A review of the use of B. melitensis Rev 1 vaccine in adult sheep and goats

The live Brucella melitensis Rev 1 strain is considered the best vaccine available for the prophylaxis of brucellosis in small ruminants. The classically recommended exclusive vaccination of young replacement animals has failed to control brucellosis in some developed countries and is frequently inapplicable in the developing world. Accordingly, whole-flock vaccination is the only feasible alternative to control B. melitensis infection in small ruminants under the extensive management conditions characteristic of these countries. This review describes the practical problems encountered and the experience acquired over the past decade (particularly in Spain) using the Rev 1 based control strategy. The vaccination of pregnant animals with full standard doses of Rev 1 administered subcutaneously is followed by abortion in most vaccinated animals. Reducing the dose of vaccine has been suggested as a method of avoiding this problem and, accordingly, a reduced-dose vaccination strategy has been widely used and has been reported as a safe and effective method of controlling small ruminant brucellosis. However, we reviewed field and experimental results supporting the fact that as a result of the induction of abortion in pregnant animals and the low degree of immunity conferred, reduced doses of Rev 1 should not be recommended as an alternative to the full standard doses. When tested in a mouse model, differences in residual virulence and immunogenicity have been demonstrated between the different Rev 1 vaccines produced world-wide. These differences could account for the discrepancies in safety results obtained in mass vaccination trials in different countries. The induction of abortions when vaccinating pregnant animals means that there is no entirely safe strategy for Rev 1 vaccination. Conjunctival vaccination is safer than subcutaneous vaccination but is not safe enough to be applied regardless of the pregnancy status of the animals, and should be used only under restricted conditions. For sheep, conjunctival administration of standard doses of Rev 1 during the late lambing season or during lactation is recommended as a whole-flock vaccination strategy.