Fungal infection has sublethal effects in a lowland subtropical amphibian population

Abstract

BackgroundThe amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), has been implicated as a primary cause of decline in many species around the globe. However, there are some species and populations that are known to become infected in the wild, yet declines have not been observed. Here we conducted a yearlong capture-mark-recapture study and a 2-year long disease monitoring study of northern cricket frogs, Acris crepitans, in the lowland subtropical forests of Louisiana.ResultsWe found little evidence for an impact of Bd infection on survival; however, Bd infection did appear to cause sublethal effects, including increased capture probability in the field.ConclusionsOur study suggests that even in apparently stable populations, where Bd does not appear to cause mortality, there may be sublethal effects of infection that can impact a host population’s dynamics and structure. Understanding and documenting such sublethal effects of infection on wild, seemingly stable populations is important, particularly for predicting future population declines.Electronic supplementary materialThe online version of this article (10.1186/s12898-018-0189-5) contains supplementary material, which is available to authorized users.

Effects of latitudinal, seasonal, and daily temperature variations on chytrid fungal infections in a North American frog

“…These observations fit well with those of other field studies that have detected strong seasonal patterns in Bd dynamics with prevalence and/or load increasing during the cool months and dropping off during the warmest months of the year (Kriger and Hero 2006, Sapsford et al 2013, Brannelly et al 2018. In Louisiana, where cricket frogs can be sampled year-round, Bd prevalence was higher from late fall through early spring than during the summer to early fall months, and across latitudes, we saw a decline in prevalence and pathogen load from spring through early fall.…”

“…In Louisiana, where cricket frogs can be sampled year-round, Bd prevalence was higher from late fall through early spring than during the summer to early fall months, and across latitudes, we saw a decline in prevalence and pathogen load from spring through early fall. These observations fit well with those of other field studies that have detected strong seasonal patterns in Bd dynamics with prevalence and/or load increasing during the cool months and dropping off during the warmest months of the year (Kriger and Hero 2006, Sapsford et al 2013, Brannelly et al 2018. We saw less monthly variation in Ohio than in our lower latitude field sites, perhaps because temperatures in the northeastern United States do not often get high enough in the summer to kill Bd directly and/or to enable the cricket frog's immune system to resist or tolerate infection (Sonn et al 2017).…”

“…However, movement between microhabitats and changes in posture and orientation allow these animals to modulate heat gain and loss and can lead to differences between environmental temperatures and body temperatures. A previous mark-recapture study (Brannelly et al 2018) supports the idea that infection with Bd can affect the behavior of A. crepitans and other studies have found correlations between other (namely reproductive) behaviors in Bd-infected frogs as well (e.g., An and Waldman 2016). If so, protection from infection and disease could be dependent upon microhabitat use and host behavior.…”

“…While some Bd exposure studies suggest that infection with Bd can lead to a decline in body condition (e.g., in A. crepitans, Sonn et al 2017) and some field studies find lower body condition in infected individuals (e.g., Richards-Zawacki 2010), other exposure experiments (e.g., in A. crepitans, Robak and Richards-Zawacki 2018) and field studies (e.g., Murray et al 2009, Whitfield et al 2012, but see Richards-Zawacki 2010) have found no relationship between body condition and Bd infection. However, given evidence for subclinical effects of Bd infection in one northern cricket frog population (Brannelly et al 2018), the absence of obvious disease in the animals captured during our field study should not necessarily be interpreted as the absence of an impact of infection on the health of these populations. We also did not observe dead or moribund animals or clinical signs of chytridiomycosis (see Voyles et al 2007) during our field study, suggesting that animals carrying Bd infections may not commonly have been suffering from disease symptoms.…”

“…We also did not observe dead or moribund animals or clinical signs of chytridiomycosis (see Voyles et al 2007) during our field study, suggesting that animals carrying Bd infections may not commonly have been suffering from disease symptoms. However, given evidence for subclinical effects of Bd infection in one northern cricket frog population (Brannelly et al 2018), the absence of obvious disease in the animals captured during our field study should not necessarily be interpreted as the absence of an impact of infection on the health of these populations.…”