High infection intensity of S. mansoni should be probably controlled by a major locus SM1 (Abel et al., 1991) mapped to chromosome 5q31-q33 that contains three cytokine genes notably Interleukin 4 (IL4), Interleukin 13 (IL13) and Interleukin 5 (IL5) that are implicated in the Th2 immune response (Marquet et al., 1996; Marquet et al., 1999; Kouriba et al., 2005). Here, IL5 is linked to infection.