What is chytridiomycosis?
Chytridiomycosis is another significant threat to amphibians both nationally and globally. A worldwide effort to understand the decline in amphibian populations, led by a group of scientists including Professor Andrew Cunningham (scientific advisor to the NGO Ranita de Darwin), succeeded in identifying this emerging infectious disease in the late 1990s. The disease, which affects amphibian skin, is caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd).
Chytridiomycosis is often regarded as one of the most devastating infectious diseases—caused by microorganisms such as viruses, fungi, and bacteria—to have ever affected any animal species on Earth.
Click to view to see the chytrid fungus by microscopy.
Chytridiomycosis in amphibians is considered one of the infectious diseases with the greatest negative impact, as it affects a large number of species (having been linked to the decline of 501 amphibian species) and has the potential to cause the extinction of many. It has been associated with the presumed extinction of 90 amphibian species worldwide. As a result, this disease has become one of the major global threats to amphibian conservation.
Olson et al. (2021)
Infectious diseases such as chytridiomycosis can be studied scientifically in a retrospective manner, meaning by looking back and analysing animals preserved in museums. These types of samples play a fundamental role in such studies and are one of the many reasons for maintaining animal specimens in museum collections.
In the case of Darwin's frog and its sister species, the Cowboy Toad (Rhinoderma rufum), numerous specimens are preserved in museums across Chile, North America, and Europe. Using these samples, a study led by scientist and academic Claudio Azat demonstrated that the first infections with the fungus responsible for chytridiomycosis appeared in both frog species in Chile during the 1970s, coinciding with the enigmatic disappearance of the Cowboy Toad.
We now have evidence that the strain of chytrid fungus present in Chile is genetically identical to strains found in Europe and various other parts of the world. Genomic analysis suggests that this strain was recently introduced to our country, likely due to the global trade of live amphibians or other aquatic animals for laboratory use, as pets, or for food (for example, the African Clawed Frog).
African Frog - Photography by Claudio Azat
We have also monitored a large number of wild Darwin's frog populations, revealing that chytridiomycosis is primarily present in northern populations (roughly between the Biobío and Los Ríos regions). This area also corresponds to where Darwin's frog has declined most dramatically over the past three to four decades.
Click to see the short documentary: 'Chytridiomycosis in Darwin's Endangered Frogs'.
In Darwin's frog, chytridiomycosis causes high mortality in infected individuals (almost 100%), but it is not capable of triggering epidemics or mass die-offs. Instead, cases of the disease occur in low numbers but are distributed throughout the year.
Unfortunately, this negative impact appears to have the potential to drive population declines and even extinction. This represents a new paradigm in the study of infectious diseases in wild animals, highlighting that even when obvious negative effects (such as mass mortalities) are not observed in nature, infectious diseases can still pose a significant threat to the survival of wild populations.
A hopeful discovery has emerged from the long-term monitoring of Darwin's frog populations conducted by the NGO Ranita de Darwin since 2014. We have observed that some populations can counteract the high mortality caused by chytridiomycosis through an increase in reproduction, leading to a greater number of juveniles being born each year.
However, it appears that not all populations of this species possess this ability to compensate for the disease's negative effects. Populations that fail to counteract these impacts are at a higher risk of local extinction.
This study highlights the importance of variation between populations in life history strategies in determining the fate of host populations following the emergence of infectious diseases.
Furthermore, one of our findings suggests that an increase in reproductive effort may be one of the key processes underlying compensatory recruitment in amphibian populations susceptible to the chytrid fungus.
Who should be more vigilant in preventing chytridiomycosis?
• Professionals working with amphibians (zoos, researchers, environmental consultants).
• Students.
• Groups and societies dedicated to the conservation of wildlife.
• People who encounter frogs, either casually or regularly.
Prevention
It is recommended that parks and other natural areas establish disinfection zones, such as covered footbaths. When moving between sites, the following hygiene measures should be taken to minimise the risk of disease transmission:
Personal hygiene: Clean areas of the body that may have come into contact with the ground, mud, leaves, or mosses (such as hands, arms, knees, etc.) using an appropriate disinfectant. It is preferable to do this before entering a vehicle or moving to another location.
Footwear and clothing: Footwear should be cleaned and disinfected at the start of fieldwork and between each sampling site. This can be achieved by first scraping the boots to remove mud and then immersing the soles in a disinfectant solution. The rest of the boot should be rinsed or sprayed with a sanitising solution. Additionally, clothing that comes into significant contact with frogs or their environment should be changed or cleaned.
Prevent disinfectants from entering any body of water, and use biodegradable disinfectants whenever possible. Carrying multiple sets of shoes and clothing for changes between sites could serve as a practical alternative to on-site cleaning. For high-value sites, it may be advisable to store equipment and clothing at the entrance (for example, in a secure locker).
Equipment: Equipment such as nets, scales, calipers, bags, scalpels, headlamps, flashlights, wetsuits, boots, and similar items used at one site must be thoroughly cleaned and disinfected before being used at another site. Disposable items should be used whenever practical or possible. Non-disposable equipment should be used only once and disinfected afterward.
Vehicles:If it is suspected that mud and water on wheels and tyres could be transferred to other bodies of water or amphibian habitats, these components should be cleaned and disinfected. This is particularly important when vehicles are used in areas not typically frequented by other vehicles. Disinfection should be carried out at a safe distance from bodies of water to minimise the risk of contamination.
Click to watch the National Geographic infographic video: The chytrid fungus: a threat to amphibians
To download the E-book Darwin's Frog and other incredible Chilean amphibians, click here
To download the book Darwin's Frog Conservation Strategy,click here
