SummaryA series of studies were undertaken in the development of a chicken model of avian pathogenic Escherichia coli (APEC) peritonitis. Once established, this model was then used to measure the effectiveness of a siderophore receptor and porin (SRP®) APEC vaccine. Initially, five pilot studies were performed on egg-laying chickens to compare Escherichia coli ( E. coli ) serotype, challenge route, and dose of inoculum that resulted in pathology characteristic of peritonitis observed in commercial layer facilities such as widespread organ infection, atrophy, discoloration, corrugation of yolk sacs and the presence of caseous exudate. Isolates of serotype O1, O2 and O78 were tested by intravenous (IV), intravaginal (IVAG), intratracheal (IT) and intraperitoneal (IP) routes and were compared at varying levels of challenge inoculum. Daily observations of mortality and morbidity were made and, at necropsy, gross lesion scores were collected and bacterial colonization of internal organs determined. Outcomes varied from a complete lack of mortality or detectable pathology and low, or no, organ colonization in the case of IVAG and IT routes with each E. coli serotype to moderate to high levels of mortality, pathology and colonization after challenge via the IV and IP routes with O2 and O78 serotypes. The O78 serotype was found to result in pathologies most consistent with field observations of peritonitis and, therefore, subsequent studies were performed only with O78. In addition to the relative failure with both the IT and IVAG routes of challenge, the IV route was found to be inconsistent and often resulted in lameness not observed with the IP route. A final pilot study confirmed that the dose (~ 8 log 10 CFU) administered by the IP route replicated peritonitis and, therefore, all vaccination/challenge studies were conducted in this manner.Five vaccination/challenge studies are reported here where variables of chicken age, vaccination interval and vaccination to challenge interval were examined. In all studies, vaccine effectiveness was dramatic and was shown to completely protect against mortality, and substantially against tissue colonization and pathology typical of APEC infections. The vaccine elicited a rapid onset of immunity with both narrow and broad vaccination intervals and in both young and mature chickens. Additionally, the vaccine was demonstrated to sustain robust effectiveness against mortality over a three-month duration. The SRP APEC vaccine should provide effective protection of young and mature chickens from E. coli under broadly flexible conditions of use in commercial operations.

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