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.

An enzyme-linked immunosorbent assay (ELISA) for canine brucellosis has been described but is not yet commercially available. It has been reported to be quite accurate, yielding a false positive rate of only 2% in one study. Polymerase chain reaction (PCR) testing has been described in the literature and is very specific and sensitive when used for evaluation of serum, semen, or vaginal swabs from potentially infected animals. Should PCR testing become commercially available, it will be the gold standard for screening and diagnosis.

Treatment recommendations vary with housing and breeding status of the animal. Treatment with antibiotics has not been shown to effect a long-term cure. It must be remembered that decreasing titres occur in natural infection, and that negligible titres are not indicative of complete clearing of infection from the body.

Pet animals that are housed singly may be neutered and treated with antibiotics to decrease bacteremia and subsequent shedding of the organism. Antibiotic regimens that have been described include minocycline (25 mg/kg po SID x 14 days) with dihydrostreptomycin (5 mg/kg IM BID x 7 days) and tetracycline (30 mg/kg po BID x 21 days) with streptomycin (20 mg/kg IM SID x 14 days). The zoonotic potential of the disease must be stressed, especially in households with young children or immunosuppressed occupants.

Control of Brucella canis in a kennel includes the following steps: (i) confirm disease, (ii) quarantine kennel; no animals allowed in or out, (iii) determine source of infection; the index case is not necessarily the source of infection, (iv) eliminate mode of transmission within kennel, (v) identify and cull infected animals, (vi) initiate practices to prevent further outbreaks. Kennels should be considered clear of disease only when all animals have tested negative for 3 consecutive months. In closed kennels, complete eradication of the disease may require 5 to 7 months of testing and culling. In open kennels, the disease may never be eradicated.

Canine brucellosis is a reportable disease in many states. Although there are only 30 cases of human infection documented worldwide since the disease was first recognized in dogs in the 1960s, there is a public health risk that must be addressed. Because of recent outbreaks in the United States, the following measures have been taken by some states to promote control of the disease by the state: All positive serologic test results must be reported to the state board of animal health by the veterinarian within 24 hours of testing. All animals are identified and individually housed to the greatest extent possible. The kennel is quarantined, with no animals allowed to go in or out. Designated personnel work with the dogs and are required to take all necessary precautions (wearing gloves, using foot baths, etc). All dogs are tested, the positive dogs euthanized, and the negative dogs retested until all dogs in the facility have had two or three negative tests (varies by state). When the kennel is opened, all incoming dogs are kept isolated from the main population of the kennel until they have had two negative tests 4-6 weeks apart. All dogs are retested yearly.

Economic losses to a kennel from brucellosis can be severe and may even close that kennel. Zoonotic potential is slight but does exist. Canine brucellosis is a disease condition all small animal veterinarians should be familiar with, ready to test for, and aware of control procedures for should an animal test positive.