Frog-killing chytrid fungus has reached Madagascar

Many of world’s amphibians are dying. The usual suspects can claim their share of the blame: habitat degradation, over-exploitation, invasive species. But one of the most important factors that’s linked to the global decline in amphibians like frogs is an infectious disease called Chytridiomycosis, which is caused by a pathogen called Batrachochytrium dendrobatidis (Bd), also known as the chytrid fungus.

The pathogen has been detected in more than 500 amphibian species across our planet and has been definitively linked to declines in at least 200 of them in the Neotropics, Australian Tropics, the western US, Europe, and east Africa. Despite its global occurrence, it’s thought that there are some regions that have remained free of chytrid, small islands of amphibian health preserved from the infection’s ravages. Despite the fact that the rainforests of Papua New Guinea and Madagascar are prime habitats for the fungus to spread, surveys conducted between 2005 and 2010 suggested that they remained uninfected.

Unfortunately, “the pathogen’s ability to infect numerous host species and spread rapidly through amphibian assemblages and on a global scale makes it the greatest disease threat to biodiversity at the current time.” That’s according to James Madison University biologist Molly C. Bletz and colleagues, writing this week in the journal Nature Scientific Reports. With the constant trade of pathogens across the world thanks to human travel and trade, it’s perhaps only a matter of time before chytrid’s footprint covers the entire world. Indeed, it might already be too late for Madagascar, according to Bletz’s research.

The island of Madagascar is home to nearly 300 described amphibian species and some 200 additional candidate species that have not yet been formally described in the scientific literature. With the exception of two introduced species, all are endemic; they’re found nowhere else on the planet. The nation’s amphibians are already threatened by deforestation and by over-exploitation for the pet trade. If a non-native chytrid virus invaded the island, it could be devastating for this unique assemblage of amphibian species.

Bletz and her colleagues tested 4,155 amphibians from 52 sites on Madagascar between 2005 and 2014 to see whether they carried the Bd pathogen.

Every specimen collected between 2005 and 2008 tested negative for the chytrid fungus. That all changed in December 2010. Three of 37 frogs tested at one particular sampling site tested positive. In 2011, the parasite was detected again, at a nearby sampling site, but didn’t seem to be present in the original 2010 location. But then in 2012 the infection exploded. The fungus was detected at three new locations. It spread to a fourth site by 2013. By 2014 the infection was detectable at all locations the researchers sampled, though it seemed to occur more often at mid to high elevations, and was more detectable during the dry season than during the wet season. The disease has now been observed in each of the four families of native frogs.

However, a genetic analysis showed that at least some of the chytrid detected on the island is quite similar to the “global panzootic lineage” that’s infecting amphibians across the planet, which at least suggests it’s been introduced.

Assuming that further analyses support the notion that chytrid was introduced to the island, Bletz argues it’s imperative to understand the mechanism through which it was introduced. It could be invasive crayfish, though that’s unlikely. It could be an invasive toad species recently detected in eastern Madagascar. It could have been brought on bird feathers, or in soil on the shoes of human tourists or researchers. It could have hitched a ride through the international trade of wildlife, plants, or machinery.

There is a silver lining to this story. “During the conducted surveys, no individuals exhibited signs of clinical chytridiomycosis, and up to now (January 2015) no mortality events associated to Bd occurrence have been reported in Madagascar,” according to the researchers. That is, the parasite has been detected but the animals don’t seem to be suffering from it. At least, not yet. It is possible, though highly speculative, that the native Malagasy amphibians could have some pre-adaptation that makes then resistant to infection or more tolerant to it, though laboratory-based research as well as anecdotal evidence from pet breeding facilities and zoos has indicated that Malagasy amphibians can become infected by the fungus.

It could also be that the strain of Bd on Madagascar is less virulent than other strains found elsewhere. Or it could be that Bd has always been on Madagascar, and it’s only been recently detected since researchers have only recently started looking for it. “With the current data it is not possible to discriminate between the opposing hypotheses that the detected Bd is introduced or endemic,” write the researchers. The truth is both could be the case.

As is typically true of such experiments, the researchers conclude by prescribing additional research to further characterize the strains of Bd present in Madagascar and to further understand how they propagate across the landscape. “In coordination with the Malagasy authorities, researchers and conservationists must [also] prepare stakeholders for an effective response to a chytridiomycosis outbreak by the development and implementation of disease mitigation strategies,” they add. Just what those strategies might be is still very much unclear. Some researchers have found success by using a probiotic therapy; their own skin bacteria could help inhibit the fungus’s growth. Elsewhere, healthy captive assurance populations are being bred. Still other researchers are focusing their efforts at understanding the micro-organismal communities in freshwater ecosystems. Hopefully someone comes up with something effective before all the frogs are gone. – Jason G. Goldman | 27 February 2015

Source: Molly Bletz et al. (2015). Widespread presence of the pathogenic fungus Batrachochytrium dendrobatidis in wild amphibian communities in Madagascar. Scientific Reports 8, 8633. DOI: 10.1038/srep08633

Header image: Boophis tasymena, via ZSL.