Skin Deep: How Bd and Bsal Fungi Threaten Amphibian Health

In the 1970s, amphibians across the world began dying en masse. The cause was a mystery until researchers made a surprising discovery: the culprit was Batrachochytrium dendrobatidis (Bd), a ubiquitous fungus that . More recently, a relative of Bd, Batrachochytrium salamandrivorans (Bsal), has begun exacting its toll on European salamanders. These fungal pathogens have decimated amphibian populations around the world. As of 2019, due to Bd and Bsal, with Bd alone linked to 90 frog and toad extinctions—and ecosystem changes may be accelerating the fungi’s spread. It is with this understanding that scientists are , such as antifungal treatments and probiotics, that may help save amphibian species from these fungal foes before it’s too late.

Under the Skin

What makes Bd and Bsal so dangerous to amphibians is that they attack one of their most important and complicated organs—the skin.

As far as barriers go, amphibian skin is particularly permeable. , resulting in unique methods of maintaining adequate hydration and . Under normal conditions, terrestrial amphibians use a highly vascularized region of their skin—the “drink patch”—to suck up water and ensure hydration, while aquatic species have adapted to avoid excess water uptake. Furthermore, frogs use the concerted action of electrolyte channels and pumps to ensure that sodium, potassium and chloride are present at the necessary physiological levels, despite their relatively low levels in aquatic environments. Perturbation of this balance can be lethal.

Enter: Bd.

Bd zoospores live in water, where they contact and colonize amphibians’ skin. , using a flagellum to to seek out susceptible hosts. Amphibians maintain in the mucus layer of their skin, such as short antimicrobial peptides (AMPs), small molecules and microbes, to keep pathogenic invaders out. But Bd is largely unphased. The fungus by transitioning from a motile zoospore to a stationary cyst with a protective, chitin-based cell wall. This allows the fungus to overcome amphibian barriers and successfully infect the skin.

Artist illustration of Bd life cycle. — The Bd life cycle. Starting out as a motile zoospore, the fungus undergoes morphological changes that allow it to invade the skin of amphibians.
Source: Buttimer S., et al./*Global Change Biology*, 2025 via a CC BY 4.0 license

While it may start out on the skin’s surface, the fungus does not remain there for long. After cyst formation, Bd penetrates host cells using a germ tube and reproduces to yield more zoospores. As this reproductive cycle continues, the fungus burrows deeper, colonizing skin layers in which occur. As colonized amphibian cells differentiate and rise to the surface through a natural cycling process, , too, until fungal sporangia——release zoospores back into the environment via fungal discharge tubes, perpetuating Bd’s life cycle.

As Bd is replicating and generally thriving, the health of its amphibious host takes a nosedive. Over the course of several days to weeks, a symptomatic amphibian will begin , the disease caused by Bd. Through mechanisms that are still largely unclear, infected amphibians exhibit abnormal skin thickening and sloughing, and their electrolyte levels tip off-balance. this disruption of electrolyte levels could ultimately be what leads to cardiac failure and amphibian death.

While much is unknown about Bsal's pathology, it is hypothesized to share similarities with Bd. Both cause lethal skin disease, but rather than hyperkeratosis (excessive skin thickening). Its host range also appears to be more limited than that of Bd. Ongoing research seeks to understand Bsal's pathology, host range and virulence determinants.

An Emerging Fungal Pathogen

Despite its relatively recent discovery, researchers have found evidence of Bd in museum specimens dating as far back as , and . These strains, as well as those found around the world today, have diverged into multiple distinct lineages, not all of which are as deadly or highly virulent as the global panzootic lineage—BdGPL—. In fact, many amphibian species infected with lineages other than BdGPL do not experience . The rise of globalization and the amphibian trade may have allowed Bd lineages that were previously geographically isolated from one another to recombine into BdGPL, which has spread to impact amphibian populations around the world.

The geographic origin of BdGPL has been a source of much debate, but and observations of global patterns of decline indicate it likely has roots in Asia. Along with Africa, North America and Europe, Asia has seen notably low levels of chytridiomycosis-related amphibian declines, whereas South America, Mesoamerica and the tropics of Australia have seen much higher levels of devastation. indicated that Asian amphibians may have evolved more robust defenses against Bd, due to their long-term coexistence with the pathogen. Reduced virulence due to historical coexistence with different Bd strains. BdGPL, however, seems able to evade many pre-established defenses. While some species have , ongoing habitat shifts and globalization may open the door to more panzootic events.

Global map of distribution of different Bd lineages. — Bd is found around the world. The BdGPL lineage is hypothesized to be causing much of the devastation in amphibian populations. (Click image for larger view.)
Source: Sewell, T.R.,et al./*Trends in Parasitology*, 2021 via a CC BY-NC-ND 4.0 license

In addition to the ongoing impact of Bd, , particularly in salamanders. Between 2010 and 2013, Bsal brought the fire salamander “” in the Netherlands, . The emergence of Bsal sparked major concern about fire salamanders and other already fragile amphibian populations. , Bsal appears to be limited to Asia, where it is , and Europe, where it was recently introduced and has caused fatal disease. Because of the discontinuity of its distribution, Bsal's spread has been attributed directly to human causes such as amphibian trade and a lack of efficient biosecurity. There is the potential that, without intervention, Bsal could spread beyond its current borders and impact global salamander populations.

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Controlling the Spread

At the global scale, mitigation efforts, such as in the U.S., and in the European Union, are underway. The World Organization for Animal Health includes both and in its animal health code, and outlines recommendations for control measures for these pathogens.

Researchers are exploring the use of antifungals, probiotics and to help combat chytridiomycosis. Antifungals may help reduce the fungal burden on impacted species, giving them a better chance of survival. that are already present in the skin of amphibians has been shown to reduce morbidity and mortality related to chytridiomycosis. These microbes produce antifungal peptides and are already a component of amphibians’ natural immune defenses. Environmental alterations, such as strategic reduction of shaded areas over water to increase water temperature and hinder Bd infection, have also been proposed as a potential intervention. But though numerous conservation strategies have been explored, eradication of Bd still seems to be a long way off, and environmental shifts may exacerbate the problem.

A Delicate Balance

While some amphibian species have from the declines caused by Bd, scientists are concerned that the changing climate, habitat alterations and amphibian trade will open the door to future panzootic events. Like their amphibian hosts, and are highly sensitive to climate, relying on specific temperature ranges to survive and thrive. Natural fluctuations in temperature can dictate both pathogen and host performance. Bd is a global pathogen and is considered enzootic in many regions. However, even within those regions, based on host factors, pathogen factors, temperature, moisture and more. But what happens when climate conditions change, opening new regions and hosts to Bd infection?

The predicts that variation in pathogen performance will correspond with variation in host performance. In an environment where the host performs well and the pathogen performs poorly, the pathogen should have less impact. The converse is also predicted to be true—if warm-adapted hosts are subjected to a cold spell, or cold-adapted hosts experience unusually warm climates, there is an increased likelihood of infection. In a world where climates are shifting, this may have devastating consequences for amphibian hosts.

A dead frog, killed by Bd, lies on its back. — A frog killed by Bd. Scientists worry that climate change could exacerbate the spread of the fungus.
Source: Gratwicke B./Wikimedia Commons via a CC BY 2.0 license

Precipitation is another important variable that can determine Bd infection dynamics. Researchers have integrated precipitation into the EMTH, developing the . They have used studies of Bd to conclude that both thermal and precipitation mismatches can increase Bd infectivity. Bd exhibits increased infection rates and mortality in wet conditions and may be less successful at colonizing hosts under dry conditions. But . This is likely due to a combination of the THMH and additional host behavior dynamics. Amphibians adapted to living in naturally wet areas experience stress when subjected to drought; they may seek out areas of moisture to relieve that stress. While the dry environment hinders Bd, the concentration of hosts in smaller spaces where water is abundant and potential shifts in immunity due to stress may be enough to cause die-offs.

There is ongoing concern that changing temperatures and precipitation patterns will lead to a new wave of extinction due to chytridiomycosis, but that isn’t the only worry. Researchers have built models to predict the potential impacts of climate change on diverse fungal diseases beyond Bd and Bsal, such as and .

Ultimately, coordinated efforts to ensure efficient surveillance, as well as exploration of ways to implement existing interventions more effectively, are critical for keeping these pathogens in check in an ever-changing world.

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