It’s the End of the Frogs as We Know it?

by BP

Figure 1:  Recorded distribution of Batrachochytrium dendrobatidis infections,
with red indicating recorded sites, white indicating negative sites, and blue
 indicating negative or unknown sites.

In recent history, worldwide amphibian populations have been on a sharp decline, in a trend that points towards extinction for many species.  In some part, this is due to habitat loss as humans use up resources and rapidly change ecosystems, but the primary cause of this extinction points towards an emerging fungal parasite with a wide host range.  In the early 1990’s, people found dead frogs infected with fungal zoospores across Australia.  An experiment in 1999 identified this fungal species in the skin of infected amphibians, and named it Batrachochytrium dendrobatidis (Bd) [1].   Since then however, more deceased amphibians were being discovered across the Americas, Europe, Africa, and other regions of the world, all infected with the same kind of fungus [Figure 1].  Bd is so frightening to modern scientists because it is highly transmittable, and has been described as the single worst infectious disease among vertebrates based on the number of species affected and the rates of population decline [2].   These massive declines in amphibian populations are a major problem because most of the species affected serve important roles to their ecosystems.  Because of this, scientists have been rushing to find the mechanism behind this disease and focus efforts on preventing its spread.

Figure 2:  Batrachochytrium dendrobatidis
zoosporangium, which can open and
release zoospores for further infection.

So what is this disturbing fungal parasite?  Well, Bd is a chytrid fungus that infects the keratin-rich skin of amphibians, and causes the disease known as chytridiomycosis. Bd has a life cycle that involves two stages [3].  First, as a zoospore, Bd is mobile, and can detect and move towards various macromolecules required for life on the skin of a frog, in a process called chemotaxis.  Once settled, it matures into a reproductive body called a zoosporangium, which releases more zoospores to either re-infect the host or infect nearby amphibians [Figure 2]. As zoospores accumulate in the host, symptoms start to appear.  These symptoms include both physical and behavioral changes, as the frog’s skin becomes irritated and start to shed unnaturally, and frogs start to behave abnormally, losing their ability to right themselves, and failing to eat or seek shelter properly [4].
        

All these symptoms can obviously leave hosts vulnerable to predators and the elements, but more often than not, amphibians die from the parasite because of how it disrupts homeostasis in the host’s skin.  A study in 2009 by Voyles et. al looked at what the mechanism is by which Bd kills [5].  Their results showed that the physiological changes in amphibian skins, caused by Bd, are what kill the hosts. Most amphibians use their skins as a sort of gateway, which they can use to either “breathe” air in, or allow transport of water across to “drink” it.  When their skins suffer from the symptoms of infection, it disrupts the ability of sodium and potassium pumps to maintain electrolyte balance.  Without the function of these pumps in their skin to maintain homeostasis, they are effectively killed off either by cardiac arrest, as happens in tree frogs, or by suffocation, as seen in some other amphibian species [5].

Not all is hopeless for our amphibian friends, however, as some species have demonstrated resistance to infections by Bd.  Originally, scientists suspected that some predisposed genetic trait may be the cause of this resistance.  Some studies, however, have discovered ways that these species have shown resistance even in the presence of lethal amount of Bd cells.  A recent study by Woodhams et. al looked at the survivability of four different amphibian species, and found two things [6].  First, they found that the number of certain innate immune system cells, rather than lymphocyte numbers, that differed between the susceptible and resistant members of one species. They also found that more members of species were resistant when there were more effective antimicrobial skin proteins.  Taken together, these results suggest that the innate immune system in amphibians plays a strong role in resistance to Bd.  Another study, by Ramsey et. al, came to a similar conclusion, finding that higher amounts of skin peptides and antibodies defended amphibians from Bd, showing that the resistant amphibian species likely have a stronger adaptive and innate immune [7].  These results suggest that the difference of amphibians in susceptibility to Bd comes from the strength of the immune response rather than a predisposed inability to be infected.

The big question circulating around concerned scientists right now is whether there is hope for amphibian populations to recover from these sharp population declines.  The answer, unfortunately, seems to be uncertain at the moment.  As previously stated, Bd is a highly transmittable fungus that has at this point already reached most corners of the globe.  This makes it very difficult for uninfected populations to escape the spread of chytridiomycosis.  Scientists are also concerned with resistant individuals carrying Bd to susceptible populations, furthering spread of the disease.  The main hope is that the number of infected amphibians in a population doesn’t cross a threshold in which recovery is nigh impossible [8].  Even though the disease can infect about a third of amphibian species, the other hope is that Bd doesn’t evolve mechanisms to infect other amphibian species as well.  Only time and further study can tell if there is hope for the future for amphibians.  In the meantime efforts focus on how resistance can be attained for susceptible species, and preserving infected species from total extinction.  The best thing that we can do, as people, is to try to prevent the spread of Bd ourselves when we are near wetland areas, where we can transfer chytrid fungi to the environments in which amphibians inhabit.  Maybe with luck, nature will help us find a way to these vulnerable amphibians

References:

1.     Longcore, J. E., Pessier, A. P., & Nichols, D. K. (1999). Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia, 219-227.

2.     Gascon, C. (2007). Amphibian conservation action plan: proceedings IUCN/SSC Amphibian Conservation Summit 2005. IUCN.

3.     Berger, L., Hyatt, A. D., Speare, R., & Longcore, J. E. (2005). Life cycle stages of the amphibian chytrid Batrachochytrium dendrobatidis. Diseases of aquatic organisms, 68, 51-63.

4.     Padgett-Flohr, G.E. (2007). "Amphibian Chytridiomycosis: An Informational Brochure" (PDF). California Center for Amphibian Disease Control. Retrieved 12 November 2015.

5.     Voyles, J., Young, S., Berger, L., Campbell, C., Voyles, W. F., Dinudom, A., ... & Speare, R. (2009). Pathogenesis of chytridiomycosis, a cause of catastrophic amphibian declines. Science, 326(5952), 582-585.

6.     Woodhams, D. C., Ardipradja, K., Alford, R. A., Marantelli, G., Reinert, L. K., & Rollins‐Smith, L. A. (2007). Resistance to chytridiomycosis varies among amphibian species and is correlated with skin peptide defenses. Animal Conservation, 10(4), 409-417.

7.     Ramsey, J. P., Reinert, L. K., Harper, L. K., Woodhams, D. C., & Rollins-Smith, L. A. (2010). Immune defenses against Batrachochytrium dendrobatidis, a fungus linked to global amphibian declines, in the South African clawed frog, Xenopus laevis. Infection and Immunity, 78(9), 3981-3992.

8.     Vredenburg, V. T., Knapp, R. A., Tunstall, T. S., & Briggs, C. J. (2010). Dynamics of an emerging disease drive large-scale amphibian population extinctions. Proceedings of the National Academy of Sciences, 107(21), 9689-9694.