| In order to develop attenuated Salmonella gallinarum strain 97A as vaccine, pathologic and bacteriologic dynamics in chickens inoculated with strain 97A was investigated, the safety and efficacy of the vaccine strain to protect chickens against Salmonellosis caused by virulent S. gallinarum strain Sg9 was evaluated while its immunobiological properties were also preliminary demonstrated.To evaluate the safety of strain 97A, Salmonella-free chickens were immunized orally with 5 l08CFU of the vaccine strain at 1-, 7-, 14-day-old. Three birds were euthanized at different times from 1 to 35 days post-immunization (PI). Clinical signs, gross lesions, microscopic changes, and bacterial counts were examined. As a result, the vaccinated chickens were all healthy and showed no gross lesions. The histopathological changes included mild hydropic and/or fatty degeneration, and mild inflammation in liver and kidney during the whole experimental period. Bacteria isolation in blood was negative, but strain 97A could be isolated from liver and spleen of immunized birds, and the time of colonization could last at least 4 weeks, that was longer enough to induce immune responses. These results indicated that the vaccine strain showed good safety.After inoculation with vaccine strain 97A, the chickens challenged with virulent S.gallinarum strain Sg9, pathologic and bacteriologic examinations were performed on chickens from immunized-challenged and challenge control groups. The results revealed that strain 97A manifested higher protective efficacy to chickens immunized at 1-, 7-, 14-day-old, the survival rate of the chickens from these three groups was 85%, 95%, and 100%, respectively. The protective rate of immunized group with strain 97A was significant different compared with that of the challenge control group. Scatteredlittle necrotic foci and/or mild to moderate inflammations were observed in liver, lung, kidney, and other parenchyma organs from the immunized-challenged chickens after challenge. While severe clinical signs and substantial death was seen in challenge control group at 6 to 10 days after challenged, the livers had typical lesions of fowl typhoid (FT) , including fibrin exudation, infiltration of inflammatory cells, such as heterophilic cells, lymphocytes, and plasma cells. Bacteria isolation in blood was negative from immunized-challenged groups, but the virulent strain Sg9 could be isolated from the internal organs hi immunized-challenged group. The duration period of Sg9 in livers and spleens was limited, and the number of Sg9 in these two organs was diminished with the days passing. The virulent strain Sg9 could be frequently isolated in blood, liver, and spleen from challenge control group. These results showed that strain 97A possessed pretty efficacy.Changes induced by strain 97A in T lymphocyte subpopulation in blood and spleens of immunized chickens were analyzed by flow cytometry (FACS) . At 10 days PI, the percentage of splenic CD4+ T lymphocytes increased compared with that of the control group, and at 15 days PI, the percentage of splenic CD8+ T lymphocytes raised.In summary, strain 97A could colonize and replicate in liver and spleen from the immunized birds, but it didn't induce bacteriemia, it showed good safety. The vaccine strain manifested higher protective efficacy to chickens immunized at 1-,7-,14-day-old, the survival rate of these chickens was 85%, 95%, and 100%, respectively, mild to moderate gross and microscopic lesions induced by strain Sg9 were seen in immunized chickens, but with the role of immune protection from 97A immunization, these lesions disappeared three weeks later. Visceral invasion by the virulent strain Sg9 was dramatically diminished in immunized birds. FACS assay indicated that strain 97A could induce cell-mediated immunity in some degree to limit the colonization and proliferation of virulent strain.It concluded that strain 97A is a good vaccine candidate for the prevention of Salmonellosis caused by S. pullonnn/gallinarum.
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