Epidemiology of crayfish plague

Crayfish plague is a severe disease of European crayfish species and has rendered the indigenous crayfish populations vulnerable, endangered or even extinct in the most of Europe. Crayfish plague is caused by an oomycete Aphanomyces astaci, a fungal-like water mould that lives its vegetative life in the cuticle of crayfish and infects other crayfish by producing zoospores. Zoospores swim around for a few days in search of crayfish, and when they find one they attach onto its surface, encyst and germinate to start a new growth cycle as new growing hyphae penetrate the crayfish tissues. Unrestricted growth of A. astaci leads to the death of the infected animal in just a few weeks. Crayfish plague induced mortalities started in Italy around 1860. Although the disease was known about since 1860 its cause remained unknown for several decades. Little was done to prevent the spread of the disease. A lively crayfish trade probably facilitated the spread of the crayfish plague, which reached Finland in 1893. The crayfish plague has remained the most important disease problem of the Finnish noble crayfish Astacus astacus, since then. The consensus was that the disease killed all infected animals in a short time, and it appeared almost impossible to restore the flourishing crayfish populations to the levels that existed before. Following the example of neighbouring Sweden, a North American crayfish species, the signal crayfish Pacifastacus leniusculus that appeared resistant to crayfish plague was introduced to Finland in 1960s. As expected, the signal crayfish slowly started to replace the lost populations of the noble crayfish to become an important part of the crayfish fisheries. The introduction of the signal crayfish significantly added to the management problems of the noble crayfish stocks left. Signal crayfish appeared to be a chronic carrier of the crayfish plague agent, and spread the disease to the dwindling vulnerable noble crayfish populations. Later research showed that the crayfish plague agent is a parasite of North American crayfish that in normal circumstances does not harm the host animal. Intriguingly, the crayfish plague agent carried by the signal crayfish, genotype Ps1, is different from the pathogen originally introduced into Europe, genotype As. The diagnosis of crayfish plague especially when based on the isolation of the pathogen is challenging and accordingly the genotype difference was mostly unrecognized until recently. In this study we determined the genotype of the causative agent from most of the detected Finnish crayfish plague cases between 1996 - 2006. It appeared that most of the epidemics in the immediate vicinity of signal crayfish populations were caused by genotype Ps1, whereas genotype As was more prevalent in the noble crayfish areas. Interestingly, a difference was seen in the outcome of the infection. The Ps1 infection was always associated with acute mortalities, while As infections were also frequently found in existing but weak populations. The persistent nature of an As infection could be verified in noble crayfish from a small lake in southern Finland. This finding explained why many of the efforts to introduce a new noble crayfish population into a water body after a crayfish plague induced mortality were futile. The main conclusion from the field study data of this research was the difference in virulence between the Ps1 and As genotype strains. This was also verified in a challenge trial with noble crayfish. While the Ps1 strains did not show much variation in their growth behaviour or virulence, there was much more variation in the As strains. The As genotype arrived in Finland more than 100 years ago, and since that date it seems to have adapted to the novel host, the noble crayfish, to some extent. In order to gain insight into a possible vector of this genotype, we studied another North American crayfish species present in Europe, the spiny-cheek crayfish Orconectes limosus from a Czech pond. This crayfish species appeared to carry a novel genotype of A. astaci, named Orconectes genotype, designated “Or”. It seems possible that many of the North American crayfish species carry their own type of crayfish plague agent, with variable features such as virulence. These differences should be further tested in the future. The results of this study alleviate the necessity to study the noble crayfish mortalities for the verification of crayfish plague, including the study for the genotype of the A. astaci strain. Crayfish fisheries and conservation management decisions should not be made without a prior control of the donating population and the receiving water body for the eventual presence of a low-virulent A. astaci

Identification and genetic characterization of Saprolegnia parasitica, isolated from farmed and wild fish in Finland

Oomycete infections in farmed fish are one of the most significant disease issues in salmonid aquaculture worldwide. In the present study, Saprolegnia spp. in different farmed fish species in Finland were identified, and the molecular epidemiology of especially Saprolegnia parasitica was examined. We analysed tissue samples from suspected oomycete-infected salmonids of different life stages from a number of fish farms, as well as three wild salmonids. From collected oomycete isolates, the ITS1, 5.8S and ITS2 genomic regions were amplified, analysed phylogenetically and compared with corresponding sequences deposited in GenBank. Of the sequenced isolates, 91% were identified as S. parasitica. Isolates of yolk sac fry were identified as different Saprolegnia spp. Among the isolates from rainbow trout eggs Saprolegnia diclina dominated. In order to determine potential dominating clones among the S. parasitica, isolates were analysed using Multi Locus Sequence Typing (MLST). The results showed that one main clone contained the majority of the isolates. The MLST analysis showed four main sequence types (ST1–ST4) and 13 unique STs. This suggests that the Saprolegnia infections in farmed fish in Finland are not caused by different strains originating in the farm environment. Instead, one main clone of S. parasitica is present in Finnish fish farms.publishedVersio

Kalojen ylisiirtojen riskit ja riskinhallinnan kehittäminen

Kalojen ylisiirrot ovat yksi kalataloudellinen hoitotoimenpide vaelluskalakantojen elvyttämisessä. Ylisiirrossa emokaloja siirretään jokeen: nousuesteen alapuolelta yläpuolisiin vesistöihin, jotta ne pääsevät lisääntymisalueilleen. Projektissa laadittiin riskiprofiili, jossa kuvataan ylisiirtoprosessi kolmella joella (Kemijoki, IIjoki ja Oulujoki) ja tarkasteluun valittujen kalatautien (IHN, IPN, ISA, SAV ja VHS) leviämisriskiin liittyviä seikkoja ylisiirroissa käytettyjen lajien (lohi, taimen, nahkiainen) osalta. Lisäksi pyrittiin tunnistamaan prosessiin liittyvät riskitekijät ja riskinhallintakeinot. Merestä sisävesistöihin tehtävät ylisiirrot ovat riippuvaisia tautitilanteesta - tällä hetkellä ylisiirrot ovat luvanvaraisia. Lohi ja meritaimen tekevät pitkän syönnösvaelluksen, minkä aikana ne voivat kohdata luonnossa esiintyviä tauteja. Lohen syönnösvaellus ulottuu pidemmälle ja se voi kohdata tauteja laajemmalla alueella kuin meritaimen, joka vaeltaa lähellä rannikkoa. Jos kala sairastuu syönnösvaelluksen aikana, sen kyky palata takaisin jokeen alenee mm. uimiskyvyn heikkenemisen vuoksi. On kuitenkin mahdollista, että jokisuuhun palaa syönnösvaellukselta kala, joka voi tartuttaa tauteja muihin kaloihin. Palaavalla kalalla ei välttämättä ole ulkoisia merkkejä kantamastaan viruksesta. Arvioinnissa mukana olleiden jokien ylisiirroissa piilevä tautiriski vaikuttaa suhteellisen pieneltä, koska kalatautiseurantaa on tehty jo pitkään eikä tautilöydöksiä ole ilmennyt. Itämeri on kuitenkin avoin järjestelmä ja uusia tauteja voi tulla Itämereen niin luonnonkalojen liikkeiden kuin kalanviljelyn kautta. Nykyiset seurantamäärät kestävät suhteellisen hyvin myös tämän tekijän tuoman epävarmuuden tautien esiintyvyyteen. Jos seuranta lopetetaan, varmuus tautivapaudesta ryhtyy alenemaan. Niinpä seurantaa ei kannata lopettaa edes näiden jokien osalta. Jotta kalatautautiriski ja sen mahdolliset muutokset tunnetaan, kalatautinäytteiden säännöllinen ottaminen ja tutkiminen on tärkeää. Näytteeksi tulee toimittaa sairaiden yksilöiden lisäksi ulkoisesti terveen näköisiä kaloja, koska virustaudit eivät välttämättä näy ulkoisesti. Myös jokisuilla, niissä vesistöissä, joissa vasta suunnitellaan ylisiirtoja, tulisi tehdä ennakkoon kalatautiseurantaa

Crayfish disease diagnostics : Towards a Nordic standard

Crayfish utilization is a long-standing Scandinavian tradition, which unfortunately has suffered a major drawback with the introduction of crayfish plague disease about a hundred years ago. In spite of intensive research many problems in management and diagnosis of this disease are not yet solved. To evaluate ongoing research and diagnostic methodology in different countries, representatives of diagnostic laboratories involved in crayfish disease diagnostics were invited to a workshop in Kuopio to discuss the problems with crayfish diseases. The workshop was held at the Kuopio unit of the Finnish Food Safety Authority Evira. Participants from Finland, Sweden, Norway, Estonia and Latvia were present, as well as the OIE expert from the reference laboratory for crayfish plague. In the programme of the first day the state of crayfish stocks, crayfish diseases and diagnostic methods used in each country were presented and discussed. During the second day diagnostic methods were discussed in detail, as well as research projects concerning crayfish diseases. The workshop has been made possible by way of a grant from the Nordic Council of Ministers