Emerging Infectious Disease as a Proximate Cause of Amphibian Mass Mortality

Abstract

A newly discovered infectious disease of amphibians, chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis, is implicated in population declines and possible extinctions throughout the world. The purpose of our study was to examine the effects of B. dendrobatidis on the mountain yellow-legged frog (Rana muscosa) in the Sierra Nevada of California (USA). We (1) quantified the prevalence and incidence of B. dendrobatidis through repeat surveys of several hundred R. muscosa populations in the southern Sierra Nevada; (2) described the population-level effects of B. dendrobatidis on R. muscosa population abundance; and (3) compared the mortality rates of infected and uninfected R. muscosa individuals from pre-through post-metamorphosis using both laboratory and field experiments. Mouthpart inspections conducted in 144 and 132 R. muscosa populations in 2003 and 2004, respectively, indicated that 19% of R. muscosa populations in both years showed indications of chytridiomycosis. Sixteen percent of populations that were uninfected in 2003 became infected by 2004. Rana muscosa population sizes were reduced by an average of 88% following B. dendrobatidis outbreaks at six sites, but at seven B. dendrobatidis-negative sites, R. muscosa population sizes increased by an average of 45% over the same time period. In the laboratory, all infected R. muscosa developed fatal chytridiomycosis after metamorphosis, while all uninfected individuals remained healthy. In the field experiment in which R. muscosa tadpoles were caged at infected and uninfected sites, 96% of the individuals that metamorphosed at infected sites died vs. 5% at the uninfected sites. These studies indicate that chytridiomycosis causes high mortality in post-metamorphic R. muscosa, that this emerging disease is the proximate cause of numerous observed R. muscosa population declines, and that the disease threatens this species with extirpation at numerous sites in California's Sierra Nevada.

Deposition and accumulation of airborne organic contaminants in Yosemite National Park, California

“…High-elevation areas of the Sierra Nevada have experienced widespread declines in amphibian populations since the 1970s [5,6]. The most dramatic decline has been observed for the mountain yellow-legged frog (Rana muscosa), which has disappeared from more than 90% of its historic range in the Sierra Nevada [7].…”

“…The most dramatic decline has been observed for the mountain yellow-legged frog (Rana muscosa), which has disappeared from more than 90% of its historic range in the Sierra Nevada [7]. Several hypotheses have been proposed to explain these declines, including the introduction of nonnative fish species [8], elevated levels of ultraviolet radiation [9], recent emergence of the chytrid disease [5], and increasing pesticide use in adjacent agricultural areas [10][11][12]. Davidson and Knapp [13] suggested that multiple stressors may be involved, although upwind pesticide application appears to have had the greatest influence on amphibian declines in the Sierras.…”

“…For example, chlorpyrifos and endosulfan, two commonly used insecticides in the San Joaquin Valley, were found to be highly toxic to two species of Sierran frogs at environmentally realistic concentrations in surface water [12]. Pesticides also might indirectly affect amphibians by compromising amphibian immune systems and making them more susceptible to disease such as the chytrid fungus [5].…”

Frog Virus 3-Like Infections in Aquatic Amphibian Communities

“…An emerging infectious disease is defined as a disease caused by a pathogen that is currently increasing in geographical range, is infecting an increased diversity of hosts, and/or has recently evolved (Daszak et al, 2000). Examples of diseases in amphibian populations around the world that have recently gained attention include chytridiomycosis, caused by Batrachochytrium dendrobatidis (Muths et al, 2003;Rachowicz et al, 2006), and the iridoviruses (family Iridoviridae) (Harp and Petranka, 2006). Iridoviruses have been associated with large-scale morbidity and mortality events of amphibians throughout North America and Europe.…”

Lack of Evidence for the Drought-linked Chytridiomycosis Hypothesis

“…On the contrary, the vast majority of studies (Lips, 1998(Lips, , 1999Bosch et al, 2001;Muths et al, 2003;Lips et al, 2006;Rachowicz et al, 2006) detail mortality events occurring at permanent water bodies, and thus there is no reason to believe that chytridiomycosis epidemics are caused by an increase in the density of breeding aggregations due to abnormally dry weather. Although the number of ephemeral water bodies certainly decreases during dry spells, amphibians that breed in these habitats are at low risk of acquiring chytrid infections (Kriger and Hero, 2007a).…”