1998: a fungus turns out to be killing amphibians

Towards the end of the twentieth century amphibian populations began disappearing in places where nothing obvious was happening: montane rainforests in Queensland and Panama, inside reserves, far from logging and pollution. The cause remained unknown until 1998, when Lee Berger, Rick Speare and colleagues published in PNAS their examination of sick and dead frogs collected during mass mortality events.

In the animals' epidermis they found changes caused by a chytridiomycete fungus, phylum Chytridiomycota. It was the first report of a member of that phylum parasitising a vertebrate. Chytrids had been known as aquatic and soil organisms that break down organic matter or parasitise algae and invertebrates — not as agents of disease in mammals, birds or amphibians.

The authors backed the observation with experimental data supporting the conclusion that cutaneous chytridiomycosis is a fatal disease of anurans, and hypothesised that it was the proximate cause of the declines being observed. The mechanism turned out to involve the skin, through which amphibians breathe and regulate water balance — infection disrupts ion transport and leads to cardiac arrest.

The paper changed how wildlife disease is understood. It showed that a fungal pathogen can rebuild the fauna of entire regions within a decade or two, and that its spread is tied to the movement of animals by people. Chytridiomycosis is now a textbook case of an emerging disease and one of the main arguments for biosecurity in the amphibian trade.

From field association to experimental infection

The 1998 paper brought together material from amphibian mortality events and population declines in Australian and Central American rain forests. Histology and electron microscopy revealed a chytrid organism in the keratinized layers of diseased skin, while small-subunit rRNA analysis placed it within Chytridiomycota. The second level of evidence was crucial: experimental exposure of healthy frogs produced a fatal cutaneous disease. The authors therefore concluded that the experimental data supported cutaneous chytridiomycosis as a lethal disease of anurans.

The paper’s title deliberately described mortality “associated with” population declines, and its conclusion framed the pathogen as the hypothesized proximate cause of the episodes examined. It did not demonstrate that one agent explained every amphibian decline worldwide. The formal binomial Batrachochytrium dendrobatidis was introduced in a subsequent publication, so the 1998 paper itself should be presented as the causal-disease study, not as the original species description.