Potential of a Live Salmonella Typhimurium Vaccine to Provide Cross-Protection against a Novel Salmonella Enteritidis Strain in Layers

Salmonella serotypes vary throughout the livestock industry. Historically Salmonella Enteritidis is not considered to be prevalent in Australian poultry flocks. As recently as 2018 a novel S. Enteritidis emerged in the industry (FSANZ, 2019). Although, there are no specific vaccines available in Australia against this serotype in poultry, the use of live attenuated Salmonella vaccines that have the potential to cross protect against several Salmonella serotypes could be a solution for the industry.

Currently, if a commercial layer flock becomes infected with serotype Enteritidis, flock depopulation is mandated, and the farm must demonstrate freedom before restocking. This has a significant animal welfare and economic impact on farms where this serotype has spread, representing a significant risk to the Australian poultry industry and human health. The benefits of an existing live attenuated S. Typhimurium vaccine to be able to protect birds against both S. Typhimurium and S. Enteritidis challenge in the field would increase confidence and security in the industry and enhance human protection against food borne salmonellosis.

116 commercial layer hens were held in floor pens much like commercial production practices. Commercial layers were assessed at point of lay at 18 weeks of age, after sexual maturity, as this is a high stress period where the layers are assumed to be most vulnerable. The hens were challenged with the novel S. Enteritidis strain (phage type 12 according to the Salmonella reference laboratory IMVS) orally at 109 .

To establish a cross-protection benefit, the vaccination program applied was as follows: day old (coarse spray), 3 weeks age (oral gavage), followed by either one or two intramuscular doses of the same vaccine at 9, or 9 and 14 weeks of age respectively. Protection was measured by a reduction in the proportion of S. Enteritidis detected in cloacal swabs, caeca, liver, spleen, and reproductive tracts of unvaccinated and vaccinated birds. Antibody titre test were conducted for Group B and Group D serovars to measure the effect of vaccination on the production of these antibodies.

Results from cloacal swabs are: in the challenged groups, all had a similar proportion of cloacal swabs positive at 3 days post challenge (83%), but this declined significantly by days 10 and 14 post challenge. The proportion of birds with positive cloacal swabs at 5 days post challenge was significantly lower in the single-injection vaccinated group and at 7 days post challenge in the double vaccinated group compared with the challenged controls. Complete results will be presented and discussed in detail during the conference.