Control of bovine brucellosis from persistently infected holdings using RB51 vaccination with test-and-slaughter : a comparative case report from a high incidence area in Portugal

Abstract

Bovine brucellosis due to Brucella abortus infection causes significant reproduc-tive and production losses in cattle and is a major zoonosis. Eradication of this disease has proved difficult to achieve in Portugal where it still occurs in some regions despite an ongoing national eradication programme. In 2004, the Alent-ejo region, a major cattle producing area, reported one of the highest levels of bovine brucellosis in the country, especially in one divisional area. In that area, bovine brucellosis was particularly problematic in a holding of ten herds, the largest extensive cattle unit in the country, which remained infected despite an extensive test-and-slaughter programme and depopulation of five herds. A 5-year programme of RB51 vaccination with biannual test-and-slaughter was thus implemented in 2004. The apparent animal seroprevalence decreased from 19% (646/3,400) to 3% (88/2930) on the third herd-level test and remained below 0.8% (27/3324) after the fourth test. After the tenth test, the holding had a prevalence of 0.1% (2/2332) and only one herd remained positive with a within-herd prevalence of 1.1% (2/177). The results were compared to all other herds (n = 10) in the divisional area that were also persistently infected but were subject only to test-and-slaughter before being depopulated. In these herds, the strategy of test-andslaughter did not reduce the prevalence, which remained significantly higher than the vaccinated group (median = 0.48% and 8.5% in vaccinated versus non-vaccinated herds; Wilcoxon rank sum test; P < 0.01). The success of this pilot programme in continental Portugal pro-vided a valuable case study to the official veterinary services by illustrating the value of RB51 vaccination with parallel testing and improved biosecurity as a comprehensive and sustainable strategy for bovine brucellosis control in persis-tently infected herds.