Genotypic variation in the Brucella melitensis hemagglutinin gene in vaccine strains and field isolates in Palestine

In Palestine, as in other countries, brucellosis is an endemic disease, even in highly vaccinated zones. Many reports suggest that a decline in vaccine productivity may be due to antigenic shifts in the circulating Brucella melitensis. To address this aspect, the hemagglutinin gene from B. melitensis field isolates was amplified by polymerase chain reaction (PCR), sequenced, digested with the EcoRV and HaeIII, and compared to the Rev.1 strain of the B. melitensis vaccine. From January to December 2008, 80 milk samples were collected from infected flocks, from different West Bank regions of Palestine. From these, 77.5% (62/80) were shown to be positive for specific B. melitensis PCR. However, from the PCR-positive milk samples, only 38 strains could be isolated by culture on Brucella agar plates. The nucleotide alignment and phylogenetic tree for the hemagglutinin gene showed a mismatch between the vaccine strain and field isolates. This is also suggested by the observation of EcoRV and HaeIII digestion profiles for the vaccine strain and field isolates. Although a limited number of isolates and genes encoding immunologically relevant proteins were analyzed, we observed antigenic divergence between the current B. melitensis field isolates and the vaccine strain, in particular with respect to the hemagglutinin gene. Therefore, more research will be necessary to rule out the possibility of reduced efficacy of Brucella whole-cell vaccines.