Environmental and physiological correlates of the severity of clinical signs of snake fungal disease in a population of pigmy rattlesnakes, Sistrurus miliarius.

Ciera M. McCoy,Craig M. Lind,Terence M. Farrell

doi:10.1093/conphys/cow077

Abstract

In the past decade, snake fungal disease (SFD) has been identified as an emerging threat to snake populations throughout the eastern USA. Snake fungal disease is caused by the fungus Ophidiomyces ophiodiicola. Little is known regarding the environmental or physiological variables that affect host vulnerability and O. ophiodiicola virulence in wild snake populations. Understanding the intrinsic and extrinsic factors that correlate with infection severity is a key first step in understanding host-pathogen dynamics. Host vulnerability may vary seasonally as a result of thermal conditions or energetic trade-offs, and pathogen growth rates or dispersal may be tied to seasonal trends in climate. To determine whether season, environmental temperature or energetic trade-offs associated with life-history stage influence an individual's susceptibility to infection, we monitored the severity of clinical signs of SFD, surface air temperature, reproductive status, body condition and serum complement activity (plasma bactericidal ability) in free-ranging pigmy rattlesnakes, Sistrurus miliarius, over the course of 18months. Seasonal increases in the severity of clinical signs of SFD were correlated negatively with monthly air surface temperature and the mean body condition of the population. Bactericidal ability varied seasonally, but pigmy rattlesnakes suffering from active SFD infections did not exhibit deficits in innate immune function. Infected snakes were in significantly lower body condition when compared with the general population, but seasonal patterns in the mean body condition of the population were not driven by seasonal patterns of infection severity. Our results highlight the potential importance of the thermal environment and energetic status in determining infection severity and outcomes and the need for managers and researchers to consider seasonality of symptom presentation when the goal is to identify the prevalence or incidence of SFD in populations.

Highlights

Fungal pathogens have emerged as a major conservation concern in recent decades (Sutherland et al, 2014)
Our results support the hypothesis that the severity of clinical signs of snake fungal disease (SFD) varies seasonally and in relationship to seasonal variation in air temperature and the mean energetic status of the population (Fig. 2)
Peaks in the severity of clinical signs of SFD in both males and females occur in months when mean energetic status and surface air temperature are lowest (Fig. 3)

Summary

Introduction

Fungal pathogens have emerged as a major conservation concern in recent decades (Sutherland et al, 2014) Diseases such as white nose syndrome (WNS) in bats and chytridiomycosis in frogs have led to widespread population declines and a loss of global biodiversity (Blehert et al, 2009; Fisher et al, 2012). Description of environmental and physiological correlates of infection severity in free-living populations is a first step towards identifying the factors that determine the spread and impacts of SFD and can inform effective management and conservation strategies (Berger et al, 2004; Rachowicz et al, 2005; Kriger and Hero, 2007; Kilpatrick et al, 2010)

Methods

Results

Conclusion