Objective—To determine the virulence of a Brucella
abortus mutant, BA25, lacking a major 25 kd outer
membrane protein ( Omp25) in cattle.
Animals—20 mixed-breed heifers in late gestation.
Procedure—10 heifers were inoculated with 1 × 107
colony-forming units of the Omp25 mutant via the
conjunctival sac, and an equal number were infected
with the virulent parental strain B abortus 2308. The
delivery status of the dams was recorded, and colonization
was assessed following necropsy. The ability
of BA25 to replicate inside bovine phagocytes and
chorionic trophoblasts was also evaluated in vitro
because of the propensity of virulent brucellae to
replicate inside these cells in vivo.
Results—The parental strain induced abortions in 5 of
10 inoculated cattle, whereas only 1 of 10 dams
exposed to BA25 aborted. Brucella abortus strain
2308 colonized all of the cow-calf pairs and induced
Brucella-specific antibodies in 100% of the dams. In
contrast, BA25 was isolated by bacteriologic cultural
technique from 30% of the calves and 50% of the
inoculated dams (n = 10). Of the 10 heifers inoculated
with BA25, 4 did not develop Brucella-specific antibodies
nor were they colonized by the mutant strain.
In bovine macrophages and chorionic trophoblasts,
BA25 replicated in significantly lower numbers than
the virulent parental strain (n = 3).
Conclusions and Clinical Relevance—The 25 kd outer
membrane protein may be an important virulence factor
for B abortus in cattle. The attenuation of the Omp25
mutant in cattle may involve the inability of BA25 to replicate
efficiently in bovine phagocytes and chorionic trophoblasts.(Am J Vet Res 2001;62:1461–1466)