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Estimates of strain effects, heritabilities and genetic correlations for pond survival, resistance to Aeromonas hydrophila and resistance to koi herpesvirus (KHV) were obtained from a diallel cross of 92 full-sib families of common carp produced from four strains (Szarvas 15, Tata, Duna and Amur) and using five females and ten males per strain. Disease resistance was obtained from survival data from challenge-tests using intraperitoneal injection for A. hydrophila and cohabitation for KHV. Two separate challenge-tests were conducted for each disease. The overall survival rates were 44% and 34% for the two tests of A. hydrophila , and 7% and 5% for the two tests of KHV. Pond survival (averaging 78%) was observed over a six months period prior to harvest (at approximately 18 months of age). The three traits were analysed jointly in a multivariate threshold model. For KHV the strain Szarvas 15 had the lowest observed (purebred) survival (0%) followed by Amur (11%), Duna (12%) and Tata (21%), while for A. hydrophila , the lowest (purebred) survival was observed for Duna (28%) followed by Amur (31%), Szarvas 15 (38%) and Tata (48%). Heterosis was not significant for KHV and A. hydrophila resistance, although highly significant for pond survival. The estimated heritability (on the underlying liability scale) was low (0.04 ± 0.03) for A. hydrophila resistance, very high (0.79 ± 0.15) for KHV resistance, and moderate (0.34 ± 0.09) for pond survival. The genetic correlation between the two challenge-tested diseases (KHV and A. hydrophila ) was moderately high (0.61 ± 0.29), although uncertain, while the estimated genetic correlations between pond survival and the two challenge-tested diseases were low and not significantly different from zero (0.01 ± 0.28 and − 0.22 ± 0.21 for A. hydrophila and KHV, respectively). The latter may be expected for KHV, as no outbreaks of the disease had been observed in Hungary. Based on the favourable heritabilities of KHV and pond survival there is good prospect for joint improvement of these two traits in common carps through genetic selection.

The ELISA technique was adapted for the rapid identification of infectious haematopoietic necrosis virus and two eel rhabdoviruses in cell cultures or directly from tissues of infected fry. Results were obtained in under 2 h. The assays were specific for the viruses when infected cell cultures were tested, but there were low-level cross-reactions with certain other rhabdoviruses when the test was applied to extracts of infected fry. Attempts to produce assays for the detection of viral haemorrhagic septicaemia virus and spring viraemia of carp virus were unsuccessful, apparently because the titres of the antisera were too low.