Experimental infection with Salmonella enterica subsp. diarizonae serotype 61:k:1,5,(7) in sheep: Study of cell mediated immune response

Abstract

The aim of the present study was to investigate the clinical, pathological and immunological changes in lambs infected experimentally with Salmonella enterica subsp. diarizonae serotype 61:k:1,5,(7) (SED) that causes chronic proliferative rhinitis (CPR) in sheep. Eleven four-month-old lambs were infected with 3.2×108UFC of SED by nasal route and four lambs with the same age were kept as placebo group. Clinical explorations were carried out monthly and vaginal, nasal swaps and fecal samples were taken for microbiological study during one year. Furthermore, a postmortem investigation was performed. Ventral turbinates of both nostrils and submandibular, parotid and retropharyngeal lymph nodes were sampled for histopathological, immunohistochemical and microbiological studies. Cell mediated immune response was assessed by lymphocyte stimulation test (Con A, PWM, PHA and LPS were used as mitogens), DTH test and by measuring IFN-γ concentration in serum and cultured supernatants.SED was isolated from nasal swabs and fecal samples of all infected lambs virtually throughout the study. No isolations were obtained from control group. At necropsy, no lesions associated with SED were recorded; however, the microorganism was isolated in regional lymph nodes in all infected animals. Moreover, infected lambs showed lower stimulation indices to Con A, PHA and PWM and higher for both LPS (P<0.001). Besides, DTH response was lower in infected lambs than in the control group, but statistical significant differences were not found (P>0.05). Serum levels of IFN-γ during the experiment were higher in infected sheep than in the control group, suggesting an active infection. Furthermore, mitogens stimulated the production of IFN-γ in lymphocyte cultures from infected sheep, and the highest level were obtained with Con A.These results demonstrate that after experimental infection of healthy lambs with S. enterica subsp. diarizonae serotype 61:k:1,5,(7) infection was maintained for at least one year in the upper respiratory tract and that digestive route should be taken into account as a possible way of SED elimination. Moreover, SED infection caused a weak response of the T cells; nevertheless, more studies are needed to understand the mechanisms involved in the immunitary response and pathology of CPR in sheep associated with Salmonella enterica subsp. diarizonae infection.