Chytridiomycosis: Will one fungus see the death of all frogs?

The presence of a looming ‘sixth mass extinction’ is currently a much debated issue. Whilst some groups of plants and animals remain well away from the 75% threshold extinction level, others are rapidly approaching. In the case of amphibians, the threat of a mass extinction has never been more imminent. And it is all down to one fungal disease: chytridiomycosis.

Chytridiomycosis is caused by one of two fungal species: Batrochochytrium dendrobatidis (Bd) and Batrochochytrium salamandrivorans (Bsal), although Bd currently poses a much greater threat. Bd is a type of zoospore aquatic fungus, meaning it is able to move in the environment to seek out new amphibian hosts such as frogs and salamanders. 

The pathogenic effects from Bd come from the way in which it interacts with the skin of the hosts. The skin is a hugely important organ for amphibians, seeing as it is used for thermoregulation and osmoregulation, which ensures the correct balance of water and salts in the body. The fungal spore infects these functionally important skin cells, causing hyperkeratinization of the epidermal layer. This disrupts normal functioning of the skin, meaning the host is no longer able to control its electrolyte balance, a hindrance great enough to send it into fatal cardiac arrest.

To date, there are 501 species reported to suffer declines from this disease, with 124 of those enduring over 90% declines. There have been an additional 91 extinctions from chytridiomycosis since the early 1970s. However, these are likely gross underestimates, seeing as it is apparent that this disease was causing damage long before we discovered it in 1998. 

Although the disease originated in Asia, it is now apparent in all continents aside from Antarctica, where no amphibians live. Humans have had a large part to play in this rapid colonisation of the globe, particularly due to the wildlife pet trade. 

The first rapid expansion of chytridiomycosis came before we were even aware of its existence. This was driven by the mass transport of African Clawed frogs across the globe, due to their use in pregnancy testing. From the 1930s to 1950s, these frogs were exported all over the world to be injected with the urine of potentially pregnant women. Should the woman have been pregnant, the hormones in her urine would have stimulated the frog to release eggs within a matter of hours, indicating a positive result. Whilst this method has been buried with time, we are still feeling the negative consequences today, seeing as such a widespread movement of the frogs introduced chytridiomycosis to regions all over the globe, an event that is very difficult to undo. 

Whilst biosecurity has improved exponentially since the 1950s, humans are still exacerbating the spread of the disease in other ways, namely through climate change and chemical pollution.

As greenhouse gas emissions increase, and more of the sun’s solar radiation is reflected back to Earth, the Earth is warming at an alarming rate. This has hugely detrimental effects on disease dynamics, as it makes the host species much more vulnerable to the disease-causing pathogens. This is because pathogens have a faster life cycle than their host, meaning they are able to adapt to a changing climate much faster, and therefore do not feel the ill-effects of climate change to the same extent.

 Furthermore, the warming poles and higher altitudes mean the area in which the pathogens can survive is rapidly expanding. This allows the fungal spores to exist in previously uncolonised areas, meaning there are whole swathes of hosts that are being introduced to the disease, who have not had the chance to build up resistance from previous exposure. 

As well as altering the climate, humans are also producing huge amounts of chemical pollutants, which get released to the atmosphere. A major air pollutant today is tropospheric ozone, which has been shown to decrease the health of amphibians by reducing their capacity for oxygen consumption and immune response. This makes them more vulnerable to the fungal disease, allowing it to take advantage of the poor-conditioned hosts. Similar patterns have been seen with microplastics, with a positive correlation between the amount of microplastics consumed, and the fungal load on the skin of frogs. 

However, with an increasing threat of chytridiomycosis comes an increasing effort to mitigate against it. Captive breeding programmes are being set up all across the globe to try to prevent the extinction of the most threatened species, and work is being done to produce probiotics that will augment the microbiome of the amphibian’s skin, in the hope of making them better able to fight off the disease. 

To conclude, the world’s amphibians are facing a huge threat to biodiversity. This has historically been facilitated by the actions of humans, but we are gradually seeing a rise in efforts to alleviate our impact. The fate of amphibians very much rests on what humans do next, so we must continue our research into protecting species and curing this disease if we are to ward off the next mass extinction.