Long-term changes in the prevalence of the crayfish plague pathogen and its genotyping in invasive crayfish species in Czechia

The widespread presence of North American alien crayfish in Europe is a major driver of native crayfish population declines, mainly because they are chronic carriers of the oomycete Aphanomyces astaci responsible for crayfish plague. Screening for the crayfish plague pathogen in host populations has become a common practice across Europe, but sampling usually covers spatial but not temporal variation. Our study focuses on the current situation in Czechia, where screening for A. astaci was first conducted in the mid-2000s. We provide data about the distribution and prevalence of this pathogen at almost 50 sites with three host crayfish: the spiny-cheek crayfish Faxonius limosus, signal crayfish Pacifastacus leniusculus, and marbled crayfish Procambarus virginalis. Among these sites were 20 localities that were resampled several years (usually more than a decade) after the original screening for A. astaci. We did not detect any A. astaci infection in two studied P. virginalis populations but documented several new hotspots of highly infected P. leniusculus in Czechia, and the first site with the coexistence of the latter with F. limosus. Our data suggest that despite some fluctuations, A. astaci prevalence in North American host populations generally does not tend to change significantly over time; we only observed two cases of a significant increase and one of a significant decrease. We no longer detected A. astaci in several originally weakly infected populations, but our data suggest it likely still persists in these areas and threatens native crayfish populations. At the single known site in the country where P. leniusculus and F. limosus coexist, we documented the presence of the same A. astaci genotype group in both crayfish species, likely due to interspecific transmission of the pathogen from the former host to the latter. However, genotyping of A. astaci in infected host individuals still supported the link between specific pathogen genotypes and crayfish hosts, suggesting that assessment of sources of mass mortalities from the pathogen genotyping is feasible in European regions where the mutual contact of different American crayfish species is uncommon.

Experimental evaluation of the potential for crayfish plague transmission through the digestive system of warm-blooded predators

The crayfish plague pathogen (Aphanomyces astaci) can be transmitted through the digestive system of fish, but its dispersal through mammalian and bird digestive tracts has been considered unlikely, and direct experimental evidence remains scarce. We present a small-scale transmission experiment with European otter and American mink fed with infected crayfish, and experiments testing survival of cultures of five A. astaci strains at temperatures corresponding to those inside mammal and bird bodies. The pathogen was neither isolated from predator excrements nor transmitted to susceptible crayfish exposed to excrements. In agar-based artificial media, it occasionally survived for 15 min at 40.5°C and for 45 min at 37.5°C, but not so when incubated at those temperatures for 45 min and 75 min, respectively. The five tested strains differed in resistance to high temperatures, two (of genotype groups E and D) being more susceptible than other three (of groups A, B and D). Their survival to some extent varied when exposed to the same temperature after several weeks or months, suggesting that some yet-unknown factors may influence A. astaci resistance to temperature stress. Overall, we support the notion that passage through the digestive tract of warm-blooded predators makes A. astaci transmission unlikely.

Hosts and transmission of the crayfish plague pathogen Aphanomyces astaci: a review

The crayfish plague pathogen, Aphanomyces astaci Schikora, has become one of the most well-studied pathogens of invertebrates. Since its introduction to Europe in the mid-19th century, it has caused mass crayfish mortalities, resulting in drastic declines of local populations. In contrast, North American crayfish usually serve as latent carriers, although they may also be negatively affected by A. astaci infections under some circumstances. Recent research benefiting from molecular tools has improved our knowledge about various aspects of A. astaci biology. In this review, we summarize these advances, particularly with respect to the host range and transmission. We highlight several aspects that have recently received particular attention, in particular newly confirmed or suspected A. astaci hosts, latent A. astaci infections in populations of European crayfish, and the relationship between A. astaci genotype groups and host taxa.

Resistance to the crayfish plague pathogen, Aphanomyces astaci, in two freshwater shrimps

Aphanomyces astaci, the causal agent of the crayfish plague, has recently been confirmed to infect also freshwater-inhabiting crabs. We experimentally tested the resistance of freshwater shrimps, another important decapod group inhabiting freshwaters, to this pathogen. We exposed individuals of two Asian shrimp species, Macrobrachium dayanum and Neocaridina davidi, to zoospores of the pathogen strain isolated from Procambarus clarkii, a known A. astaci carrier likely to get into contact with shrimps. The shrimps were kept in separate vessels up to seven weeks; exuviae and randomly chosen individuals were sampled throughout the experiment. Shrimp bodies and exuviae were tested for A. astaci presence by a species-specific quantitative PCR. The results were compared with amounts of A. astaci DNA in an inert substrate to distinguish potential pathogen growth in live specimens from persisting spores or environmental DNA attached to their surface. In contrast to susceptible crayfish Astacus astacus, we did not observe mortality of shrimps. The amount of detected pathogen DNA was decreasing steadily in the inert substrate, but it was still detectable several weeks after zoospore addition, which should be considered in studies relying on molecular detection of A. astaci. Probably due to moulting, the amount of A. astaci DNA was decreasing in N. davidi even faster than in the inert substrate. In contrast, high pathogen DNA levels were detected in some non-moulting individuals of M. dayanum, suggesting that A. astaci growth may be possible in tissues of this species. Further experiments are needed to test for the potential of long-term A. astaci persistence in freshwater shrimp populations.