Fungal Invasion of Crayfish
Category 4 (Negligible Regulatory Significance in Canada)
Common, generally accepted names of the organism or disease agent
Fungal disease, Saprolegniasis, Fungal shell disease, 'Brown abdomen disease' in Astacus leptodactylus associated with Fusarium solani, 'Black gill disease' in Austropotamobius pallipes caused by Fusarium tabacinum, Burn spot disease BUT not including crayfish plague "fungus".
Scientific name or taxonomic affiliation
Various species of fungi (the Eumycota) and morphologically similar Oomycetes (commonly called water moulds but more closely related to brown algae and diatoms than to the true fungi, the Eumycota) have been reported in crayfish. The Eumycota include; Fusarium solani, Fusarium oxysporum, Fusarium tabacinum, Fusarium roseum var. culmorum, Trichosporon beigelii, and trichomycete fungi: and Oomycetes include; Achlya sp., Saprolegnia spp., Saprolegnia parasitica (a pathogen of fish especially salmonids and infectious to crayfish), Aphanomyces frigidophilus and Aphanomyces laevis, BUT not including Aphanomyces astaci. Royo et al. (2004) proposed the name Aphanomyces repetans for isolates that like A. astaci, had repeated zoospore emergence and lacked sexual reproduction. However, the isolates of A. repetans were significantly different from A. astaci in three characteristics: 1) A. repetans was not capable of killing susceptible crayfish (the Australian crayfish Cherax destructor and the European noble crayfish Astacus astacus) following standardised experimental infection, 2) it had randomly amplified polymorphic DNA - polymerase chain reaction (RAPD-PCR) and internal transcribed spacer (ITS) region sequences different from the A. astaci reference strains and 3) it did not express chitinase constitutively during growth or sporulation (Royo et al. 2004).
Probably ubiquitous but some species may have limited distribution.
Probably not host specific. Fusarium spp. are widespread and often found in soil and plants. In crayfish, they have been reported infective to Astacus leptodactylus, Austropotamobius pallipes, Pacifastacus leniusculus and Procambarus simulans simulans (Chinain and Vey 1988c, Alderman and Polglase 1985, Edgerton et al. 2002). Trichosporon beigelii was found in the cuticle of Astacus astacus that had been stressed for a period of time in an experiment which was testing the effectiveness of MgCl2 for preventing the transmission of A. astaci between crayfish (Söderhäll et al. 1993). Trichomycete fungi are commonly found in the intestines and sometimes the cuticle of various freshwater crayfish and are considered to be non-pathogenic (Edgerton et al. 2002). Achlya sp. were reported in the soft cuticle of A. astacus (Vey 1986), on gill filaments of Cambarus affinis and A. leptodactylus in Europe (Edgerton et al. 2002 ), and infect the gill filaments of moribund Cherax quadricarinatus from Queensland, Australia (Herbert 1987). Saprolegnia parasitica has been isolated from A. leptodactylus in Turkey (Söderhäll et al. 1991) and experimentally found infective and lethal for A. astacus, P. leniusculus and Procambarus clarkii (Diéguez-Uribeondo et al. 1994). Aphanomyces laevis was demonstrated to infect wounds in P. clarkii. Aphanomyces repetans was isolated from crayfish (P. leniusculus and P. clarkii) in Spain and Italy, experimentally infectious to Cherax destructor and A. astacus (Royo et al. 2004) and later detected in A. pallipes in Italy (Cammà et al. 2010).
Impact on the host
Many species of fungi including Fusarium spp. and Saprolegnia spp. are opportunistic saprophytes which will invade dead or damaged tissues (i.e., crayfish that are wounded or stressed by poor water quality), but some species may also be primary pathogens. Examples of saprophytic oomycetes and fungi which have caused lesions in freshwater crayfish are Saprolegnia parasitica (pathogens of fish) and Trichosporon beigelii (pathogens of mammals), respectively. However, crayfish may act as vectors by transmitting the oomycetes/fungi to susceptible species (Söderhäll et al. 1993, Diéguez-Uribeondo et al. 1994). Usually the invading organism triggers a strong cellular defence reaction that appears as a dark patch (melanization especially around the hyphae). Infection may become systemic and lethal. Generally, the rate of mortality varies between crayfish species, species of fungus and whether crayfish are injured or uninjured at the time of infection. Secondary bacterial infection can also contribute to the death of the crayfish (Vey, 1986). Mortality under experimental conditions has been up to 100% (Maestracci and Vey, 1988). Death was attributed to physiological disturbances resulting from interference with moulting, exotoxin production by the fungus and disturbances of osmotic pressure and sodium and chloride ion concentration in the haemolymph (Maestracci and Vey 1988, Chinain and Vey 1988c, Edgerton et al. 2002). Exotoxins produced by F. solani were found responsible for some of the pathology associated with infection (Chinain and Vey 1988b). Chinain and Vey (1988a) found significant differences in pathogenicity among strains of F. solani which cause disease in freshwater crayfish.
Twenty percent of the healthy (non-injured) crayfish died after experimental challenge with zoospores of S. parasitica and there was no significant differences in susceptibility between three species of crayfish (A. astacus, P. leniusculus and P. clarkii) tested (Diéguez-Uribeondo et al. 1994). Various ubiquitous fungi may have a role in the regulation of crayfish populations. Fungus also infects dead eggs and moves onto live ones (Herbert 1987). Active females may remove dead eggs.
Gross Observations: Hyphal mass around a wound, or on eggs. Melanized spots may mark focal sites of infection in some crayfish.
Wet Mounts: Confirm by microscopic examination for hyphae. Shell lesions may also be colonized by bacteria (Geasa 2003).
Histology: Cuticular erosion to severe ulceration accompanied by epicuticle and endocuticle degeneration, melanization, and haemocyte infiltration into the epithelium below the cuticular lesion. Fungal hyphae that reach the haemocoel elicit a strong haemocytic reaction leading to the formation of large granulomas with a melanotic deposit formed in areas of haemocyte contact with the wall of the hyphae.
Culture: Surface disinfection of infected tissue with sodium hypochlorite and antibiotic solutions followed by incubation on malt agar at 25°C was used by Chinain and Vey (1988c) to isolate F. solani. The colonies of F. solani were initially cottony white and later developed a rose violet pigmentation. RGY agar with streptomycin sulphate and penicillum G incubated at 16°C was used by Alderman and Polglase (1985) to isolate F. tabacinum. Other species of fungi and Oomycetes can be cultured using the same methods as for A. astaci (Cerenius et al. 1988, Söderhäll et al. 1991).
DNA Probes: Royo et al. (2004) used the sequences of products from randomly amplified polymorphic DNA - polymerase chain reaction (RAPD-PCR) and the internal transcribed spacer (ITS) region in comparison to those from A. astaci reference strains as partial support for proposing a new species Aphanomyces repetans. Similarly, Ballesteros et al. (2006) used the sequence of the ITS regions of nuclear ribosomal DNA (ITS1+5.8S+ITS2) to identify A. frigidophilus (initially described from eggs of Japanese char, Salvelinus leucomaenis) as a parasite of crayfish in Europe. The polymerase chain reaction (PCR) diagnostic procedure designed to amplifies DNA from the ITS region of A. astaci (Oidtmann et al. 2004) was capable of detecting other species of Aphanomyces (A. frigidophilus and A. repetans) as well as Saprolegnia parasitica from infected crayfish and pure culture isolates of S. parasitica and Saprolegnia australis (Ballesteros et al. 2007). Because some Aphanomyces isolates from Austropotamobius pallipes associated with a crayfish plague outbreak in Italy produced negative results with the specific PCR assay for A. astaci, Cammà et al. (2010) used universal primers for amplification of ITS1 and ITS2 in fungal ribosomal RNA genes and detected A. repetans by sequencing the DNA product.
Methods of control
Infection probably results from poor water quality or poor husbandry. No confirmed therapeutic treatment, but superficial infestations may respond to saline baths. Lilley and Inglis (1997) indicated that hydrogen peroxide and Proxitane 0510 (containing 5% peracetic acid in hydrogen peroxide) have some potential as fungicidal treatments and disinfection, respectively.
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Alderman, D.J. and J.L. Polglase. 1988. Pathogens, parasites and commensals. In: D.M. Holdrich and R.S. Lowery (eds.). Freshwater Crayfish - Biology, Management and Exploitation. Timber Press, OR., p. 175.
Ballesteros, I., M.P. Martín and J. Diéguez-Uribeondo. 2006. First isolation of Aphanomyces frigidophilus (Saprolegniales) in Europe. Mycotaxon 95: 335-340.
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Cammà, C., N. Ferri, D. Zezza, M. Marcacci, A. Paolini, L. Ricchiuti and R. Lelli. 2010. Confirmation of crayfish plague in Italy: detection of Aphanomyces astaci in white clawed crayfish. Diseases of Aquatic Organisms 89: 265-268.
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Bower, S.M. (2012): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Fungal Invasion of Crayfish.
Date last revised: January 2012
Comments to Susan Bower
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