Geographic and Individual Determinants of Important Amphibian Pathogens in Hellbenders (Cryptobranchus Alleganiensis) in Tennessee and Arkansas, Usa

Abstract

Wildlife diseases are a major threat for species conservation and there is a growing need to implement disease surveillance programs to protect species of concern. Globally, amphibian populations have suffered considerable losses from disease, particularly from chytrid fungi ( Batrachochytrium dendrobatidis [Bd] and Batrachochytrium salamandrivorans [ Bsal]) and ranavirus. Hellbenders ( Cryptobranchus alleganiensis) are large riverine salamanders historically found throughout several watersheds of the eastern and midwestern United States. Populations of both subspecies (Ozark hellbender, Cryptobranchus alleganiensis bishopi, and eastern hellbender, Cryptobranchus alleganiensis alleganiensis) have experienced precipitous declines over at least the past five decades, and emerging pathogens are hypothesized to play a role. We surveyed Ozark hellbender populations in Arkansas (AR) and eastern hellbender populations in Middle Tennessee (MTN) and East Tennessee (ETN) for both chytrid fungi and ranavirus from swabs and tail tissue, respectively, from 2011 to 2017. Overall, we detected Bd on hellbenders from nine out of 15 rivers, with total prevalence of 26.7% (54/202) that varied regionally (AR: 33%, 28/86; MTN: 11%, 4/36; ETN: 28%, 22/80). Ranavirus prevalence (9.0%, 18/200) was comparatively lower than Bd, with less regional variation in prevalence (AR: 6%, 5/85; MTN: 11%, 4/36; ETN: 10%, 8/79). We did not detect Bsal in any hellbender populations. We detected a significant negative correlation between body condition score and probability of ranavirus infection (β=−0.13, SE=0.06, 95% CL: −0.24, −0.02). Evaluation of infection load of positive individuals revealed different trends than prevalence alone for both ranavirus and Bd, with Middle Tennessee having a significantly greater average ranaviral load than both other regions. We documented a variety of lesions that likely have multiple etiologies on hellbenders located within all geographic regions. Our data represent a multiyear pathogen dataset across several regions of C. alleganiensis, and we emphasize the need for continued pathogen surveillance.

The fungal pathogen Batrachochytrium salamandrivorans is not detected in wild and captive amphibians from Mexico

“…The non-detection of Bsal in those areas suggests that to date, Bsal has not yet arrived to these areas. In agreement with our results, previous studies have not detected Bsal in the USA and Northern Mexico ( Bales et al, 2015 ; Klocke et al, 2017 ; Parrott et al, 2017 ; Newman et al, 2019 ; Hardman et al, 2020 ; Waddle et al, 2020 ; Hill et al, 2021 ), which could indicate that Bsal is still absent in North America, or that Bsal is at very low prevalence within these populations and has a low detection probability. Because the risk of this pathogen is high for amphibians and its arrival to America is imminent, we highlight the importance of continuous surveillance of Bsal in areas of potential risk and new areas where the pathogen arrival is more likely to occur.…”

“…Thus, Bsal surveillance directed to areas of high suitability for Bsal and high amphibian diversity could be a good strategy to take immediate conservation actions. Recent efforts to monitor the presence of Bsal in the USA and Northern Mexico have not detected the pathogen ( Bales et al, 2015 ; Klocke et al, 2017 ; Parrott et al, 2017 ; Newman et al, 2019 ; Hardman et al, 2020 ; Waddle et al, 2020 ; Hill et al, 2021 ). However, surveillance of Bsal presence in amphibians across Mexico is still lacking.…”

Dorsal Skin Microbial Assemblages in Free-ranging Hellbenders, Cryptobranchus Alleganiensis, Associated With Subspecies and Chronic Toe Lesions, but Not With Chytrid Fungal Infection.

“…We immediately rubbed a sterile cotton swab (Fisherbrand product# 23400111) over the dorsal skin surface for 30 seconds and placed it in a sterile 1.5 mL tube on dry ice for transport to a -80°C freezer. For individuals in AR only, we also obtained a swab for Bd load analysis for a concurrent pathogen prevalence study (see Hardman et al 2020b), as well as, lesion severity score (scored from 0-7) for toe lesions of unknown etiology (Hardman et al 2020a). All individuals sampled from AR for the microbiome analysis had toe lesions, but because of low sample size within AR, and subsequent inability to compare across all 8 lesion scores, we binned scores of 0-7 into three biologically relevant severity levels of mild (score 0-2; toe swelling only), moderate (3-4; toe swelling and some shortened) and severe (5-7; most toes shortened and/or missing).…”

Disease state associated with chronic toe lesions in hellbenders may alter anti-chytrid skin defenses

“…Chytridiomycosis (disease due to chytrid infection) has been hypothesized to contribute, but previous studies have been inconclusive in support of this. Bd has been detected in hellbenders as early as 1969 19 , and in the past decade, it has been found in both healthy and unhealthy hellbender populations ranging from 0 to 33% prevalence 20 – 24 . Chytridiomycosis has only been documented in captive and captive-raised populations with associated mortalities as high as 100% 25 , 26 .…”

“…Chytridiomycosis has only been documented in captive and captive-raised populations with associated mortalities as high as 100% 25 , 26 . Like Bd , ranavirus has largely unknown effects in wild hellbenders, having only been detected subclinically 20 , 22 , however, infection is significantly associated with a decrease in body condition 20 suggesting sublethal effects. One study showed that at least one strain of ranavirus can cause mortality in juvenile hellbenders 27 .…”