The major causes of leg weakness/lameness were investigated in two male commercial broiler flocks. The numbers of dead and lame birds culled from the flocks each day were recorded by the flock managers. Forty-four lame birds and 22 sound birds were examined postmortem during a period of six weeks and the proximal and distal end of each femur, tibiotarsus and tarsometatarsus were examined histologically. Attempts were made to isolate bacteria and viruses from the proximal end of each femur. Blood samples were examined for antibodies to chicken anaemia virus (CAV), infectious bursal disease virus (IBDV) and Mycoplasma species. Bacterial chondronecrosis with osteomyelitis was identified in the proximal end of the femur of eight of the 44 lame birds, and in the proximal end of the tibiotarsus of a further bird (20.4 per cent). Gram-positive bacteria were present in all the lesions. Staphylococcus aureus was recovered from 62.5 per cent of the lesions confirmed by histology. Bacterial chondronecrosis associated with S aureus has thus been identified as an important cause of leg weakness in these commercial broilers. Lesions suggestive of the condition were visible macroscopically in only 11.1 per cent of the cases subsequently diagnosed by histology and bone histology is therefore required before a diagnosis can be excluded. Angular limb deformities (13.6 per cent) and spondylolisthesis (11.4 per cent) were the most common macroscopic lesions identified as causes of lameness. The overall incidence of tibial dyschondroplasia was similar in both the lame and sound broilers, but severe lesions were found only in lame birds (4.5 per cent).

A series of experiments was designed in an attempt to reproduce bacterial chondronecrosis with osteomyelitis in broiler chickens using a natural route of infection. Birds in isolators were exposed to a suspension of Staphylococcus aureus by aerosol or exposed to S. aureus and subsequently inoculated with chicken anaemia virus (CAV) alone, or with CAV and infectious bursal disease virus (IBDV). Subsequently, S. aureus was recovered and bacterial chondronecrosis with osteomyelitis was diagnosed, by histology, in the proximal end of the femur and/or tibiotarsus of lame birds exposed to S. aureus with and without CAV and IBDV infections. Birds fed 60% of the recommended feed intake for the breed developed a lower incidence of S. aureus infection and/or bacterial chondronecrosis (P < 0.05) than birds fed 100% of the recommended intake. A significantly lower incidence of S. aureus was recovered (P < 0.05) in birds simultaneously exposed to S. aureus and inoculated with CAV and IBDV at day 21, than in birds exposed to S. aureus at day 10, and inoculated with CAV and IBDV at day 21. With the exception of birds exposed to S. aureus at 1 day old, a higher incidence of bacterial chondronecrosis was diagnosed in birds exposed to S. aureus and inoculated with CAV and IBDV than in birds exposed to S. aureus alone. It is hypothesised that inoculation with CAV and IBDV at day 21 enhanced the development of bacterial chondronecrosis in birds exposed to S. aureus at day 10 and fed 100% of the recommended feed intake or ad libitum.

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