Evaluation of protective efficacy of irradiated Salmonella Gallinarum vaccine against fowl typhoid in Sasso breed chicken

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Addis Abeba University


Fowl typhoid is worldwide distributed septicemic disease of chicken, turkeys, ducks, pheasants, guinea fowl, peafowl, sparrow, goose, and quail caused by Salmonella enterica subspecies enterica serovar Gallinarum. Live attenuated 9R strain of Salmonella Gallinarum (SG) is commonly used vaccine for the control of fowl typhoid. However, its persistence in vaccinated chickens causes vertical transmission through eggs and the residual virulence inducing lesions in the liver and spleen in some breeds of chicken are the drawbacks of this vaccine. In recent vaccine development efforts, alternative methods to develop a variety of vaccine types have been attempted of which radiation inactivated pathogens are some of them. Irradiation can avoid chemical contaminants from chemical inactivation and penetrate pathogens to destroy nucleic acids without damaging the pathogen surface antigens. The objective of this study was to evaluate protective efficacy of gamma irradiated Salmonella Gallinarum vaccine against fowl typhoid in poultry. After the strain identification test, inoculum of approximately 109 cfu/ml of field strain of Salmonella Gallinarum was prepared and exposed to series of radiation dose ranging from 1.1-3 kilo gray (kGy) for inactivation of which 2.6 KGy was found to be minimum lethal dose and it was used for final irradiation dose in this study. Forty two (42) days old Sasso breed of chickens were allocated randomly to six groups having 10 chickens each: G1 (vaccinated with irradiated Salmonella Gallinarum vaccine but not challenged), G2 (vaccinated with irradiated Salmonella Gallinarum vaccine and challenged), G3 (vaccinated with irradiated Salmonella Gallinarum vaccine, provided with booster dose 21 days later and challenged), G4 (vaccinated with irradiated Salmonella Gallinarum containing 20% trehalose and challenged), G5 (vaccinated with 9R commercial Salmonella Gallinarum vaccine produced at NVI, Bishoftu, Ethiopia, and challenged) and G6 (unvaccinated but challenged group as a control). Prior to the immunization process, all chickens were assessed for the presence of antibody against Salmonella Gallinarum on the 7 th weeks of age and none of them were found serologically positive using slide agglutination test (SAT). The homologous challenge infection experiment was conducted using the standard field dose of (~5.3 x 107 cfu/ml) ~ with optical density value of 0.6. On day 21, G4 and G5 showed strong antibody production than other groups, 80% and 90%, respectively. Fifty (50%) of G1 showed strong antibody production and 50% of them moderate reaction. However, only 20% and 30% of G2 and G3 respectively produce strong reaction on day 21 post vaccination. As G6 was unvaccinated group, there was no reaction throughout the experiment. But G3 on booster dose after 2 weeks of vaccination showed 60% strong agglutination on day 35 of first vaccination. Up on the challenge, chickens in G3 and G4 showed significant difference in survival rate (70%) over G2 and G6 which only 20% of them survived. Survived and sacrificed chickens at the end of experiment showed significantly lower lesions and bacterial re-isolation from the liver, spleen and gizzard as compared to those birds died during challenge experiment. There was significantly higher number of survivors among vaccinated G3 & G4 as compared to non-vaccinated group (G6) (p<0.0001). There was significant difference (p<0.05) in level of protection between G2 and G4 as well as between G2 and G5. Survivors in G5 were 100% that showed commercial 9R vaccine conferred strong protection as compared to G2, G3 and G4 and their protection was 20%, 70% and 70% respectively. There was no significant level of protection in chickens in G2 compared to unvaccinated control group (p>0.05). Addition of 20% trehalose and booster dose improved protection of irradiated vaccine by50%. In conclusion, subcutaneously administered irradiated SG candidate vaccine with 20% trehalose and booster dose of irradiated vaccine without 20% trehalose showed promising safety, immunogenicity and protective efficacy. Further studies on safety, shelf life, radiation dose optimization for trehalose added irradiated Salmonella Gallinarum vaccine and quantification of antibody response using ELISA and other immunological methods are recommended



Fowl typhoid, 9R vaccine,, Irradiated vaccine, Salmonella Gallinarum, Trehalose