Batrachochytrium salamandrivorans sp. nov. causes lethal chytridiomycosis in amphibians

Abstract

The current biodiversity crisis encompasses a sixth mass extinction event affecting the entire class of amphibians. The infectious disease chytridiomycosis is considered one of the major drivers of global amphibian population decline and extinction and is thought to be caused by a single species of aquatic fungus, Batrachochytrium dendrobatidis. However, several amphibian population declines remain unexplained, among them a steep decrease in fire salamander populations (Salamandra salamandra) that has brought this species to the edge of local extinction. Here we isolated and characterized a unique chytrid fungus, Batrachochytrium salamandrivorans sp. nov., from this salamander population. This chytrid causes erosive skin disease and rapid mortality in experimentally infected fire salamanders and was present in skin lesions of salamanders found dead during the decline event. Together with the closely related B. dendrobatidis, this taxon forms a well-supported chytridiomycete clade, adapted to vertebrate hosts and highly pathogenic to amphibians. However, the lower thermal growth preference of B. salamandrivorans, compared with B. dendrobatidis, and resistance of midwife toads (Alytes obstetricans) to experimental infection with B. salamandrivorans suggest differential niche occupation of the two chytrid fungi.

Keywords: amphibian decline; ecosystem health; emerging infectious disease.

Fig. 1.

Maximum likelihood tree (−Ln L = 9,562.04266) for the analysis of a 1,513-bp data matrix of partial 18S + 28S rRNA genes. Together with B. dendrobatidis, B. salamandrivorans sp. nov. forms a well-supported clade [maximum parsimony bootstrap support = 100; maximum likelihood bootstrap support (MLBS) = 100; Bayesian posterior probability (BPP) = 100] of Chytridiomycota that parasitize amphibians with potentially lethal consequences. Squares on branches indicate MLBS > 70 and BPP > 95; triangles indicate MLBS < 70 and BPP > 95.

Fig. 2.

In vitro culture of B. salamandrivorans in TGhL broth at 15 °C. (A) Monocentric thalli predominate, with the rare presence of colonial thalli (black arrow). Sporangia develop discharge tubes (white arrow) to release zoospores (Scale bar, 100 µm.) (B) Scanning electron microscopic image of a mature sporangium with rhizoids (R), discharge tubes (D), and germ tube formation (arrow) (Scale bar, 10 µm.)

Fig. 3.

Microscopy of the skin of a fire salamander that died due to infection with B. salamandrivorans. (A) Immunohistochemical staining of a 5-µm skin section. Intracellular colonial thalli abound throughout all epidermal cell layers and are associated with erosive lesions. (Scale bar, 20 µm.) (B) Transmission electron microscopy picture of an intracellular colonial thallus of B. salamandrivorans inside a keratinocyte (Scale bar, 4 µm.)

Fig. 4.

Growth of B. salamandrivorans in TGhL broth at different temperatures. (A) Growth was quantified by calculating the average percentage ± SD of the surface area of three wells covered by the fungus after 10 d of incubation at a given temperature. Motile zoospores were present at 5–20 °C, but not at 22, 24, and 30 °C. (B) B. salamandrivorans growth after 10 d at 4 °C (a), 15 °C (b), 20 °C (c), and 30 °C (d) (Scale bar, 200 µm.)