Amphibian Disease Ecology: Are We Just Scratching the Surface?

Abstract

Pathogen-induced population declines and extinction events have been recognized as main threats to amphibian species around the globe. However, the ecological drivers underlying epidemiological patterns are still poorly understood. In an attempt to assess the current knowledge on the ecological drivers of amphibian diseases, we identified 832 peer-reviewed publications on the ecology of amphibian pathogens and diseases published between 2009 and 2019. The vast majority of publications investigated either chytrid or ranavirus infections (79% of the articles), whereas other pathogens such as bacteria and helminths received considerably less attention. Just over half of the studies we reviewed included field research and 40% were experimental in nature, yet only 8% combined field and experimental approaches. More than half of the literature (56%) investigated postmetamorphic stages, whereas premetamorphic stages were considered in 23% of the reviewed studies, and only 13% included both life stages. Susceptibility and mortality have been assessed in almost every study (91%) whereas 37% of them tested for cellular, physiological, or immunological responses. However, other host characteristics such as growth/development, behavior, and specific mucosome/microbiome were considered in only one of four studies. Most research included at least one biotic factor (e.g., host and pathogen identity, species diversity, genetic adaptations), but only one-third considered environmental factors (e.g., temperature, landscape features, inorganic chemicals). Furthermore, there is no general consensus about the factors driving epidemiological patterns of pathogens in amphibian communities, and it is clear that the complexity and specificity of interactions between ecological factors and host-pathogen dynamics make conservation implications difficult and management decisions challenging. To this end, our review identifies some research gaps and proposes future directions to better understand one of the major threats to this class of vertebrates.

Gaps in global wildlife trade monitoring leave amphibians vulnerable

“…Trade can enable pathogen spread (O'Hanlon et al, 2018), which has facilitated devastating amphibian species loss (Scheele et al, 2019; but see Lambert et al, 2020 for concerns over the number of species). Invasive amphibians (often linked to trade; Lockwood et al, 2019;Stringham and Lockwood, 2018) can be vectors for pathogen spread (Bienentreu and Lesbarrères, 2020;Feldmeier et al, 2016), but also can compete with native species for resources such as space and prey (Falaschi et al, 2020). Wild collection (directly taking animals from the wild) occurs at several scales: on local levels, humans collecting species for trade, consumption and medicine (Ribas and Poonlaphdecha, 2017;Van Vliet et al, 2017;Onadeko et al, 2011); whereas more widely amphibian trade is augmented by demand for pharmaceutical products, pets and even fashion (Auliya et al, 2016;Xiao et al, 2011).…”

Research on helminths from Mexican amphibians: gaps, trends, and biases

“…Worldwide, most of the research on amphibian pathogens has focused on chytrid fungus and on ranaviruses that have been recognized as factors associated with amphibian declines, while the role of helminth parasites in such declines has been poorly explored (Bienentreu & Lesbarrères, 2020). Regarding pathogenic helminths, research elsewhere has mostly been performed on nematodes of the genus Rhabdias and trematodes of the genera Ribeiroia and Echinostoma , which usually affect performance and/or development of amphibians (Koprivnikar et al ., 2012).…”

Ranavirus Amplification in Low-Diversity Amphibian Communities

“…Unfortunately, the effects of species identity and species richness are not commonly studied in wild amphibian host-pathogen systems [see ( 34 , 48 )]. For instance, RVs are globally distributed multi-host pathogens, infecting a variety of different host species across three classes of ectothermic vertebrates [amphibians, reptiles, and fish; ( 43 , 49 )] but studies investigating RV dynamics tend to focus on selected subsets or single amphibian hosts (and life-history stage), often within a community of several host species [reviewed in ( 50 )]. Susceptibility and infection outcome vary greatly and typically depend on host identity as well as the life-history stage of the infected individual ( 11 , 12 ).…”