The effect of a second dexamethasone treatment on turkeys previously challenged in an experimental Escherichia coli respiratory model of turkey osteomyelitis complex

Abstract

In two separate experiments, turkeys that had survived immunosuppression with dexamethasone (DEX) and air sac inoculation with low numbers of Escherichia coli at 5 wk of age were maintained until 13 wk of age, at which time they were given a second treatment with DEX. All mortalities and birds that were necropsied 8 and 15 d (Experiment 1) and 21 d (Experiment 2) after the last DEX injection were scored for air sacculitis/pericarditis and turkey osteomyelitis complex (TOC). In both experiments, all of the lesions that characterize TOC were reproduced, including osteomyelitis of the proximal tibia, synovitis/tendonitis, abscesses in the soft tissues, and green liver. In Experiment 1, all mortalities after Day 7 had TOC lesions, whereas 44% of mortalities had green livers. Staphylococcus aureus was isolated from 90% of all TOC lesions cultured. In Experiment 2, the incidence of mortality, air sacculitis, TOC, and green liver as well as the heterophil:lymphocyte ratio were significantly higher in birds that had previously been treated with DEX but had never been challenged with E. coli than in birds that had survived both treatment with DEX and challenge with 25 or 50 cfu of E. coli. Staphylococcus aureus was isolated from 73% of TOC lesions cultured, whereas E. coli was isolated from only 5.4% of the lesions. These studies suggest that TOC incidence may be related to a stress-induced susceptibility to opportunistic infection.