Killer frog fungus could actually help amphibians survive disease

Killer frog fungus could actually help amphibians survive disease



The loss of amphibian species across the world from chytridiomycosis, an infectious disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has been described
as “the most spectacular loss of vertebrate biodiversity due to disease
in recorded history”. So it’s of grave concern that the pathogen has
been discovered in Madagascar, an incredibly biodiverse region
previously thought free of the fungus.



Madagascar has the 12th highest rate of amphibian species richness in
the world, with more than 400 species, 99% of which are indigenous to
the region. But this biodiversity hotspot is already under severe
pressure – a quarter of its species are under threat, according to the
latest Global Amphibian Assessment. It’s rightly feared that the arrival of Bd, as reported
in the journal Scientific Reports, could bring about mass amphibian
decline – and even extinctions – as has been seen elsewhere.

An scanning-electron micrograph image of a Chytrid fungus (Bd) spore.
Alex Hyatt/CSIRO, CC BY

Testing of the samples of the Bd fungus found in Madagascar reveals the strain is closely related to BdGPL,
the hyper-virulent lineage behind all the known outbreaks of the
chytrid fungus pathogen that have decimated amphibian populations.
However what’s interesting is that the rate of infection is extremely
low and there’s no clinical signs of chytridiomycosis: the frogs have
the fungus, but they haven’t developed the disease.

What could this mean?

This discovery presents us with a number of scenarios, which need further investigation.



Perhaps the comprehensive monitoring plan put in place by A Conservation Strategy for the Amphibians of Madagascar (ACSAM) has worked as planned, in that the presence of the Bd pathogen has been detected – for the first time in 2010 – before amphibian declines have occurred.



Perhaps the strain of Bd detected in Madagascar is not a
virulent kind that poses a serious threat to amphibians. This was seen
with the introduction of the BdCape fungus lineage into Mallorca, where it had little effect on the population of Alytes muletensis toads there.



It’s possible that the Bd detected in Madagascar has been
present on the island for a long time, but undetected. It may be an
endemic, non-virulent lineage as seen in Brazil and Asia, where certain
lineages endemic to the regions appear to have evolved alongside the
native amphibians.



Or perhaps there is an endemic, previously undetected chytrid fungus on the island, related or not to Bd, which could be acting as a buffer for local amphibians against the invasion of BdGPL – acting, in effect, as a natural vaccine.



Alternatively, Malagasy amphibians may have developed some intrinsic resistance to Bd,
for example through protective bacteria in their skin. This could
explain the low infection rates and the ambiguous test results reported
in the paper showing that some Bd-positive samples did not conform to
any known lineage of the fungus. Although rare, resistance to BdGPL is not unprecedented – this has been seen and documented in Brazil.

The last known surviving Rabb’s Fringe-limbed Treefrog, a species ravaged by the Bd fungus.
briangratwicke, CC BY

A potential threat or a potential benefit

The first scenario would be a disaster – and should be a priority. If
this turns out to be the case, the survival of Malagasy amphibians
could depend on the conservation and scientific groups involved in ACSAM
managing to restrict the spread of the disease. Tackling invasive
species such as the Asian Toad that might spread the disease and
ensuring tourists and researchers stick to strict hygiene protocols
would be necessary. Perhaps even more drastic conservation measures,
such as capturing animals from particularly vulnerable species for
raising in captivity.



On the other hand, the fourth scenario presents an intriguing
possibility: if it’s the case that Malagasy amphibians are resisting a
fungal invasion, discovering how this works could provide crucial
information to help save amphibians elsewhere from the disease.



The research on the amphibian skin microbiome, for example, and its
role in the creature’s immune system is producing some exciting results.
It’s also apparent that the diversity of the Chytrid fungus species as a
whole, and in particular of Bd, has not been appreciated. It’s
possible there are many types of chytrid fungus associated with
amphibians that we’re not yet aware of which provide some protection
against BdGPL.



So without a doubt, this report will sound warning bells loud and clear for conservationists, and Bd’s
appearance in Madagascar could still result in a huge loss of
amphibians. However, the lack of chytridiomycosis symptoms also suggest
there’s something special in Madagascar that could yield a breakthrough
in how the disease spreads – something that may not only benefit
Malagasy amphibians, but those throughout the world.

Widespread presence of the pathogenic fungus Batrachochytrium dendrobatidis in wild amphibian communities in Madagascar : Scientific Reports : Nature Publishing Group

The chytrid fungus, which is fatal to amphibians, has been detected in Madagascar for the first time. This means that the chytridiomycosis pandemic, which has been largely responsible for the decimation of the salamander, frog and toad populations in the USA, Central America and Australia, has now reached a biodiversity hotspot. The island in the Indian Ocean is home to around 290 species of amphibians that are not found anywhere else in the world. Another 200 frog species that have not yet been classified are also thought to live on the island. Researchers from the Helmholtz Centre for Environmental Research (UFZ) and TU Braunschweig, together with international colleagues, are therefore proposing an emergency plan. This includes monitoring the spread of the pathogenic fungus, building amphibian breeding stations and developing probiotic treatments, say the scientists, writing in Scientific Reports, the acclaimed open-access journal from the publishers of Nature.


The entire amphibian class is currently afflicted by a global pandemic that is accelerating extinction at an alarming rate. Although habitat loss caused by human activity still constitutes the main threat to amphibian populations, habitat protection no longer provides any guarantee of amphibian survival. Infectious diseases are now threatening even seemingly secluded habitats. The most devastating of the known amphibian diseases is chytridiomycosis, which is caused by a deadly chytrid fungus (Batrachochytrium dendrobatidis, or Bd). The fungus attacks the skin, which is particularly important in amphibians because they breathe through it. A large number of species have already been lost in this way - particularly in tropical Central America, where two-thirds of the colourful harlequin frog species have already been decimated across their entire area of distribution. Bd has now been identified in over 500 amphibian species, 200 of which have seen a significant decline in numbers. The pathogen is therefore classified worldwide as one of the greatest threats to biodiversity.

Until now, however, a few islands like Madagascar were thought not to have been affected. The last series of tests from 2005 to 2010 found no trace of the pathogenic fungus there. However, an analysis of the latest series of tests shows that the chytrid fungus also poses a threat to amphibians in Madagascar. "This is sad news for amphibian-lovers around the world," says Dr Dirk Schmeller of the UFZ, who was involved in analysing the samples. "Firstly, it means that an island that is home to a particularly high number of amphibian species is now at risk. Several hundred species live only on this island. And, secondly, if the pathogen has managed to reach such a secluded island, it can and will occur everywhere."


For the study that has just been published, the research team analysed samples from over 4000 amphibians from 50 locations in Madagascar taken since 2005. Samples from four species of Madagascan frog (Mantidactylus sp.) taken in 2010, and from one Mascarene frog (Ptychadena mascareniensis) taken in 2011 from the remote Makay massif tested positive for the fungus. In samples from 2013 and 2014 the pathogen was found in five different regions. Prof. Miguel Vences from TU Braunschweig says, "The chytrid fungus was found in all four families of the indigenous Madagascan frogs, which means it has the potential to infect diverse species. This is a shock!" The study also shows that the disease affects amphibians at medium to high altitudes, which ties in with observations from other parts of the world, where the effects of the amphibian epidemic have been felt primarily in the mountains.

The fact that the fungus has been identified in a very remote part of the island has puzzled the researchers. There is some hope that it may prove to be a previously undiscovered, native strain of the pathogen, which may have existed in the region for some time and have gone undetected because of a lack of samples. In this case, Madagascar's amphibians may have developed resistance to it. However, further research is needed to confirm this hypothesis before the all-clear can be given. It is also possible that the fungus was brought to the island in crustaceans or the Asian common toad (Duttaphrynus melanostictus), carried in by migratory birds or humans. "Luckily, there have not yet been any dramatic declines in amphibian populations in Madagascar," Dirk Schmeller reports. "However, the pathogen appears to be more widespread in some places than others. Madagascar may have several strains of the pathogen, maybe even the global, hypervirulent strain. This shows how important it is to be able to isolate the pathogen and analyse it genetically, which is something we haven't yet succeeded in doing." At the same time, the researchers recommend continuing with the monitoring programme across the entire country to observe the spread of the disease. The scientists also suggest setting up extra breeding stations for key species, in addition to the two centres already being built, to act as arks, so that enough amphibians could be bred to recolonise the habitats in a crisis. "We are also hopeful that we may be able to suppress the growth of the Bd pathogen with the help of skin bacteria," says Miguel Vences. "It might then be possible to use these bacteria as a kind of probiotic skin ointment in the future." A high diversity of microbial communities in the water could also reduce the potential for infection, according to earlier investigations conducted by UFZ researchers and published in Current Biology.


The outbreak of amphibian chytridiomycosis in Madagascar puts an additional seven per cent of the world's amphibian species at risk, according to figures from the Amphibian Survival Alliance (ASA). "The decline in Madagascan amphibians is not just a concern for herpetologists and frog researchers," says Dr Franco Andreone from the International Union for Conservation of Nature (IUCN), who is one of the study authors. "It would be a great loss for the entire world." In the coming months, the scientists therefore plan to work with the government to draw up an emergency plan to prevent this scenario.

Widespread presence of the pathogenic fungus Batrachochytrium dendrobatidis in wild amphibian communities in Madagascar : Scientific Reports : Nature Publishing Group