Canine Brucellosis

Canine brucellosis is caused by Brucella canis. B. Canis is a gram-negative, intracellular bacterium. Dogs are also susceptible to infection with Brucella abortus (the Brucella bacterium infecting cattle) and Brucella suis (the Brucella species infecting pigs).

Adult dogs are rarely seriously ill and most female dogs that are not pregnant will not have clinical signs of illness. Canine brucellosis should be suspected in cases of canine abortions and reproductive failures. Abortions after 45 - 55 days of gestation occur in 75% of cases. Aborted puppies are usually dead and may be partly autolyzed (decomposed). Some puppies may be born stillborn or weak. Most other cases involve early embryonic death and fetal resorption at 10 - 20 days following breeding. An early abortion may go unnoticed. Brucella canis does not interfere with normal estrous cycles and the chief complaint by an owner may be a failure to conceive.

In the male dog, the primary clinical sign is that of infertility. The epididymitis may be inflamed and swollen, the testicles may appear shrunken, and a moist scrotal dermatitis or skin infection may occur on the scrotum. Semen from infected male dogs may contain large numbers of abnormal sperm and inflammatory cells. Changes in the semen are especially common during the first three months of infection and are typically seen on microscopic examination as abnormal sperm in the presence of large numbers of inflammatory or white blood cells. Chronically infected males may have a very low sperm count or have immature sperm when their semen is evaluated. The production of anti-sperm antibody by the infected male is believed to contribute significantly to infertility.

Other nonspecific clinical signs may include the dog being lethargic, loss of libido (sexual desire), premature aging, recurrent uveitis (inflammation of the iris in the eye), diskospondylitis (inflammation of the vertebrae and disks), glomerulonephritis (inflammation of the kidneys), meningoencephalitis (infection of the brain and spinal cord), generalized lymph node enlargement and prostatitis (inflammation of the prostate).

Brucella canis may be inactivated by common disinfectants and does not persist in the environment for long once outside the host. Man is relatively resistant to infection but human infections have been reported as a result of laboratory accidents or contact with infected dogs or tissues. Dogs and other canine species are believed to be the only true hosts of B. canis. The disease is primarily seen in commercial or research breeding kennels and stray dogs.

In the dog, the most common route of infection is by venereal transmission during breeding. The disease may also be contracted by ingestion of contaminated placental materials or aborted fetuses, and exposure to vaginal discharges from infected dogs, cattle or pigs.

B. canis may be shed for several weeks or intermittently for months following an abortion. Males may also shed organisms in the urine, but bacterial numbers are so low in urine that transmission through the urine is deemed unlikely, unless the urine is contaminated with seminal or prostatic fluids.

The disease is most prevalent in Central and South America where 20 to 30% of stray dogs were found to be serologic positive for Brucellosis. In the southern United States and Japan, 7 - 8% of stray dogs are estimated to be serologic positive.

There are several screening tests used for diagnosis of Brucella canis. The Indirect Fluorescent Antibody Test (IFA), the Rapid Slide Agglutination Test (RSAT), and the Agar Gel Immunodiffusion (AGID) Test. Each of these screening tests may have false positive results. In fact, with the RSAT, a negative slide test is strong evidence that the dog is not infected but about 60% of dogs who test positive are actually negative for the bacteria. An AGID test currently being used by Cornell University makes use of a protein antigen to the cell cytoplasm of Brucella and is considered to be highly specific for B. canis. The Cornell AGID test thereby diminishes the problem with false positive testing. Blood cultures are recommended to confirm a positive diagnosis, regardless of the screening test used. This is due to the dire consequences a positive infection will have on the animal being tested.

Treatment is not usually recommended for breeding dogs or when infected animals cannot be isolated or monitored following treatment. It is common practice to euthanize infected animals. Treatment is expensive and cures are difficult to achieve especially in chronically infected male dogs. Recurrence of infection is common once antibiotic therapy is discontinued. Sterilization of infected animals may reduce the risk of transmission from infected dogs.

Treatment is most successful during the first three months of infection. The best-known regimen is minocycline plus dihydrostreptomycin. Unfortunately, dihydrostreptomycin is unobtainable in many countries. Because of the restricted availability of dihydrostreptomycin, a combination of minocycline and gentamycin is often substituted. The new combination of minocycline and gentamycin appears to be less effective than the original treatment regimen. The minocycline is given for a four-week period and is combined with gentamycin on weeks one and four. Doxycycline is often substituted for minocycline due to the lower cost of doxycycline. Renal function tests should be monitored during gentamycin treatment phases. Repeated blood cultures and serologic monitoring is required for at least three months post treatment before a dog should be considered negative for the infection. It is not unusual for dogs to require more than one course of treatment.

Infection may be prevented through the use of serologic testing of dogs prior to breeding. Dogs in endemic areas should not be allowed to roam freely. Animals should be determined to be serologic negative for B. canis prior to introduction into a breeding kennel environment.

Brucella canis is a reportable disease in CO, MN and GA.

References:

"Antech Diagnostic News", January 2007.

Ettinger, Stephen and Edward Feldman. Textbook of Veterinary Internal Medicine. Sixth Edition. Vol. l. Elsevier Inc. 2005. pp 625-627.