Dexamethasone triggers lameness associated with necrosis of the proximal tibial head and proximal femoral head in broilers

Bacterial chondronecrosis with osteomyelitis (BCO) and turkey osteomyelitis complex (TOC) are characterized by bacterial infection and necrotic degeneration within the tibiae and femora. Stress and immunosuppression have been implicated in the pathogenesis of BCO and TOC. Immunosuppressive doses of dexamethasone (DEX) trigger high incidences of TOC in turkey poults. The present study was conducted to determine if DEX injections or heat stress can trigger BCO and lameness in broilers. In 3 independent experiments, broilers were weighed and either remained uninjected or received repeated injections of 0.9% saline or DEX dissolved in saline (0.45 to 1.5 mg of DEX/kg of BW). Across all 3 experiments, the incidences of lameness were 0% for uninjected controls, 0 to 8% in saline-injected groups, and 24 to 68% in groups injected with 0.9 to 1.5 mg of DEX/kg of BW. Growth was inhibited by DEX injections regardless of whether the birds became lame or survived. When compared with saline-injected groups, DEX injections consistently increased the incidence of severe proximal tibial head necrosis in lame birds as well as in survivors. The DEX injections also triggered a subset of lesions that are not considered pathognomonic for BCO (for example, avascular femoral head necrosis and fatty necrosis of the tibiae). In a fourth experiment, repeated episodes of heat stress did not trigger lameness, although the subclinical incidence of tibial head necrosis was substantially higher at 28 and 35 d of age in heat-stressed broilers when compared with broilers reared under thermoneutral conditions. Accordingly, stress and immunosuppression must be considered contributing factors in the pathogenesis of tibial and femoral lesions associated with lameness in broilers. A subset of the lesions triggered by repeated DEX injections did not precisely mimic the pathogenesis of BCO in broilers, and DEX consistently inhibited growth whereas BCO is associated with rapid growth. These caveats must be acknowledged when DEX is used to trigger lameness in broilers.