Amphibians in Plight
Chytrid Fungus, Batrachochytrium dendrobatidis
Tropical Ecology
By Clarence Wright

Introduction:

Global decline of amphibians around the world has been contributed to many factors from global warming to pollution and evidence indicates another factor that has appeared in the last few years is also contributing significantly to this problem. Batrachochytrium dendrobatdis,(Bd) or commonly known as Chytrid Fungus has been devastating populations of tropical frogs and now other temperate frogs and amphibians in general are causing wide spread mortality. This paper covers the history and current status of this disease as well as the various theories as to how the evolution of this fungus began.

History:
It was not until the late 1980’s that the world became aware of the decline of amphibians around the world where at the First World Congress of Herpetology in Kent Canterbury, England that scientist heard papers presented from delegates that amphibians in their country were declining rapidly. At this meeting a special symposium was put together at the last minute so that scientist could confer about this mounting global problem. At this meeting it was decided to form a group of scientist to monitor the disappearance of amphibians globally and report back the next year. During the convening years reports flowed in that frogs were dying from what most scientist believe was due to radiation from the sun or genetic deformities related to it. It is now known that amphian populations are declining globally due to pollution and environmental degradation. It wasn't until much later on that the discovery of Bd was discover. Most scientist believe that Bd is not the only factor contributing to the decline of amphians in the world

Many scientist believed that Bd has been prevalent in the past hundred years and most recently with the development of new techniques in pathology have we revealed this unique fungus. In museum specimens of Xenopus (African Clawed Frogs) are able to be tested for the fungus and it is believed this may of been a primary source for introduction of this disease. Frogs as early as 1879 through 1999 were analyzed with the earliest known specimen testing positive was a specimen from 1938. (Weldon, 2004) It is believed Africa was the origin for Bd and is pathogenicity evolved relatively recently from the other 1000 chytrid species. It is believed that we are still not sure of the source of contagion for Bd around the world. Bd is found in and on non-organic life forms in pristine environments where survival of this fungus must depend on non-amphibian organic materials as nutrient resources. (Rosenblum, 2010).

Pathology

As with any disease understanding the dynamics and pathological function of a disease is important if we are to understand how to resolve the outcome of it’s’ effect. Bd in amphibians at first attacks the epidermis and causes a thickening of the outer layer of skin. In most other animals skin fungus does not have much effect on the outer layer of skin, but the amphibian skin with it physiologically active and tightly regulating exchange of gases, water, and electrolytes within the epidermis has a devastating effect. Evidence suggests that electrolyte depletion and osmotic imbalance caused by Bd is primarily responsible for the high mortality. Infection with Bd is transmitted by a form of the fungus called a “zoospore”. The zoospore (Fig. 1) has a very distinctive appearance with a single flagellum that helps the spore swim through water or moist environments. Zoospores require moisture and cool temperatures and can persist in moist environments for several months (Johnson and Speare, 2003), but do not tolerate conditions that are warm or dry for more than a few hours (Johnson and Speare, 2005). Therefore, the most common and successful ways that Bd zoospores spread from place to place are in water, moist or wet materials (including soil or equipment) or on the skin of infected amphibians. (Amphibian Ark, 2013)

external image 175px-Chytridiomycosis2.jpgFig 1

Chytridiomycosis in Atelopus varius - two sporangia containing numerous zoospores are visible
Global Perspective of Batrachochytrium dendrobatidis, Amphibian Chytrid Fungus

Since the discovery in the late 1980’s and early 1990’s of Bd most attention has been given to regional research and spread of this fungus. No matter how the Bd fungus got started or how ubiquitous it currently is, it is relevant at this time to look at the disease as a global problem for far reaching implications it may have on the amphibians of the world. Tracking diseases especially the Bd fungus allows us to oversee the history, present and future of the disease. By maintaining our tracking of this fungus will help us to keep our fingers on the pulse of the environment for each of these species and the vigor of the niche they inhabit. Each host range can be monitored as the disease spreads through the amphibian population and to evaluate the impact it has on the environment and its’ stability.

We still don’t know how the disease pathways of transmission occur and what vectors of transmission occur within species specific susceptibility to the disease. (Olsen, 2013) Only recently do we believe there are many strains or species of this fungus making its’ way around the world. In the last four years a team of scientist has been compiling a digital map(s) of where the Bd fungus has been located. The purpose of this team’s effort focuses on providing new insights into both Bd occurrence patterns and the development of hypotheses for the study of Bd ecology and epidemiology, and inform species and land-management planning efforts they have reported: 1) spatial and taxonomic patterns of infection, including amphibian families that appear over and under-infected; 2) relationships between Bd occurrence and declining amphibian species, including an analysis of Bd occurrence, species richness, and occurrence of enigmatic population declines and 3) patterns of environmental correlates including climate metrics for all species and for three families (Hylidae, Bufonidae, Ranidae) at the global scale and the U.S.A. scale. (Olsen, 2013) (www.Bd-maps.net)

Hypothesis of the development and spread of Batrachochytrium dendrobatidids
There have been many hypotheses and theories as to what brought about the development of Bd in the first place and how did it spread so rapidly in the last few years. Early in the history of Bd was that it was introduced by the African Clawed Frog that was imported into the United States for pregnancy testing on women. (Weldon, 2004) Some scientist believes global warming in some way triggered the transition of a plant fungus to jump the species barrier and transition into our current epidemic. (Pounds, 2007) No matter how it began it is believed by this scientist that it has always existed and like most diseases given the right circumstances and opportunity developed into the present day scourge.

Watch Frogs: The Thin Green Line on PBS. See more from Nature.





Despite the devastation of amphibian populations around the world we are seeing resurgence of some populations of amphibians. It is believed that some Bd isn’t as virulent as other, even though genetically they are the same. (Berger, 2005) Still in question is the potency of these different strains of Bd or is it the genetic makeup of the different taxon of amphibians? Is it the environment that challenges the fungus survivorship or is it simply genetic viability of the strain of Bd?
To address the issue surrounding the genomic changes that accompanied the evolution of pathogenicity scientist compared Bd with Homolaphlyctis polyrhiza (Hp) a closely related chytrid fungus that is known pathogen of vertebrates. It was discovered that Bd and Hp had different nutritional modes in which Bd was only capable of growing in amphibian skin alone. Most chytrids are saprobes like Hp, and Bd’s ability to infect vertebrate’s skin likely arose after the divergence of Bd and Hp from their common ancestor. (Joneson, 2011). Though lacking in evidence it is believe by some scientist a genetic factor may be involved. It was observed by scientist conducting experiments on frogs infected with Bd that several hundred immune gene function responses found in vertebrates were noted. (Rosenblum, 2012)

Conclusion: New Horizon

Since Darwin’s time it has been known that life is ever changing to accommodate nature and that it reveals itself when the environment challenges the survival ship of a species or taxon. Though some species of amphibians may disappear it is in nature that some will survive. As with most diseases scientist learn much from the outcome and in the case of the Bd fungus we are and will be learning how genes work to response to this environmental challenge. Some scientist are noting that after the Bd fungus passes through an area that some of the amphibians survive. There are several hypotheses regarding these phenomena but it is this author’s hope and belief that genetic factors are involved that will save some of the species of frogs in our world.

Citations:

Bd Mapping Group: www.Bd-maps.net
Berger, Lee, Marantelli, Gerry, Skerratt, Lee F. and Speare, Rick. (2005) Virulence of the Amphibian Chytrid Fungus
Johnson, M.L., R. Speare. 2003. Survival of Batrachochytrium dendrobatidis in water: Quarantine and disease control implications, Emerging Infectious Diseases 9: 922-925.
Johnson, M.L., R. Speare. 2005. Possible modes of dissemination of the amphibian chytrid Batrachochytrium dendrobatidis in the environment, Diseases of Aquatic Organisms 65:181-186.
Joneson, S., Stajich, J.E., Shlu, Shin-Han, and Rosenblum, E. B. (2011) Genomic Transition to Pathogenicity in Chytrid Fungi, PLoS Pathogens, Vol. 7, Issue 11, e1002338.
Olson, Deanna H., Aanensen, David M., Ronnenberg, Kathryn L., Powell, Christopher I., Walker, Susan F., Bielby, Jon, Garner, Trenton W. J., Weaver, George, and Fisher, Matthew C. (2013) Mapping the Global Emergence of Batrachochytrium dendrobatidis, the Amphibian Chytrid Fungus. PLoS One, Vol. 8, Issue 2, e56802. Retrieved from: www.plosone.org
Pounds, J.A. (2007) Global Warming and Amphibian Losses, Nature: Vol 447. Doi:10.1038/nature05942
Rosenblum, E.B., Poorten, T.J., and Settles, M., (2012) Only Skin Deep; shared genetic response to the deadly chytrid fungus in susceptible frog species. Molecular Ecology. Blackwell Publishing Ltd., doi: 10.111/j.1365-294X.2012,05481.x
Rosenblum, Erica Bree, Voyles, Jamie, Poorten, Thomas J. and Stajch, Jason E., (2010), The Deadly Chytrid Fungus: A Story of an Emerging Pathogen. PLoS Pathogens, Vol. 6, Issue 1, e1000550. Retrieved from: www.plospathogens.org.
Weldon, Che, Louis du Preez H., Hyatt, Alex D., Muller, and Speare, Rick. (2004), Origin of the Amphibian Chytrid Fungus, Emerging Infectious Diseases. Retrieved from: www.cdc.gov/eid - Vol. 10. No. 12