Canine Brucellosis: An Underrecognized Zoonotic Disease

Brucellosis is a worldwide zoonotic disease caused by a gram-negative coccobacillus with 12 known species. Brucella melitensis, Brucella abortus, and Brucella suis are well-known causes of a flulike infection in humans.

Brucella canis has been reported primarily in domestic dogs but is known to infect humans. It is endemic to the Americas, Asia, and Africa, but data on current seroprevalence are lacking. In a recent Emerging Infectious Diseases article, the Centers for Disease Control and Prevention (CDC) highlighted B canis as an underrecognized zoonotic pathogen.

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Canine Infection

Late-term abortion and reproductive failure are the most common clinical manifestations of B canis infection in dogs. Abortion occurs during days 45 to 59, followed by 1 to 6 weeks of vaginal discharge. In males, chronic inflammation of the testes, epididymis, and prostate can lead to infertility.

Another common presentation is diskospondylitis. Additionally, many cases are subclinical, making identification of the disease difficult.

Treatment of infected dogs should be multimodal. Because the bacteria may be sequestered intracellularly, antibiotics alone are unlikely to clear infection. Surgical sterilization should be used, but neutered animals may still shed bacteria as the organism can persist in the prostate and lymphoid tissues.

Zoonotic Risk

Infected dogs may shed B canis intermittently in reproductive fluids (vaginal secretions and semen), aborted fetuses, placentas, saliva, nasal secretions, and urine. Reproductive fluids, especially postabortion vaginal secretions, are a common source of infection.

Clinical signs in humans are nonspecific and include flulike symptoms, an undulant fever, splenomegaly, and lymphadenopathy. Those at increased risk for the disease include veterinarians, animal caretakers, and laboratory workers.

In 2014, researchers found a B canis seroprevalence of 3.6% in humans with occupational exposure to dogs. Only 2 of the 306 exposed humans displayed clinical signs of disease, suggesting that healthy adults may be resistant to clinical disease. However, multiple case reports have shown that children and immunocompromised individuals are at higher risk.

A Diagnostic Challenge in Dogs and People

Serologic testing for antibodies against Brucella spp cell wall antigens is used as a screening tool for breeding dogs, but available human serologic tests cannot detect B canis. The limitations of the human testing come from the fact that Brucella species have 2 cell wall morphologies: smooth (ie, B abortus) and rough (ie, B canis). Tests designed for one type of cell wall will not detect organisms with the other. Available human serologic tests only detect antibodies to smooth cell walls. Some studies have used canine serologic tests on human samples, but more reliable testing is needed.

Culture is the gold standard test for B canis, but false-negative results are possible due to intermittent bacteremia and shedding of the organism.

Public Health Implications

The CDC identifies B canis as a zoonotic risk but highlights the lack of data and surveillance worldwide. Further studies are needed to determine the true prevalence and zoonotic risk of this disease.

Breeding and stray dog populations are suspected to pose the highest risk to humans. Breeding dogs and their offspring commonly move between states and countries without any testing requirements. Large stray populations are known to have a higher level of B canis and may pose a risk of transmission to those living in close proximity.

The CDC recommends implementation of mandatory testing for dogs prior to interstate and international movement as a first step to minimize transmission. Additionally, sterilization or testing of stray populations and development of a vaccine for B canis are identified as potential future steps to control infection.

Dr. Boatright, a 2013 graduate of the University of Pennsylvania, is an associate veterinarian in western Pennsylvania. She is actively involved in her state and local veterinary medical associations and is a former national officer of the Veterinary Business Management Association.