A deadly amphibian fungus

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One of the most prominent diseases linked to mass die-offs in amphibians is chytridiomycosis (chytrid). This disease is caused by a single celled fungal pathogen, Batrachochytrium dendrobatidis (Bd) which has spread globally from its origin in East Asia (O’Hanlon et al. 2018). Bd is highly transmissible, posing a significant threat to many amphibian species across the globe and has been described as the most lethal disease to infect vertebrates. Nine species of amphibian have gone extinct since 1980, and 113 are suspected to be extinct.

Increases in temperature across the globe has been suggested to facilitate the spread of chytrid as it provides optimum conditions for its dispersal. Bd has a thermal optimal range of 17-25°C (Rohr et al. 2013), and prefers moist, damp environments.

Some of the key characteristics of climate change includes shifts in temperatures and changes in annual rainfall. In optimal temperatures, the fecundity of zoospores is enhanced, and the pathogen is able to establish itself in various environments by increasing efficacy of transmission (Rödder et al. 2010).

The extent of Bd spread, globally. From http://www.bd-maps.net/maps/

The immune response of amphibians can also be altered by fluctuating temperatures. A paper by Raffel et al. (2012) described the change in immune system cells and proteins circulating the body of amphibians as a result of temperature change. Particularly of lymphocytes, which are the main immune response in the body to infection and of which counts are significantly reduced in lower temperatures.

Temperature fluctuations can therefore increase susceptibility of amphibians to infection by increasing the abundance of the pathogen itself in the environment and the immune response in amphibians, particularly lymphocyte circulation (Raffel et al. 2012).

Overall, various cofactors are able to facilitate the spread of disease in amphibians as they create increased environmental stress, which leads to vulnerability in the immune system, allowing for disease to infect greater numbers of amphibians.

Chytrid fungus infograph.

This is a small summary of the current threat posed by Bd to amphibians and the influence current climate change has on its spread. Although there are more than just climatic factors affecting its spread, the effect and relevance of climate change is very prominent.

References

O’Hanlon, S., Rieux, A., Farrer, R., Rosa, G., Waldman, B., Bataille, A., Kosch, T., Murray, K., Brankovics, B., Fumagalli, M., Martin, M., Wales, N., Alvarado-Rybak, M., Bates, K., Berger, L., Böll, S., Brookes, L., Clare, F., Courtois, E., Cunningham, A., Doherty-Bone, T., Ghosh, P., Gower, D., Hintz, W., Höglund, J., Jenkinson, T., Lin, C., Laurila, A., Loyau, A., Martel, A., Meurling, S., Miaud, C., Minting, P., Pasmans, F., Schmeller, D., Schmidt, B., Shelton, J., Skerratt, L., Smith, F., Soto-Azat, C., Spagnoletti, M., Tessa, G., Toledo, L., Valenzuela-Sánchez, A., Verster, R., Vörös, J., Webb, R., Wierzbicki, C., Wombwell, E., Zamudio, K., Aanensen, D., James, T., Gilbert, M., Weldon, C., Bosch, J., Balloux, F., Garner, T. and Fisher, M. (2018). Recent Asian origin of chytrid fungi causing global amphibian declines. Science. 360(6389), 621-627.

Raffel, T., Romansic, J., Halstead, N., McMahon, T., Venesky, M. and Rohr, J. (2012). Disease and thermal acclimation in a more variable and unpredictable climate. Nature Climate Change. 3(2), 146-151.

Rohr, J., Raffel, T., Romansic, J., McCallum, H. and Hudson, P. (2008). Evaluating the links between climate, disease spread, and amphibian declines. Proceedings of the National Academy of Sciences, 105(45), 17436-17441.

Rödder, D., Kielgast, J. and Lötters, S. (2010). Future potential distribution of the emerging amphibian chytrid fungus under anthropogenic climate change. Diseases of Aquatic Organisms, 92(3), 201-207.