The Influence of Temperature and Salinity Conditions on Chytridiomycosis in Anurans

The influence of temperature and salinity conditions on chytridiomycosis in anurans

Abstract

Chytridiomycosis is an infectious disease contracted by anurans through the waterborne chytrid fungus (Batrachochytrium dendrobatidis). The disease is responsible for a biological epidemic causing amphibian species extinction and global population declines. Environmental factors affecting the growth and spread of the fungal infection have been investigated using field experiments, laboratory tests and observational studies. Research indicates habitat conditions can influence disease prevalence and severity, in particular the temperature and salinity of water bodies. The fungal pathogen’s optimal and limiting ranges, naturally occurring refuge areas and habitat manipulation as a conservation measure, are reviewed.

Keywords: amphibian decline, chytrid, fungal pathogen, frog disease, habitat management

Introduction

The waterborne chytrid fungus (Bactachochytrium dendrobatidis) present in aquatic habitats is transmitted to anuran species through the skin and causes the infectious disease chytridiomycosis. Affected frogs and toads lose their ability to osmoregulate and the disease is eventually fatal by cardiac arrest (Heard et al. 2014). The pathogen is transmitted through contact with an infected animal (host) or fungal zoospores in aquatic environments (Scheele et al. 2014). Chytridiomycosis is present on every continent that amphibians inhabit and has been responsible for the extinction or severe decline of two hundred species globally (Scheele et al. 2014). The loss of anuran species is likely to cause trophic cascades with major consequences on entire ecosystems (Whiles et al. 2006). An understanding of how chytridiomycosis is influenced by environmental conditions is paramount to reducing the rate of anuran decline and conserving threatened populations. This review aims to assess how natural and man-made variations in temperature and salinity impact the chytrid fungus and infection rates. Firstly the relationship between temperature and chytridiomycosis will be discussed, followed by the impact that different salt levels can have on infection rates and finally creating refuge areas with ideal heat and sodium chloride levels will be examined as a management strategy.

Temperature and chytridiomycosis

The chytridiomycosis causing pathogen has physiological intolerances and optimum conditions that influence its ability to develop, reproduce and live (Heard et al. 2014; Stockwell et al. 2015). One of the most important abiotic factors that can enhance and inhibit Batrachochytrium dendrobatidis functionality is temperature (Venseky et al. 2014). Increasing temperatures are expected to lower the survival and prevalence of the chytrid fungus. This reduced fungal fitness provides frogs and toads with a greater chance of avoiding the disease, or at least lessens the severity of infection if they do contract it (Venseky et al. 2014). Past research on the chytrid fungus has determined that it experiences optimal growth and virulence between 17-25˚C and is killed at high temperatures above 30˚C (Scheele et al. 2014). Cooler climates have found to prompt the production and release of more zoospores (Woodhams et al. 2008) and thus increasing the risk of disease in lower temperatures.

Multiple studies have produced observational evidence for the negative association between temperature rise and chytridiomycosis occurrence. Murray et al. (2013), Heard et al. (2014) and Scheele et al. (2015) linked cool weather periods, lower water temperatures and colder habitats with increased probability of infection in toads and frogs. Another study compiled global records and data of known chytridiomycosis cases to map the distribution of the disease and find patterns with external variables (Olson et al. 2013). Results showed that the likelihood of detecting infected hosts decreased by 8.8% for each degree increase in temperature on average. An American study on the effect of wildfire on chytridiomycosis infection rates supports that high heat environments kill off the chytrid fungus, with the amount of infected toads in recently burnt areas being half that found in non-burnt land (Hossack et al. 2013).

The microclimate of the surrounding region is not the only influence of temperature on a chytrid fungus. Rowley and Alford (2013) reported on infection rates and the thermal body temperature of three species: Stony creek frogs (Litoria lesueuri), green eyed tree frogs (Litoria serrata) and waterfall frogs (Litoria nannotis). Research on these tropical rainforest species correlated individuals’ body heat with chytrid fungus loads and found that frogs able to maintain their temperature above 25˚C had a significantly lower probability of chytridiomycosis (Figure 1). This attests that high temperatures decrease the effects of Batrachochytrium dendrobatidis and help protect anurans. However, it should be noted that this study is specific to species in warm, tropical environments and is not a survival technique for frogs and toads in temperate regions.

Increased body temperatures in tropical frogs may not remain a way to subdue infection as recent experimental results have suggested that the chytrid fungus is capable of adapting tolerate higher temperatures. Stevenson et al. (2013) tested Batrachochytrium dendrobatidis samples from Tasmania, New South Wales and Queensland for their ability to resume growth after being exposed to high temperatures that halt development. The Queensland isolate was able to continue functioning after it was taken from the 26-28˚C to 23˚C, whilst samples from the Tasmania and New South Wales did not. Laboratory analysis by Voyles et al. (2012) also found that the chytrid fungus is capable of evolving to tolerate new thermal conditions and suggests that this mechanism enables its widespread distribution. Changing temperature limitations of the pathogen is an important area to focus further research, as understanding its physical restrictions and suitable environments is crucial for disease management.

Salinity and chytridiomycosis

Sodium chloride (salt) has fungicidal properties that inhibit the function and development of Bactrachochytrium dendrobatidis by altering osmotic gradients (Stockwell et al. 2012). Berger et al. (2009) tested for the quantity of sodium chloride required to inhibit zoospores of the chytrid fungus. A solution with 6.25 milligrams of salt per millilitre reduced zoospore motility and double this concentration (12.5 mg/mL) resulted in death of the zoospores in laboratory conditions. The osmotic gradients are increasingly altered with the addition of more salt and the organism is required to expend more energy on osmoregulation processes, until expenditure is too high and the organism dies (Stockwell et al. 2012).