No abstract

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.

1. Several studies have shown that genetic variation exists in response to various Salmonella strains in mammals and poultry. In the current study immunocompetence traits related to natural resistance to Salmonella were measured in 7 genetic groups of meat-type chickens (in total 296 chickens involved). 2. Variables were measured of both innate (phagocytic activity) and adaptive immune responses that are important after a natural or experimental Salmonella enteritidis infection. Two traditional Old Dutch Breeds (groups 1 and 2), four commercial broiler groups (groups 3 to 6), and one experimental broiler group (group 7) were used. In two periods, birds of each group were killed for examination at ages between 14 and 35 d post hatch. 3. Significant differences between groups were found for most immune variables measured, with significant correlations between several of them. All groups produced an adequate immune response, of either the innate or the adaptive type. 4. In the current study, group 2 showed the highest overall natural resistance, though none of the groups was uniformly superior with respect to all traits measured. 5. In conclusion, for reliable measurements of general immunocompetence or resistance to Salmonella, for example, it is important to measure several aspects of the immune system.

No abstract