Disarticulation-Associated Femoral Head Separation in Clinically Normal Broilers: Histologic Documentation of Underlying and Predisposing Cartilage Abnormalities

Downregulation of growth plate genes involved with the onset of femoral head separation in young broilers

Femoral head separation (FHS) is characterized by the detachment of growth plate (GP) and articular cartilage, occurring in tibia and femur. However, the molecular mechanisms involved with this condition are not completely understood. Therefore, genes and biological processes (BP) involved with FHS were identified in 21-day-old broilers through RNA sequencing of the femoral GP. 13,487 genes were expressed in the chicken femoral head transcriptome of normal and FHS-affected broilers. From those, 34 were differentially expressed (DE; FDR ≤0.05) between groups, where all of them were downregulated in FHS-affected broilers. The main BP were enriched in receptor signaling pathways, ossification, bone mineralization and formation, skeletal morphogenesis, and vascularization. RNA-Seq datasets comparison of normal and FHS-affected broilers with 21, 35 and 42 days of age has shown three shared DE genes (FBN2, C1QTNF8, and XYLT1) in GP among ages. Twelve genes were exclusively DE at 21 days, where 10 have already been characterized (SHISA3, FNDC1, ANGPTL7, LEPR, ENSGALG00000049529, OXTR, ENSGALG00000045154, COL16A1, RASD2, BOC, GDF10, and THSD7B). Twelve SNPs were associated with FHS (p < 0.0001). Out of those, 5 were novel and 7 were existing variants located in 7 genes (RARS, TFPI2, TTI1, MAP4K3, LINK54, and AREL1). We have shown that genes related to chondrogenesis and bone differentiation were downregulated in the GP of FHS-affected young broilers. Therefore, these findings evince that candidate genes pointed out in our study are probably related to the onset of FHS in broilers.

Differentially expressed genes in the femur cartilage transcriptome clarify the understanding of femoral head separation in chickens

Locomotor problems are among one of the main concerns in the current poultry industry, causing major economic losses and affecting animal welfare. The most common bone anomalies in the femur are dyschondroplasia, femoral head separation (FHS), and bacterial chondronecrosis with osteomyelitis (BCO), also known as femoral head necrosis (FHN). The present study aimed to identify differentially expressed (DE) genes in the articular cartilage (AC) of normal and FHS-affected broilers by RNA-Seq analysis. In the transcriptome analysis, 12,169 genes were expressed in the femur AC. Of those, 107 genes were DE (FDR < 0.05) between normal and affected chickens, of which 9 were downregulated and 98 were upregulated in the affected broilers. In the gene-set enrichment analysis using the DE genes, 79 biological processes (BP) were identified and were grouped into 12 superclusters. The main BP found were involved in the response to biotic stimulus, gas transport, cellular activation, carbohydrate-derived catabolism, multi-organism regulation, immune system, muscle contraction, multi-organism process, cytolysis, leukocytes and cell adhesion. In this study, the first transcriptome analysis of the broilers femur articular cartilage was performed, and a set of candidate genes (AvBD1, AvBD2, ANK1, EPX, ADA, RHAG) that could trigger changes in the broiler´s femoral growth plate was identified. Moreover, these results could be helpful to better understand FHN in chickens and possibly in humans.

A Field Study of Histologic and Bacteriologic Characterization of Femoral Head Separation and Femoral Head Necrosis

Histologic and bacteriologic features for groups of average 31-day-old broilers displaying three gross categories of femoral head alterations were documented. Categories included simple femoral head separation (FHS), femoral head transitional changes (FHT), and femoral head necrosis (FHN). Groups with grossly normal (NORM) femoral heads and cull birds with FHN and having gross signs of sepsis (Cull-FHN) were also included in the study. There was a 10% occurrence of positive bacterial cultures for all birds tested. Most positive cultures (33%) were found in the Cull-FHN group, while only a 12% occurrence was seen in the FHS group, and no positives were present in the FHT or FHN groups. A 14% total occurrence of femoral bacterial chondronecrosis with osteomyelitis or simple osteomyelitis (BCO-O) was observed. A progressive increase in the prevalence of BCO-O was apparent between groups going from NORM (0%), FHS (4%), FHT (14%), FHN (13%), and reaching a maximum of 67% in the Cull-FHN group. Minimal to mild femoral head cartilage necrosis was present in 40% of NORM broilers and 100% of the FHS, FHT, and FHN groups, but at moderate severity in 20% of the Cull-FHN group. Thus, the majority of FHN cases were associated with aseptic cartilage necrosis rather than BCO-O. These findings suggest that aseptic cartilage necrosis may be as important as septic necrosis as a cause of gross femoral head disease. A 26% overall occurrence was seen for hip synovitis-arthritis, but group differences were not statistically significant. Synovitis was not seen in the NORM group and was present in some (12%) of the FHS group but was observed at a high rate in both the FHN (43%) and the Cull-FHN (50%) groups. Morphometric measurements demonstrated that the area size of femoral fibrous cortical defects or "cutback zones" were generally larger for all gross categories relative to NORM, with a significant difference between NORM and FHS groups. This study underscores the multifactorial etiology of FHN and the importance of conducting both histologic and bacteriologic evaluations in which gross evidence of FHN or BCO-O occurs.