SUMMARY.

Major disease outbreaks caused by Streptococcus equi subsp. zooepidemicus seldom are reported in poultry. Besides acute septicemia, infection can result in a subacute or chronic form of disease with described mortality rates of 11% to 80%. Previously, the source of infection in poultry was linked to horses in which this bacterium can be present as an opportunistic pathogen on mucus membranes. The main route of spreading and being maintained within a poultry flock, after entering the stable, however, remains unclear. This case report describes an outbreak associated with S. zooepidemicus affecting a flock of 28 500 layer hens housed in an aviary system with free range. Besides sudden deaths, clinical signs of depression were noticed. Between 44 and 61 wk of age a total mortality of 23% was observed. Egg production dropped from 92% to 83%. Bacterial titration revealed substantial numbers of S. zooepidemicus present in the ceca of a healthy chicken. This novel finding hypothesizes that transmission of the infection within the flock might occur through the fecal route.

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