Attempts to Stimulate Immunology Responses by Intramuscular and Intraperitoneal Delivery of EPS-Adjuvanted Mannheimiosis Vaccine

Abstract

This study was conducted on 48 male Sprague Dawley rats to determine the immunological responses of experimental adjuvanted vaccines of Mannheimia haemolytica A2 (M. haemolytica A2) injection and to observe their protection level upon live M. haemolytica A2 challenge. Forty-eight clinically healthy Sprague Dawley rats were divided into four groups; Groups 1 and 2 were vaccinated intramuscularly and intraperitoneally, respectively, with an Exo-Polysaccharide (EPS)-adjuvanted vaccine prepared from formalin-killed M. haemolytica serotypes A2, Group 3 with formalin killed M. haemolytica seed. At the same time, Group 4 received intraperitoneally Phosphate Buffer Saline (PBS) as a control. After the first vaccination dose, Groups 1,2, and 3 showed a gradual increase in IgM, IgG, and IgA levels significantly (p<0.05). However, their level started to decline six weeks post-vaccination, indicating that the booster dose was needed. Upon the delivery of the second booster dose, antibodies titer showed a persistent increase significantly (p<0.05), especially the serum IgG level. All groups were challenged with live virulent Mannheimia haemolytica A2 after the level of antibodies declined twice after the second booster was delivered. No rat deaths were found in the Combined EPS - M. haemolytica adjuvant vaccine after 14 days following the challenge (0%). In unvaccinated rats, higher mortality and morbidity were noted (100%) and less was reported in rats receiving M. haemolytica A2 Vaccine seed (8%). Histologically at two weeks, post-challenge revealed negligible lung lesions in groups 1 and 2 and mild lesions in group 3. Lung lesions were recorded in all unvaccinated control rats. Furthermore, M. haemolytica A2 re-isolated successfully from lung samples in groups 3 and 4. In conclusion, Rats receiving adjuvant-M. haemolytica vaccine confers reasonably high preventive efficacy compared to M. haemolytica A2 by itself. Further studies should be conducted on measuring the antibody titer in different vivo, such as goats or sheep.