Serological monitoring of Enterococcus cecorum specific antibodies in chickens

Pathogenic Enterococcus cecorum (EC) has gained increasing importance as the cause of skeletal infections in meat-type chicken production. Since effective intervention strategies are scarce, it must be focused on preventive measures. Vaccination of meat-type breeder chicken flocks is common practice to protect the progeny against infection with EC. However, no data are available on seroconversion after infection or vaccination. The aim of the present study was the serological monitoring of chickens for EC-specific immunoglobulin Y (IgY) using a newly established EC-specific, indirect ELISA for chickens. Sera from previous infection studies were used for the establishment of the assay. Serum samples from confirmed EC-positive meat-type chicken flocks, vaccinated, and non-vaccinated meat-type chicken breeder flocks were analyzed for EC-specific IgY. Comparison of ELISA results with results from real-time PCR and/or bacteriological examination via culture revealed fair to substantial agreement. In infected chickens, more samples were classified as positive via ELISA than via real-time PCR and/or bacteriological examination via culture. Focusing on chickens experimentally infected at 1 day post-hatch (dph), the highest proportion of positive results and highest S/P ratios were found at 42 dph (p < 0.05). A similar trend was observed for the samples from naturally infected chickens (p < 0.05). Adjustment of the secondary antibody against immunoglobulin M (IgM) may open possibilities to use the assay during the early phase of the growing period, when there is still a chance to treat the infection. The examination of samples from vaccinated and non-vaccinated meat-type breeder chickens revealed no significant differences of S/P ratios independent of farm and autogenous vaccine used. In addition to that, monitoring of a non-vaccinated meat-type breeder chicken flock at 4, 10, 15, and 19 weeks post-hatch showed a continuous increase of ELISA-positive serum samples associated with an increase of S/P ratios. This may be explained by cross reactivity with antibodies to Enterococcus hirae or natural antibodies. The usage of EC-specific, recombinant proteins for coating of the plates may help to reduce unspecific background and increase the assay's specificity in future applications. In conclusion, the newly developed ELISA provides a suitable tool for serological monitoring of meat-type chickens during experimental studies with EC under standardized conditions. Remarkably, the assay is able to detect a higher proportion of EC-positive chickens than other methods, which are currently available. However, the assay is not yet suitable for the monitoring of breeder flocks due to high background.