Introduction
A line (F) of turkeys selected long-term for increased 16-week body weight was more susceptible to Pasteurella multocida (fowl cholera) (Saif et al., 1984; Sacco et al., 1991; Nestor et al., 1996ab), Erysipelas rhusiopathiae (erysipelas)(Saif et al., 1984); Newcastle disease virus (Tsai et al., 1992; Nestor et al., 1996b), and Bordetella avium (Sacco et al., 2000) than RBC2. Although the F and RBC2 lines differed in frequency of Class II major histocompatibility complex haplotypes (MHC; Zhu et al., 1995) and MHC genotypes differed in resistance to P. multocida and Newcastle disease virus (Nestor et al., 1996b), the line difference in resistance to these diseases could not be explained by changes in frequency of MHC haplotypes.
In order to determine if the disease susceptibility was specific to only the F line or was a general occurrence in most large-bodied turkey lines, the resistance of three large-bodied commercial sire lines, the F line, and the RBC2 line to challenge with P. multocida was compared (Nestor et al., 1996a). Two of the commercial sire lines had similar susceptibility to P. multocida as the F line while one commercial sire line was more resistant than the F line. All commercial sire lines were more susceptible to P. multocida than the RBC2 line. This paper summarizes several studies designed to evaluate changes in the immune system that might be responsible for the differences between the F and RBC2 lines in disease resistance.