Chlorpyrifos toxicity and its association with Salmonella gallinarum infection in broiler chickens: an immunotoxicological and patho-logical analysis

A study was conducted on 270 broiler chicken to elucidate the immuno-toxicological and pathological effects of chlorpyrifos (CPF) toxicity and its interaction with Salmonella Gallinarum (S. Gallinarum) infection. Out of 270, forty chicken were used for calculating the Maximal tolerated dose (MTD) of CPF. Two hundred and thirty (230) chickens at age of 14 days were divided into three groups viz. A (n=80), B (n=80) and C (n=70). All the birds of groups A and B were orally given CPF @ 1.5 mg/kg b. wt. (1/5 of MTD) and 0.75 mg/kg b. wt. (1/10 of MTD), respectively throughout the experiment. CPF was not given in group C. Chickens of groups A, B and C were again subdivided into 02 groups at the age of 21 days. A₁ and A₂ (n=40 chicks each), B₁ and B2 (n=40 chicks each) and C1 (n=30) and C2 (n=40), respectively. Each bird of group A2, B2 and C2 was intraperitoneally injected with S. Gallinarum (@10⁷ CFU). Blood was collected from five birds of each subgroup directly from heart just prior to infection and subsequently on 3^rd, 7^th, 14^th, 21^st and 28^th day post infection in sterile tubes for serum separation. Serum samples were stored in aliquots at -20°C until its use for the study of humoral mediated immune response. Blood samples were also collected in hep-arinized vials for cell mediated immune response. After collection of blood, these birds were sacrificed and they were subjected to thorough post-mortem examination. Representative tissues of the different immune organs such as spleen, thymus and bursa of Fabricius were collected in 10% buffered formalin for histopathological studies. S. Gallinarum infection clinical signs were more severe in group A₂, followed by B₂ and C₂. In S. Gallinarum infected group C₂ on 3 DPI, gross lesions noticed were bronze discolouration of liver with large necrotic foci, small raised necrotic foci and misshapen heart and congestion in thymus, enlarged spleen, congested lungs and catarrhal exudate in the intestine. Birds in the infected group A₂ (1/5^th MTD of CPF+S. Gallinarum) showed similar lesions in different organs but were more severe as compared to group C₂. The gross changes in group A₂ were noticed upto 28 DPI. Regarding the birds of group B₂ (1/10^th MTD of CPF+S. Gallinarum), gross changes mentioned above were of mild magnitude as compared to group A₂ but of higher magnitude as compared to group C₂. In chicks of chlorpyrifos treated group A₁ (1/5^th MTD of CPF), lesions observed were anaemic carcass, mottled liver; congestion and haemorrhages in intestine, pancreas was swollen and congested, lungs, kidneys and brain were also found congested from 7 days post chlorpyrifos treatment (DPCT) onwards. Gross pathological changes in chicks of group B₁ (1/10^th MTD of CPF) were similar to that of group A₁ but they were of lesser severity. The histopathological lesions observed were congestion, haemorrhage, depletion of lymphocytes in spleen, multiple necrotic foci and reticulo-endothelial cells proliferation in spleen; congestion, haemorrhage and multiple small necrotic areas of lymphocytes in thymus; bursa of Fabricius showed mild depletion of lymphocytes in the follicles in all the S. Gallinarum infected and CPF intoxicated groups. However, severity of these lesions in group B₁ and B₂ was lesser than group A₁ and A₂, respectively. Mean reciprocal log₁₀ antibody titer difference between all the infected groups was found to be non-significant. Intoxication with CPF caused significantly lower values of lymphocytes proliferation assay (LPA) and phenotypic expression of avCD4+ and avCD8+ T-lymphocyte subpopulation. Present study finally elucidated that chlorpyrifos toxicity amplify the severity of the S. Gallinarum infection as well as impaired cell mediated immune response in broiler chicken.