THE EFFECTS OF DEXAMETHASONE IMMUNOSUPPRESSION ON TURKEY OSTEOMYELITIS COMPLEX IN AN EXPERIMENTAL ESCHERICHIA-COLI RESPIRATORY-INFECTION

Six hundred male turkeys were maintained in floor pens for 5 wk at whi ch time half of the birds were given three intramuscular injections of 2 mg/kg BW of dexamethasone (DEX) on alternating days. On the day of the third DEX injection, the left thoracic air sac of each bird was in jected with sterile tryptose phosphate broth (TPB) or with TPB contain ing approximately 1 x 10(2), 1 x 10(3), 1 x 10(4), or 1 x 10(5) cfu of Escherichia coli. All mortalities and birds necropsied at 14 and 15 d postchallenge were scored for air sacculitis/pericarditis (AS) and tu rkey osteomyelitis complex (TOC). Cumulative mortality and AS score we re both increased by either DEX treatment or E. coli. Although TOC inc idence was significantly increased by the lowest titer of E. call inoc ulation, increasing the number of bacteria inoculated did not increase TOC incidence due to increased mortality before TOC lesions developed . The DEX treatment by itself increased TOC incidence and there was a synergistic interaction between DEX treatment and E. coli on TOC incid ence. Both DEX treatment and E. coil significantly decreased BW. Relat ive weights of liver, heart, and spleen were significantly increased b y both E. coli and DEX, whereas both treatments significantly decrease d relative weight of the bursa of Fabricius. The number of positive ba cterial isolations from tissue and the heterophil to lymphocyte ratio were increased by both DEX treatment and E. coli challenge. These resu lts suggest that stress-induced immunosuppression may be involved in t he etiology of TOC, and that bacterial respiratory infection can lead to the development of TOC lesions.