Epidemics vary in their dynamics. Sometimes they spit fast and disappear gradually; sometimes they rise slowly; sometimes they have several waves. What they all eventually do is an end, usually without either the pathogen or the host dying out. But we do not usually understand how or why.
Chytridiomycosis is an epidemic that has been killing amphibians around the world for at least a decade. It is caused by a fungal pathogen called Batrachochytrium dendrobatidis (Bd) which can lead to deformities of the limbs and skin anomalies. Because amphibians absorb water and electrolytes through their skin rather than through their mouth, such abnormalities can be fatal. But now we have some evidence that, as with previous epidemics, Bd is not a one-way ticket to extinction.
Between 2004 and 2007 Bd decimated frog populations in Panama. However, about five years later, the frog populations recovered again. Bd was still there, though it was not nearly as widespread. Scientists collected frog and fungal specimens from the onset of the 2004 outbreak and after the recovery in 201
The later Bd samples were no less pathogenic. They grew at the same rates, reached the same size and produced a comparable number of spores. They had the same responses to inhibitory factors in amphibian skin secretions and inhibited amphibian immune cells to the same extent. And when two different frog species were exposed to the batches of 2004 Bd and 2013 Bd in the lab, all the frogs became infected and died.
The researchers looked at how well frog skin secretions can inhibit Bd . The inhibitory effect differed between species and place. But frogs collected in 2013 – after the epidemic subsided – had more inhibiting material on their skin than frogs that had been captive since the outbreak.
Since there was no difference in the fungus, the researchers concluded that "postpones host resistance could contribute to the recovery of some amphibian species." But they have not performed any genetic or biochemical studies to explore the fundamentals of this shift.
Developing countries are currently experiencing a resurgence of mumps, especially in vaccinated populations. The virus did not mutate to avoid the vaccine, and the vaccine is still effective; The effect has dropped after 27 years. Now we know that we need boosters in adulthood. Discovering the factors that influence the different lines of development that can trigger different epidemics – even amphibious epidemics – can provide us with the information we need to implement public health measures like these. It is an added bonus to understanding how to preserve our amphibians.
Science 2018. DOI: 10.1126 / science.aao4806.1720692115 (About DOIs).