Abstract
Invasive species can facilitate the spread of pathogens by first providing asymptomatic host reservoirs, and then driving disease outbreaks in native populations through pathogen spillover. An example of this are invasive crayfish species in Europe (Faxonius limosus, Pacifastacus leniusculus, Procambarus clarkii), which carry the deadly plague agent (Aphanomyces astaci). Effective disease management requires comprehensive monitoring, however, pathogen detection in carrier populations with low pathogen prevalence and intensities is challenging. We simultaneously collected and analysed crayfish tissue samples of invasive crayfish populations and water samples to compare A. astaci detection in different sample types using quantitative PCR. Combined, the two sampling methods revealed A. astaci presence with DNA concentrations above limit of detection (LOD; the lowest concentration which can be detected with reasonable certainty) in 13 of 23 invasive crayfish populations. In four additional sites, A. astaci DNA concentrations below LOD were found in water. In four populations only were A. astaci concentrations above LOD detected in both sample types and in three populations in concentrations above LOD in tissue but below LOD in water. The likely reason for these discrepancies is the low A. astaci prevalence and concentration in resistant invasive crayfish, which limit detection reliability. Consistency may be improved by timing surveys with seasonal periods of high A. astaci abundance and by increasing water sampling effort. Considering the ease of collecting eDNA samples, compared to crayfish tissue sampling, eDNA methods would facilitate frequent and comprehensive surveys. However, remaining uncertainties in eDNA-based detection reveal the relevance of combining monitoring tools to improve detection of invasive pathogens and their management.
Highlights
Invasive species can disrupt the structure and functioning of communities and ecosystems, threatening the survival of endangered species (Strayer 2010)
Aphanomyces astaci DNA was detected in quantities above limit of detection (LOD) in water from five and in tissue samples from twelve out of 23 sites with invasive crayfish (Fig. 1, Table 1)
The water samples revealed the presence of A. astaci in one site where it was not detected in crayfish tissue
Summary
Invasive species can disrupt the structure and functioning of communities and ecosystems, threatening the survival of endangered species (Strayer 2010). The spread of a species carrying parasites into new territory can lead to novel combinations of parasites and hosts, i.e. to a spillover event, where the parasite acquires a new host species in its invasive range (Strauss et al 2012) In their introduced range, invasive parasites can mediate the competition between species (Price et al 1988; Dunn and Hatcher 2015). If the new native host is more susceptible to the parasite than its original, non-native host, the nonnative host can acquire a competitive advantage, which promotes its spread and its chances of becoming invasive (Strauss et al 2012) Invasive species and their parasites can become serious threats to highly susceptible native species, since invasive species can act as an asymptomatic carrier and reservoir species for the parasite. Such reservoir species can be crucial for the persistence of an invasive parasite (Reynolds 1988)
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