Genetic parameters for resistance against Flavobacterium columnare in Nile tilapia Oreochromis niloticus (Linnaeus, 1758)

Columnaris disease is a major cause of mortality in tilapia hatcheries and commonly occurs during the summer season in Thailand. One way of reducing the problem is by selective breeding for increased disease resistance. The objective of this study was to estimate quantitative genetic parameters for resistance against columnaris in the Chitralada 4 strain of Nile tilapia. Data from 43 full-sib families (2,580 records) of fry (age = 32 +/- 4 days post-hatch) were used in the analyses. Initially, fry were subjected to bath challenge with Flavobacterium columnare (LD50 concentration = 1.2 x 10(6) CFU/ml) for 14 days. Disease resistance was defined as the number of days from challenge until death (DD) or as a binary trait (dead/alive) on day 14. Linear animal and sire-dam models were used for DD, while threshold animal, threshold sire-dam, binary linear animal and binary linear sire-dam models were used for binary outcomes. Covariate effect of age, fixed effect of challenge day and random effects of the individual animals or sires and dams were included in the models. Mean survival was 32.4 +/- 11.6%, and survival rates of the best and poorest families were 70% and 8%, respectively. The highest estimate of heritability (0.30 +/- 0.025) was obtained under the threshold sire-dam model. Heritability estimates for DD (0.16 +/- 0.034 and 0.17 +/- 0.046) were comparable to those obtained from the threshold animal (0.15 +/- 0.031) and the binary linear (0.14 +/- 0.045 and 0.15 +/- 0.044) models. The linear animal and sire-dam models for DD and the threshold sire-dam models performed equally with similar values of r(EBV) (0.629, 0.628 and 0.627) and accuracy of selection (0.793, 0.793 and 0.791). This study reveals the potential of selective breeding to increase disease resistance to F.columnare in the studied population of Nile tilapia.