Immune and protective role of vaccines against salmonella enteritidis and salmonella typhimurium in laying hens

Resumen

Salmonellosis is a zoonosis caused by non-adapted bacterial strains of the genus Salmonella. Salmonella is commonly found in the intestines of healthy birds and mammals and can enter the food chain through contaminated meat, eggs and their products. Consumption of food or water contaminated with Salmonella causes gastroenteritis in humans. The last report from the European Food Safety Authority indicates that this bacterium was the second cause of human zoonosis in the European Union in 2018. Particularly, Salmonella caused one third of all foodborne outbreaks (30.7%). From these outbreaks almost half were caused by eggs and egg products (45.6%), and most (70%) were caused by only two serovars: Salmonella Enteritidis and Salmonella Typhimurium. In 2003 the EU started to implement the Salmonella National Control Programmes in all member states to reduce Salmonella prevalence in the poultry and pig industry. These plans consisted in measures of biosafety and additional tools like vaccination against Salmonella, which achieved an important reduction in prevalence and the consequent reduction of reported cases of human salmonellosis. Non-adapted serovars do not cause clinical disease in hens older than 3 days. Therefore, the main objective of vaccines against these Salmonella serovars in poultry is to increase resistance of animals against infection in order to reduce the contamination of poultry derived products. The main objective of this thesis was to determine the protective function of a live attenuated Salmonella Typhimurium strain (alone or combined with a Salmonella Enteritidis strain) against field strains infections and to evaluate the associated immune response. Three studies were carried out at three different production times in hens. In the first study the objective was to determine the efficacy of a first vaccine dose at day-old after challenge at 14 days comparing oral and spray administration. The oral vaccination partially protected and reduced colonisation in internal organ, indicating the need of additional vaccine boosters to develop a stronger immunity against Salmonella. The protection was not correlated with humoral or cellular immune response. The spray administration failed, and animals were not protected, indicating the importance of the administration route. In the second study, the efficacy of the vaccine was tested after 2 vaccine doses (challenged at 16 weeks of life) or 3 vaccine doses (challenged at 35 weeks of life). Both vaccine groups had reduction of Salmonella excretion and colonisation of internal organs. This adaptive protective immune response was not correlated with levels of antibodies (which were especially higher in non-vaccinated animals in the intestine), but was correlated with a general increase in CD3+ T cell population in the intestine and a IFNγ up-regulation, indicating a possible drift to a protective Th1 immune response. In the third study, the efficacy of vaccination during rearing with a combined Salmonella Enteritidis and Salmonella Typhimurium vaccine was determined at the end of the laying period. Vaccinated animals had reduced rates of Salmonella Enteritidis and Salmonella Typhimurium excretion in cloacal swabs and were protected against internal organ colonisation. The protection was not related with humoral response but with a cellular response in the intestine including the infiltration of macrophages, CD4+ and CD8+ T cells. The results extracted from this thesis indicate that the tested vaccines could be a useful tool to decrease vertical transmission to eggs (reducing infection of internal organs) and horizontal transmission to eggs and poultry products (reducing excretion of bacteria) to ultimately minimise the risk of salmonellosis in humans.