Deadly viruses don’t usually kill every person they infect, even when vast numbers of people die, as in the Spanish flu or Black Plague. Frog and salamander populations have been decimated by a fungal disease that seems to have stopped short of eliminating them entirely. New work reported in Science, by researchers at the Smithsonian Tropical Research Institute (STRI) in Panama, has assessed the epidemic in frogs caused by chytridiomycosis, and the broader implications for diseases that have the potential to be pandemics.
"Imagine a deadly disease that affects not only humans but other mammal species like dogs, cats, and cows," said STRI staff scientists Roberto Ibáñez, the in-country director of the Panama Amphibian Rescue and Conservation Project. "Chytridiomycosis kills off most of the individuals in many different species of amphibians, but for some species, it stops short of driving them to complete extinction." Learn more about the disease from the National Geographic video.
"We were lucky that Karen Lips, now at the University of Maryland in College Park and colleagues saw this epidemic coming into Panama from Costa Rica, and researchers were able to study both the frogs and the disease before, during and after the peak of the epidemic," Ibañez added.
Pathogens rely on their hosts for survival, so disease outbreaks often don’t wipe out the host species; they keep the pathogen going. The researchers have been able to track how amphibian species have changed to see whether resistance to the pathogen was emerging. They looked at the numbers in communities, the patterns of infection and virulence of the pathogen for decades.
"Because we have pathogen and host samples from before, during and after the epidemic, we can ask whether some frogs survived because the pathogen grew weaker through time, or because the frogs' immune systems or resistance increased through time," explained first author Jamie Voyles, disease ecologist at the University of Nevada, Reno.
The fungus that causes the disease, Batrachochytrium dendrobatidis, emerged from El Cope, Panama, in 2004, from El Valle in 2006 and from Campana National Park in 2007. In the five to thirteen years following the epidemic, the investigators saw that nine species were experiencing some recovery while the fungus was still present. Not all frog species have rebounded though, and some species have not yet been found.
The pathogen wasn’t growing more slowly, and it had not become less virulent; it was not responding to secretions from frogs in new ways either. The researchers saw that wild frog survivors of the epidemic had done a better job of inhibiting the growth of the fungus, much better than frogs in captive breeding programs. That
suggested that the wild frogs had built some resistance to the disease.
In the short video, a frog is shown getting swabbed for science.
"The Panama Amphibian Rescue and Conservation Project collected healthy frogs before the outbreak," Ibañez said. "We learned to breed them in captivity and are now releasing Atelopus varius in areas where the epidemic has passed, so it is extremely important for us to realize that the defenses of these frogs may be weaker than the defenses of frogs that survived the epidemic in the wild. Captive breeding programs must consider breeding and releasing frogs with stronger defenses, and testing their skin secretions against the fungus is one useful tool to see which frogs are more resistant."
It is vital to understand how disease transitions work--from outbreak, to epidemic, to coexistence--and our results have implications for a skyrocketing human population facing emerging diseases with the potential to cause global pandemics," concluded Ibañez.