The Great Global Die-Off: Frogs and Lymphocytes
By Roberta Attanasio
Frogs and other amphibians – salamanders and caecilians – have been declining worldwide during the past few decades at an alarming rate. According to a June 2012 assessment by the International Union for the Conservation of Nature and Natural Resources (IUCN), about 41 percent of amphibian species are at risk of extinction, and some are already extinct.
Like many other inhabitants of our planet, amphibians have been hit hard by climate change and habitat loss – and not only. Amphibians have also been decimated by the spread of chytridiomycosis, which is defined by the IUCN as the single most devastating infectious disease of vertebrate animals. In a historical article published in 2008, David Wake and Vance Vredenburg state: “A general message from amphibians is that we may have little time to stave off a potential mass extinction.”
The deadly chytridiomycosis is caused by Batrachochytrium dendrobatidis (Bd), a fungus found on all continents except Antarctica. According to epidemiological evidence described in an article published in 2004, the Bd fungus was present in African clawed frogs (Xenopus laevis) in their native South Africa as early as the mid-1930s. African clawed frogs infected with Bd do not show any clinical sign of disease and can carry the fungus for long periods of time without dying. Therefore, these frogs are Bd carriers and transmit the fungus to vulnerable species.
Worldwide dissemination of Bd started in the 1930s because of the international trade of African clawed frogs. From the 1930s to 1950s, large numbers of these frogs were caught in the wild in southern Africa and exported across the world, mostly for use in the first human pregnancy tests and scientific research. Results from a study recently published in the journal PLOSone confirm that Bd was present as a stable, endemic infection in Xenopus populations in Africa prior to their worldwide distribution, which likely occurred via international live-amphibian trade. Vance Vredenburg, lead author of the study, said: “Today, these frog populations are often found in or near urban areas, probably because hospitals released them into the wild when new pregnancy testing methods were invented in the 1960s.”
Bd proliferates in skin cells and rapidly kills amphibians by disrupting skin function – it impairs the skin’s ability to absorb electrolytes. A few years ago, a group of scientists from Australia found that, in diseased frogs, the skin’s ability to take up sodium and potassium ions from the water decreases by more than 50 percent, leading to a sharp decline of these ions in the blood and eventually causing cardiac arrest and death.
Despite these recent discoveries, there was an unanswered question: “Why is the amphibian immune system so inept at clearing the fungus?” Now, a team of investigators from Vanderbilt University reports, in an article published in the journal Science and entitled “The Invasive Chytrid Fungus of Amphibians Paralyzes Lymphocyte Responses” (October 18, 2013), that Bd paralyzes the immune system of the affected amphibians by releasing a toxic factor.
The researchers found that white blood cells called macrophages and neutrophils, which are responsible for the first line of immune defenses, are not affected by the toxic factor. However, the toxic factor inhibits the proliferation of other types of white blood cells called lymphocytes. In addition, the toxic factor induces lymphocyte cell death. Lymphocytes are responsible for the adaptive function of the immune system, which is essential in protecting against infections when the first line of defenses are not sufficient to control the spread of micro-organisms. Because of lymphocyte loss, frogs are unable to eradicate Bd before it induces irreversible damage to their skin.
Results of the study suggest that the toxic factor is not a protein, as it is resistant to heat and to the action of proteases (enzymes that cut proteins into pieces). However, drugs that interfere with cell wall synthesis reduce the killing of lymphocytes. In addition, the zoospore — an immature form of the fungus that lacks a cell wall — does not produce the toxic factor. Thus, the toxic factor is likely to be a component of the fungus cell wall.
Louise Rollins-Smith, senior author of the study, said: “The new findings suggest the possibility that toxic factors — in addition to acting locally to inhibit the immune response — might get into the circulation and have neurotoxic effects. Fungal infection causes rapid behavioral changes — frogs become lethargic and start to crawl out of the water — suggesting that even though the fungus stays in the skin, the toxic material is having effects elsewhere.”
For the past few years, researchers have been trying different approaches to treat existing Bd infections, as for example administering anti-fungal drugs to tadpoles or using “probiotic” bacteria that naturally secrete anti-fungal compounds. However, albeit a leading one, Bd is only a contributor to the global decline of amphibian population.
The fact that frogs are declining so rapidly is very disturbing. The interesting fact that I read is how the African clawed frogs are not affected by this fungus. Which makes me wonder what is it about the African clawed frogs adaptive immune system that prevents the fungus from killing them. Could it be that the other frogs adaptive immune system are lacking something that the African clawed frogs have?
I can also see how this is such a problem because of how difficult it would be to treat all the wild amphibians in the natural environment with antifungal medications.The amount of work that would took is unimaginable! Although I remember learning in Microbiology of how certain bacteria can kill fungus which makes me wonder if researchers are looking into how bacteria could be use to help treat a population.
I learned how important frogs truly were to the environment and science upon taking animal biology. According to amphibianark.org, the American bullfrog is also resistant to Bd. The information in the article above regarding the lifespan of the African clawed frogs is interesting in that they do not show any clinical signs of Bd and can carry the fungus for a long time before death occurs. But that specific point is key – their death is prolonged, not avoided. I would expect that some of the underlying reasons that African clawed frogs and American bullfrogs are not affected due to reasons related to their ion stabilization. I say this because it is known that the fungus impairs the frog’s ability to obtain and maintain the essential ions potassium and sodium. Both of these ions are pertinent for appropriate muscle and heart function (but most importantly heart function). This may be a superficial approach at finding a solution, but I would first study the differences in how the carriers and the frogs that are susceptible regulate both potassium and sodium.
Another solution to this fast spreading fungus are “frog farms”. These “frog farms” are a quick, yet risky, short term solution to the mass extinction of frogs. They are great in that the farms can foster an environment for healthy frogs to reproduce safely. That being said, they are also risky in that if the farms become infected, the fungus would likely spread quickly.
Can you clarify what you mean by “death is prolonged, not avoided”?
By prolonged I meant that the indicated species can live longer after being infected by the fungus. I may have misinterpreted the above article, but based on the quote from above, “African clawed frogs infected with Bd do not show any clinical sign of disease and can carry the fungus for long periods of time without dying.”, I assumed that the carrier species responds to the fungus more effectively than others given they can carry the fungus for a long time. I also inferred from the quote that because their immune system responds more effectively, the frogs likely take a longer period of time to die as a result of their exposure. Please correct me if I misinterpreted.
I’m sorry for the confusion. I meant that the African clawed frogs would eventually die as a result of complications from the fungus. Many comments of the article seem to allude that the African clawed frogs are resistant to the fungus. I simply wanted to stress that (based on inferences made from the article) the African clawed frogs are not just carriers. Their immune system becomes compromised as well. It is also important to note that while the African clawed frog’s immune system succumbs to the fungus, it does so at a much slower pace than other amphibian species.
As Tylah pointed out, amphibians are important indicators of the changing conditions in the environment. It is possible that an increase in temperature due to climate change, as well as an increase in habitat loss due to pollution and habitat destruction, have facilitated fungal proliferation. What is interesting about Bd is that it produces flagellated spores that can spread in aquatic and moist environments. In “Survival of Batrachochytrium dendrobatidis in Water: Quarantine and Disease Control Implications, ” researchers found that Bd can survive outside of a host in tap, deionized, and lake water for weeks. This knowledge is critical in creating strategies to contain the spread of the Bd in the amphibian habitats.
From an immunological perspective, cells likely targeted by the toxic factor are cytotoxic T cells, T helper 1 cells, and B cells, which are all used by the adaptive immune system during intracellular infections. Studies on the immune response on Xenopus laevis have been conducted and found that antimibrobial peptides are released in the skin as part of the innate immune response. However, little is known about the adaptive immune response against Bd. As the article mentions, X. laevis has been able to live with the fungus without dying, thus making this species a good model for investigations on the immune response. Nevertheless, it is important to remember that these frogs are in fact infected by Bd and research to prevent infection should be conducted.
Why T helper 1 cells and not T helper 2 cells? And…. do frogs have these two subsets of T helper cells?
Frogs have both subsets of T helper cells. T helper 1 cells help clear intracellular infections through the activation of macrophages and B cells. T helper 2 cells are involved in the activation of B cells to make antibodies and fight an extracellular infection. If BD is proliferating inside of skin cells, the former response would be initiated by the adaptive immune response instead of the latter.
I first heard about the decline of frogs in Marine Biology and their importance in world. Tadpoles keep the water clean by consuming algae, frogs eat vectors like mosquitoes, frogs are food for many predators, and frogs are important for scientific research. So I’m glad there are scientific experiments being done trying to fix this problem. In an experiment scientists added an anti-fungal bacterial species, Janthinobacterium lividum, on frogs and found that it prevented death from Bd. Janthinobacterium lividum showed that the skin microbes are part of the frog’s immune system and the relations of the skin microbes can prevent death from Bd.
“Janthinobacterium lividum showed that the skin microbes are part of the frog’s immune system and the relations of the skin microbes can prevent death from Bd” This is a very interesting concept. Can you clarify and elaborate on it?
Janthinobacterium lividum are skin microbes that are gathered from frog skins and cultured in a lab. So amphibians with extremely high amounts of Janthinobacterium lividum showed the best immunity to Bd. Amphibians with more skin microbes like Janthinobacterium lividum creates a barrier and a stronger innate immune systems against pathogens like Bd.
I found it very interesting on how ” high amounts of Janthinobacterium lividum showed the best immunity to Bd.” After reading your post I did more research and have come across this article called “Skin Fight: Could Bacteria Carried by Amphibians Save Them from Extinction?” by Erica Rex in which she wrote about how researchers have found this symbiotic bacteria that lives on amphibians’ skins and protects them from Bd. They have found that “Wild mountain yellow-legged frogs and redback salamanders host naturally occurring bacteria whose metabolites are toxic to Bd.” Amazing isn’t it! So what they did was captured and bred these mountain yellow-legged frogs,collected their eggs and found that the bacterium Janthinobacterium lividum, which they cultured from wild frogs,was protective against Bd,as you said. It is great to know researchers are making progress and that there is hope to help a population of frogs. This also answers my question on whether scientist were looking into how bacteria could help kill fungus.
It is essential that scientists get this infectious disease under control. Frogs are an important species to scientists and researchers. The skin of frogs can tell scientists and researchers a lot about changes to the environment. Frogs can become affected even by the slightest change to their environment. The skin of frogs is very thin and they can easily absorb dangerous toxins through it. The skin is important in respiration and temperature control. The reason Batrachochytrium dendrobatidis or Bd is dangerous is because it disrupts one of the important functions of the skin. Bd makes it difficult for frogs to absorb important ions into the skin which is life threatening to the frogs. I think the African clawed frogs may have developed an immunity to the toxic Bd. Scientists should study the African clawed frogs more closely and see if they can find the exact reason why the Bd does not affect these frogs and devastates other frog populations. They should take a close look at what the immune system of an African clawed frog does when the Bd fungus infects them. Scientists know that the toxin does not affect the innate immune system but affects the adaptive immune system. This is bad because the frogs can never develop memory of the Bd toxin. This means that the innate immune system may become overwhelmed by the fungus and the adaptive immune system will always take a long time to “kick in” because it will not be able to recognize the Bd. Also, the adaptive immune system never gets a chance to really fight the fungus because the toxins prevent the frogs from making essential cells used to fight off infections. Scientists need to find a better and more effective way at treating the frogs who have been affected or could possibly be affected by the Bd. There is no way that scientists can treat every frog that is infected by the disease. Even if scientists do find a medication that is extremely effective at inhibiting the cell wall synthesis of the fungus, there needs to be a way to get it out there in the environment.Unless scientists can find a way to treat large numbers of frogs or why the African clawed frogs are not affected negatively by the Bd fungus, then there will be a mass extinction of frogs.
“This is bad because the frogs can never develop memory of the Bd toxin” – this is a very important point. What is known about memory responses in frogs?
From what I have been reading and looking up, it seems that frogs’ memory responses are similar to humans. If a frog never encounters a pathogen, it will not have immunity to the bacteria/virus. However, if a frog has encountered a pathogen before, the response time for the immune system will be less than the first time it encountered a pathogen. Not having immunity means that it will take longer for the immune system to fight off the infection and can possibly increase the chances of death.
Last year I read an interesting story on ancient Russians putting frogs in buckets of milk so the milk did not go sour, and now scientists discovered that there are a lot of antibiotic substances in the skin of the Russian Brown frogs, so probably because of the frogs bacteria don’t grow in the milk. It’s sad to think that if the frogs are disappearing, a new source of antibiotics will be disappearing too. There is a lot to lose here. I hope we can keep all the frogs in the world.