• Dirk

Fighting an emerging invasive pathogen from Asia

We live in, to say the least, interesting times. We have a pandemic at hand, which causes havoc in many countries around the world. Hundreds of thousands infected and a global death toll of many thousands. At least that is what we know at the moment. But do you know that there is another ongoing pandemic, a disease epidemic that has spread across multiple continents?



In that pandemic many millions of individuals got infected, and millions died. We do not even know the full numbers. At first, the spread of the disease went unseen, many deaths went unrecognized, until a few concerned people sounded the alarm. But, the pandemic was there already. The pathogen had already reached all continents and was further spreading, causing one of the most devastating pandemics known to us. Because the pathogen was new, because the infected were naïve to it, they had no immunity to it. The spreading of the pathogen happened quickly within a group. On a local scale, within less than 2 weeks, almost every single individual was infected (infection rate 99%). Mortality rates were very high and recovery was rarely observed. Whole populations vanished and whole regions became quiet. Still, even today, the most devastating pandemic the world has seen goes largely unrecognized and that even despite far reaching impacts that pandemic might have on your lives. You now are wondering: How is that possible?


The pathogen came from Asia and likely from the Korean Peninsula. Wait – The Korean Peninsula? Not Wuhan, China? Yes. I am not talking here about the Corona virus. I am telling the story of a pathogenic fungus, one of the most devastating pathogens known to us. The pathogen is called Batrachochytrium dendrobatidis or Bd, is infecting amphibians and causes a disease called chytridiomycosis, we could call it ChM-87, as it was first discovered in 1987. Now, more than 20 years later, it is touching all kinds of amphibians on all continents they inhabit, and is a still on-going but largely ignored global pandemic.


from O'Hanlon et al. 2018


Many amphibian species have been lost. Lost, because they were not prepared for that pathogen. Lost, because they had no strategy to cope with a pathogen they have never encountered before. Lost, because the pathogen found ideal conditions in many regions of the world to thrive and infect. In many cases, 14 days after first symptoms were observed adults died! Sounds familiar? There is not much of a difference between a wildlife pathogen and a human pathogen. If our immune-system has not be challenged by a pathogen, it has no antibodies yet and cannot fight the new pathogen. As the human population is widespread a pathogen like SARS-Cov2 can easily find a host and can spread very quickly. A wildlife pathogen might spread more slowly, but it does spread whatsoever.



Bd has e.g. taken from 1987 to 2006 to spread from Costa Rica to Panama, but with the help of humans it jumps, as any other pathogen can jump and as we can see currently in the Corona-Virus pandemic. It jumps from Wuhan China, to Ischgl Tirol, and from there to many other countries in a matter of days. Once the pathogen has been introduced its spread rate is highly linked to the host density. The higher the density of hosts, the higher its rate of spreading.



In such a scenario, super-spreaders are an enormous risk. They are contagious individuals which are in contact with many others, infecting them and making them new vectors of the pathogen. These super-spreaders stand out of the crowd by having more contacts to others than a normal individual would have, e.g. religious gurus, barmen, or in nature, aggressive males, fighting off other males to mate with a maximum number of females. Those super-spreaders are extremely important to control to stop or at least reduce pathogen spread.


Fighting for amphibians, mitigating the effects of the pathogen Bd, and saving populations has been the aim of hundreds of disease ecologists around the world. It is impossible to treat wild animals with medications and we cannot simply use fungicides to kill Bd in a wetland or pond, as many other benign fungi would be killed along side. We used quarantine to distance infected individuals from others, particularly in zoos and we had some success with that strategy. Many other strategies failed.


We were also able to show that zooplankton in lakes can protect amphibians by eating the infective stages of that Bd fungus. However, not all plankton species can feed on the pathogen and zooplankton is particularly sensitive to a range of factors, including increasing temperatures due to climate change, disturbance due to chemical pollution, changes of nutrient levels and many more. The human influence is everywhere, impacting on biodiversity and also on zooplankton. With increasing human influence the capacity of zooplankton to protect amphibians is reducing.



Microbial and disease ecologists have also teamed up to understand what the role of microorganisms are to fight pathogens. Microorganism live everywhere and most of them are benign and actually are little helpers in many different ways. On amphibian skin they may be able to fight pathogens, but we have not yet fully understood how. They either secret metabolites which have antifungal properties, hamper or stop the pathogen invasion of an amphibian skin by building a barrier called mucus, or simply feed on the pathogen.


from Campbell et al. 2019 - https://doi.org/10.3389/fmicb.2019.01245


But also microbial communities are impacted by human induced changes and a change may open the door for other pathogens to enter, if that is your digestive tract, your lungs or amphibian skin does not matter to a pathogen. Changing nature is a risky business for everyone.


Despite our meticulous research, we still see many dead amphibians at the shores of mountain lakes, we still realize that populations vanish and tropical forest become bewilderingly quiet, we observe the amphibian hecatombs around the world. Finding a cure is difficult in a human context, which means for ONE species, and it is even more difficult in the wild with millions of different species and at least 8000 species of amphibians.



You may now say, its only amphibians, or even wonder what amphibians are. It is not important, or is it? Humans are so much more important, so why care with those cold, gluey little critters? As an ecologist I can tell you this: Each and every species on our planet has its small job to do. It has its small function to fulfill in an incredibly complex machinery called ecosystem. Any loss of a species increases the stuttering of that machinery biodiversity, impacted by pollution, climate change, and destruction. Each stuttering increases the risks for human well-being. Each stuttering could lead to conditions that allow pathogens to thrive, and with increasing pathogen numbers the probability of a transfer to the human population increases, as it has in amphibians, but also in other species groups like bats and trees.


Pathogens are an important selective force, they are natural, but they can cause havoc when introduced to populations not having encountered that pathogens before and then they can also mutate. Mutate into something more virulent, more deadly. This can currently be seen in the human population around the world and we are far away from the numbers we observe during disease outbreaks in the wild. Loss of any being is tragic, especially when it goes unrecognized. The increasing human impact on nature is not only increasing the risk for pathogens in amphibians or wildlife, but actually also zoonoses, pathogens which can jump from wildlife to a human host.


Amphibians, bats, bees, trees and all the other species groups play crucial roles. Even if you may not know much about them, they do a daily job from which you profit, which makes it possible that you can live. Respecting nature means respecting your life and that of every being on the planet. Loose that respect and you will quickly learn that humans are not that great and may never be great again.

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