Canine brucellosis (Proceedings)
Brucella canis is a non-motile, Gram negative coccobacillus with a host range limited to domestic and wild canids. Stray dog populations may act as a reservoir. There is a higher reported prevalence in purebred animals but few studies have attempted to compare populations of owned and stray dogs. Highest prevalence in the United States is in the rural southeast. Other areas of the world reporting cases of canine brucellosis include Canada; Mexico; South America; Europe, including Great Britain; and southeast Asia.
Infected animals shed organisms in urine, vaginal discharge, aborted materials and semen, and to a lesser extent in salivary, nasal and non-estrous vaginal secretions and milk. In urine, the highest number of organisms is secreted in the first 3 months post-infection. Aborted materials and fluid may contain as many as 500 oral infective doses per milliliter. Transmission occurs primarily by ingestion or inhalation. Research studying transmission is contradictory in its conclusions, with some studies showing minimal transfer from positive to negative animals housed in same-sex groups, and other studies showing definite evidence of transmission from positive animals to negative animals. Transplacental and venereal transmission also occur. A slight but real zoonotic potential exists.
The organism is phagocytized by macrophages and initially transported to the lymph nodes. Transient lymphadenopathy is followed by a prolonged bacteremia of 6 to 64 months duration. The organism replicates within lymphocytes and lymphoreticular tissues for an indefinite period of time. Antibody titres will rise after initial infection but will fall as bacteremia abates; decreasing serum antibody titres are not indicative of clearing of infection from the body. Organs most commonly infected are the uterus of all intact female dogs and the placenta of gravid female dogs, causing signs of infertility, persistent vaginal discharge and late-term abortion, and the testes and epididymes of male dogs, causing signs of orchitis/epididymitis, scrotal dermatitis, oligozoospermia and infertility. Brucella canis infection also has been reported as a cause of uveitis, meningitis, diskospondylitis, glomerulonephritis and pyogranulomatous dermatitis in the dog. Definitive diagnosis requires culture of Brucella canis from blood, lymph node aspirates, or infected tissues or discharges. However, Brucella canis is a fastidious organism; one negative culture does not rule out the disease. Blood culture is the most accurate test available in the first 8 weeks post-infection.Serologic testing is readily available and diagnostic once significant antibody titres develop, at greater than 8 weeks post-infection. Routine testing of asymptomatic bitches may best be performed just prior to estrus, as it is thought that the hormone influence of estrus may increase bacteremia and subsequently enhance the immune response. Agglutination tests identify presence of antibodies to cell wall antigens of Brucella canis. A rapid slide agglutination test (RSAT, card test) and tube agglutination test (TAT, tube test) are commercially available. The RSAT gives a positive/negative result while the TAT yields a titre. A titre of greater than 1:200 is considered a positive result. Agglutination tests are sensitive but not specific. Negative test results are 95% accurate. Positive test results may be inaccurate as many common organisms share cell wall antigens with Brucella canis; false positive results of 20 to 50% have been reported. Specificity of the test may be enhanced with addition of 2-mercaptoethanol to the test serum. Common cross-reacting organisms include Pseudomonas aeruginosa, Staphylococcus sp. and Bordetella bronchiseptica. Agglutination tests will remain positive in infected animals from 8-12 weeks post-infection to 3 months after the animal becomes abacteremic. These are excellent screening tests but positive results should always be verified with another testing method before diagnosis of Brucella canis infection is considered definitive.
The most accurate serologic test currently available is the agar gel immunodiffusion test (AGID) available through Cornell University, the University of Georgia, and the University of Florida. This test identifies antibodies to cytoplasmic antigens of Brucella canis, which are very specific to the Brucella sp. As dogs are rarely infected with other species of Brucella (e.g. B. abortus, B. melitensis) a positive result is considered definitive for a diagnosis of canine brucellosis. AGID tests will be positive in infected animals from 12 weeks post-infection to 36 months after the animal becomes abacteremic.