The fungus Batrachochytrium dendrobatidis (Bd) has been blamed for many declines in amphibian populations. One of these cases was the Eleutherodactylus frogs from Puerto Rico. However, a new paper shows one of the major problems with the current research on the fungus: lethality is assumed even when the data shows otherwise.
The paper by Langhammer et al. (A Fungal Pathogen of Amphibians, Batrachochytrium dendrobatidis, Attenuates in Pathogenicity with In Vitro Passages. PLoS ONE. 10.1371/journal.pone.0077630) worked on the question of the pathogenicity of Bd once it has been frozen in the lab. This is a worthwhile question since it is easier to work with lab samples, that could have lost some pathogenicity in the lab, than from fresh field samples.
The study found that for E. coqui:
[…] there was no significant difference in mortality between the three treatment groups, including the controls (Log-rank test, p=0.37). Only 1 frog died during the experiment, from the JEL427-P39 group. The frog may have succumbed to chytridiomycosis given its relatively high pathogen load (12,816 zoospore genomic equivalents), but all other Bd-exposed frogs cleared infection within 80 days. (emphasis mine)
A single frog died. This is in no way evidence that would support saying that the cause may have been the infection. Maybe the animal was too old, after all they kept them for a year in the lab before the tests. This is my major annoyance, that no matter what the data shows, they insist that Bd can kill an E. coqui frog. What if it can kill one? At the most, this would call for a large-scale study to determine probability of lethality in the species. But from this data we can conclude that Bd, both fresh and lab-stored, is not lethal to E. coqui.
The data showed that E. coqui cleared the infection from both strains used in less than two months, while the other species, Atelopus zeteki, did not:
As a species that does die from Bd, A. zeteki individuals started dying less than a month and a half after infection:
The authors justify talking about lethality in E. coqui because they found in another paper two animals in the field that apparently died from the disease. Two animals from a species with thousands of individuals per hectare. This hardly seems like something to worry about. At of today, Bd is still listed as a threat in the Red List entry of the species. Several years ago I tried to make the case to change this, but was met with a wall of credulity to the claim and incredulity to what the data actually showed. I eventually gave up.
Perhaps it is time to start testing the assumption that Bd is lethal but that it can be treated as just a disease in some species. Then, we can start focusing on what determines this difference between species and discard it as a threat in the species that resist it.