Erysipelothrix rhusiopathiae in Laying Hens

The bacterium Erysipelothrix rhusiopathiae can infect a wide range of mammals (including humans) and birds. Disease outbreaks (erysipelas) have been considered unusual in chickens internationally, but outbreaks with high mortality and egg production losses have been diagnosed in Swedish laying hen flocks every year since 1998. Different aspects of E. rhusiopathiae infection in chickens were examined in this thesis with the aim of preventing future outbreaks. These aspects included determining occurrence of the bacterium in different housing systems for laying hens and the potential of the poultry red mite (Dermanyssus gallinae) to carry it, characterization of isolates of the bacterium from different hosts using pulsed-field gel electrophoresis (PFGE), serotyping, antimicrobial susceptibility testing and 16S rRNA gene sequencing, and determining the incidence of E. rhusiopathiae in the environment on affected organic laying hen farms by the use of selective culture and PCR. The results showed an association between erysipelas outbreaks and housing system. Flocks in free-range systems appeared to be at a higher risk than flocks in indoor litterbased systems, while flocks in cages appeared to be at the lowest risk. Dermanyssus gallinae collected from affected flocks was shown to carry E. rhusiopathiae externally and internally, but the reservoir potential of the mite could not be proven. When characterizing E. rhusiopathiae isolates, PFGE proved to be a suitable method for genotypic studies. Investigations of isolates from affected flocks showed that outbreaks appeared to be of a clonal nature and thus caused by introduction of E. rhusiopathiae from a single extraneous source. An external source of infection was also indicated by outbreaks in consecutive flocks caused by different PFGE types. Manure and dust samples collected in affected flocks were shown to contain E. rhusiopathiae and may therefore represent sources of transmission.