponding to the period depicted by the yellow background band) may either (1) recover (B₁), or (2) survive long enough to develop an adaptive immune response (orange background band), from which they might then (3) recover (B₂) or (4) remain infected (B₃). (5) Finally, if their adaptive immune response is ineffectual, their infection trajectory may continue to increase (B₄) until they succumb to the infection. Type C individuals do not possess effective constitutive defences; therefore, their native infection trajectory follows an exponential pathogen growth rate. If and when their induced innate and/or adaptive responses occur, similarly to type B, they may depart from this trajectory and either recover or remain infected (C_1–3). Alternatively, if the immune responses are entirely ineffective, the infection burden will increase exponentially until the host succumbs to infection (C₄). Importantly, individuals may die from infection at any point during any of the above-described infection trajectories due to immunopathology (e.g., it is entirely possible for individuals of type C to die at point X, despite a declining infection load).