The Spread of an Amphibian Epidemic, Bd in Mesoamerica

Male (top) and female (bottom)
Golden Toads, Incilius periglenes
(Savage, 1967). Monteverde, Costa
Rica. Described by science in 1967
it is likely now extinct, the victim of
 Bd. This was a lower montane
rainforest species. JCM
Evidence suggests that we are seeing the start of the 6th mass extinction event in Earth's history, and some of the evidence comes from amphibians, 40% of which are considered endangered. Hypotheses have implicated habitat destruction, overexploitation, pollution, and climate change in the loss of amphibians, but the infectious fungal disease, chytridiomycosis, has been the main suspect in the disappearance of many different species in many different families in geographically distant locations. The chytridiomycete fungus Batrachochytrium dendrobatidis (Bd) has a flagellated infective life stage called the zoospore that penetrates the skin of amphibians causing hyperkeratosis, this results in a loss of skin function, osmoregulatory failure, and death. First reported in 1999, Bd has been shown to be closely associated with the collapse of amphibian populations in Australia, Panama, California, and Peru and has been linked to amphibian declines that occurred decades ago. Bd is unusual because multiple host species in at least one locality have been extripated before density-dependent factors could slow the spread of disease. Now, Tina Cheng and colleagues (2011) present evidience on how Bd has spread using museum specimens from vanished populations. Using two well-studied cases of amphibian decline in Mesoamerica: the decline and disappearance of anurans from Costa Rica’s Monteverde Reserve in the late 1980s, and the decline and disappearance of plethodontid salamanders from the mountains of southern Mexico and western Guatemala in the 1970s and 1980s, the authors test retroactively whether Bd emergence was linked to earlier declines and extinctions. They use a noninvasive PCR sampling technique that detects Bd in formalin-preserved museum specimens and found Bd using the PCR technique in 83–90% (n = 38) of samples that were identified as positive by histology. They examined specimens collected before, during, and after major amphibian decline events at established study sites in southern Mexico, Guatemala, and Costa Rica and found a pattern of Bd emergence coincident with decline at these localities. The absence of Bd over multiple years at all localities followed by the concurrent emergence of Bd in various species at each locality during a period of population decline. The geographical and chronological emergence of Bd at these localities suggests a southward spread from southern Mexico in the early 1970s to western Guatemala (1980s/1990s), and to Monteverde, Costa Rica (1987); thus the authors found evidence of a historical “Bd epidemic wave” that began in Mexico and subsequently spread to Central America. The full article is available on-line, follow the link below.



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