Bd found in Madagascar

A paper in Nature Scientific Reports today (co-authored by our very own Prof. Mat Fisher) reveals that Bd has been detected in native, wild populations of Malagasy amphibians. This is a huge development in the area of Chytrid research and amphibian conservation – but there’s no way of knowing right now how serious it is, or even whether it could have positive implications for amphibian conservation globally.

Below is a summary of our lab group’s thoughts on the paper, and it’s implications for amphibian conservation and Chytrid research.

The impact of chytridiomycosis, an emerging infectious disease of amphibians 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” (Skerratt, et al., 2007). Detection of the pathogen in Madagascar, published in Nature Scientific Reports today, is therefore of huge concern to the conservation community. Madagascar has an amphibian genera endemism of 88% and the 12th highest amphibian species richness in the world (Andreone, et al., 2008) making it megadiverse for amphibians. However, this amphibian biodiversity hotspot is already under severe pressures, with 25% of species threatened according to the latest Global Amphibian Assessment (Andreone, et al., 2012). A well-founded fear is the possibility that the detection of Bd could herald the coming of mass amphibian declines, such as those seen in the Neotropics and Western USA (Lips, et al., 2006; Vredenburg, et al., 2010).

Lineage specific testing of the Bd positive samples has suggested that the Bd present is most closely related to BdGPL, the hyper-virulent lineage behind all known chytrid-driven declines to date. As yet however, no clinical signs of chytridiomycosis are apparent in Madagascar, and the infection intensities reported today are extremely low (about 0.1 zoospore genomic equivalent). This presents four scenarios to be investigated as a priority:

  • That Bd has recently arrived in Madagascar, that it is invasive, and is the hypervirulent lineage Bd The comprehensive monitoring plan set in place by the ACSAM (A Conservation Strategy for the Amphibians of Madagascar) has worked as planned, and the pathogen has been detected before amphibian declines have occurred.
  • That the Bd detected has recently arrived in Madagascar, but is a hypovirulent lineage and is unlikely to cause mass amphibian declines. This was seen with the introduction of the hypovirulent lineage BdCape into Alytes muletensis populations on Mallorca.
  • That the Bd detected in Madgascar has been present on the island for a long time already, but was not previously detected. The Bd may be an endemic, hypovirulent lineage as seen in Brazil and Asia, where endemic lineages appear to have long established evolutionary relationships with native amphibians.
  • There is an endemic, previously undetected chytrid on the island, which may or may not be a new lineage of Bd, which could be buffering local amphibians against a new invasion of BdGPL by acting as a natural vaccine. Alternatively, the amphibians of Madagascar could have some intrinsic resistance to Bd, for example through protective bacteria in their skin microbiome. This could explain the low infection intensities seen and the ambiguous diagnostic results where some Bd positive samples did not test positive with any lineage specific diagnostic tests. Although rare, resistance to BdGPL where the pathogen is emerging is not unprecedented, as documented in Brazil (Lips, 2014).

Scenario 1 would be a potential environmental disaster for Malagasy amphibians, and as such this possibility must be investigated as a top priority. If this scenario turns out to be true, the various conservation and scientific partners involved in ACSAM will need to focus on disease mitigation by restricting further spread of the fungus, identifying and closing potential routes of fungal spread (invasive species, such as the Asian Toad, ensuring tourists and researchers stick to strict hygiene protocols) and consider more drastic conservation measures such as taking individuals from particularly vulnerable populations into captivity.

Scenario 4, on the other hand, is an intriguing possibility. Understanding how Malagasy amphibians are resisting a BdGPL invasion, if this turns out to be the case, could provide valuable information to help mitigate the impact of Bd in other areas. The pace of research on the amphibian skin microbiome, for example, and its’ role as part of the amphibian immune system is increasing and producing some exciting results. It is also becoming apparent that the diversity of the Chytrids as a whole, and in particular of Bd, has not previously been appreciated. It is possible that there are many amphibian-associated Chytrids we are currently unaware of which may affect BdGPL’s ability to infect an individual.

This report that Malagasy amphibians are now known to be infected with Bd will rightly sound loud and clear warning bells for amphibian conservation, and could be harbinger for yet another blow to the already challenged amphibians in Madagascar. However, the lack of clinical signs of chytridiomycosis and the low levels of infection intensity also indicate that Madagascar could yield exciting new information about Bd infection dynamics which not only could mean that the amphibians of Madagascar are less likely to experience the same fate as those in the Neotropics and other areas of Bd-associated declines, but could yield more information to aid our understanding and mitigation of Chytridiomycosis and Bd in other naïve and threatened populations.

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