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Using experimental infections, three traits for salmonella resistance were studied: mortality, survival time (in animals that died by infection), and quantitative cecal salmonella carriage at the end of the rearing period (in animals that did not die). In total, 548 animals were used; mortality was 29.2%, mean survival time was 5.97 d (n = 160), and the mean 10log of colony forming units per gram of cecal contents was 1.62 (n = 387). Genetic parameters were evaluated in bivariate threshold-linear models to account for the selective measurement of survival time and cecal carriage. Heritabilities were .06 for survival time, .09 for cecal carriage, and .12 for mortality. The genetic correlation between mortality and cecal carriage was weak (.26), which suggests that these traits are largely controlled by different genes. The genetic correlation between mortality and survival time was relatively strong (-.68). Simultaneous study of multiple traits seems to be of particular importance in judging epidemiological consequences of a possible selection for resistance. Results here indicate that selection on decreased mortality could be unfavorable for the spread of salmonella because the resulting correlated increase in survival time, implying longer shedding by infected animals, is relatively stronger than the correlated decrease in level of cecal carriage. Selection to reduce the level of cecal salmonella carriage could be done while keeping survival time constant, if so desired, because the correlation between these traits is weak (-.15).

Disease is a major issue in animal production systems and society demands that the use of medicines and vaccines be reduced. This review describes the breeding approaches that could be used to improve disease resistance and focuses especially on their application to pigs. Disease reduction by genetic means has certain advantages through cumulative and permanent effects, and direct and indirect selection methods are available. Direct selection for disease incidence requires, besides a unique pig identification and disease registration system, challenge routines that are inconvenient in intensive pig production. Indirect selection for the expression of immune capacity may be an alternative but requires detailed knowledge of the different components of the immune system. There is ample opportunity for genetic improvement of the immune capacity because immune traits show substantial genetic variation between pigs. We therefore conclude that indirect selection via immune traits is very interesting, also for practical implementation, and that there is an urgent need for knowledge, within lines, about the genetic relationships between immune capacity traits and resistance to specific diseases or to disease incidence in general. Furthermore, knowledge about the relationship between immune system traits and production traits is needed as well as knowledge about the effect of selection on the epidemiology of disease at a farm/population level and on the host-pathogen interaction and coevolution.

Salmonella infections originating from poultry are one of the major causes of food-borne disease. For the control of salmonella in poultry a multifactorial approach is more likely to be effective, and the genetic resistance of poultry breeds to salmonella infections may be a valuable contribution. Experimental Salmonella enteritidis infections were examined in three different broiler outbred lines: the FC line, which had been selected for feed conversion efficiency; the R line, which had been selected for growth rate; and the C line, a commercially available line. The FC line had the highest mortality rate after intramuscular inoculation with 5 3 10 6 colony forming units (CFU) of S. enteritidis at 2 weeks of age (40% versus 21 and 20% in the other lines). However, at slaughter age, the number of birds carrying salmonella in caecal contents, and the concentration of salmonella in the caecal contents, was lowest in the FC line. The FC and R lines were compared by inoculation with doses ranging from 10 2 to 10 7 CFU S. enteritidis. At sublethal doses (10 5 CFU or less), the FC line carried significantly less salmonella in caecal contents and the rate of systemic infection was lower. The start of shedding was also delayed compared with the R line. At doses of 10 6 CFU S. enteritidis or higher, there were no differences in salmonella carriage between the lines, and the FC line showed higher mortality. In conclusion, resistance to mortality and resistance to the carriage of S. enteritidis do not necessarily coincide within lines, as the FC line showed high mortality but low carriage, both in survivors of high infection doses and in all birds at lower infection doses.