Differences in lesion development of slow- and fast-growing meat-type chickens after experimental challenge with <i>Enterococcus cecorum</i>

Field observations suggest genotype-associated differences in susceptibility to infection with <i>Enterococcus cecorum</i> (EC). We hypothesized that slow-growing (SG) chickens show less clinical signs and reduced lesions compared to fast-growing (FG) chickens after experimental challenge with EC. At 1-day post-hatch (dph), 97 FG and 97 SG chickens were randomly assigned to four groups: control, SG (CSG), control, FG (CFG), EC-inoculated, SG (ESG), and EC-inoculated, FG (EFG). After oral inoculation with 10⁷ colony forming units EC or mock inoculation with physiological saline, chickens were monitored for clinical signs and samples were collected weekly for further analysis via ELISA, real-time PCR, and flow cytometric analysis. The final necropsy of 40 chickens per group was carried out at 43/44 dph. Clinical signs and pathological findings were observed in a reduced number of chickens in group ESG compared to EFG. Extra-intestinal colonization with EC at 43/44 dph was significantly reduced in group ESG compared to EFG (<i>P</i> < 0.05). Circulating heterophils and monocytes were significantly increased in groups ESG and EFG compared to the control groups (<i>P</i> < 0.05). Additionally, circulating monocytes were increased and heterophil/lymphocyte (H/L) ratios were decreased in SG chickens compared to FG chickens. We may speculate that SG chickens were less affected by the EC-associated disease due to a more efficient innate immune response and may more vigorously control extra-intestinal colonization after translocation from the intestine. Overall, the use of slow-growing chickens may reduce the incidence of the EC-associated disease in meat-type chickens and therefore increase overall health performance.