Embryo lethality assay as a tool for assessing virulence of isolates from bacterial chondronecrosis with osteomyelitis in broilers

We used an embryo lethality assay (ELA) to assess virulence for different isolates from cases of bacterial chondronecrosis with osteomyelitis (BCO) in broilers. Lameness is among the most significant animal welfare issues in the poultry industry. Bacterial infections are a major cause of lameness and different bacterial species have been obtained from lame broilers. Reliable lab-based assays are required to assess relative virulence of bacteria obtained from lame broilers. ELA has been used to assess lethal dosage of Enterococcus faecalis and Enterococcus cecorum. We hypothesized that ELA could substitute for more laborious and costly assessments of BCO isolate pathogenicity using live birds. We evaluated 2 different levels of bacteria injected into eggs from layer and commercial broiler embryos. Significant findings include 1) Escherichia coli from neighboring farms operated by the same integrator had very different embryo lethality, 2) isolate Staphylococcus agnetis 908 had low virulence in ELA, even though this isolate can induce more than 50% BCO lameness, 3) Enterococcus cecorum 1415 also had low pathogenicity; even though it was recovered from severe bilateral tibial dyschondroplasia, 4) human and chicken BCO isolates of S. aureus had significant pathogenicity, 5) virulence for some isolates was highly variable possibly corresponding with quality of the embryos/fertile eggs used, and 6) ELA pathogenicity was much lower for our BCO isolates than previous reports which may reflect maternal environment. Overall, ELA virulence and BCO virulence are not always concordant indicating that ELA may not be an effective measure for assessing virulence with respect to BCO.