FAO Fisheries Technical Paper 402/2 - Library - Network of ...

C.11 Crayfish Plague• growth within the agar medium (unless culturedat < 7°C, which promotes superficialgrowth);• colourless colonies;• aseptate, highly branching, vegetative hyphae,7-9 µm in diameter (min-max 5-10 µm);• young hyphae are densely packed withcoarse, granular cytoplasm and containhighly refractile globules;• older hyphae are highly vacuolated and theoldest hyphae appear to be emptyWhen thalli are transferred from the culturemedium to sterile distilled water, they developsporangia within 12-15 h (20°C) or 20-30 h(16°C). Elongate, irregularly amoeboid shapedspores are released and rapidly encyst as amass around the sporangial tip(Fig.C.11.3.2.1a). Encysted primary sporesmeasure 9-11 µm in diameter (min-max 8-15µm). Release of the secondary zoospores occursfrom papillae that develop on the surfaceof the primary spore cyst. This occurs at temperaturesas low as 4°C, peaking at 20°C andstopping at temperatures >24°C. Thezoospores have lateral flagella and measure 8x 12 µm. More details on culture media, techniquesand developmental stage morphologyare provided in the OIE Manual (OIE 2000).(EAFP/DJ Alderman)Fig. C.11.3.2.1a. Fresh microscopic mount ofa piece of infected exoskeleton showing fungalspores.(EAFP/DJ Alderman)aC.11.3.2.2 Bioassay (Level I/II)Confirmation of crayfish plague can be doneusing zoospores cultured from fungal isolatesfrom suspect crayfish tissues. Rapid mortalitiesin the susceptible crayfish, along with reisolationof the fungus as described above,should be considered conclusive for A. astaci.C.11.4 Diagnostic MethodsMore detailed information on methods for diagnosisof crayfish plague can be found in theOIE Diagnostic Manual for Aquatic Animal Diseases(OIE 2000), at http://www.oie.int or selectedreferences.There is no other disease, or pollution effect,that can cause total mortality of crayfish butleave all other animals in the same water unharmed.In such situations and with known susceptiblespecies, presumptive diagnosis can befairly conclusive. In first-time cases or in situationswith resistant species, however, confirmatoryisolation of the pathogen is recommended.bFig. C.11.4.1.1a,b. Clinical signs of infectedcrayfish showing whitened necrotic musculaturein the tail, and often accompanied inchronic infections by melanisation (blackening)of affected exoskeleton.C.11.4.1 PresumptiveC.11.4.1.1 Gross observation (Level I)Large numbers of crayfish showing activity duringdaylight should be considered suspect,since crayfish are normally nocturnal. Somemay show uncoordinated movement, easily tiponto their backs, and be unable to right themselves.Gross clinical signs of crayfish plague vary fromnone to a wide range of external lesions. White212