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SummaryThe crayfish plague ( Aphanomyces astaci ) susceptible freshwater crayfish Astacus astacus and the resistant species Pacifastacus leniusculus were compared with respect to differential haemocyte count and expression of prophenoloxidase and peroxinectin. A major difference found was that resistant crayfish continuously produced high levels of prophenoloxidase (proPO) transcripts and that these levels could not be further increased, whereas in susceptible crayfish proPO transcript levels and resistance were augmented by immunostimulants. In As. astacus this could be registered as higher proPO transcript levels in the semigranular population of haemocytes and to an increased survival time after experimental infections with A. astaci .

The signal freshwater crayfish Pacifastacus leniusculus was found to be susceptible to infection with white spot syndrome virus (WSSV). Histopathological observations of various tissues of virus-injected crayfish showed similar symptoms to those from WSSV-infected penaeid shrimp, but no appearance of white spots on the cuticle or reddish body colour were observed, although these are the prominent gross signs of white spot disease in shrimp. A gene probe for detecting WSSV was developed in order to detect the virus in affected cells and tissues using in situ hybridisation. Strong signals were observed in cells of virus-injected crayfish, but not in control-injected crayfish. The number of granular haemocytes in virus-injected crayfish was significantly higher than in sham-injected and non-injected crayfish from Days 5 to 8 (p ≤ 0.05) and Days 3 to 8 (p < 0.01) postinjection, respectively. The proportion of granular haemocytes in virus-injected crayfish was also significantly higher than in sham-injected controls from Days 3 to 8 (p < 0.01). These results indicate that WSSV has a significant effect on the proportion of different haemocyte types in the freshwater crayfish.

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Nineteen channel catfish isolates of Saprolegnia sp. obtained from 5 separate fish farms in Mississippi, which became affected by winter kill syndrome during 1991 and 1996, were investigated with respect to physiological characteristics and genetic variation. Isolates of S. parasitica from crayfish and S. diclina were included for comparison. Most strains of catfish isolates grew well at 20 and 30°C. Repeated zoospore emergence was found in catfish isolates of Saprolegnia sp. and S. parasitica, but not in S. diclina. Random amplification of polymorphic DNA polymerase chain reaction (RAPD-PCR) was applied for a further characterisation of the isolates. The RAPD analysis among Saprolegnia spp. isolates was constructed from 686 amplified products in 67 separable positions and indicated that the catfish isolates of Saprolegnia sp. are composed of 3 genetically distinct groups.KEY WORDS: Saprolegnia · Saprolegniosis · Fish disease · Random amplification of polymorphic DNA (RAPD) Resale or republication not permitted without written consent of the publisherDis Aquat Org 45: [53][54][55][56][57][58][59] 2001 signs of winter saprolegniosis (formerly termed winter kill syndrome). In the case of one fish, isolates were obtained from the eye and fin. All catfish were obtained from catfish farms, at least 50 miles (~80 km) apart, but located within the Mississippi Delta, USA. Isolates obtained in 1991 are designated CF91 and comprise CF91-1 (now deposited in the American Type Culture Collection; ATCC # 200048), CF91-2, CF91-4, CF91-5, CF91-6 and CF91-9. All 1991 isolates were collected from different fish in one pond at Dyche Plantation catfish farm (designated Farm 1, Table 1; Bly et al. 1992, wherein these isolates are referred to as CF1 through CF9). All other isolates collected from catfish were obtained in 1996 and are designated as CF96. CF96-1 through CF96-13 were obtained from different places of origin (see Table 1) in order to determine if geography had an effect on Saprolegnia spp. genetic variation. Four farms were sampled in 1996, TAM-AN (Farm 2), Holly Ridge (Farm 3), Topcat (Farm 4) and Higg (Farm 5). Other strains, i.e. S. parasitica (Spt) isolated from freshwater crayfish Astacus leptodactylus (Söderhäll et al. 1991) and S. diclina (Sdi; ATCC # 42062), were included for comparison. All isolates were maintained on PG-1 agar medium (Unestam 1965).Hyphal radial growth rate. The inoculum was prepared by cutting at the advancing edges of a young colony grown on PG-1 agar with a 6 mm diameter borer. The different Saprolegnia isolates were inoculated on a fresh PG-1 agar dish. The colony hyphal radial growth rates were determined as described by Trinci (1969) at 20 and 30°C. Comparisons of hyphal growth rate between isolates were made using a Student's t-test. Differences were considered significant at p ≤ 0.05.Repeated zoospore emergence. Repeated zoospore emergence was performed according to the method described by Diéguez-Uribeondo et al. (1994). Briefly, mycelia were grown in PG-1 drop cultures ...