The Big Bad Bd: The Rush to Save Amphibians from Extinction

Rebecca SE Tan

29 July 2022

Introduction

Amphibians are at most risk of a potential sixth extinction, with 41% already at risk (IUCN Red List, 2020; Jackson, 2008; McCallum, 2007; Parry et al., 2007; Stuart et al., 2005; Thomas et al., 2004; Wake & Vredenburg, 2008). This seemingly mysterious and rapid decline of amphibians has been primarily attributed to the emergence of a virulent skin pathogen, Batrachochytrium dendrobatidis (Bd) (Gascon et al., 2005; Skerratt et al., 2007). Bd is a highly transmissible soil-borne fungus which causes the infectious disease chytridiomycosis in amphibians (Longcore et al., 1999; McMahon et al., 2012; Poon et al., 2007; Skerratt et al., 2007; Stuart et al., 2005; Wake & Vredenburg, 2008). This fatal disease is thought to disrupt the vital process of osmoregulation and the transport of water, oxygen, and salts (Kärvemo et al., 2018; Voyles et al., 2007). Already, Bd has dwindled the populations of 700 species of amphibians and is likely to get worst as global warming shifts temperatures closer to the optimal temperature for Bd growth (Lips, 2016; Piotrowski et al., 2004; Pounds et al., 2006; Skerratt et al., 2007). In this paper, I explore the different solutions proposed to counter this disease and save amphibians from extinction.

Solution 1: Amphibian introduction and reintroduction programs

Non-native amphibian introductions have been considered as some amphibians are able to survive chytridiomycosis, while others may not even be susceptible to the disease (Daszak et al., 2004; K. McDonald, pers. comm., as cited in Poon et al., 2007; McDonald et al., 2005; Retallick et al., 2004). For example, certain resistant and adaptive species such as the American Bullfrog (R. catesbieana), the Cane Toad (B. marinus), and the Clawed Frog (X. laevis) can be introduced into the environment to increase amphibian populations (Wake & Vredenburg, 2008). The problem with such non-native introductions, however, is that they may increase Bd emergence in susceptible native amphibians (Fisher & Garner, 2007; Weldon et al., 2005). Introduced amphibians may also be invasive and damage the ecosystem, as in the American Bullfrog example (Roach, 2004).

Hence, the reintroduction of native amphibians may be the less risky choice. Susceptible native species can be kept in captivity and protected until chytridiomycosis is mitigated and the environment is deemed safe for reintroduction (Gewin, 2008; Speare et al., 2004). These reintroduction programs are already being carried out in Panama, as well as by The Amphibian Conservation Action Plan (Gascon et al., 2005; Wake & Vredenburg, 2008). With a better understanding of the environmental factors associated with Bd infections[1], the location and timing for reintroduction can be better determined (Kärvemo et al., 2018).

However, there are still feasibility challenges to amphibian reintroduction. They are incredibly costly – for one, the Northern Queensland Threatened Frogs Recovery Team’s five-year Recovery Plan estimates the expenditure of about $1.523 million (Poon et al., 2007). Further, institutions may also lack expertise in amphibian care and reproduction (Garner et al., 2009). Most importantly, since Bd can persist without amphibian hosts, such as in water and other animals, amphibian reintroductions may scarcely be successful (Johnson & Speare, 2003; Longcore et al., 1999; McMahon et al., 2012; Mitchell et al., 2008; Skerratt et al., 2007).

Solution 2: Research to Reduce the Impact of Bd on Amphibians

A complementary solution is the treatment of amphibians. While community participation in monitoring and treating amphibians is helpful – such as the treatment of sick frogs brought in by the public in the Cairns Frog Hospital in Australia – superficial treatment of individual amphibians may not be feasible on a large scale (Poon et al., 2007). While it is possible to use formalin or antifungals to remove Bd from individual frogs, they may simply be reinfected again (Bosch et al., 2015; Parker et al., 2002). Hence, instead of aiming to spray chemicals at every single location and frog, research is needed to target the possibility of strengthening the natural defence of amphibians.

One such research focuses on the antifungal properties of symbionts on amphibians. As a proof of concept, anti-Bd skin bacteria have been successfully added to skins of the salamander Plethodon cinereus, though much needs to be done to expand this project (Harris et al., 2009).

Another research project focuses on strengthening the immune response of amphibians themselves. On the one hand, Bd-infected frogs produce more enzymes like chitinase[2] and serine-type proteases[3], which can kill Bd and the associated chytridiomycosis disease (Ellison et al., 2014). On the other hand, Bd-infected frogs have also shown suppressed immunity levels, leading to a lower expression of their lymphocyte genes (Ellison et al., 2014). More research must be done on the interaction of the amphibian immune system and Bd in order to find solutions to tackle Bd (Fites et al., 2014).

Thankfully, research on Bd is being consolidated to hasten the progress towards reducing Bd. Under Australia's 2016 Threat Abatement Plan, researchers can upload and access data to create a coordinated national database (Commonwealth of Australia, 2016). We would do well, however, to have a global one as well.

Solution 3: Trade Regulations and Bans

The international trade of amphibians has exacerbated both fungal and pathogen spread, especially since Bd can even be spread through the water containing amphibians for trade (Fisher & Garner, 2007; Kolby et al., 2014; Schloegel et al., 2012; Weldon et al., 2005). Hence, an amphibian trade regulation or ban can cull the spread of Bd to new geographic locations (Kriger & Hero, 2009).

Indeed, following a global outbreak of Batrachochytrium salamandrivorans (Bs), a fungus very similar to Bd, the trade in salamanders to the U.S. was banned (Yap et al., 2015). This measure was highly effective – no salamander in the U.S. has yet been found with Bs (Bales et al., 2015; CABI, n.d.).

Despite this success, however, a ban on all amphibians is tricky. In just five years, the U.S. imported more than 3.6 million live amphibians for research, pets, food, or entertainment (Chinnadurai & Kane, 2014; Fisher & Garner, 2007; Jancovich et al., 2005; Mohanty & Measey, 2019; Picco & Collins, 2008). With such prominence, outright bans may simply drive the trade underground, where the lack of regulations may exacerbate the spread of disease. Already, the black-market trade of wildlife is estimated to be second only to drugs (Garner et al., 2009). Illegal amphibian trade is particularly rife because they are easy to smuggle in large numbers and are very lucrative – one frog could potentially fetch up to £500 (Garner et al., 2009).

In addition, a blanket ban on amphibian trade can hurt scientific research and use. In 2005, licensed procedures in the United Kingdom required 21,000 amphibians (Home Office, 2005). Such use of amphibians in research, especially that of clawed frogs (Xenopus), has increased significantly in past years (Garner et al., 2009). Amphibians are also used for other scientific purposes – such as pregnancy diagnosis – which is the primary use of amphibian trade in South Africa (Measey, 2017).

In view of these considerations, perhaps regulations rather than bans would be a better approach. The biosecurity[4] of amphibians can be improved through the monitoring, quarantine, screening, and treatment of amphibians in the pet trade, such that the health status of the amphibians is known (Measey, 2017; Skerratt et al., 2007). For large-scale implementation, the partnership of public and private organizations is crucial (Garner et al., 2009). One example is the regulation of amphibians and their eggs by the Commonwealth of Australia, which requires compliance with the Animal Biosecurity Policy Memorandum to provide information on the amphibian’s health and disease-free status (Poon et al., 2007).

Discussion

Perhaps, community help and reintroduction programs are beneficial as immediate but short-term solutions. There is still a need for longer-term solutions such as immunity research and better monitoring of amphibian trade. With proper complementation of these measures, the fate of amphibians need not look bleak. What are your thoughts on these solutions? Are there other drawbacks that I have lacked in considering? Let me know in the comments below!

Footnotes

[1] see: Brem & Lips, 2008; Kriger et al., 2007; Longo et al., 2010; Puschendorf et al., 2009 [2] Chitinase degrades chitin, which makes up the cell wall of fungi (Vega & Kalkum, 2012). [3] Serine-type proteases regulate the inflammatory process and kill bacteria. Hence, these proteases help to inhibit the Chytridiomycosis bacterial disease caused by Bd (Pham, 2006; Rollins-Smith et al., 2006). [4] Biosecurity refers to measures that protect populations against harmful biological or biochemical substances (Lexico, n.d.). In this case, biosecurity refers to the protection of amphibians from Bd.

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