Outbreaks of salpingitis and peritonitis cause major economic losses due to high mortality, reduced egg-production, and culling. The aim of the present study was to characterize, in detail, lesions associated with increased mortality in layers due to avianpathogenic Escherichia coli (APEC) and to investigate the population structure of the E. coli involved, which is important for selection of optimal treatment and prophylactic strategies. Among 322 layers received from eight farms with increased mortality due to E. coli, three lesion types were observed; sepsis-like lesions, chronic salpingitis and peritonitis, and chronic salpingitis and peritonitis associated with sepsis-like lesions. One hundred isolates of E. coli obtained in pure culture from the different lesion types were selected for genetic characterization. Six out of 10 submissions (two farms with two submissions) were considered clonal as defined by more than 85% of the typed isolates of E. coli belonging to the same sequence-type (ST). B2 was the most-prevalent phylogroup, including the clonal complex of ST95. The most-important virulence genes of E. coli were demonstrated from both clonal and nonclonal outbreaks, and major differences as to phylogeny and virulence genes were not observed between the lesion types. Cannibalism was more-often observed during polyclonal outbreaks. A new pathotype of APEC is suggested based upon lesions and route of infection, high similarity of virulence genes including plasmid-associated genes, and high frequency of ST95 and other isolates belonging to phylogroup B2. Compared to the best-known pathotypes of E. coli, this needs further investigations, including infection experiments to show if single virulence factors can be pointed out that are specific for the salpingitis-peritonitis pathotype and possibly not found in other pathotypes of E. coli.
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