Canine brucellosis is caused by Brucella canis (B. canis), a gram-negative, intracellular bacterium. Other Brucella species, such as B. abortus and B. suis, have occasionally caused canine infections. Canine brucellosis was first recognized in 1966 as a cause of abortions and reproductive failures. The disease is common in Central and South America, and in the southern United States, and has been diagnosed in commercial or research breeding kennels of beagles in several other countries, including Japan and China. It has been reported sporadically in Europe. Although humans can be infected, dogs and other canine species are believed to be the only true hosts. Natural infections occur most often after ingestion of contaminated placental materials or aborted fetuses, exposure to vaginal discharges from infected bitches that are in estrus or abort, and during natural breeding. The organisms may be shed for several weeks or intermittently for months following an abortion. Males also may shed organisms in the urine, but bacterial numbers are relatively low, unless urine is contaminated with seminal or prostatic fluids. Sero-prevalence estimates appear high (20-30%) in Mexico and Central/South America, whereas estimates in stray dogs for the southern United States and Japan have been reported as 7-8%. Food-producing animals are highly resistant.

Clinical signs principally involve the reproductive tract. In females, the most prominent sign in ~ 75% of cases is abortion after 45-55 days of gestation. Early embryonic death and resorption, or abortion 10-20 days after mating, may occur. These signs may go unnoticed and the female may present with the chief complaint of "failure to conceive." In males, the main sign is unilateral or bilateral epididymitis and infertility. Testicular atrophy and moist scrotal dermatitis may be present, and organisms can persist in the testes and prostate for several months. Semen from infected males usually contains large numbers of abnormal sperm and inflammatory cells, especially during the first 3 months post-infection. Chronically infected males may be aspermic or have immature oligospermia. The presence of anti-sperm antibodies probably contributes to male infertility. Both sexes may exhibit nonspecific signs including lethargy, loss of libido, premature aging, recurrent uveitis, diskospondylitis, and generalized lymph node enlargement.

B. canis is short-lived outside the dog and is readily inactivated by common disinfectants.

The diagnosis of canine brucellosis requires laboratory confirmation. Blood cultures are strongly recommended before declaring an animal infected. The currently available serological tests are imprecise since surface antigens of Brucella spp, such as B. canis, cross-react strongly with antibodies to several other nonpathogenic bacterial species. As veterinarians may be unfamiliar with the interpretation of canine brucellosis diagnostic procedures, destruction of non-infected dogs solely on the basis of false-positive agglutination test results frequently still occurs.

The most commonly used tests include:
Serological Tests
Indirect Fluorescent Antibody Test (IFA)
The IFA is a very sensitive screening test but is not highly specific for B. canis. Positive tests should always be confirmed by AGID/RSAT testing.

Rapid Slide Agglutination Test (RSAT)
The RSAT requires brief treatment of serum with 2-mercaptoethanol (0.2M), and is readily available as a commercial screening test. A negative slide test is strong evidence that the dog is not infected, but only about 40% of dogs whose sera agglutinate in the slide test antigen are actually positive for canine brucellosis. Thus, dogs positive on the slide agglutination test should not be considered infected until additional serological tests and blood cultures are performed. More than 50% of infected dogs have a bacteremia lasting 1 year or longer.

Agar Gel Immunodiffusion (AGID) Test
The AGID test utilizes cell wall antigens and is more specific than IFA tests but also is flawed by false-positive reactions and difficulties in interpretation. Thus, positive results obtained by the RSAT and AGID tests should be confirmed by culture.

Cultures
Despite these improvements in serologic methods, blood cultures should always be performed when the disease is suspected. B. canis is readily isolated from the blood on tryptose or trypticase soy media. Brucella also can be cultured from semen, disc or bone marrow specimens from dogs with diskospondylitis, and necropsy tissue (lymph nodes, spleen, liver, male reproductive organs, uterus, and placenta).