Epidemiology of crayfish plague

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http://urn.fi/URN:ISBN:978-952-225-156-5
Title: Epidemiology of crayfish plague
Author: Viljamaa-Dirks, Satu
Contributor: University of Helsinki, Faculty of Veterinary Medicine, Department of Veterinary Biosciences
The Finnish Food Safety Authority Evira
Thesis level: Doctoral dissertation (article-based)
Abstract: 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.Rapurutto on eurooppalaisten rapulajien vakavin tautiongelma, joka on saattanut kotoperäiset rapulajit uhanalaisiksi lähes kaikkialla Euroopassa. Rapuruton aiheuttaa leväsieni Aphanomyces astaci, joka elää sienenkaltaisena rihmastona ravun kuoressa, ja siirtyy isäntäeläimestä toiseen siimallisten uimaitiöiden välityksellä. Taudille herkät rapulajit eivät pysty rajoittamaan rihmaston kasvua tehokkaasti ja kuolevat yleensä tartuntaan parissa viikossa. Ensimmäiset rapujen joukkokuolemat raportoitiin Italiassa 1860-luvulla, ja Suomeen tartunta levisi 1893. Siitä lähtien rapurutto on tuhonnut merkittävän osan Suomen alkuperäisen rapulajin jokiravun (Astacus astacus) kannoista. Tuhoutuneita kantoja korvaamaan tuotiin Pohjois-Amerikasta täplärapuja, jotka eivät olleet herkkiä taudille. Ne kuitenkin kantoivat taudinaiheuttajaa kuoressaan ja tartuttivat sitä edelleen jokirapuihin. Geneettisesti täplärapujen kantama rapurutto, genotyyppi Ps1, eroaa alun perin Suomeen tulleesta taudinaiheuttajasta, genotyypistä As. Tässä tutkimuksessa määritettiin Suomessa vuosina 1996-2006 todettujen rapuruttotapausten genotyyppi. Täplärapujen esiintymisalueen läheisyydessä Etelä-Suomessa jokiravuilla esiintyi yleensä Ps1-tyypin aiheuttamia tartuntoja, kun taas muualla Suomessa As-tyypin rapurutto oli yleisempi. Ps1-tyyppi aiheutti aina rapukuolemia, kun taas As-tyyppiä löytyi usein myös heikoista jokirapukannoista. Rapukuoleman jälkeen heikkoon rapukantaan piileväksi jäänyt As-tyypin rapuruttotartunta todennettiin pienessä eteläsuomalaisessa järvessä. Genotyyppien välinen taudinaiheutuskyvyn ero todettiin myös kokeellisesti. Ps1-rapuruttokantojen välillä ei ollut merkittäviä eroja taudinaiheutuskyvyssä tai kasvunopeudessa, toisin kuin As-kannoilla. As-tyypin rapurutto näyttää sopeutuneen jossakin määrin uuteen isäntälajiinsa jokirapuun. Sen alkuperäistä isäntälajia ei tunneta. Tutkimme sen selvittämiseksi Eurooppaan tuotua toista rapulajia amerikkalaista kääpiörapua (Orconectes limosus), joka kuitenkin osoittautui aiemmin kuvaamattoman rapuruttotyypin Or kantajaksi. On mahdollista, että pohjoisamerikkalaisilla rapulajeilla esiintyy paljon enemmänkin lajinomaisia rapuruttotyyppejä, joiden ominaisuudet kuten taudinaiheutuskyky voivat poiketa toisistaan. Tämä tutkimus osoitti, että rapuruttotapaukset on varmistettava laboratoriotutkimuksella, joka sisältää myös rapuruton tyypin määrittämisen. Jokirapukantojen hoidossa on otettava huomioon piilevän rapuruton mahdollisuus. Siksi istutussuunnitelmiin täytyy aina sisällyttää sekä istukkaiden että vastaanottavan vesistön rapuruttotilanteen tutkiminen.
URI: URN:ISBN:978-952-225-156-5
http://hdl.handle.net/10138/168313
Date: 2016-11-18
Subject: eläinlääketiede
Rights: This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.


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