What’s in store for the future of BD research?

Now that more and more of the scientific community are coming to release the significance of BD, the next question is what happens next?

Firstly, there needs to be an increase in the understanding of its origin and spread. This will help to improve treatment, control and prevent further introduction to more areas. Furthermore, the lessons learnt from BD could be applied to other emerging infectious diseases which affect ecosystems, food security and biodiversity.

Additionally, scientists would like to improve the survival of frogs within the wild. This can be achieved by a number of methods, such as habitat manipulation, bioaugmentation (addition of nutrients to the environment) or selection for resistant frogs. Unfortunately, these measures are often very expensive, have a lasting ecological impact or are just difficult to implement within the environment.

How has the disease spread around the world?

There is no doubt that humans have massively been responsible for the spread of BD around the world. Fossil and old museum records show that disease has been in Asia and Africa for at least 100 years, but has no caused declines in these countries.

Yet, human frog trade has led to BD introduction elsewhere. Bull frogs for mean, African clawed frogs for pregnancy testing, pet trade, or through accidental spread when frogs stowaway on vessels are all examples of how we as humans have contributed to BD global distribution.

The arrival of BD to naive populations who have never been exposed to the pathogen leads to the massive loss in amphibians.

Genetics

The origin of BD is not fully understood, neither is its transmission around the globe. One thing is for sure though, that humans have only made the problem worse. Transporting frogs around the world, for research or as pets has helped BD establish residence all over the globe.

3 genetically different lineages are found known:

1. From Switzerland – BD-CH
2. From Mallorca (originally from South Africa) – BD-CAPE
3. All over the world – Global Panzootic Lineage – BD-GPL

BD-GPL is extremely aggressive and kills the Amphibians much more quickly compared to the others. It occurred as a result of mixing of a previously geographically isolated populations, all as a result of human frog trade.

Treatment Options

Unfortunately, frogs heavily infected with BD are likely to die. However, scientists are studying various methods to try and treat the disease and prevent this from happening. At present, there are 3 pathways for treatment:

1. Drugs – 2 drugs can be used to try and treat the infection: Itraconazole (kills the fungus) and Chloramphenicol (an antibiotic, kills the secondary bacterial infections)
2. Bacterial Skin top-up – Larger amounts of the bacteria Janthinobacterium lividum on the frogs skin has been shown to interfere with the fungi’s ability to reproduce
3. Redbush tea – Compounds within the drink appear to have anti-fungal abilities

Global Prevalence

BD can be found in every continent apart from Antarctica. It cannot survive in extreme temperature environments, such as deserts or the cold tundra of Greenland for example. Yet those with a temperate to tropical climate, with free flowing water, are susceptible.

Two maps are shown below (click to enlarge). The first shows the recorded instances where BD has been observed infecting frogs in the wild. Follow the ‘BD-Maps’ hyperlink under the ‘useful links’ page to see an up to date version for yourself.

The second shows the global environmental susceptibility to BD. However, this is a current estimation. With climate change and global warming, this is likely to change dramatically in the future.

Global Prevalence (BD-maps.net)

Global Susceptibility (Fisher et al. (2009) Annu.Rev.Microbiol., 63, 291-310)

Causative Agent

Scientists quickly realized that this was a new, previously unreported species of fungi attacking the frogs. But the researchers did know that it was a non-hyphal aquatic species and therefore belong the the fungal Phylum Chytridiomycetes.

Specimens were loaded up into a DNA sequencer to allow identification. The machine revealed that the fungi belong to no recognized genus, so a new one was created – Batrachochytrium. Fitting into this genera, the species was eventually dubbed Batrachochytrium dendrobatidis (or BD for short) – the causative organism of Amphibian Chytridiomycosis.

Researchers now set out to study the ecology of BD, how it infects frogs and how it is spread throughout the environment. Now the culprit had been identified, the task now was to study the fungi in order to treat and prevent further loss to amphibians.

Histological Examination

Below is section of skin that you would see under a microscope from a wild Australian green tree frog (Litoria caerulea) from Queensland that was killed in 1998

The skin is clearly thickened (Hyperplasia) – shown by the red marker – and hardened (Hyperkeratosis). Additionally fungal features like Sporangia (S) and discharge tubes (D), which are used in their reproduction, were identified.

 

Berger et al. (1998) PNAS, 95 (15), 9031-9036

Skin use their skin for breathing and ions in their skin were unregulated as a result of the fungal presence, multiple organ failure rapidly followed.

The next question was, what is this fungi?

What is amphibian chytridiomycosis?

In the late 1990’s, scientists in Australia started noticing that frogs were dying of a new, strange disease. Once reported, other researchers from all over the world started noticing the problem as well. It was particularly prevalent in areas of high frog populations, such as South America and South Africa. Eventually, the disease was noticed on every continent bar Antarctica. It wasn’t even wild frogs, captive frogs in zoo’s all over the planet were succumbing to the disease as well. Scientists rapidly collected the dead frogs and took a look under the microscope …

Is there cause for concern?

32% of amphibian species are now on the IUCN Red List, compared to 23% of mammals and 12% of birds. 486 species of amphibians are now critically endangered, and things only set to get worse. Many factors – such as a climate change, UV, pollution, habitat destruction etc. have roles in loss of amphibian biodiversity, yet a large percentage of the cause is unknown. Since 1999, a new emerging infectious disease – Amphibian Chytridiomycosis – is believed to play a substantial part is this massacre.  This blog will discuss the disease, the infectious agent and control of this fungal predator wiping out frogs all over the globe.

Stevens et al. (2008) Threatened Amphibians of the World, Lynx Edicions, Barcelona, Spain