An outbreak of Brucella melitensis infection by airborne transmission among laboratory workers

An outbreak of acute brucellosis infection was detected among the employees of a biologicals manufacturing laboratory located in Girona, Spain. The first cases appeared six weeks after a vaccine with attenuated Brucella melitensis, Rev-1 had been produced for one week. A clinical and epidemiologic investigation conducted among the 164 employees found 22 patients with clinical symptoms and positive serology, and six patients detected by serology only (attack rate: 17.1 per cent). Blood cultures were obtained from two patients and Brucella melitensis was isolated. Employees working in areas with open windows above the laboratory air extracting system had an attack rate of 39.5 per cent, substantially higher than those working in other locations. When vaccine was manufactured again, an electric oven reaching 300 degrees C had been installed in the air extracting system just before its exit to the exterior. Appropriate culture medium plates were exposed to the laboratory air before and after passing through the oven. Brucellae were isolated from the plates exposed to the air before passing through the oven but not after doing so.