The amphibian chytrid fungus Batrachochytrium dendrobatidis in Sweden

Abstract

The amphibian chytrid fungus Batrachochytrium dendrobatidis in Sweden

Emerging infectious disease is an important source of wildlife population declines and loss of biological diversity. Many of these emerging wildlife diseases are caused by fungi. The chytrid fungus Batrachochytrium dendrobatidis (Bd) infects amphibians and has caused severe population declines in all continents where amphibians occur. While Bd is widespread in southern and central Europe, its occurrence and distribution in northernmost Europe is unknown. Bd was first found in Sweden in 2010. To get a more complete picture of the distribution of the chytrid we conducted a larger survey during 2015 and 2016 centered round the areas of the original findings in southern Sweden and at five new sites in central Sweden. In total, we collected samples from 1144 amphibians from 40 localities. Bd was detected in 47.5 % of the 40 localities studied with an overall prevalence of 17%. In the southern sites Bd was found in all seven species studied, pool frog Pelophylax lessonae (63.64%) and green toad Bufotes variabilis (61.43 %) having highest prevalence. In central Sweden (Uppsala) Bd was found in three of five localities studied, representing the northernmost records of Bd in Europe. Here three species were sampled and the common toad Bufo bufo was the only species infected (prevalence 9.5 %). No individuals showing signs of chytridiomycosis were found in either of the areas. However, in inoculation trials using Bd isolated and cultured from green toads in southern Sweden, high mortality rates were found in infected B. bufo with the highest mortality in individuals from northern populations. The same pattern but with lower mortality rates was shown in Rana arvalis. The isolated strain of Bd belongs to the global pandemic lineage (GPL). An additional study showed that northern populations of R. arvalis have a lower variation in a major histocompatibility (MHC) class II gene –implicated to play a role in Bd resistance - than southern populations. These results indicate that Bd is much more widespread and common in Scandinavia than previously thought. They also suggest that high-latitude - amphibians may be more sensitive to infectious disease than those from more southern populations. This can be of considerable conservation concern if Bd is becoming more common at high latitudes.