The effects of methyl prednisolone immunosuppression on Khazak bird osteomyelitis complex in an experimental Escherichia coli respiratory infection in Zabol

Material methods: Onehundred male Khazakbirdwere maintained in floor pens for 5 wk at which time half of the birds were given three intramuscular injections of 2 mg/kg BW of methylprednisolone on alternating days. On the day of the third methylprednisolone injection, the left thoracic air sac of each bird was injected with sterile tryptose phosphate broth (TPB) or with TPB containing approximately 1x10(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 Khazakbird osteomyelitis complex (KOC). Cumulative mortality and AS score were both increased by either methylprednisolone treatment or E. coli. Although KOC incidence was significantly increased by the lowest titer of E. coli inoculation, increasing the number of bacteria inoculated did not increase KOC incidence due to increased mortality before KOC lesions developed. Results: Themethylprednisolone treatment by itself increased KOC incidence and there was a synergistic interaction between methylprednisolonetreatment and E. coli on KOC incidence. Both methylprednisolone treatment and E. coli significantly decreased BW. Relative weights of liver, heart, and spleen were significantly increased by both E. coli and methylprednisolone, whereas both treatments significantly decreased relative weight of the bursa of Fabricius. The number of positive bacterial isolations from tissue and the heterophil to lymphocyte ratio were increased by both methylprednisolone treatment and E. coli challenge. These results suggest that stress-induced immunosuppression may be involved in the etiology of KOC, and that bacterial respiratory infection can lead to the development of KOC lesions.