Additive genetic variation for resistance to clinical and sub-clinical diseases (i.e., lameness, respiratory diseases, diarrhoea, and arthritis) exists in growing pigs (Smith et al., 1962 ; Lundeheim, 1988 ; Lingaas and Rønningen, 1991 ; Henryon et al,. 2001). This finding suggests that selecting pigs for resistance to clinical and sub-clinical disease could be successful. However, the level of success is largely dependent upon the reliability with which breeding values of the pigs can be estimated for resistance. Using the incidence of clinical and subclinical disease prior to selection as the sole selection criterion often results in unreliable breeding value estimates. Consequently, selective breeding would be more successful if other traits, which reflect resistance of the pigs, could be identified and used as additional selection criteria. Such traits may include total and differential numbers of white blood cells (i.e., leukocytes). Total and differential numbers of leukocytes may reflect resistance, as the immune system, with its complex interactions of innate and adaptive response mechanisms, depends upon the activities of leukocytes. In its simplest form, innate response largely depends upon granulocytes (i.e., neutrophils, eosinophils, and basophils) and monocytes, while adaptive response involves lymphocytes. 

M. Henryon, H. R Juul-Madsen, P. Berg

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 2002. Session 13, , 13.02, 2002
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