Absence of the crayfish plague pathogen (Aphanomyces astaci) facilitates coexistence of European and American crayfish in central Europe

Abstract

Summary 1. Most European crayfish species are strongly threatened, mainly as a result of the introduced pathogen, Aphanomyces astaci, transmitted by invasive North American crayfish. Long‐term coexistence of American and European crayfish species is therefore regarded as almost impossible, even though some coexisting populations have been observed. 2. In this study, crayfish were collected from presently coexisting populations of the introduced spiny‐cheek crayfish (Orconectes limosus) and the native noble crayfish (Astacus astacus) from nine standing waters in central Europe. Our aim was to resolve whether the coexistence resulted from reduced virulence in local strains of A. astaci, increased immunity in the native crayfish or an absence of the pathogen in these populations. We used highly sensitive A. astaci‐specific real‐time PCR to evaluate the crayfish latent carrier status, combined with transmission experiments to further validate the molecular results. 3. From the total of 523 crayfish tested (490 spiny‐cheek crayfish, 33 noble crayfish), none positive for A. astaci was detected. Transmission experiments confirmed these results: No abnormal mortality or behavioural changes were seen in noble crayfish kept together with American crayfish from the coexisting populations. If we assume a very low prevalence of A. astaci of 10% in a carrier population, there is a 98% probability of disease being absent in five of the nine coexisting populations tested. Hence, a consistent absence, or an extremely low prevalence, of A. astaci seems to allow the coexistence of European and American crayfish in these central European populations. 4. The results are important for native crayfish conservation and management and demonstrate that disease transmission risk may vary substantially between the different populations of spiny‐cheek crayfish in central Europe.

Prevalence of the Crayfish Plague Pathogen Aphanomyces astaci in Populations of the Signal Crayfish Pacifastacus leniusculus in France: Evaluating the Threat to Native Crayfish

“…In Finland, Westman & Savolainen [56] reported a 30-year coexistence of P. leniusculus with another native species, Astacus astacus , although the latter was finally outcompeted. In these cases, it has been suggested that the invasive species was not infected by the pathogen, similarly as in some Central European sites where native crayfish coexisted with O. limosus [49]. The present situation in Lake Geneva, where we demonstrated the presence of A. astaci in five out of 16 analysed signal crayfish, seems thus different, although we cannot rule out that the pathogen has been introduced to the local population of P. leniusculus only recently.…”

“…As only a small number of individuals could be tested from some sites, lack of detection of A. astaci cannot be interpreted as the absence of the pathogen from the population (see the 95% confidence intervals for the prevalence in Table 1). Providing conclusive evidence that some populations of potential A. astaci hosts (North American invasive crayfish) are likely to be uninfected requires much more intensive sampling effort [49]. Temporal fluctuations of the pathogen prevalence (or detectability) could also influence our results, as was shown for a Czech Orconectes limosus population studied over several seasons [54].…”

“…The pathogen prevalence in samples from the studied populations was highly variable, ranging from 0% to 80%; however, due to often low number of individuals analysed per populations, the confidence intervals for the prevalence estimates remain wide (Table 1). Thus, lack of unambiguous detection of the pathogen at a particular site cannot be considered as an evidence for its absence from the population; substantially higher sample sizes are needed to test for such scenario [49].…”

Detection of crayfish plague spores in large freshwater systems

“…Schrimpf et al . ). However, the diagnosis of crayfish plague in susceptible species would still require tissue analysis (OIE ).…”

First evidence of crayfish plague agent in populations of the marbled crayfish (Procambarus fallaxformavirginalis)

“…However, the number of samples was relatively small (N = 11 to 28) and due to environmental stochasticity (Dwyer et al, 2004) or recent molting events (Oidtmann, 2012), the infection status with A. astaci may have been underestimated. The analysis of 28 individuals seems to indicate that the population from Lake Singliser is A. astaci free (Schrimpf et al, 2013b). However, a more intensive sampling could possibly still increase detection probability (95% confidence interval ranged from 0 to 12%) of A. astaci.…”