Brucellosis in dogs is a bacterial infection caused by Brucella canis, a organism first identified in 1966 that lives inside cells and primarily attacks the reproductive system. It’s one of the most common infectious causes of infertility and pregnancy loss in dogs, and it can spread to humans. The infection is notoriously difficult to eliminate once established, making prevention and early detection critical for any dog owner, especially breeders.
How Dogs Get Brucellosis
Brucella canis spreads through direct contact with infected body fluids. Vaginal discharge, semen, and the fluids and tissues associated with birth or miscarriage carry the highest bacterial loads. But urine, blood, milk, saliva, and feces can also contain the organism. In practical terms, this means a dog can pick up the infection through mating, sniffing or licking contaminated discharge, or simply sharing close quarters with an infected animal during or after a birthing event.
The bacteria enter through mucous membranes, the moist tissue lining the mouth, nose, eyes, and reproductive tract. Once inside, Brucella canis hides within the dog’s own immune cells, which is part of what makes it so persistent. The immune system struggles to clear an organism that’s essentially using its own defenses as a hiding place.
Symptoms in Female Dogs
Reproductive failure is the hallmark of canine brucellosis, and in females it often appears as pregnancy loss. A female dog may experience early embryonic death, where she appears to conceive but the pregnancy quietly fails within the first few weeks. Late-stage abortions, typically around 45 to 55 days of gestation, are also common and produce large amounts of highly infectious discharge. Some females carry to term but deliver weak or stillborn puppies.
Between pregnancies, females may develop uterine infections. Persistent infertility in a previously fertile dog is a red flag, especially if she’s been bred to a proven male.
Symptoms in Male Dogs
Males often show visible changes to the reproductive organs. Early in the infection, the testicles and scrotum may become swollen and irritated. Over time, chronic inflammation damages the tissue, and the testicles actually shrink. Sperm quality deteriorates significantly, leading to infertility even when the dog appears otherwise healthy. Inflammation of the prostate gland can also occur.
Signs Beyond the Reproductive System
Brucellosis doesn’t always announce itself clearly. Many infected dogs show vague, nonspecific symptoms that are easy to dismiss: a dull or dry coat, low energy, decreased appetite. Some dogs develop eye inflammation, which can look like a routine eye infection. One of the more serious complications is diskospondylitis, an infection of the spinal discs that causes back or neck pain, stiffness, and reluctance to move. In some cases, a dog may show no obvious symptoms at all while still shedding bacteria and infecting others.
How Brucellosis Is Diagnosed
Testing for Brucella canis relies primarily on blood tests that detect antibodies the dog’s immune system produces in response to the bacteria. The most widely used screening method is the rapid slide agglutination test, which gives results quickly and is relatively inexpensive. However, no single test is perfect. A 2025 study evaluating several commercially available tests found sensitivity ranging from about 87% to 97% and specificity from roughly 79% to 98%, depending on the test used.
What that means in practice: a negative result on one test doesn’t guarantee a dog is clean, and a positive result sometimes needs confirmation. Veterinarians typically follow a positive screening test with a more specific confirmatory test to reduce the chance of a false alarm. Blood cultures, where a lab tries to grow the actual bacteria from a blood sample, provide the most definitive answer but take longer and aren’t always successful, since the bacteria can circulate intermittently.
Why Treatment Rarely Cures the Infection
This is the hardest part of a brucellosis diagnosis. Because Brucella canis hides inside cells, antibiotics have trouble reaching it. Treatment typically involves long courses of multiple antibiotics, often lasting six weeks or more. Even with combination therapy, relapse rates with a single antibiotic can approach 40%. Adding a second drug improves outcomes, but no antibiotic regimen reliably eliminates the infection entirely.
Dogs that appear to respond to treatment can still harbor the bacteria and begin shedding it again weeks or months later. For this reason, many veterinary authorities consider infected dogs to be lifelong carriers. Spaying or neutering is strongly recommended to remove the reproductive organs where bacteria concentrate, and it reduces (though doesn’t eliminate) the amount of bacteria the dog sheds into the environment. Treated dogs need ongoing monitoring with repeat blood tests to watch for relapse.
In breeding kennels, the standard recommendation for confirmed positive dogs has historically been euthanasia, because the risk of ongoing transmission to other dogs and to humans is difficult to manage. This is a painful reality, and individual pet owners working with their veterinarian may choose long-term treatment and management instead, but they should understand the limitations.
Prevention and Screening for Breeders
For anyone breeding dogs, routine screening is the single most important protective measure. The USDA recommends that kennels that have never tested for brucellosis start by testing every dog, then repeat testing every four weeks until all dogs have two consecutive negative results. After that baseline is established, every new dog should be tested before entering the property. If immediate testing isn’t possible, the new dog should be quarantined in a separate building as far from the existing kennel as possible until results come back.
Before any planned breeding, both the male and female should be tested. This applies even to stud dogs with a clean history, since dogs can be infected without showing symptoms. The cost of a blood test is negligible compared to the cost of losing a litter or dealing with a kennel-wide outbreak.
Risk to Humans
Brucella canis is zoonotic, meaning it can jump from dogs to people. Humans become infected through contact with contaminated body fluids, especially reproductive discharge and birthing materials. A 2023 CDC case report from South Carolina documented a family that handled aborted materials and newborn puppies without gloves or other protection. The three family members with direct contact received preventive antibiotics and were monitored for symptoms for 24 weeks.
In humans, brucellosis causes nonspecific symptoms: fever, joint pain, and fatigue that can drag on for weeks. Severe cases can lead to heart valve infection, an enlarged spleen, or neurological problems. Diagnosis is particularly challenging because no blood tests for Brucella canis are currently approved for use in humans, meaning cases can go unrecognized.
People most at risk include breeders, veterinary staff, and anyone who assists with dog births or handles reproductive fluids. Basic precautions make a significant difference: wearing gloves when handling newborn puppies or cleaning up after a birth, washing hands thoroughly, and keeping pregnant or recently whelped dogs in areas that can be easily cleaned and disinfected. Immunocompromised individuals should be especially cautious around dogs with unknown brucellosis status.