I. DEVELOPMENT

Infected dogs shed Brucella organisms in all body fluids, including urine, saliva, semen, vulvar discharge, and milk. Transmission occurs by several routes. The most common route of transmission is ingestion or inhalation of infectious organisms from urine, vulvar discharges, or aborted materials. Venereal transmission (transmission during breeding) also may occur, as may infection of puppies across the placenta before birth or through the dam’s milk after birth. Because ingestion is a common form of transmission, it is important to remember that dogs that have not been bred still might have been exposed and should be tested for the disease.

After the organism is introduced into the body, it moves into the tissues of the immune system and the reproductive tract, where it multiplies. Although the organism grows best in tissues dependent on testosterone, estrogen, or progesterone, B. canis can also infect other tissues, such as the vertebrae, eyes, and kidneys. Bacteria will be free in the bloodstream as soon as 1 to 4 weeks after exposure and can remain in the bloodstream for up to 5 years after exposure. Bacteria are first shed in urine a few weeks after bacterial concentrations in the bloodstream increase and might be shed in the urine for months. The largest number of organisms is usually found in the uterus and vulvar discharge of bitches. After a variable amount of time, antibodies produced by the animal promote decline of the number of organisms present in the bloodstream. Bacteria may persist in tissues and within the cells of the immune system, even after treatment.

II. SIGNALMENT

There is no recognized age or breed predisposition for infection with B. canis in dogs. The disease presumably is more likely to infect intact animals than neutered animals.

III. HISTORY AND CLINICAL SIGNS

Many dogs carrying the Brucella organism have no clinical signs of disease. Infected bitches may show no change in estrous cycling or type of vulvar discharge produced during heat. The classic clinical sign in bitches is spontaneous abortion at about day 40 to 50 of gestation. Aborted pups usually have died well before abortion, and greenish gray vulvar discharge may persist for weeks after abortion occurs. Pups may be born live but do poorly, generally dying well before weaning. In males, brucellosis causes epididymitis (see Chapter 25). Again, dogs may show no clinical signs of disease. Semen quality declines after infection, and agglutination, or clumping, of spermatozoa may be evident.

Dogs with Brucella infection in tissues outside of the reproductive tract present with clinical signs referable to the site of infection. Examples include lameness and pain in dogs with vertebral infection; eye pain in dogs with eye infection; wobbly gait in animals with infection of the spine; and signs of urinary tract disease in animals with infection of the kidney.

IV. DIAGNOSIS

Several types of diagnostic tests are commercially available. The two broad categories are culture and serologic tests (Table 20-1).

Table 20-1 Diagnostic Scheme for Canine Brucellosis

Culture of the bacterial organism itself is the definitive test for canine brucellosis. However, the organism is difficult to culture because it is easily overgrown with other bacteria that may have contaminated the sample. Animals with obvious clinical signs of Brucella infection yield good diagnostic samples, such as vulvar discharge or aborted puppies. Animals with no obvious clinical signs yield poor diagnostic samples. Cells drawn from lymph nodes or samples of blood can be cultured. It has been suggested that 10 negative cultures are required before an animal can definitively be called clear of infection; this is not a cost-effective diagnostic technique.

Serologic testing, or assessment for infection in blood samples by identification of antibodies against B. canis, is more cost-effective and, although it is not definitive, is very accurate when used properly. Most serologic tests are negative until at least 6 to 8 weeks after infection and become negative as the number of organisms in blood—and subsequently the concentration of antibodies—declines. Thus testing more than once might be required to ensure accuracy. Animals to be tested for brucellosis should not be treated with antibiotics before testing because this may alter test results.

The most common type of serologic test is the agglutination test. This test identifies antibodies against Brucella spp. in the animal’s serum. Two types of agglutination test exist: the rapid-slide agglutination test (RSAT) and the tube agglutination test (TAT).

The RSAT is the test commonly performed in the veterinarian’s office (Figure 20-1). Blood is drawn from the animal to be tested and centrifuged, and the serum is drawn off. A drop of serum is placed on the test card. A drop of positive control from the test kit is placed on an adjoining circle on the test card. A drop of “agglutinating agent,” which will form clumps with antibody and is stained purple to increase the visibility of agglutination, is added to each and mixed. The card is slowly rocked back and forth, and the area where the mixture sheets down the card is observed for agglutination, comparing the test sample to the positive control. All samples begin to clump after 2 minutes. This test is very sensitive; any animal that tests negative either has not been exposed to B. canis or is in the very early stages of infection. This is not a specific test. Other bacterial organisms elicit formation of antibodies that resemble Brucella, such that false-positive results may occur. If the animal has recently been exposed to another organism and has mounted an appropriate immune response to it, that may be recognized by the RSAT. Vaccination has not been demonstrated to cause false-positive results.

Figure 20-1 Rapid-slide agglutination test.