Canine Brucellosis: Symptoms, Diagnosis, and Management of Brucella canis

TL;DR. Canine brucellosis is a highly contagious bacterial infection that causes reproductive failure, joint pain, and eye inflammation in dogs, carries a guarded prognosis for a complete cure, and can spread to humans.

Lethargy and subtle discomfort can be early, non-specific signs of systemic Brucella canis infection.

What is it?

Canine brucellosis is a contagious, infectious venereal disease of dogs caused by the bacterium Brucella canis. This bacterium has a unique and highly effective survival strategy: it is an intracellular pathogen, meaning it lives and multiplies inside the host dog's cells, particularly within white blood cells called macrophages. This intracellular lifestyle shields the bacteria from the dog's immune system and makes it incredibly difficult for standard antibiotics to reach and eliminate the infection.

While the disease primarily targets the reproductive organs, it is also a systemic infection. Once the bacteria enter a dog's body, they quickly spread to the lymph nodes and travel through the bloodstream to various organs. This can lead to widespread inflammatory complications, such as diskospondylitis (a painful infection of the spinal discs) and uveitis (severe inflammation inside the eye).

For pet owners, breeders, and veterinary professionals, canine brucellosis is a disease of profound concern. It is highly contagious among dogs, difficult to diagnose, and notoriously resistant to complete cure. Furthermore, Brucella canis is zoonotic, meaning it can be transmitted from dogs to humans, making it a significant public health concern alongside its devastating veterinary impacts.

Causes & risk factors

Canine brucellosis is caused by the bacterium Brucella canis. The primary route of transmission is venereal, occurring during mating between an infected and a non-infected dog. However, sexual contact is far from the only way this disease spreads. The bacteria are shed in massive quantities in reproductive fluids, including vaginal discharge following an abortion or pregnancy loss, aborted fetuses, placenta, and semen.

Non-sexual transmission occurs when dogs sniff, lick, or ingest these contaminated materials. Additionally, other bodily fluids serve as significant sources of environmental contamination. According to a leading veterinary internal medicine reference:

"Urine can serve as a contaminated vehicle because of the proximity of the urinary and genital tracts in the dog, with shedding present for months to years; this is more prevalent in males. Organisms can also be shed in milk." [2]

Because the bacteria can persist in urine for years, especially in male dogs, any environment where multiple dogs mingle—such as breeding kennels, shelters, or dog parks—presents a risk if an infected dog is present.

There are no specific breed predispositions for canine brucellosis; any dog, regardless of breed, can contract the infection if exposed. However, intact breeding dogs, dogs imported from regions with high infection rates, and dogs kept in crowded, unscreened kennel environments are at the highest risk of contracting and spreading the disease.

Signs to watch for

Canine brucellosis can be a silent and deceptive disease. Many infected dogs appear completely healthy, showing no obvious signs of illness while actively shedding the bacteria into their environment. When clinical signs do emerge, they most commonly relate to reproductive failure, though systemic signs can also occur.

Cardinal Signs

• Pregnancy loss: This is the hallmark sign of the disease in female dogs. Affected pregnant dogs typically experience spontaneous abortion late in gestation, usually between 45 and 59 days of pregnancy.
• Abortion: The sudden loss of a litter, often with no warning signs of illness in the mother beforehand.

Common Signs

• Infertility: Female dogs may fail to conceive despite successful matings, or male dogs may experience a decline in fertility.
• Stillbirth: Puppies may be carried to term but are born dead or die shortly after birth.
• Vaginal discharge: Female dogs often exhibit a persistent, dark, or greyish-brown vaginal discharge for several weeks following an abortion.
• Orchitis: Inflammation and painful swelling of one or both testicles in male dogs.
• Epididymitis: Inflammation of the coiled tube at the back of the testicle that stores and carries sperm.

Occasional Signs

• Lumbar pain: Severe lower back pain caused by diskospondylitis (infection of the spinal discs).
• Lameness: Limping or difficulty walking, which can stem from joint inflammation or spinal pain.
• Uveitis: Inflammation of the inner structures of the eye, causing redness, cloudiness, squinting, and pain.
• Lethargy and weight loss: A general decrease in energy levels and unexplained weight loss.
• Splenomegaly: Enlargement of the spleen, detected during a physical exam by your vet.
• Regional lymphadenomegaly: Swollen lymph nodes, particularly those in the groin or neck area.
• Pyospermia: The presence of pus or inflammatory cells in the semen of male dogs.
• Suboptimal athletic performance: A noticeable decline in stamina or ability in working, hunting, or agility dogs.
• Scrotal dermatitis: Skin irritation, redness, and self-trauma on the scrotum due to licking and pain.
• Testicular atrophy: Shriveling and hardening of the testicles, which typically occurs in chronic, long-term cases after the initial swelling subsides.

Rare Signs

• Dermatitis: General skin inflammation or lesions unrelated to the scrotum.
• Meningoencephalitis: A rare but life-threatening inflammation of the brain and its surrounding membranes, leading to neurological signs.

Uveitis, or inflammation inside the eye, is an occasional but serious systemic manifestation of canine brucellosis.

How vets diagnose it

Diagnosing canine brucellosis is a complex process. Because the bacteria live inside cells and are not always consistently present in the bloodstream, a single test is rarely sufficient. Your vet will typically use a combination of screening tests, confirmatory tests, and direct detection methods.

Screening Tests

• Rapid Slide Agglutination Test (RSAT): This is a quick, highly sensitive blood test performed in the clinic. Because it is highly sensitive, a negative result is highly reliable, meaning your dog is almost certainly free of the disease. However, it can yield false positives by reacting to other, non-harmful bacteria.
• 2-Mercaptoethanol Modified RSAT (ME-RSAT): If a dog tests positive on the standard RSAT, your vet will perform this modified version. The addition of 2-mercaptoethanol helps eliminate false-positive reactions, providing a more accurate picture.

Confirmatory and Definitive Tests

• Tube Agglutination Test (TAT): A laboratory-based blood test used to measure the level of antibodies against Brucella canis.
• Agar Gel Immunodiffusion (AGID) Assay: Often considered one of the most reliable serological tests for confirming a diagnosis, particularly in chronic cases.
• Polymerase Chain Reaction (PCR) Assay: This molecular test detects the DNA of Brucella canis in blood, semen, vaginal discharge, or tissue samples.
• Blood, Urine, or Tissue Culture: The gold standard for diagnosis. Your vet will attempt to grow the bacteria from blood, urine, or tissue samples (such as aborted placenta). While a positive culture is definitive proof of infection, the bacteria can be slow and difficult to grow, and a negative culture does not entirely rule out the disease.
• Histopathology: Microscopic examination of affected tissues, such as placental tissue or testicular biopsies, to look for characteristic cellular changes caused by the infection.

Treatment options

Treating canine brucellosis is exceptionally challenging. Because Brucella canis resides inside host cells, standard antibiotic courses are ineffective. There is no treatment protocol that is guaranteed to completely cure the infection, and lifelong carrier status is common.

Antibiotic Therapy

Your vet will prescribe long-term, multi-drug antibiotic protocols to suppress the infection. Single-antibiotic treatments are highly prone to failure. The primary classes of antibiotics used include:

• Tetracycline Antibiotics (Doxycycline or Minocycline): These form the backbone of most treatment protocols, often administered for several weeks.
• Aminoglycoside Antibiotics (Gentamicin Sulfate): Often given in repeating cycles alongside tetracyclines to help target extracellular bacteria.
• Fluoroquinolone Antibiotics (Enrofloxacin): May be utilized in combination protocols due to their excellent tissue penetration.

Surgical and Management Measures

Because antibiotics rarely clear the infection completely, medical treatment must be combined with strict management practices:

• Spaying or Neutering: Surgical sterilization is highly recommended for any infected pet. While it does not cure the disease, it eliminates the primary source of bacterial shedding (reproductive fluids) and prevents the dog from passing the infection via mating or pregnancy.
• Quarantine and Isolation: Infected dogs must be strictly isolated from all non-infected dogs to prevent transmission.
• Removal from Breeding Programs: Any dog diagnosed with brucellosis must be immediately and permanently retired from breeding.

As noted in a leading veterinary internal medicine reference:

"Infected dogs and bitches should be removed from breeding programs and quarantined. Eradication of the disease in kennel situations has not been successful without removal (culling) of all infected (current or historically) dogs." [3]

Prognosis

The prognosis for dogs diagnosed with canine brucellosis is guarded for a complete cure. While adult mortality from the disease is very low—meaning the infection itself is rarely directly fatal to adult dogs—the likelihood of completely clearing the bacteria from the body is minimal. Relapses are common, even after months of aggressive antibiotic therapy.

For breeding kennels, the prognosis for the facility's operations is severe, often requiring complete depopulation or rigorous, expensive testing and quarantine protocols to eradicate the disease. For individual household pets, a diagnosed dog can live a relatively normal lifespan, but they must be managed as a lifelong carrier. This means they must be spayed or neutered, kept isolated from other dogs, and monitored closely for signs of relapse, such as returning joint pain or eye inflammation.

Furthermore, because of the zoonotic risk, owners of infected dogs must take strict hygiene precautions, particularly when handling urine or cleaning up after their pet. Immunocompromised individuals, young children, and pregnant women should not live in the same household as a Brucella canis-positive dog.

Prevention

Prevention is the single most effective tool against canine brucellosis. Because the disease is so difficult to treat, keeping it out of your home or kennel is paramount.

Pre-Breeding Screening

Every dog intended for breeding should be screened for Brucella canis prior to every mating. This applies to both the male and female dog. As a leading veterinary textbook emphasizes:

"Screening for B. canis infection is an important part of the prebreeding evaluation of any dog and should be included in the initial diagnostics in any case of canine abortion, orchitis, epididymitis, and apparent infertility in bitches or dogs." [1]

Kennel and Rescue Management

• Quarantine: Any new dog entering a breeding kennel or a multi-dog household should be quarantined and tested twice, 30 days apart, before being allowed to mingle with the resident dogs.
• Sanitation: Brucella canis is easily killed by common household disinfectants, including diluted bleach. Regular, thorough cleaning of kennels, run areas, and bedding is essential.
• Responsible Adoption: Ensure that any rescue dog, especially those from high-risk environments or imported from other regions, has been screened for brucellosis before adoption.

When to call your vet

If you own an intact breeding dog or have recently adopted a dog with an unknown medical history, you should contact your vet if you notice any reproductive abnormalities.

Contact your vet immediately if you observe any of the following emergency red flags:

• A pregnant dog experiences a sudden late-term abortion or pregnancy loss.
• Your dog develops sudden, severe back pain, an arched back, or an inability to walk comfortably.
• Your dog's eye suddenly becomes red, cloudy, squinted, or highly painful.
• A male dog develops sudden, severe, and painful swelling of the scrotum or testicles.

Sources

• Small Animal Internal Medicine, 5th Edition, pages 959–960.