Summary1. Indigenous European freshwater crayfish (ICS) are threatened due to invasive North American freshwater crayfish that are natural carriers of Aphanomyces astaci which causes crayfish plague. Infectious A. astaci zoospores are released from carrier crayfish, but little is known about the spore abundance in water systems that either host non-indigenous crayfish species (NICS) or experience crayfish plague outbreaks. We tested two large-scale filtering approaches to generate new insight about the abundance and dynamics of A. astaci spores in natural freshwater systems. 2. Depth filtration (DF) and dead-end ultrafiltration (DEUF) followed by A. astaci-specific quantitative real-time PCR was used to monitor A. astaci spores in large Nordic lakes hosting A. astaci-positive Pacifastacus leniusculus, the dominating NICS in Northern Europe. Crayfish and water were sampled together to compare the A. astaci pathogen load in tissues, A. astaci prevalence in the population and the corresponding spore density in water. Samples were also obtained from a river where indigenous noble crayfish suffered from acute crayfish plague. The sensitivity of the filtering techniques was evaluated using simulation of random events. 3. We detected A. astaci spores in lakes hosting NICS with both filtering methods but predominantly at concentrations below c. 1 spore L À1 . We found a significant positive association between A. astaci spore density in water, the A. astaci prevalence in the corresponding NICS population and the tissue pathogen load. Water from the river with the ongoing crayfish plague outbreak contained overall c. 43 times more spores L À1 than water hosting NICS. Both filtering techniques proved suitable and equally sensitive, but simulations suggest that an optimization of the spore recovery could yield a 10-fold increase in the DEUF-method sensitivity. 4. Synthesis and application. Our study demonstrates a low amount of pathogen spores are present in aquatic environments with non-indigenous crayfish species, emphasizing the need for large-volume filtering techniques for successful detection. The approach can be used for risk assessments and to improve conservation and management strategies of crayfish in Europe. Applications of this method include targeted disease surveillance, habitat evaluation prior to crayfish re-stockings and water monitoring that can minimize disease transmission and spread, for example in crayfish farms and prior to fish movements for stocking purposes.

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The crayfish plague agent Aphanomyces astaci was isolated from 69 noble crayfish Astacus astacus samples in Finland between 1996 and 2006. All isolates were genotyped using randomly amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR). Altogether, 43 isolates belonged to the genotype group of Astacus strains (As), which is assumed to represent the genotype originally introduced into Europe around 1860 and into Finland in 1893. There were 26 crayfish plague isolates belonging to the group of Pacifastacus strain I (Ps1), which appeared in Europe after the stocking of the North American species signal crayfish Pacifastacus leniusculus. The geographical distribution of the 2 genotypes in Finland corresponded with the stocking strategies of signal crayfish. The majority of Ps1-strains (83%) were associated with a classical crayfish plague episode involving acute mortality, compared with only 33% of the As-strains. Asstrains were found more often by searching for reasons for population declines or permanently weak populations, or through cage experiments in connection with reintroduction programmes. In some water bodies, isolations of the As-strains were made in successive years. This study shows that persistent crayfish plague infection is not uncommon in noble crayfish populations. The described epidemiological features suggest a difference in virulence between these 2 genotypes. KEY WORDS: Crayfish plague · Aphanomyces astaci · RAPD-PCR · Oomycete · VirulenceResale or republication not permitted without written consent of the publisher Dis Aquat Org 103: 199-208, 2013 Although some introductions of signal crayfish were made previously in the middle, eastern and northern parts of Finland, there was later a proposal (Kirjavainen 1989) that signal crayfish stocking should be restricted to a distinct region of southern Finland. This area, with some minor changes, was approved by the fisheries authorities in a national crayfish strategy agreement (Mannonen & Halonen 2000). As the carrier of crayfish plague, the signal crayfish has been shown or suspected to be the source of native crayfish mortalities in numerous reports. There are also numerous reports that invading North American crayfish species have been shown or suspected to be the source of native crayfish mortalities (e.g. Huang et al. 1994, Vennerström et al. 1998, Lilley et al. 1997, Oidtmann et al. 1999a, Pöckl & Pekny 2002, Bohman et al. 2006.In spite of the long history of crayfish plague in Europe, relatively little is known about the behaviour of Aphanomyces astaci in natural epidemics involving the highly susceptible European species. It took over 50 yr before the oomycete was accepted as the etiological agent for crayfish plague, illustrating the difficulties in the isolation and identification of the organism (Schäperclaus 1935, Nybelin 1936, Rennerfelt 1936. Subsequent research has provided improved methods for isolation (Alderman & Polglase 1986, Cerenius et al. 1988, Oidtmann et al. 1999b, Viljamaa-Dirks & Heinikainen 2006, and...

Infectious pancreatic necrosis (IPN) is a contagious viral disease of fish that causes economic losses in aquaculture worldwide. In Finland, IPN virus (IPNV) has been isolated since 1987 from adult fish showing no signs of clinical disease at fish farms located in the coastal areas of the Baltic Sea. The inland area of Finland, however, remained free of IPN until 2012, when fish on several rainbow trout farms were diagnosed IPNV-positive. The fish mortalities detected at the farms were low, but clinical signs and histopathological changes typical for IPNV infection were seen in juvenile salmonids. IPNV was isolated at high water temperatures up to 22°C. In 2013 and 2014, IPNV detections continued at inland farms, indicating that infections have spread. The aim of this study was to describe the epidemiology of the outbreak and to characterise the Finnish inland IPNV isolates using histopathological, immunohistochemical and genetic approaches. In order to determine the epidemiological origin of the inland IPNV infections, the partial viral capsid protein (VP2) gene sequences of the inland IPNV isolates were compared with the sequences of the isolates from the coastal farms. Based on the genetic analysis, the inland isolates belong to IPNV Genogroup 2 (Serotype A3/Ab), and the origin of the isolates appears to be one or several coastal fish farms.

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