Co-infections of <i>Enterococcus cecorum</i> and various avian pathogens resulted in varying rates of SPF broilers with an <i>E. cecorum</i> infection

<i>Enterococcus cecorum</i> infections can be experimentally reproduced after oral inoculation. Co-infections of <i>E. cecorum</i> with other avian pathogens might increase the proportion of broilers with <i>E. cecorum</i> infections. The aim of the study was to examine via which infection routes <i>E. cecorum</i> is capable of causing infections, and which co-infections exacerbate <i>E. cecorum</i> infections. Two experiments, each with 12 groups, were conducted. Per group, 40 SPF broilers were inoculated intravenously, orally or via aerosol with <i>E. cecorum</i> alone, a negative control group was included, and in eight groups the effect of co-infections was studied. In experiment 1, infectious bronchitis virus (IBV), Newcastle disease virus (NDV) vaccine, reovirus, and chicken anaemia virus (CAV) were inoculated before oral or aerosol challenge with <i>E. cecorum</i>. In experiment 2, <i>E. cecorum</i> was given after the administration of dexamethasone, or inoculation of CAV, or combinations of IBV or NDV with <i>Mycoplasma synoviae</i> (M.s.). Typical <i>E. cecorum</i> lesions were reproduced via all applied infection routes. The proportion of lesions was highest in the intravenously inoculated groups, being 0.25 (95% confidence interval (CI):0.127–0.412) and 0.275 (95% CI:0.146–0.439) in experiments 1 and 2, respectively. More lesions were observed in experiment 2, while the proportion of lesions after oral and aerosol inoculation did not differ in either experiment. The proportion of lesions was not exacerbated after co-infections of <i>E. cecorum</i> with various avian pathogens. Only the proportion of <i>E. cecorum</i> reisolations was exacerbated after inoculation with CAV or M.s. in combination with IBV or NDV. Typical <i>E. cecorum</i> lesions can be reproduced in SPF broilers after intravenous, aerosol and oral inoculations.The respiratory route is potentially an infection route for pathogenic <i>E. cecorum</i> bacteria.Co-infections tested in this study or dexamethasone do not exacerbate the proportion of <i>E. cecorum</i> lesions.M.s. in combination with IBV or NDV vaccines exacerbates the proportion of positive reisolations.Immunosuppression induced by early CAV infection increases the proportion of positive reisolations. Typical <i>E. cecorum</i> lesions can be reproduced in SPF broilers after intravenous, aerosol and oral inoculations. The respiratory route is potentially an infection route for pathogenic <i>E. cecorum</i> bacteria. Co-infections tested in this study or dexamethasone do not exacerbate the proportion of <i>E. cecorum</i> lesions. M.s. in combination with IBV or NDV vaccines exacerbates the proportion of positive reisolations. Immunosuppression induced by early CAV infection increases the proportion of positive reisolations.