Brucellosis: RB51 Exposures

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What is Brucella abortus RB51?

Brucella abortus is a bacterium that causes disease in cattle (and other animals), and also in humans. B. abortus RB-51 (RB51) is a strain of this bacterium developed specifically for an immunization for cattle against brucellosis. RB51 was selected because it is less likely to cause severe disease in cattle than other strains of B. abortus and is presumed to be less likely to cause severe disease in humans. Cattle immunizations against brucellosis started in1941. The RB51 immunization represents a safer immunization both for cattle and for the veterinarians administering it. The vaccine was conditionally licensed for use in cattle in 1996 and received full licensure in March 2003 and is used in 49 states, Puerto Rico, and the US Virgin Islands. RB51 does not induce an antibody response measurable with standard assays. This means that testing cattle for brucellosis will correctly identify naturally occurring cases that could affect other cattle and humans, and not identify antibody response resulting from immunization.

Can RB51 cause infections in people?

Accidental human exposure to RB51, although uncommon, has resulted in disease. Exposures have included needle sticks, eye and wound splashes, and exposure to infected material. In a series of exposures reported to CDC, described below, most of those exposed developed local symptoms of brucellosis infection; and of those ill, most exhibited some systemic symptoms. In the CDC review, 26 individuals reported exposure to the RB51 vaccine from January 1998 through December 1999. Of these, 21 (81%) reported needlestick injuries; of 5 patients (19%) who were splashed with the RB51 vaccine, 4 were splashed in their conjunctiva and 1 was splashed in an open wound. Even though most individuals (69%) reported receiving prophylactic antibiotics, 19 (73%) reported at least 1 systemic symptom. Of those ill, three (12%) reported persistent local reactions with systemic involvement, and 7 (27%) had persistent symptoms for 16 months. One patient required surgery and RB51 was isolated from the patient’s surgical wound. Very limited published information is available regarding laboratory exposures to RB51.

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What laboratory activities are considered high risk exposures to RB51?

Laboratories working specifically with RB51 isolates should perform all manipulations in a class II biological safety cabinet, utilizing biosafety level 3 (BSL-3) practices as described in Biosafety in Microbiological and Biomedical Laboratories, 5th edition. The risk for accidental exposure is highest for procedures or manipulations that would have occurred outside the class II biological safety cabinet and that have the potential for creating aerosols or splashes; examples of such procedures include pipetting, centrifuging, grinding, blending, shaking, mixing, sonicating, and opening containers of infectious materials.

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What can those exposed do to decrease their risk of developing brucellosis?

Although RB51 was developed as a less pathogenic strain, it does retain pathogenicity for humans so exposures can pose a human health risk. Post-exposure prophylaxis should therefore be considered for persons with RB51 exposure. RB51 was derived by selection in rifampin-enriched media and is resistant to rifampin in vitro. Use of rifampin for post-exposure prophylaxis or treatment, a recommended choice for other pathogenic bacteria, will not be effective for RB51. Post-exposure prophylaxis for those at high risk of exposure should include doxycycline 100 mg orally twice daily for at least 21 days. For those with contraindication to doxycycline, trimethoprim-sulfamethoxazole 160 mg/800 mg orally twice daily for at least 21 days may be used. Persons with contraindications to these antimicrobial agents should consult with their health care provider for alternative post-exposure prophylaxis. Should brucellosis occur despite prophylaxis, treatment regimens would need to be selected based on antimicrobial susceptibility results. Those with low risk exposures may also opt to take post-exposure prophylaxis.

Both those with high and low risk exposures should be monitored for the development of symptoms of brucellosis. Routine serologic tests for brucellosis will not be effective in monitoring for infection. From the last exposure, temperature should be actively monitored for fever for four weeks. Broader symptoms of brucellosis should be passively monitored for six months from the last exposure. Persons potentially exposed to RB51 who develop symptoms should consult with a health care provider. These symptoms include: