Etiology

Bacteria in the genus Brucella are nonmotile, nonencapsulated, non-spore-forming, facultatively intracellular Gram-negative coccobacilli or short rods . Four of the six classical Brucella species are known to cause disease in dogs and humans: Brucella canis (natural reservoir animal is the dog), Brucella melitensis (sheep, goats), Brucella suis (pigs), and Brucella abortus (cattle, bison, buffalo) . The remaining 2 of the 6 classical Brucella species [Brucella neotomae (rodents, desert rats) and Brucella ovis (sheep)] are not associated with disease in dogs. Additional Brucella species including both terrestrial forms (B. microti, B. inopinata) and marine forms (B. maris, B. pinnipediae, B. ceti) are of uncertain pathogenicity to dogs.

Brucella canis was discovered in 1966–1967 during an investigation of abortion in beagles, in which the organism was isolated from aborted tissues and vaginal discharge . Brucella canis was initially thought to be a biotype of Brucella suis based on genotypic and phenotypic similarities . The significance of this distinction is paramount to the Canadian swine industry and the Canadian Food Inspection Agency (CFIA) as Canada is considered free of Brucella suis biovar 3 . Differentiation between Brucella canis and Brucella suis biovar 3 can be challenging . A multiplex conventional polymerase chain reaction (PCR) has been optimized to differentiate between these Brucella species .

The host range for Brucella canis is predominantly domestic dogs, but other species have been investigated. Serologic studies of wild canids have documented positive antibody titers in foxes and coyotes . Experimental studies involving conjunctival and oral inoculation of cattle, swine, and sheep with B. canis showed that these host species were highly resistant to B. canis infection, despite 2 field reports of B. canis in cattle . Similarly, oral experimental infection of cats documented transient bacteremia in 3/14 but none developed agglutinating antibody titers .

Transmission Major routes of transmission for this venereally transmitted agent are genital, conjunctival, and oronasal mucosae, as occurs during normal reproductive, social, and grooming activities in dogs .

The primary sources of transmission are reproductive fluids: vaginal discharges and semen. Tissues and fluids associated with the fetus, the placenta, and the vagina after abortion or stillbirth have approximately 10⁶ organisms/mL . Shedding of the organism occurs in vulvar secretions for up to 6 wk after abortion and during estrus . In males, semen has high concentrations of the bacterium for 6 to 8 wk after infection . The agent is then shed intermittently for up to 2 y at lower concentrations in semen which remains an important source of infection for other dogs . The minimum infectious dose is approximately 10⁶ organisms/mL via the oral route and 10⁴ to 10⁵ organisms/mL by the conjunctival route . Minor routes of transmission include in utero, broken skin, blood transfusions, feces, milk, and fomites such as contaminated syringes, vaginoscopes, and artificial insemination equipment .

Brucella bacteria attach to mucous membranes, penetrate the epithelial barrier, and are taken up by the mononuclear phagocytic system, where they reside intracellularly. This is accomplished by utilizing virulence factors presumably via the type IV secretory system, and inhibiting the bactericidal myeloperoxidase-peroxide-halide system through the release of 5-guanosine and adenine .