Abstract
Background: Brucella abortus strain RB51 is a rough strain that was derived after multiple passages of virulent strain 2308. Levamisole, a potent anthelmintic that is widely used in veterinary medicine, is also known as an immune stimulant.

Aim and Objectives: Determine the capability of levamisole to enhance the humoral antibody response and cell-mediated immunity as well as protective capacity of RB51 vaccine in mice against challenge with virulent Brucella melitensis strain.

Materials and Methods: Six groups of 7–8-week-old female Bulb/C mice were used. Group I was vaccinated I/P with 5 × 108 colony-forming units (CFU) of RB51 strain. Group II was vaccinated I/P with 5 × 108 CFU of RB51 strain simultaneously injected (at 0 day) s/cu with 12.5 mg/Kg levamisole. Group III was vaccinated I/P with 5 × 108 CFU of RB51 strain and injected at 7-day postvaccination with 12.5 mg/Kg levamisole. Group IV was vaccinated I/P with 5 × 108 CFU of RB51 strain and injected at the day of vaccination (0 day) and 7-day postvaccination with 12.5 mg/Kg levamisole. Group V was injected with levamisole alone. Group VI was kept as control group. 8-week postvaccination; all vaccinated as well as control animals groups received I/P challenge of 2 ml dose of 2 × 104 CFU/ml of virulent strain of B. melitensis biovar 3.

Results: (Groups III and IV) At the day of vaccination (0 day) and 7-day postvaccination showed the highest significant serum antibody titer measured by ELISA all over the experimental period as well as increase in delayed type hypersensitivity (DTH) response 48 h after elicitation by intradermal inoculation of B. abortus soluble antigen compared to vaccinated control group (Group I). While RB51 vaccinated and simultaneously treated with levamisole (Group II) showed mild elevation in antibody titer as well as the lowest significant increase in DTH response compared to vaccinated control group. Bioassay test of challenged mice groups showed significant improvement in the resistance of vaccinated groups treated with levamisole either 7-day post-vaccination (Group III) or 0 and 7 days postvaccination (Group IV) which showed 10% bacteriologically positive mice, whereas only 30% of the vaccinated control mice (Group I) and vaccinated simultaneously treated with levamisole (Group II), were bacteriologically positive.

Key words: Levamisole; Brucella; Immunogenic Capacity; Protective Capacity; RB51