Spontaneous and stimulated chemiluminescence responses of blood leukocytes from healthy and infected tilapia

A bimodal distribution in the spontaneous chemiluminescence (SpCL) measurements in naive tilapia leads us to divide the fish into low and high responder groups. Because of the homogeneity in both differential phagocyte cell counts and stimulation indexes, it is suggested that differences between the responder groups are due to innate intrinsic factors. Difference between these two groups is also displayed, in a less significant manner, in zymosan stimulated chemiluminescence (zStCL) that showed a weak linear regression factor (R²=0.4756). In naive tilapia, Streptococcus difficile bacterin-stimulated chemiluminescence (bStCL) values were much lower than those obtained in zStCL When sibling fish were infected with live S. difficile bacteria, a spectacular increase in bStCL values, with a high stimulation index of 42, was displayed on day 6 after infection. The SpCL and bStCL values increased considerably in diseased fish which displayed a serious symptomatology on day 36 after infection. However, fish that developed resistance to the disease showed lowered values in both SpCL and bStCL. The implication of these findings in the in vivo production of reactive oxygen radicals by phagocytic cells and their connection to resistance and susceptibility to disease are discussed in comparison to mammals.