An old unknown: 40 years of crayfish plague monitoring in Switzerland, the water tower of Europe.

The oomycete Aphanomyces astaci is the causative agent of crayfish plague, a disease threatening susceptible freshwater crayfish species in Europe. To detect its spatiotemporal occurrence in Switzerland, we reviewed (1) the literature regarding occurrence of crayfish plague and North American crayfish carrier species and (2) the necropsy report archive of the Institute for Fish and Wildlife Health (FIWI) from 1968 to 2020. In the past, crayfish plague was diagnosed through several methods: conventional PCR, culture, and histology. When available, we re-evaluated archived Bouin's or formalin-fixed, paraffin-embedded samples collected during necropsies (1991-2020) with a recently published quantitative PCR. Literature research revealed putative reports of crayfish plague in Switzerland between the 1870s and 1910s and the first occurrence of three North American crayfish species between the late 1970s and 1990s. Finally, 54 (28.1%) cases were classified as positive and 9 (4.7%) cases as suspicious. The total number of positive cases increased by 14 (14.7%) after re-evaluation of samples. The earliest diagnosis of crayfish plague was performed in 1980 and the earliest biomolecular confirmation of A. astaci DNA dated 1991. Between 1980-1990, 1991-2000 and 2001-2010 crayfish plague spread from one to two and finally three catchment basins, respectively. Similar to other European countries, crayfish plague has occurred in Switzerland in two waves: the first at the end of the 19th and the second at the end of the 20th century in association with the first occurrence of North American crayfish species. The spread from one catchment basin to another suggests a human-mediated pathogen dispersal.

Detection of novel RNA viruses in wild noble crayfish (Astacus astacus): A virome analysis in Swiss water bodies.

European native crayfish populations are undergoing a strong decline due to environmental factors and the introduction of highly competitive non-native species. Pathogens are an additional threat to native crayfish. However, aside from the crayfish plague, other infectious diseases are still widely unknown. This study aimed to investigate viruses present in seven populations of wild noble crayfish (Astacus astacus) in Switzerland, through high-throughput sequencing. Sequence analysis revealed the presence of 11 novel RNA viruses (one bunya-like, four hepe-like, two dicistro-like, three picorna-like, and one permutotetra-like) in the samples. The discovery of a novel bunya-like virus in noble crayfish without associated mortality or macroscopical alterations is of particular interest since it is closely related to the bunya-like brown spot virus, a virus described in 2019 from diseased native white-clawed crayfish (Austropotamobius pallipes) during a mass mortality event in France. It seems that these two closely related viruses have very different impacts on their respective hosts, raising the need for further investigations on virulence factors and host susceptibility towards these viruses. This study provides a basis for future investigations, permitting to gradually fill the knowledge gap in crayfish viral diseases.

Further spread of perch rhabdovirus on European percid farms.

Variants of perch rhabdovirus (PRV) circulate across European percid farms via the fish trade. To trace their circulation, they are usually isolated by cell culture and subsequently identified genetically by sequencing partial or complete genes. Here, a newly developed nested PCR-based method was used to amplify and sequence the complete N and P genes directly from clinical samples obtained during an outbreak on a farm as well as from four batches of fish sampled from two other farms in another country. In an attempt to trace the origin of the five detected viruses, their N and P sequences were concatenated and compared with related viruses. One virus found in pike-perch was highly related to a virus isolated in 2016 in Belgium. Two other viruses detected on a single farm were distinct from one another, with one being almost identical to another virus isolated in 2016 in Belgium and the other being more closely related to a subgroup with different origins, France and Belgium. Two other viruses found in perch from a third farm were identical and were more related to a subgroup of viruses isolated in France. Identifying variants by a direct PCR approach will help to prevent further dissemination in farms.

Deformed wings of small hive beetle independent of virus infections and mites.

Insect wing deformities can be caused by viruses, mites and other environmental stressors during development. Here we conducted differential diagnostics of deformed wings in small hive beetles, Aethina tumida (SHB). Adult SHB with and without deformed wings from individual and mass reared scenarios were evaluated for the mite Tyrophagus putrescentiae and for deformed wing virus. Viral load and mite number were similar for SHB with deformed wings and unaffected beetles. Because deformed wings were only observed in individually pupating SHB, a humidity challenge most likely explains the observed clinical symptoms. Our observations support the importance of differential diagnostics.