The old menace is back: Recent crayfish plague outbreaks in the Czech Republic E. Kozubíková a, ⁎ , A. Petrusek a,b , Z. Ďuriš c , M.P. Martín d , J. Diéguez-Uribeondo d , B. Oidtmann e a Charles University in Prague, Faculty of Science, Department of Ecology, Viničná 7, Prague 2, CZ-12844, Czech Republic b University of South Bohemia in České Budějovice, Research Institute of Fish Culture and Hydrobiology, Zátiší 728/II, Vodňany, CZ-38925, Czech Republic c University of Ostrava, Department of Biology, Chittussiho 10, Ostrava, CZ-71000, Czech Republic d Real Jardín Botánico CSIC, Departamento de Micología, Plaza Murillo 2, 28014 Madrid, Spain e CEFAS Weymouth Laboratory, Barrack Road, The Nothe, Weymouth, Dorset DT4 8UB, United Kingdom Received 5 September 2007; received in revised form 2 November 2007; accepted 12 November 2007 Abstract The crayfish plague pathogen, Aphanomyces astaci (Saprolegniales, Oomycetes), is lethal for native European crayfish species and eliminated a number of their populations during the second half of the 19th and throughout the 20th centuries. The disease is spread by non-native North American crayfish species, known to be carriers of the pathogen, which are currently thriving in many European countries. This represents one of the major threats to the recovery of populations of native crayfish (the noble crayfish Astacus astacus and the stone crayfish Austropotamobius torrentium) in the Czech Republic. While only two suspected plague-caused crayfish mass mortalities were reported in the second half of the 20th century, five mass mortalities involving both native species were found to be caused by A. astaci between 2004 and 2007, and several additional ones, though unconfirmed, are likely. We discuss possible reasons for this apparent increase in the number of crayfish plague outbreaks, and we report details of the four most recent mass mortalities in which the presence of A. astaci has been confirmed by molecular methods (PCR with species-specific primers and sequencing of the pathogen DNA) and microscopic analysis, and of additional suspicious cases of crayfish disappearance. As these plague outbreaks have been found only accidentally, it is likely that their actual frequency is higher. The Czech Republic is probably not exceptional in this respect, and plague monitoring within the whole Central Europe deserves more attention. © 2007 Elsevier B.V. All rights reserved. Keywords: Aphanomyces astaci; Oomycete; Orconectes limosus; Astacus astacus; Austropotamobius torrentium; Mass mortalities 1. Introduction Crayfish plague, the most harmful disease of European freshwater crayfish, is caused by the parasitic saprolegnious oomycete Aphanomyces astaci Schikora, 1903 (Cerenius et al., 1988). This parasite is endemic to North America and is highly specialized to penetrate into and live in the crayfish cuticle. In European and Australasian freshwater crayfish species, the infection leads to the death of affected animals (Söderhäll and Cerenius, 1999), and due to the high infectivity of the pathogen, whole populations of susceptible crayfish can be lost in a short time. North American crayfish species are usually able to limit an A. astaci infection (Unestam, 1969, 1972), as they can inhibit the growth of hyphae in the cuticle by fast and heavy melanization. Such an immune response is possible due to their ability to continuously produce prophenoloxidase, an enzyme involved in melanin formation (Cerenius et al., 2003). In European species, the process of melanin formation in response to A. astaci infection is not as efficient, leading to a higher susceptibility to the crayfish plague (Cerenius et al., 2003). In North American species, the parasite survives despite being inhibited in growth, and its infective units – zoospores – may be released into the water (mostly during moulting of the crayfish host), so infected American crayfish serve Available online at www.sciencedirect.com Aquaculture 274 (2008) 208 – 217 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. E-mail address: evikkk@post.cz (E. Kozubíková). 0044-8486/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2007.11.015