SPECIES.

Brucella canis is one of six species of this nonmotile, gram-negative coccobacillus with a host range limited to domestic and wild canids. Of the six genus Brucella species that have been identified, dogs have been demonstrated to become infected by four: B. melitensis, B. abortus, B. suis, and B. canis. Dogs may occasionally be mechanical or biologic vectors for transmission to other animals or to humans. Except for B. canis infection, these infections are generally self-limiting.

INFECTION WITH B. MELITENSIS, B. ABORTUS, AND B. SUIS.

These three species of Brucella have been reported to cause infections in the dog through either natural or experimental exposure. Natural infection occurs with ingestion of contaminated milk, meat, or aborted fetuses or the fetal membranes of infected livestock. Dogs appear to be relatively resistant to brucellosis because infection may result in no, few, or mild clinical signs. These infections may persist without clinical signs for varying periods after confirmation of the diagnosis (Pidgeon et al, 1987). Infected dogs usually show positive results on the conventional serologic tests used to diagnose brucellosis in livestock, such as the card or tube agglutination test (Barr et al, 1986; Nicoletti, 1989). Natural and experimentally induced infections have been transmitted to livestock (Kormendy and Nagy, 1982). Infected dogs therefore have the potential to infect not only cattle but also humans, and they pose a threat of longer duration of disease transmission in cattle than was once assumed (Johnston et al, 2001).

INFECTION WITH B. CANIS.

Natural infection with B. canis appears to be limited to the Canidae family. Humans are accidental hosts, and infections are usually relatively mild compared with the clinical signs in humans infected with other Brucella species. Livestock, primates, cats, and rabbits are quite resistant to experimental infection (Greene and George, 1984; Nicoletti, 1989).

TRANSMISSION BETWEEN DOGS.

Infection can occur readily across mucous membranes, causing dogs to be infected by oronasal, conjunctival, or vaginal exposure. Transmission occurs readily when an uninfected bitch is mated with an infected male. Males can also acquire the disease from the female (Greene and George, 1984). Interestingly, in one study, when infected and uninfected dogs of the same gender were housed together for as long as 10 months, transmission did not occur (Hubbert et al, 1980). This suggests that urine and mucous membrane secretions are less important factors in the natural transmission of the bacteria. However, in a subsequent study, transmission between sexually mature male dogs was reported to occur after 4 to 6 months of cohabitation (Carmichael and Joubert, 1988).

Dogs without clinical signs can harbor Brucella organisms for prolonged periods. The time from exposure to bacteremia is usually 21 days (Meyer, 1983). After initiation of bacteremia, the organisms can become localized, causing continuous or recurrent bacteremia that lasts a few months to 3 to 4 years. Bacteria can be localized in the prostate or epididymis (or both) of an asymptomatic male. These focal infection sites can serve as a source of widespread dissemination if such a male is actively used in breeding. Venereal transmission probably occurs readily, even when low numbers of organisms are shed. Venereal transmission appears to occur most frequently when infected males are bred to susceptible females and somewhat less often when susceptible males are bred to infected females (Carmichael and Joubert, 1988).

In a kennel situation, a Brucella-infected aborting bitch is highly dangerous to Brucella-free dogs. Aborted placental tissues and fluids may contain huge numbers of organisms. Spread throughout a kennel is rapid, and persistent discharge of infected uterine secretions may continue for 4 to 6 weeks after a single abortion. Milk from infected bitches, which has been shown to contain abundant numbers of organisms, serves as an additional environmental contaminant. The effect on a breeding kennel can be devastating. With the potential for asymptomatic and prolonged bacteremia, blood transfusions also function as vectors for dissemination of brucellosis (Zoha and Walsh, 1982).

TRANSMISSION TO HUMANS.

During the years since the initial isolation of B. canis, an extremely small number of cases have been reported in humans. Most of these individuals were laboratory personnel or kennel attendants who had repeated and/or massive exposure to the organism. However, a few people who had contact only with infected pets have contracted the organism. Therefore it is wise to advise but not frighten owners regarding the zoonotic potential of the disease (Johnston et al, 2001).

NONSPECIFIC SIGNS.

Dogs infected with B. canis are not always seriously ill. Deaths from such an infection have not been reported. Fever is an uncommon finding, presumably because the organism lacks endotoxin. Generalized lymph node enlargement, as a result of diffuse lymphoid and reticular cell hyperplasia, is common. The spleen may become enlarged, firm, and nodular. Inflammation of the liver may also occur.

Other systemic problems may be seen with Brucella infection. Discospondylitis of the thoracic and/or lumbar vertebrae has been reported (Smeak et al, 1987; Kerwin et al, 1992). Endophthalmitis and recurrent uveitis have been associated with brucellosis (Gwin et al, 1980; Johnson and Walker, 1992). A low-grade nonsuppurative meningitis (without clinical signs) was observed in dogs that were experimentally infected. Dogs with brucellosis can have arthritis or polyarthritis as the only clinical signs (Forbes, 1990). Most dogs show no clinical signs, or owners complain of subtle signs such as a poor hair coat, listlessness, and exercise intolerance (Gordon et al, 1985).

ABORTION AND VAGINAL DISCHARGE.

The most common and obvious clinical sign of brucellosis in an otherwise healthy bitch is abortion between days 45 and 59 of gestation. The bitch may resorb fetuses and appear to be infertile because other clinical signs are absent. A bitch with brucellosis may abort puppies as early as day 30 of gestation or may carry pups nearly to term. A bitch with brucellosis also may deliver both living and dead fetuses. Surviving puppies are bacteremic for at least several months. Aborting bitches may lose two or three litters in succession (Nicoletti, 1989).

Aborted puppies usually appear to be partly autolyzed. The vaginal discharge present in some bitches aborting their litters may be brown or greenish gray (Purswell, 1992). This discharge usually contains large numbers of organisms, and extreme caution should be taken to prevent humans or dogs from coming into contact with it. Prolonged vaginal discharge after abortion is characteristic of this condition. In addition to the infected vaginal discharge, mammary secretions, urine, saliva, nasal secretions, and semen may contain organisms and may cause transmission (Carmichael and Greene, 1990).

INFERTILITY.

Apparent failure to conceive may be caused by canine brucellosis. In most cases bitches do conceive, but the fetuses are resorbed early or embryonic death and abortion go unnoticed by the owner. Upon examination, the placentas reveal focal coagulative necrosis of the chorionic villi, and numerous bacteria are present in the trophoblastic epithelial cells. Although none of the clinical signs of canine brucellosis is pathognomonic for the disease, it should always be considered in dogs examined for reproductive failure.

NONSPECIFIC FINDINGS.

Hematologic and serum chemistry abnormalities in dogs with brucellosis are nonspecific, and the urinalysis results are usually unremarkable. Any inflammatory condition (involving the testes, intervertebral discs, joints, and the like) may be the result of brucellosis, but the disease is relatively uncommon. Even semen analyses and vaginal discharges from known diseased dogs show nonspecific changes, such as the head-to-head sperm agglutination with inflammatory cells described in the semen analyses from infected dogs (Greene and George, 1984).

SEROLOGIC TESTS

Background.

Serologic tests for the presence of antibodies to cell wall and cytoplasmic antigens of B. canis are considered reliable, but such titers may not be detectable for 8 to 12 weeks. When bacteremia subsides after 30 to 60 weeks of infection, the serologic titer results may decline or become equivocal (Johnson et al, 1991). Therefore serologic titers, like many tests, are not perfect. Titers may fluctuate even with persistent bacteremia, and titer magnitudes do not reflect the stage of disease. Declining titers may not correctly reflect success of therapy (Table 26-1). Thus serologic testing may provide presumptive, but not definitive, diagnosis of canine brucellosis.

TABLE 26-1 SEROLOGIC TESTS FOR BRUCELLA CANIS

Two serologic tests are widely used in screening for the diagnosis of canine brucellosis: the slide agglutination test (SAT) and the tube agglutination test (TAT). Less commonly used tests are the agar gel immunodiffusion (AGID) test and the modified mercaptoethanol tube agglutination test. Direct culture of Brucella organisms is proof of infection, but bacteriologic confirmation is difficult.