Comparison of Living Vaccines in Producing Immunity Against Natural Brucella Melitensis Infection in Sheep and Goats in Iran

Rough mutants defective in core and O-polysaccharide synthesis and export induce antibodies reacting in an indirect ELISA with smooth lipopolysaccharide and are less effective than Rev 1 vaccine against Brucella melitensis infection of sheep

Classical brucellosis vaccines induce antibodies to the O-polysaccharide section of the lipopolysaccharide that interfere in serodiagnosis. Brucella rough (R) mutants lack the O-polysaccharide but their usefulness as vaccines is controversial. Here, Brucella melitensis R mutants in all main lipopolysaccharide biosynthetic pathways were evaluated in sheep in comparison with the reference B. melitensis Rev 1 vaccine. In a first experiment, these mutants were tested for ability to induce anti-O-polysaccharide antibodies, persistence and spread through target organs, and innocuousness. Using the data obtained and those of genetic studies, three candidates were selected and tested for efficacy as vaccines against a challenge infecting 100% of unvaccinated ewes. Protection by R vaccines was 54% or less whereas Rev 1 afforded 100% protection. One-third of R mutant vaccinated ewes became positive in an enzyme-linked immunosorbent assay with smooth lipopolysaccharide due to the core epitopes remaining in the mutated lipopolysaccharide. We conclude that R vaccines interfere in lipopolysaccharide immunosorbent assays and are less effective than Rev 1 against B. melitensis infection of sheep.

Immunological responses and protective efficacy against Brucella melitensis induced by bp26 and omp31 B. melitensis Rev.1 deletion mutants in sheep

The commonly used live attenuated vaccine in ovine brucellosis prophylaxis is Brucella melitensis Rev.1. This vaccine is known to induce antibody responses in vaccinated animals indistinguishable by the current conventional serological tests from those observed in challenged animals. Brucella BP26 and Omp31 proteins have shown an interesting potential as diagnostic antigens for ovine brucellosis. Accordingly, the bp26 gene and both bp26 and omp31 genes have been deleted from the vaccine strain Rev.1. Immunogenicity and vaccine efficacy of the parental Rev.1 strain and of both mutants in protecting sheep against B. melitensis strain H38 challenge was evaluated by clinical and bacteriological examination of ewes. They were conjunctivally or subcutaneously vaccinated when 4 months old and then challenged with B. melitensis H38 at the middle of the first pregnancy following vaccination. Deletion of bp26 and omp31 genes did not significantly affect the well recognised capacity of Rev.1 to protect sheep against B. melitensis challenge. However, the protection conferred by the CGV2631 mutant was significantly lower than that conferred by the CGV26 mutant or the Rev.1 strain. Vaccinated and challenged animals were detected positive in classical serological tests and in the IFN-gamma assay. A BP26-based ELISA was investigated to discriminate between ewes vaccinated by the mutants and ewes challenged with B. melitensis H38. The cut-off which was chosen in order to have 100% specificity resulted in a moderate sensitivity for the detection of challenged ewes. The use in the field of one of the mutants as vaccine against a B. melitensis infection, combined with classic diagnostic tests and a BP26 ELISA, could thus give an improvement in the differentiation between vaccinated and infected animals and contribute to the objective of eradication of brucellosis in small ruminants.

Comparison of the efficacy of Brucella suis strain 2 and Brucella melitensis Rev. 1 live vaccines against a Brucella melitensis experimental infection in pregnant ewes

The comparative efficacy of Brucella suis strain 2 (S2) and Brucella melitensis strain Rev. 1 (Rev. 1) live vaccines in protecting sheep against B. melitensis infection was evaluated by clinical and bacteriological examination of ewes vaccinated conjunctivally with a dose of 1 x 10(9) c.f.u. when 4 months old and then challenged with 5 x 10(7) c.f.u. of the B. melitensis virulent strain 53H38 (H38) at the middle of the first or second pregnancy following vaccination. Animals were considered to be protected when no abortion, no excretion of the challenge strain and no infection at slaughter occurred. The percentages of protection in Rev. 1-vaccinated groups challenged during either first (80%) or second (62%) pregnancy were significantly different (p < 0.001 and p < 0.05, respectively) compared with those of the relevant unvaccinated control groups. In contrast no significant difference in protection was found between the S2-vaccinated and control groups.

Preliminary studies on the use of carbon substrate utilization patterns for identification of Brucella species

A commercial system (Biolog, Hayward, CA) that uses reduction of an indicator dye to determine the oxidation of 95 different carbon substrates contained in a defined minimal medium was tested with 35 isolates of Brucella spp. to determine if the system could be used in place of respirometric methods to identify species. Of 95 substrates contained in this system, three were oxidized by all the Brucella strains tested, 48 were oxidized by none of the strains tested, and 44 were oxidized differentially. Brucella melitensis, B. abortus, and B. suis could be distinguished from each other on the basis of their oxidation reactions in seven in these substrates; epidemiologically related strains could not be unambiguously differentiated. This carbon substrate utilization method may prove to be a useful alternative to respirometry as a means to identify strains of Brucella spp. to species level, provided that personnel are protected from exposure to this highly infectious agent.

Phage typing reactions on Brucella species

The nature of the phage typing reactions on Brucella species was determined by rates of adsorption and infection, one-step growth experiments, and susceptibility to lysis from without. The highest rates of adsorption and infection were obtained on smooth B. abortus cultures, and large clear plaques were produced. One or a few phage particles per B. neotomae cell killed about one-half of the cells, but some went through an infective cycle and released mature phage that resulted in production of small clear plaques. With B. suis, more phage particles per cell were required to kill, replication did not occur, and plaques were not observed. Still greater numbers of phage particles were required to cause some inhibition of growth of B. melitensis lawns. Rough Brucella cultures and species, such as B. ovis and B. canis, were not affected by the highest concentrations of phage. B. abortus cultures of intermediate colonial morphology adsorbed phage, but only a few infected cells (after a delayed latent period) released mature phage. An infected culture or colony appeared normal until spontaneous phage mutants appeared which could penetrate the cell wall more effectively than the parent phage. The mutant phage multiplied more rapidly, and the colony changed to a sticky white form.