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.

Amoeba species colonizing the gills of rainbow trout (Oncorhynchus mykiss) in Swiss aquaculture.

Nodular gill disease (NGD) is an infectious condition characterized by proliferative gill lesions leading to respiratory problems, oxygen deficiency and mortality in fish. Globally, NGD primarily impacts freshwater salmonids in intensive aquaculture systems. In recent years, numerous outbreaks of severe gill disease have affected more than half of the larger rainbow trout (Oncorhynchus mykiss) farms in Switzerland, mainly during spring and early summer. Mortality has reached up to 50% in cases where no treatment was administered. Freshwater amoeba are the presumed aetiologic agent of NGD. The gross gill score (GS) categorising severity of gill pathology is a valuable first-line diagnostic tool aiding fish farmers in identifying and quantifying amoebic gill disease (AGD) in farmed marine salmonids. In this study, the GS was adapted to the NGD outbreak in farmed trout in Switzerland. In addition to scoring disease severity, gill swabs from NGD-affected rainbow trout were sampled and amoeba were cultured from these swabs. Morphologic and molecular methods identified six amoeba strains: Cochliopodium sp., Naegleria sp., Vannella sp., Ripella sp., Saccamoeba sp. and Mycamoeba sp. However, the importance of the different amoeba species for the onset and progression of NGD still has to be evaluated. This paper presents the first description of NGD with associated amoeba infection in farmed rainbow trout in Switzerland.

An understated danger: Antimicrobial resistance in aquaculture and pet fish in Switzerland, a retrospective study from 2000 to 2017.

Aquaculture is a rapidly growing field of food production. However, morbidity and mortality are higher in aquaculture species than in domestic animals. Bacterial diseases are a leading cause of farmed fish morbidity and are often treated with antimicrobials. Since most Swiss fish farms release effluents directly into surface water without treatment and since aquaculture fish are consumed by humans, antimicrobial resistance (AMR) and multi-resistance in aquaculture fish are important for environmental and public health. In this study, AMR tests for 14 antimicrobials were performed on 1,448 isolates from 1,134 diagnostic laboratory submissions from farmed and ornamental fish submissions for the period from 2000 to 2017. Amoxicillin, gentamycin and norfloxacin had the lowest proportion of resistant samples. However, AMR was highly variable over time. Resistance proportions were higher in: (a) ornamental fish compared with farmed fish, (b) fish from recirculation systems compared with those from other farming systems and (c) isolates originating from skin compared with those originating from inner organs. Multiple resistances were common. The results of this study provide useful data for Swiss fish veterinarians and some interesting hypotheses about risk factors for AMR in aquaculture and pet fish in Switzerland. However, further research is needed to define risk factors.