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Applying Thermography Methods to Determine Chytrid Prevalence in Threatened Cloud Forest Amphibians.

by Dr Tom Martin – Operation Wallacea

Image 1. Credit@ George Lonsdale – Operation Wallacea – On location in Honduras.

Amphibians are the most threatened class of vertebrates worldwide, with more than one-third of species considered by the International Union of Conservation of Nature (IUCN) to be at risk of extinction. So many species are now imperilled that many conservation biologists believe we have entered a global “amphibian extinction crisis”. There are several drivers of this crisis, including habitat loss, hunting, pollution, and the impacts of global warming. However, perhaps the most concerning of all is the emergence of a new infectious disease; Chytridiomycosis, which is spread by the fungal pathogen Batrachochytriumdendrobatidis (Bd).

Chytridiomycosis causes mortality in amphibians due to Bd infecting keratin in their bodies. In the case of tadpoles, this can lead to the deformation of mouthparts which makes feeding difficult. In adult-stage amphibians, the fungus causes a thickening of the skin (hyperkeratosis) which can lead to difficulties in osmotic regulation – particularly concerning electrolyte exchange – ultimately leading to a fatal cardiac arrest.

Not all amphibian species are equally affected by Chytridiomycosis. Relatively small Bd loads can be fatal to some species, while others are highly tolerant of the fungus, showing little adverse effects. Some of these less-impacted species have become important vectors for the disease, spreading Bd into habitats where vulnerable species occur (although the fungus is also spread via many other means, including on peoples boots/clothing).

While Chytridiomycosis is not fatal to all amphibians, the net impacts of the disease have been devastating. To date, it has caused population declines in over 500 species and is thought to have driven nearly 100 species extinction. As such, it is now responsible for more significant biodiversity loss than any other disease in recorded history, and it is continuing to spread.

Mitigating the impacts of Chytridiomycosis is a challenging and complicated task. It involves a wide range of conservation strategies, including the development of ex-situ assurance populations of vulnerable species, ‘head-starter’ programs (while tadpoles are raised in captivity and released into the wild as sexually mature frogs), and enhanced biosecurity measures. However, we need informed strategies with necessary information on where chytrid occurs and which species are particularly vulnerable to infection – information that is lacking in many areas, particularly in the tropics.

Image 2. Photo Credit @George Lonsdale – Operation Wallacea – On location in Honduras.

As Bd spores are microscopic, determining whether an individual frog is infected or not is, at present, a difficult task. The primary methodology for assessing chytrid prevalence rates involves taking a swab from a living amphibian (Image 2.) and completing genetic analyses on these swabs to determine whether Bd is present. The process is difficult, expensive (as a DNA laboratory is required), and challenging to achieve in some countries given such swabs often require export permits, which are not always easy to obtain.

A new potential means of determining whether a frog is infected or not with Bd is through the use of thermography. While currently untested, it could be the case that infected amphibians initiate a fever response – as many organisms do when infected with a disease – and that such a response may be visible and measurable on their thermal signature. If this is the case, it may be that thermography offers an efficient means to determine chytrid prevalence in amphibians that bypass the need for genetic laboratories and biological exports.

Despite being exotherms, amphibians show clear thermal signatures (Image 3). If it can be determined what the thermal signature of a healthy amphibian looks like, any fever response in a chytrid-infected amphibian that deviates from the ‘normal’ signature may be relatively straight- forward to determine. We aim to test this in the summer of 2021 by completing thermography trials in the cloud forests of Cusuco National Park, Honduras, the location of a long-running Operation Wallacea research programme. Cusuco National Park supports many threatened amphibian species, some of which are found no- where else in the world, such as the Exquisite Spike-thumbed Frog (Plectrohyla Exquisita) (Image 1.).

Sadly, most of these are threatened with extinction, mainly due to Chytridiomycosis. Amphibian populations in Cusuco have been subject to a long-running Operation Wallacea research program using DNA swabs, which will continue in 2021.

Image 3. Photo credit @Skins – Amphibian Research – Shuttleworth College 2020

We will be running a thermography experiment in tandem with the swabbing methodology, taking thermal scans of all individuals from which a DNA swab is obtained. As data from the swabs will be able to show with certainty which individuals are infected, it will be possible to cross-reference this with the thermal scans to see whether infected individuals show a traceable difference in their thermal signature. If differences in thermal signatures between infected and uninfected amphibians can be reliably quantified, thermography methods could then be employed in the future as a faster, less expensive method of determining chytrid presence and prevalence. Thermography can then be applied in any ecosystem in any country, regardless of export permit barriers. It would be a powerful tool for herpetologists seeking to determine how far chytrid has spread, and which species are most vulnerable to its effects, in turn allowing for better-informed conservation strategies for dealing with the looming amphibian extinction crisis.

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