Considered by the Amphibian Conservation Summit (2005) to be "the worst infectious disease ever seen among vertebrates, in terms of the numbers of species affected and which are likely to become extinct", panzootic chytridiomycosis is a skin disease that endangers amphibian populations all over the world and has already caused the decline and extinction of countless species. Batrachochytrium dendrobatidis, the cause of chytridiomycosis, is an aquatic fungus with a worldwide distribution, capable of growing over a wide temperature range (4-25°C). The pathogen has two life stages typical of chytrids: a sessile reproductive zoosporangium and motile zoospores. The zoosporangium develops on the skin of amphibians, produces and releases new zoospores into the water, which can lead to reinfection or the infection of new animals - both adults and larvae. The fungus is keratotrophic and in tadpoles, it lodges in the oral cavity, as this is the only keratinized area in this stage of amphibian life. In adult animals, the infection becomes much more dangerous, as the entire skin becomes keratinized. Fungal infection leads to keratosis and hyperplasia of the keratinous cells, directly affecting the osmoregulation, thermoregulation and respiration of infected animals, since they depend on the skin to perform these functions. Ion transport in affected amphibians is drastically altered, leading to a severe depletion in plasma electrolytes and consequent cardiac arrest. Likewise, water absorption and loss are disrupted, unbalancing the osmotic balance of these animals and predisposing them to severe dehydration and its pathophysiological consequences. In addition to all this, impaired gas exchange alters the amphibians' metabolism due to hypoxia and the animal enters a state of apathy and torpor, becoming even more susceptible to recontamination by the fungus, secondary infections and predation. The inflamed skin colonization points caused by the fungus, together with the drop in immunity, also predispose the animal to secondary skin infections by other species of fungi and bacteria, which can lead to the formation of dermatosepticemias and even sepsis. If left untreated, the animal will die in different ways.
Figure 1 - Thickened keratin layer of an affected amphibian, with numerous sporangia indicated by the arrows. Source: jbernard.
Although the worldwide decline in amphibian populations has been noted since the mid-1970s, chytridiomycosis was only described in 1998, when factors such as habitat loss and anthropogenic action alone became insufficient to explain the phenomenon. Recent studies point to East Asia as the likely place of origin of the fungus, and attribute the emergence of the pathogen to the global commercialization of amphibians, especially for the trade in meat and products derived from these animals. However, it is known that trafficking to keep animals as pets or for the use of traditional medicine techniques also has a major influence on the spread of the fungus globally Today it is known that there are different strains of the fungus spread in different locations, including a Brazilian strain. There are already records of Batrachochytrium dendrobatidis infection in more than 500 species of amphibians, and it is estimated that more than 200 have already been wiped out by the fungus - 15 of them in Brazil. Despite this, much is still unknown about the pathogen and what could be done to prevent its catastrophic consequences. The fungal disease has already caused and continues to cause immeasurable losses in the biodiversity of amphibians, and all the natural cycles in which these animals are inserted, putting the subtle balance of natural life at risk. The mass extinction of amphibians, as well as representing a huge loss in biodiversity, could lead to major problems for human beings, since these animals play an important role in regulating insect populations. An imbalance in this regulation could lead to outbreaks of diseases transmitted by these animals - such as dengue fever and yellow fever. Various studies and actions have been carried out in an attempt to mitigate the consequences of the disease, but combating the fungus has proved to be a major challenge for researchers all over the world. Attempts to reintroduce amphibian species into the wild often fail, as the chytrid re-emerges in the reintroduced species. The use of probiotics to prevent the disease and the application of fungicides in the environment have also been studied, but much research is still needed to assess the feasibility and impact of these measures. At the moment, the most effective measure is the implementation of policies to regulate the world trade in amphibians. Although this won't actually solve the problem, such measures could reduce the spread of the fungus. It is also important to say that there are several conservation programs and associations such as Amphibian Ark and the Amphibian Specialist Group working in Brazil and around the world for the conservation and research of amphibian species, fighting daily to prevent even more of these animals from becoming extinct.
Figure 2 - Atelopus limosus killed by the disease. Source: Brian Gratwicke.
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