Vaccine protocols to optimise the protective efficacy of BCG.

SETTING

A deer model has been developed to study protection produced with BCG vaccination, against infection and the development of pathology, following experimental intratonsilar infection with virulent Mycobacterium bovis.

OBJECTIVE

To determine how the dose of vaccine, the route of vaccination, the viability of the vaccine and exposure to glucocorticoids at the time of vaccination, may affect the protective efficacy of BCG vaccines.

DESIGN

Deer were vaccinated with BCG and later challenged with virulent M. bovis via the tonsilar route. Protection against infection and development of disease was evaluated at necropsy six months after challenge with M. bovis, by histological examination and microbial culture.

RESULTS

Significant protection against infection and disease were obtained following boosting with two low doses (5 x 10(4) cfu) or moderate doses (5 x 10(7) cfu) of live (freshly cultured and lyophilized) BCG. Inferior levels of protection were obtained with high dose (5 x 10(8) cfu) of live BCG. Similar levels of protection were found with vaccines given subcutaneously or via the tonsilar route. Killed vaccine in a mineral-oil adjuvant did not evoke protective immunity and treatment with dexamethasone prior to vaccination with live BCG ablated its efficacy. Protection against infection did not correlate with skin test delayed type hypersensitivity (DTH) or lymphocyte transformation to tuberculin.

CONCLUSIONS

Two doses of live BCG gave significant protection against experimental infection and disease caused by virulent M. bovis. Single dose vaccine protected against disease but not infection. Vaccines administered at a dosage which did not evoke DTH, provided protection against tuberculosis infection and disease.