A severe hemorrhagic pneumonia with respiratory distress prevalent has been observed in broilers with approximately 80% morbidity and 50% mortality. In our initial studies, Ornithobacterium rhinotracheale (ORT), Enterococcus faecalis (E. faecalis) and Streptococcus zooepidemicus were isolated from the diseased birds. However, the respective contribution of these organisms in the pathogenesis of hemorrhagic pneumonia is unclear. The objective of this study was to evaluate the role of ORT-E. faecalis co-infection in hemorrhagic pneumonia in chickens. Biochemical assays and 16S rRNA-based PCR were used to identify 5 E. faecalis isolates. Subsequently, forty-eight 21-day-old SPF chickens were divided randomly into six groups, 8 birds in each group. Chickens were co-infected intraperitoneally with ORT and E. faecalis isolates simultaneously, or inoculated with ORT first, then with E. faecalis three days later, and vice versa. Control groups consisted of chickens inoculated with ORT alone or E. faecalis alone. Post inoculation, the mortality was 87.5, 62.5, 37.6, 62.5 and 50% in birds co-infected with ORT and E. faecalis, infected with E. faecalis first followed by ORT, with ORT first followed by E. faecalis, and mono-infected with ORT and E. faecalis, respectively. Moreover, serum specimens from 60 out of 194 randomly selected chickens (30.9%) were positive for antibodies against E. faecalis. Our co-infection studies suggest that E. faecalis infection is able to trigger hemorrhagic pneumonia with a rapid mortality, while ORT infection was able to prolong pathological lesions. Therefore, the occurrence of ORT and E. faecalis co-infection may be associated with the outbreak of chicken hemorrhagic pneumonia.

Key words: Chickens, Con-infection, Enterococcus faecalis, Hemorrhagic pneumonia, Ornithobacterium rhinotracheale