A study on subcutaneous infection model of colibacillosis in broilers: Selected immunological and hematological aspects

•Induction of colibacillosis by sc route in inguinal region is associated with a systemic response.•Severe changes in number and morphological features of blood cells (WBCs and thrombocytes) are observed.•Liver is more severely affected than the lung tissue in histopathological examination.•Lymphoid tissue depletion is observed in thymus, bursa and spleen.•Changes in serum cytokine levels (IL-6, IL-10 and IFN-γ) were not detected in this model, 3 days after inoculation. Colibacillosis is still a major challenge to poultry industry. This study examined selected immune and hematological features of broilers experimentally infected with E. coli via subcutaneous (sc) route. Fifty-one 5-week-old chickens were randomly assigned to three groups: two controls (NC and SCC) and one experimental (SC) group. The SC group was inoculated by O2 E. coli suspension via sc route in the inguinal region. Three days after inoculation, blood sampling was performed. Bacterial count of liver was determined. Histopathological samples of liver, lung, spleen, thymus, and bursa were also collected. Serum total protein and globulin significantly increased in birds of SC group as compared to control birds (P < 0.05). Birds in SC group also showed decreased albumin/globulin ratio. Serum levels of IL-6, IL-10, and IFN-γ remained statistically the same among groups. Lymphocytosis, monocytosis, thrombocytosis and heterophilia were present in SC group. Heterophils/lymphocytes ratio increased in SC birds. Left shift was observed in SC group with cells showing vacuolated cytoplasm that contained toxic granules. Histologically, lung involvement was less severe than liver. Lymphoid depletion was present in all examined lymphoid organs. E. coli growth was observed in liver samples from SC group but not in control birds. In conclusion, using sc route for induction of colibacillosis was associated with a systemic inflammatory response in broilers and histopathological lesions of thymus, bursa, and spleen three days post-infection. It seems that detection of changes in serum cytokine levels (IL-6, IL-10, and IFN-γ) should be performed in earlier hours of the disease in this model.