New vaccine design techniques have allowed the development of effective vaccine strains against Salmonella infections inwhich the risks of reversion to the wild type and virulence is null. The mutant strain Salmonella Gallinarum ΔcobSΔcbiA was previously shown to be avirulent in chickens. In this study, this strain was tested as a vaccine against Salmonella Gallinarum (SG) and S. Enteritidis (SE) infections, and its protection levels, safety and possible risks of reversion to virulence after vaccination of layers were evaluated. Birds were vaccinated at five days of age or at five and 25 days of age. At 45 days of age, brown and white layers were challenged with SG and SE wild strains, respectively. Two assays to test the possibility of reversion to virulence were performed. Five successive bacterial passages in brown layers were carried out in the first assay. In the second assay, brown layers received a ten-fold concentrated inoculum of the SGΔcobSΔcbiA strain and were evaluated for clinical signs and mortality. In both experiments, no birds that received the inoculation of the attenuated strain died. Additionally, the use of the mutant strain as a vaccine provided good protection levels against both challenge strains.

Salmonellosis is a worldwide disease caused by bacteria of the genus Salmonella. Currently, there are over 2,500 identified serovars of Salmonella. A reduced number of these serovars, about eighty, are implicated in most animals and human diseases. Most cases of salmonellosis in humans are associated with the consumption of contaminated food products such as beef, pork, poultry meat, eggs, vegetables, juices and other kind of foods. It may also be associated with the contact between humans and infected pet animals. Therefore, the chain of human salmonellosis is very complex and in most cases the origin of the infection is difficult to establish. The use of antimicrobial agents to treat and to prevent bacterial infections in humans and animals, as well as as growth promoters in animal production, has favoured the selection and transference of resistance genes between different bacteria, including Salmonella serovars. Many studies have confirmed the role of foods of animal origin as a source of multi drugresistant Salmonella serovars. For this reason, continuous surveillance of these pathogens along the food chain together with the responsible use of antimicrobial agents is necessary.

The protective effect of various Salmonella vaccines regimens against an experimental Salmonella Gallinarum challenge (SGNalr strain at 12 wk of age) was evaluated in two experiments. In Experiment 1 commercial brown layers were vaccinated according to one of the following programs: (i) two doses of a SE bacterin (Layermune SE; group 1); (ii) a first dose of a live SG9R vaccine (Cevac SG9R) followed by a SE bacterin (Layermune SE; group 2); (iii) one dose of each of two different multivalent inactivated vaccines containing SE cells (Corymune 4 & Corymune 7; group 3) or (iv) not vaccinated (group 4). In Experiment 2, broiler breeders were given the same vaccination treatments except for the group vaccinated with the multivalent vaccines. Overall, in both experiments, all vaccination schemes were effective in reducing mortality after challenge with a SG field strain. Primary vaccination with an initial dose of a live SG9R vaccine followed some weeks later by a dose of an inactivated SE bacterin was the most effective (p<0.05) vaccination program against mortality induced by field SG experimental challenge in both experiments. In conclusion, Salmonella vaccination programs containing SE bacterins alone or in combination with a live SG9R vaccine are effective in preventing mortality induced by infection of field SG. Nevertheless, it is important to emphasize that any vaccination program against any Salmonella serotype will only be effective if it is part of a sound and comprehensive biosecurity program designed for Salmonella control in poultry farms.

Competitive exclusion (CE) has been designed to accelerate the colonization of the alimentary tract of young commercial birds and it has been also used to repopulate the digestive tract after antibiotic therapy. The method has been successfully adopted as a means to prevent enteric salmonellosis. The present study was carried out to evaluate if CE is able to prevent this kind of infection. Newly hatched chicks were given a CE culture and at different intervals of time birds infected with Salmonella Enteritidis were placed together with the group of treated birds. CE culture was prepared from feces of adult laying hens incubated overnight at 37C under aerobic conditions. Birds were killed 4 and 8 days after challenge and viable counts of Salmonella Enteritidis were assessed in the cecal contents. The results showed that Salmonella infection was reduced even if CE culture administration was concomitant with the inclusion of the infected bird in the group.