HISTOPATOLOGICAL FINDINGS IN ATTENUATED Salmonella Typhimurium MONOPHASIC VARIANT VACCINATED PIGLETS

Salmonella Typhimurium and its monophasic variant are increasingly responsible of food borne infections in humans. The main source of infection is pork meat. Indeed, the subclinical disease in swine is the cause of the transmission of the infection into the human food chain [1]. Aim of this study was to evaluate the alterations of the intestinal mucosa of pigs immunized with an attenuated vaccine against S. Typhimurium monophasic variant ΔznuABC (mSTΔznuAB) and experimentally infected with homologous and heterologous strains of S. Typhimurium. A histopathological assessment was performed in order to evaluate the effects of vaccination and infection on different intestinal tracts. Twenty weaned piglets were divided in 4 groups: 5 piglets were vaccinated with mSTΔznuAB and infected with virulent S. Typhimurium monophasic variant (mST) (group A), 5 piglets were vaccinated with mSTΔznuABC and infected with virulent S. Typhimurium (ST) (group B), 5 piglets were infected with virulent ST (group C) and 5 piglets were infected with virulent mST (group D). At day 20 after infection, piglets were euthanized and samples of tonsils, ileocecal lymph nodes, spleen, ileum, caecum and colon were collected for histological evaluation. A numerical value based on the degree of lesions was assigned to each examined intestinal tract, considering epithelium, submucosa and Peyer's patch conditions, congestion and lesion patterns. Haemorrhages, congestion and lymphoid tissues were also scored in tonsils, ileocecal lymph nodes, and spleen. Statistical analysis was then performed by Two-Way Anova for Repeated Measures or by Kruskal-Wallis tests, assuming statistically significant differences when p<0.05. Histologically, a diffuse epithelial conglutination was revealed in all the examined intestinal tracts and in all groups, associated to vascular congestion and lymph nodes depletion. Only in one case (group C, ileum) necrosis was revealed. Mucosal lesions were significantly more severe in caecum and colon of groups B and C. Differences were detected neither in lymphocyte amount and distribution, nor in mucosa and submucosa hemorrhages. No differences were revealed in spleen or lymph nodes for each of the examined parameters. In tonsils, a significantly higher congestion, as well as a higher activation of lymphoid follicles, were revealed in group A in comparison to group D. Although mSTΔznuABC vaccine is able to reduce immune system colonisation and faecal shedding of homologous and heterologous virulent strains, the pattern of histological lesions was not clearly altered in the examined tissues. Only in group A tonsils appeared more reactive than in unvaccinated group.