Crayfish plague Aphanomyces astaci detected in redclaw crayfish, Cherax quadricarinatus in Taiwan

The functional role of Daphnia in the host-pathogen interaction of crayfish and the crayfish plague disease agent (Aphanomyces astaci)

Pathogen spores have been recognized as prey with implications for resource dynamics, energy transfer and disease transmission. In aquatic ecosystems, filter-feeders are able to consume such motile forms of pathogens that can cause severe disease in susceptible hosts. The interactions between European crayfish and the crayfish plague pathogen Aphanomyces astaci are of particular conservation interest. In this study, we aim to evaluate the ecological interactions between Ap. astaci, its host Astacus astacus and individuals of the genus Daphnia, filter-feeding planktonic crustaceans. Our focus was on the consumption of the motile zoospores by Daphnia individuals, but we also considered the potential of Daphnia as non-target hosts. We conducted a series of infection and life-history experiments with Ap. astaci, three Daphnia species (D. magna, D. galeata, and D. pulex) and the noble crayfish As. astacus. We did not observe any lethal effects in the infection experiments involving Ap. astaci and Daphnia. Only D. pulex showed differences in some life-history traits. The feeding experiment using the motile zoospores of Ap. astaci as alternative food source or as supplement to different amounts of algal food revealed their nutritional value: D. magna individuals survived, grew, and reproduced on a zoospore diet alone. When zoospores were supplemented to the regular algal diet, all life-history parameters have been significantly improved. However, this successful consumption of zoospores did not result in a reduced mortality of the susceptible crayfish As. astacus during the infection experiment. Nevertheless, the pathogen load of Ap. astaci in the tissues of As. astacus was significantly reduced as a consequence of the feeding activity of Daphnia. Our results indicate that an abundant filter-feeding community can reduce the amount of infective zoospores in the water body and thus be beneficial to susceptible crayfish hosts, potentially acting as a general buffer against zoospore-transmitted diseases in lentic waters.

Evaluation of Black Soldier Fly Larvae Meal on Growth, Body Composition, Immune Responses, and Antioxidant Capacity of Redclaw Crayfish (Cherax quadricarinatus) Juveniles

This study investigated the effects of substituting fish meal (FM) with black soldier fly (Hermetia illucens) meal (BSM) on the growth performance, body composition, immune response, and antioxidant enzyme activity of juvenile redclaw crayfish (Cherax quadricarinatus). Four isonitrogenous (41%) and isolipidic (11%) diets (i.e., FM substitutes) were formulated from BSM prepared using larvae that were fed soybean meal (BSM-S), fishery byproducts (BSM-F), or pitaya (BSM-P). The experimental diets were fed twice daily to triplicate groups of juvenile redclaw crayfish (0.56 ± 0.04 g). After the feed trial, the FM and BSM-F groups exhibited significantly lower feed conversion ratios and significantly higher weight gain; specific growth rates; and concentrations of saturated fatty acids, highly unsaturated fatty acids, eicosapentaenoic acid, and docosahexaenoic acid in the muscle. Among the tested groups, the BSM-F group exhibited significantly enhanced immune responses and increased antioxidant enzyme activity (i.e., superoxide dismutase, phenoloxidase, and glutathione peroxidase); the BSM-P group exhibited a significantly higher feed intake and hepatopancreatic index; and the FM group exhibited a significantly higher muscle body index and apparent digestibility for the dry matter of crude protein. The findings indicate that the juvenile redclaw crayfish fed BSM-F achieved the highest weight gain among the groups.

Temporal dynamics of the immune response in Astacus astacus (Linnaeus, 1758) challenged with Aphanomyces astaci Schikora, 1906

One of the main drivers of biodiversity loss in freshwater ecosystems are alien invasive species. In Europe, pathogen Aphanomyces astaci Schikora, 1906, is considered as one of the most problematic invasive species, as its introduction caused the severe decimation of the European freshwater crayfish stocks. The most affected are the populations of noble crayfish, keystone species native to European freshwaters. Unfortunately, even after decades of research, we do not understand the temporal dynamics of the noble crayfish immune response during Ap. astaci infection. Here, we studied the changes in the immune response of the noble crayfish during a time course challenge with a highly virulent strain of Ap. astaci. We recorded gross symptoms of the disease, changes in the total haemocyte count (THC), gene expression profiles of putative immune response regulators and pathogen load. Additionally, we conducted a preliminary histological analysis of the pathogen dissemination in host tissues. Based on the occurrence of symptoms we propose three stages in the crayfish plague disease progression: asymptomatic stage, symptomatic stage, and finally death of infected individual. Furthermore, based on the qPCR analysis we could differentiate three Ap. astaci growth phases: initial lag phase, followed by exponential growth phase and finally sporulation phase. We observed that all measured immune response parameters were significantly correlated to the observed increase in the pathogen load (qPCR). Altogether, our results point to the absence of a successful immune response in the noble crayfish to a challenge with a highly virulent strain of Ap. astaci. The noble crayfish immune system was not able to suppress the growth of the intruding pathogen. In general, our observations have to be considered in the context of the specific combination of crayfish plague pathogen virulence and disease resistance of the challenged crayfish population, which defines the temporal dynamics of their interaction.

Analysis of the Differences in Muscle Nutrition among Individuals of Different Sexes in Redclaw Crayfish, Cherax quadricarinatus

Redclaw crayfish (Cherax quadricarinatus) was introduced to China many years ago. In recent years, a breeding boom for C. quadricarinatus has been set off in China due to a breakthrough in key technology of seedling breeding. The size and growth rate of C. quadricarinatus vary greatly between female and male individuals, usually the size and growth rate of male individuals are bigger than that of female individuals. There is usually a certain linkage relationship between the sex traits of crustaceans and their own nutrition. In order to explore the linkage relationship between the sex traits of C. quadricarinatus and its nutritional components, this study measured and analyzed the muscle nutritional components of female and male individuals. The results showed that the meat yield rate of male individuals was significantly higher than that of females (p < 0.05), and the crude fat content was significantly lower than that for females (p < 0.05). The ratios of essential amino acids to total amino acids for females and males were 39.61% and 38.49%, respectively. The ratios of essential amino acids to non-essential amino acids were 79.69% and 75.66%, respectively, which far exceed FAO/WHO standards and both belong to high-quality protein. The total amount of flavor amino acids of male individuals was significantly higher than that of female individuals (p < 0.05). The total amount of polyunsaturated fatty acids and the polyunsaturated fatty acid eicosapentaenoic acid of males are both significantly higher than that of females (p < 0.05). Studies have shown that there are certain differences in nutrition between male and female individuals. Compared with female individuals, the meat yield rate, crude protein content, and edible value of the muscles of male individuals is higher.

Threats to UK freshwaters under climate change: Commonly traded aquatic ornamental species and their potential pathogens and parasites

The aquatic ornamental industry, whilst providing socio-economic benefits, is a known introduction pathway for non-native species, which if invasive, can cause direct impacts to native species and ecosystems and also drive disease emergence by extending the geographic range of associated parasites and pathogens and by facilitating host-switching, spillover and spill-back. Although current UK temperatures are typically below those necessary for the survival and establishment of commonly-traded tropical, and some sub-tropical, non-native ornamental species, the higher water temperatures predicted under climate-change scenarios are likely to increase the probability of survival and establishment. Our study aimed primarily to identify which of the commonly-traded non-native ornamental aquatic species (fish and invertebrates), and their pathogens and parasites, are likely to benefit in terms of survival and establishment in UK waters under predicted future climate conditions. Out of 233 ornamental species identified as traded in the UK, 24 were screened, via literature search, for potential parasites and pathogens (PPPs) due to their increased risk of survival and establishment under climate change. We found a total of 155 PPPs, the majority of which were platyhelminths, viruses and bacteria. While many of the identified PPPs were already known to occur in UK waters, PPPs currently absent from UK waters and with zoonotic potential were also identified. Results are discussed in the context of understanding potential impact, in addition to provision of evidence to inform risk assessment and mitigation approaches.

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.

Shrimp bacterial and parasitic disease listed in the OIE: A review

Shrimp aquaculture industry has steadily increased with demand and development of aquaculture technology. In recent years, frequent diseases have become a major risk factor for shrimp aquaculture, such as a drastically reduced the production of shrimp and causing national economic loss. Among them, shrimp bacterial diseases such as hepatopancreatic necrosis disease (AHPND) and necrotizing hepatopancreatitis (NHP-B) and parasitic disease such as Aphanomyces astaci (crayfish plague) are emerging and evolving into new types. OIE (World Organization for Animal Health) regularly updates information on diseases in the Aquatic Code and Aquatic Manual, but in-depth information on the shrimp diseases are lacking. Therefore, the purpose of this review is to provide information necessary for the response and prevention of shrimp diseases by understanding the characteristics and diagnosis of shrimp diseases designated by OIE.

Real-Time PCR Assays for Rapid Identification of Common Aphanomyces astaci Genotypes

The oomycete Aphanomyces astaci is the etiologic agent of crayfish plague, a disease that has seriously impacted the populations of European native crayfish species. The introduction of non-indigenous crayfish of North American origin and their wide distribution across Europe have largely contributed to spread of crayfish plague in areas populated by indigenous crayfish. Tracking A. astaci genotypes may thus be a useful tool for investigating the natural history of crayfish plague in its European range, as well as the sources and introduction pathways of the pathogen. In this study, we describe the development of real-time PCR TaqMan assays aiming to distinguish the five genotype groups of A. astaci (A–E) previously defined by their distinct RAPD patterns. The method was evaluated using DNA extracts from pure A. astaci cultures representing the known genotype groups, and from A. astaci-positive crayfish clinical samples collected mostly during crayfish plague outbreaks that recently occurred in Central Italy and Czechia. The assays do not cross-react with each other, and those targeting genotype groups A, B, D, and E seem sufficiently specific to genotype the pathogen from infected crayfish in the areas invaded by A. astaci (particularly Europe). The unusual A. astaci genotype “SSR-Up” documented from crayfish plague outbreaks in Czechia and chronically infected Pontastacus leptodactylus in the Danube is detected by the group B real-time PCR. The assay originally developed to detect group C (one not yet documented from crayfish plague outbreaks) showed cross-reactivity with Aphanomyces fennicus; the A. astaci genotype “rust1” described in the United States from Faxonius rusticus is detected by that assay as well. Analyses of additional markers (such as sequencing of the nuclear internal transcribed spacer or mitochondrial ribosomal subunits) may complement such cases when the real-time PCR-based genotyping is not conclusive. Despite some limitations, the method is a robust tool for fast genotyping of A. astaci genotype groups common in Europe, both during crayfish plague outbreaks and in latent infections.

First Detection of the Crayfish Plague Pathogen Aphanomyces astaci in Costa Rica: European Mistakes Should Not Be Repeated

The crayfish plague pathogen Aphanomyces astaci is one of the main factors responsible for the decline in European and Asian native crayfish species. This pathogen was transported to these regions through its natural carriers, North American crayfish species, which were introduced during the last century. Since then, the carrier species and the pathogen have spread worldwide due to globalization and the highly invasive nature of these species. In Europe, five carrier species have been categorized as high-risk as they are responsible for the loss of provisioning services, which endangers freshwater ecosystems. The red swamp crayfish Procambarus clarkii, in particular, is currently one of the most concerning species as its spread threatens crayfish biodiversity and freshwater ecosystems worldwide. In this study, we describe the first detection of A. astaci in an introduced population of P. clarkii in Central America, specifically in Costa Rica. Using molecular approaches, we analyzed 48 crayfish samples collected from Reservoir Cachí and detected the presence of A. astaci in four of these samples. The introduction of P. clarkii and the incorrect management of the species (related to its fishery and the commercialization of live specimens) over the past decades in Europe are mistakes that should not be repeated elsewhere. The detection of the pathogen is a warning sign about the dangerous impact that the introduction of this invasive crayfish may have, not only as a carrier of an emerging disease but also as a direct risk to the invaded ecosystems. Our results may serve to (1) assess current and future consequences, and (2) direct future research activities, such as determining the potential impacts of A. astaci on native decapod species, or on other introduced crayfish species that are used for aquaculture purposes, such as Cherax quadricarinatus.

Tracing the origin of the crayfish plague pathogen, Aphanomyces astaci, to the Southeastern United States

The oomycete Aphanomyces astaci is an emerging infectious pathogen affecting freshwater crayfish worldwide and is responsible for one of the most severe wildlife pandemics ever reported. The pathogen has caused mass mortalities of freshwater crayfish species in Europe and Asia, and threatens other susceptible species in Madagascar, Oceania and South America. The pathogen naturally coexists with some North American crayfish species that are its chronic carriers. Presumptions that A. astaci originated in North America are based on disease outbreaks that followed translocations of North American crayfish and on the identification of the pathogen mainly in Europe. We studied A. astaci in the southeastern US, a center of freshwater crayfish diversity. In order to decipher the origin of the pathogen, we investigated (1) the distribution and haplotype diversity of A. astaci, and (2) whether there are crayfish species-specificities and/or geographical restrictions for A. astaci haplotypes. A total of 132 individuals, corresponding to 19 crayfish species and one shrimp species from 23 locations, tested positive for A. astaci. Mitochondrial rnnS and rnnL sequences indicated that A. astaci from the southeastern US exhibited the highest genetic diversity so far described for the pathogen (eight haplotypes, six of which we newly describe). Our findings that A. astaci is widely distributed and genetically diverse in the region supports the hypothesis that the pathogen originated in the southeastern US. In contrast to previous assumptions, however, the pathogen exhibited no clear species-specificity or geographical patterns.

Aphanomyces astaci mtDNA: insights into the pathogen's differentiation and its genetic diversity from other closely related oomycetes

The causative agent of crayfish plague, Aphanomyces astaci (Saprolegniales, Oomycota), is one of the 100 world's worst invasive alien species and represents a major threat to freshwater crayfish species worldwide. A better understanding of the biology and epidemiology of A. astaci relies on the application of efficient tools to detect the pathogen and assess its genetic diversity. In this study, we validated the specificity of two recently developed PCR-based approaches used to detect A. astaci groups. The first relies on the analysis of mitochondrial ribosomal rnnS (small) and rnnL (large) subunit sequences and the second, of sequences obtained by using genotype-specific primers designed from A. astaci whole genome sequencing. For this purpose, we tested the specificity against 76 selected isolates, including other oomycete species and the recently described species Aphanomyces fennicus, which, when used in nrITS-based specific tests for A. astaci, is known to result in a false positive. Under both approaches, we were able to efficiently and accurately identify A. astaci and its genetic groups in both pure cultures and clinical samples. We report that sequence analysis of the rnnS region alone is sufficient for the identification of A. astaci and a partial characterization of haplogroups. In contrast, the rnnL region alone is not sufficiently informative for A. astaci identification as other oomycete species present sequences identical to those of A. astaci.

May atyid shrimps act as potential vectors of crayfish plague?

The causative agent of crayfish plague, Aphanomyces astaci Schikora, was long considered to be a specialist pathogen whose host range is limited to freshwater crayfish. Recent studies, however, provided evidence that this parasite does not only grow within the tissues of freshwater-inhabiting crabs but can also be successfully transmitted by them to European crayfish species. The potential to act as alternative A. astaci hosts was also indicated for freshwater shrimps. We experimentally tested resistance of two freshwater atyid shrimps: Atyopsis moluccensis (De Haan, 1849) and Atya gabonensis Giebel, 1875. They were infected with the A. astaci strain associated with the globally widespread North American red swamp crayfish, Procambarus clarkii (Girard, 1852), the typical host of the A. astaci genotype group D. As popular ornamental species, both shrimps may get in contact with infected P. clarkii not only in the wild but also in the aquarium trade. We assessed the potential of shrimps to transmit A. astaci to susceptible crayfish by cohabiting A. gabonensis previously exposed to A. astaci zoospores with the European noble crayfish, Astacus astacus (Linnaeus, 1758). In both experiments, the presence of A. astaci infection was analysed with species-specific quantitative PCR. We detected A. astaci in bodies and exuviae of both shrimp species exposed to A. astaci zoospores, however, the intensity of infection differed between the species and analysed samples; it was higher in A. moluccensis and the exuviae of both species. A. astaci was also detected in one A. astacus individual in the transmission experiment. This finding reveals that freshwater shrimps may be able to transmit A. astaci to crayfish hosts; this is particularly important as even a single successful infection contributes to the spread of the disease. Moreover, our results indicate that the tested shrimp species may be capable of resisting A. astaci infection and reducing its intensity through moulting. Although their potential to act as prominent A. astaci vectors requires further research, it should not be ignored as these freshwater animals may then facilitate A. astaci spread to susceptible crayfish species in aquarium and aquaculture facilities as well as in the wild.

Crayfish plague in Japan: A real threat to the endemic Cambaroides japonicus

Global introductions of aquatic species and their associated pathogens are threatening worldwide biodiversity. The introduction of two North American crayfish species, Procambarus clarkii and Pacifastacus leniusculus, into Japan in 1927 seems to have negatively affected native Japanese crayfish populations of Cambaroides japonicus. Several studies have shown the decline of these native populations due to competition, predation and habitat colonization by the two invasive North American crayfish species. Here, we identify an additional factor contributing to this decline. We report the first crayfish plague outbreaks in C. japonicus populations in Japan, which were diagnosed using both histological and molecular approaches (analyses of the internal transcribed spacer region). Subsequent analyses of the mitochondrial ribosomal rnnS and rnnL regions of diseased specimens indicate that these outbreaks originated from a P. clarkii population and identify a novel haplotype of Aphanomyces astaci, d3-haplotype, hosted by P. clarkii. Overall, our findings demonstrate the first two cases of crayfish plague in Japan, and the first case in a non-European native crayfish species, which originated from the red swamp crayfish P. clarkii. This finding is a matter of concern for the conservation of the native freshwater species of Japan and also highlights the risk of introducing crayfish carrier species into biogeographic regions harboring species susceptible to the crayfish plague.