The Lemur leaf frog (Agalychnis lemur) is critically endangered in their native Costa Rican forests partially due to the devastation by Chytridiomycosis.
PHOTOGRAPH BY Jeroen Mikkers
The planet’s most invasive species is invisible to the naked eye however that does not halt its devastating consequences on the amphibian world. Chytrid fungus threatens more species than any other disease on earth and has already caused more biodiversity loss than any disease in history. Is there any hope for our amphibian species?
Our modern human habits have undoubtedly assisted in leading us towards the ongoing 6th mass extinction event we face today. Predominantly, we focus upon deforestation, pollution and hunting as human actions that have helped shaped the new believed era, of the Anthropocene. Whilst people worry about the continual ensuing wave of global covid-19 cases, they forget, or never knew, about how we as a species have already spread other deadly pathogens worldwide. The spread of highly virulent wildlife diseases is a consequence of human’s ever more globalised world.
It is common to see news stories relating to the decline in large species such as rhinos, due to trade in horns and illegal poaching. However, we rarely focus or grant adequate media attention to population declines in smaller animals such as insects or amphibians. A lack of media attention creates less urgency and overall funding to tackle problems such as the current devastation seen by chytrid fungus.
Chytridiomycosis is an infectious disease caused by certain strains of chytrid fungus called Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). The disease has been linked with causing immense population die-offs in frog and salamander species. Once an amphibian is infected, the disease targets a protein called keratin. This is an abundant key component in the skin that is vital to allow the skin to carry out respiration and water regulation. Without this crucial organ, amphibian species succumb swiftly to the disease.
Believed to have originated in Asia on the Korean peninsula, the fungus has spread to over 50 countries and is found on every continent except Antarctica causing widespread damage. The disease has rapidly taken hold of certain areas especially Central America, Australia, South America, and pockets of northwest America.
Infecting amphibian species living in deserts, to lowland rainforests and cold mountain tops, the fungal disease has undeniably become a worldwide panzootic. This is the animal equivalent to a pandemic in a human population.
Chytridiomycosis, like most newly found diseases, poses us with numerous mysteries. The disease has been seen to wipe out certain amphibian species with an almost 100% mortality rate whereas, in other species, die-offs aren’t seen in the population. Additionally, we see that in parts of Asia amphibians live side by side with the fungus with no impact on their health.
The chytridiomycosis panzootic is responsible for the greatest recorded loss of biodiversity attributable to a single disease and threatens more species than any other disease known so far in science. Now that we face this silent global killer, we have to answer the questions about how we accidentally spread it, how it will affect our ecosystem mechanics and how do we stop it to preserve our wild environments which we all depend upon.
The Panamanian golden frog, Atelopus zeteki, is believed to be extinct now due to the impacts of chytrid fungus Bd. Species have declined rapidly since the 1980s with a study showing declines in 501 species. These numbers are likely to be underestimated due to the lack of long-term population surveys to back up observations.
PHOTOGRAPH BY Brian Gratwicke
There are hundreds of species of chytrid fungus, most of them acting as decomposers in our ecosystems helping with the breakdown of dead and decaying matter. However, Bd and Bsal don’t fall into that category, due to their appetite for amphibian skin.
These fungal species typically live in water and soil. They reproduce asexually by using spores that ‘swim’ through the water as a result of their flagellum, allowing them to be motile. A flagellum is a whip-like organelle found on a zoospore which allows it to move towards a host. Amphibian species associated with permanent water, such as streams, moist bogs and ponds suffer mostly from chytridiomycosis, due to the aquatic nature of chytrid fungus.
Bd which mainly affects frog species, and Bsal which generally targets salamanders, are very effective fungal species at spreading. Fungal reproductive cells called zoospores have been seen to live outside of a host for weeks to months.
Once an individual is infected, the fungus progresses through its life cycle to the stage where it makes new infectious zoospores to be released into the environment. This creates a hugely problematic situation when you get a species that doesn’t get impacted healthwise but still is infected and can aid in spreading the disease. You can almost think about it as an infected asymptomatic coronavirus case in humans; someone is aiding the spread unbeknown to themselves and others.
Chytridiomycosis in amphibians leads to their skin becoming reddened along with excessive shedding and the appearance of ulcers in certain species. An amphibian’s skin is an extremely vital organ due to it being uniquely physiologically active. Its skin is responsible for the regulation of water content, electrolytes, and respiration. By disrupting the normal functions of the skin, amphibians face electrolyte depletion losing essential ions such as sodium and magnesium which are necessary for metabolic processes.
The knock-on effects of damaged skin are detrimental to survival leading to reduced reflexes, poor swimming, and compromised foraging abilities. Additionally, the fungus affects tadpoles, damaging the keratin found around the mouth subsequently causing problems feeding. The fungus is impactful throughout a frog’s whole lifespan.
The Lemur Leaf frog, Agalychnis lemur, is classed as Critically Endangered and threatened by the fungal disease chytridiomycosis.
PHOTOGRAPH BY Brian Gratwicke
Genetic studies predict that Bd originated in Korea in the 1950s, but how has a microscopic fungus spread so far? The majority of all evidence links to us. Our massively globalised world has accelerated the accidental spread of pathogens, as well as directly.
With chytrid, the direct amphibian pet trade has unquestionably assisted its spread. Now it’s a very possible reality that the exotic frog living in your room, is housing the culprit responsible for the amphibian decline. In Japan, random sample studies of pet shops showed that over 25% of tested pet shops had amphibians positive for the disease. Without testing and regulation, those frogs could go anywhere.
Varying species of bullfrogs are imported into the USA for food, especially in the southern states. Certain species of bullfrogs have been identified as being resistant to Bd acting as potential spreaders of the disease. They are rarely tested when imported.
PHOTOGRAPH BY Bill Buchanan
On the other hand, globalised trade can spread the fungus completely accidentally. Infected frogs have been commonly found in banana shipments as they are very hard to spot and remove. Madagascar, a recently untouched spot from chytrid, has seen substantial increases of infected Asian common toads being transported accidentally in mining equipment. Scientists substantially worry about the impacts that chytrid will unleash on the country’s unique biosphere already massively threatened by rampant deforestation.
All of this trade has allowed the spread of the fungus, due to an overall poor biosecurity effort globally. Testing of amphibians being traded is rarely carried out along with screening shipments of raw materials for their presence.
Green tree frogs are the common species found within banana exports. Worldwide trade has been blamed for spreading chytrid fungus worldwide. There are also suggestions that the Korean War was responsible for the spreading of the fungus. As soldiers and equipment moved, the fungus followed, as chytrid was believed to have originated in Korea.
PHOTOGRAPH BY Judy Gallagher
Anthropogenic warming, driving global climate change, has been argued to increase the proliferation, range, and virulence of chytridiomycosis. Rising temperatures lead to an increase in evaporation, creating more cloud formation which blocks the sun’s UV radiation. Ultimately this leads to cooler days and warmer nights in some locations which have been suggested to be better conditions for the fungus which has an optimum of 17-25 degrees Celsius.
It’s still a very debatable topic. Studies have revealed that higher temperatures can kill the fungus providing the notion that increased temperatures in some places may help reduce chytrid. However, as temperatures fluctuate, so does amphibians’ efficiency in their immune response. With climates shifting, amphibians’ natural defences could suffer allowing for more die-offs in species.
Climate change can indirectly aid the spread of chytrid fungus. As species expand their range and elevation upwards to accommodate for rising temps, they can harbour chytrid infections introducing them to new populations. The impacts, positive or negative of rising global temperatures, are yet to be fully discovered.
The green-and-black poison dart frog, Dendrobates auratus, lives within Central America, a hotspot for chytrid.
PHOTOGRAPH BY Christina Carter
Now that the fungus has spread to all reaches of our planet, how do we stop it? Tighter biosecurity measures need to be implemented to curb the spread. Since Australia now has a high distribution of Bd nationwide, efforts need to be focused on protecting uninfected regions along with stopping more chytrid fungus from entering the country through the trade of pets and food.
In addition, the pet trade of amphibians needs to be altered and reformed. Internationally traded amphibians should be tested for these harmful chytrid fungus strains, but currently today this does not consistently happen. Now in the wake of the covid-19 pandemic, when wildlife trade has been noticeably highlighted, is the time for governments, industry, and trade to come together to establish safer restrictions.
Once a population becomes infected with Bd or Bsal, there needs to be a treatment available to try to save amphibian species. Scientists have attempted to formulate treatments but with such a vast number of species affected, with varying susceptibility to the disease, it’s challenging to create general treatments for all.
Antifungal drugs, such as itraconazole, have been seen to eradicate Bd infection in some amphibian species in captivity. However only limited research has been carried out. Antifungal treatments are expensive and can’t physically be applied worldwide to stop infections. Additionally, the research that has come out, reveals that these drugs are toxic to some amphibian species including their larvae. It’s a work in progress but these drugs will have a future role in conserving our amphibian species.
Alternatively, heat therapy, raising the surrounding temperature for a certain period of time, has had success in healing infected amphibians. Due to the fungal strains having an optimum temperature of 17-25 degrees Celsius, increasing the heat of an amphibian’s enclosure above those desired conditions has killed Bd and Bsal. Once again though, this is only controllable in lab conditions and couldn’t be applied in the wild.
Heat treatment could be used more readily in the pet trade. As frogs are traded, they could be tested for Bd and go through heat therapy if they are infected. The heat therapy technique also allows for enclosures to be cleaned to kill any zoospores present.
Vivariums could also be used in conjunction with heat therapy to restore frog numbers. Any infected species that come to the vivarium can be quarantined and treated with heat therapy. Post-treatment, breeding could be promoted and then eventually reintroduction back into wild habitats.
One of the most promising ideas on how to treat chytrid fungus comes from understanding why some amphibians have considerable immunity to the disease. Beneficial bacteria that naturally live on the skin of certain amphibians have been seen to inhibit chytrid fungus strains. At such an early stage of study, scientists don’t quite know how to use the bacteria to aid amphibians in the wild. With more time and research this will undoubtedly become a tool to help frogs fight back.
Amphibians that are found to be infected with chytrid fungus can be quarantined and treated with new emerging methods such as heat therapy and antifungal drugs.
PHOTOGRAPH BY Brian Gratwicke
Individually losing amphibians from our ecosystem is not a plausible reality. Unfortunately, as certain species rapidly decline, animals that rely on them for prey also whittle in numbers. Within Panama, a country struck by the chytrid outbreak, studies are showing declines in tropical snake species as a result of fewer amphibians to feed on. Losses of Amphibian permeate throughout ecosystems disrupting the overall food chain and seemingly decreasing biodiversity.
As frog species populations decline across Australia, all aspects of the food chain will be affected. The red-bellied black snake, Pseudechis porphyriacus, is a species of venomous snake found only in Australia, feeding predominately on frogs. Their existence could be threatened by the continuing spread of Bd.
PHOTOGRAPH BY Paul Balfe
Chytridiomycosis is only recently being studied revealing the frightening impacts upon our 8000 amphibian species. Whilst the deadliest pathogen to the planet’s biodiversity continues to spread, we don’t seem to acknowledge its importance or what it represents for our future. If the next pandemic had the capability of infecting us as humans, alongside hundreds of mammals that we deem more meaningful, our attitude would be completely the opposite and one of urgency to act.
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