Bacterial chondronecrosis with osteomyelitis ('femoral head necrosis') of broiler chickens: a review

Bacterial chondronecrosis with osteomyelitis (BCO) in chickens was first re ported in 1972 and is now recognized as an important cause of lameness in b roiler chickens. Recent systematic studies of causes of lameness in birds r eared in Northern Ireland have shown that it was the most common cause of l ameness, being present in 17.3% of lame birds. Furthermore, it was also det ected in birds presented as "found dead". Overall losses in male birds due to BCO were estimated to be 0.75% of all birds placed, which, in addition t o welfare concerns, represents considerable economic loss. The disease has been seen in birds ranging from 14 to 70 days of age, but most cases occurr ed around 35 days old. It is most commonly caused by Staphylococcus aureus, but Escherichia coli, coagulase-negative staphylococci and Enterococcus sp p. are sometimes involved, as are, rarely, other bacteria. The lesions are most commonly found associated with the growth plates of long bones, partic ularly the proximal growth plate of the femur and tibiotarsus, but other bo nes may also be affected. Since lesions were visible to the naked eye in on ly 40 to 67% of cases, histological examination is recommended where no les ions are visible macroscopically. As the lesion may be present in only one growth plate, and because histological examination is often not carried out , BCO is almost certainly underdiagnosed. The exact pathogenesis of the con dition is unknown, but it is thought that adherence of blood-borne bacteria to exposed cartilage at the tips of metaphyseal blood vessels is fundament al. Under controlled experimental conditions, infection of birds with the i mmunosuppressive viruses chicken anaemia virus and infectious bursal diseas e virus increased the incidence of the disease, while restricting feed inta ke reduced the incidence of disease. S. aureus strains identical to, or clo sely related to, isolates recovered from naturally occurring cases of the d isease (as determined by pulsed-field gel electrophoresis) have been recove red from fluff-debris in hatcheries, and also from the environment of breed ing flocks, indicating that infection in the breeding farm and in the hatch ery could be an important source of infection. It has also been shown that humans can carry poultry strains of S. aureus on their hands. There is a hi gher incidence of BCO in birds hatched from floor eggs. Thus, hygiene and m anagement practice on breeder farms and in the hatchery may influence the o ccurrence of the disease. Bacteraemia is a prerequisite for BCO. Indeed, in some flocks suffering losses due to BCO, there are also losses due to stap hylococcal septicaemia. Thus, appropriate treatment of affected flocks shou ld reduce losses due to septicaemia. It should also reduce the occurrence o f bacteraemia and the development of further cases of BCO. However, birds i n which BCO has already developed, are unlikely to respond to treatment. Co ntrol of BCO by vaccination seems unlikely in the short term. Simple bacter ins have not been effective and much basic research is needed to identify t he important virulence factors. Furthermore, more than one type of bacteriu m is capable of causing the disease. Bacterial interference has been used s uccessfully in humans and turkeys to prevent staphylococcal diseases, and w arrants investigation for the prevention of BCO in chickens. This may have an advantage in that the interfering bacterium may also exclude some of the other bacteria that can cause BCO. The recent development of a disease mod el in which S. aureus is given by a natural route allows the potential for further investi gation of the role of predisposing factors, and intervention strategies, in cluding vaccination and bacterial interference, for the prevention of BCO.