Brucellosis in dogs often looks like a reproductive problem first. Female dogs typically lose pregnancies late in gestation, between days 45 and 59, while males develop swollen or shrinking testicles and become infertile. But the disease doesn’t always announce itself through breeding failures. Many infected dogs appear completely healthy for weeks or months, making brucellosis one of the more deceptive infections a dog can carry.
Signs in Female Dogs
The hallmark sign in females is late-term abortion. A pregnant dog that loses her litter roughly six to eight weeks into a 63-day pregnancy, especially more than once, is a strong candidate for brucellosis testing. After the abortion, you’ll typically notice a brown-to-yellow vaginal discharge that’s oddly odorless. This discharge can persist for one to six weeks.
Not every infected female will visibly abort. In some cases, the embryos die early and are reabsorbed by the body. From the outside, this looks like a dog that simply failed to conceive after a seemingly successful mating. If your female dog has been bred multiple times without producing puppies, brucellosis is one possible explanation. Stillborn puppies or puppies that are born alive but weak and die within days are also common outcomes.
Signs in Male Dogs
Males show a different pattern. Early in the infection, the testicles and the tissue surrounding them become inflamed and swollen. The scrotal skin may look irritated or red. Over time, the inflammation causes permanent damage, and the testicles actually shrink. This progression from swelling to shrinkage can happen over weeks to months, and it leads to infertility. Prostate inflammation is also common but harder to spot without veterinary examination.
Semen quality drops sharply. Males may still attempt to breed but produce sperm that are abnormal or absent entirely. If you’re a breeder noticing a previously fertile male suddenly failing to produce litters, that warrants testing.
Symptoms Beyond Reproduction
Brucellosis isn’t strictly a reproductive disease. The bacteria travel through the bloodstream and can settle in the spine, eyes, and lymph nodes. Spinal infections (called discospondylitis) cause back pain, stiffness, reluctance to jump or climb stairs, and sometimes a hunched posture. Eye inflammation can cause redness, cloudiness, or squinting. Enlarged lymph nodes, particularly those in the groin and under the jaw, may be noticeable as firm lumps under the skin.
Some dogs simply seem “off” without dramatic symptoms: low energy, mild weight loss, a dull coat. These vague signs are easy to attribute to aging or minor illness, which is part of why brucellosis goes undiagnosed so often in pet dogs that aren’t being bred.
How Dogs Get Infected
The bacteria spread primarily through reproductive fluids. Vaginal discharge, especially after an abortion, contains extremely high concentrations of the organism, roughly a million bacteria per milliliter. Semen carries high loads for six to eight weeks after infection and continues to shed the bacteria intermittently for up to two years afterward, even at lower concentrations.
Dogs don’t need to mate to become infected. Normal social behaviors like sniffing, licking, and grooming can transmit the bacteria through the mouth, nose, or eyes. The conjunctival route (through the eye membranes) requires a lower dose of bacteria to cause infection than the oral route. Minor transmission routes include contact with contaminated urine, feces, milk, and even shared veterinary equipment. In utero transmission from mother to puppies also occurs.
Why It’s Hard to Diagnose
Testing for brucellosis is frustratingly imperfect. The most commonly used screening blood test has a sensitivity of only about 71%, meaning it misses roughly three out of every ten infected dogs. A more specific version of the test catches fewer false positives but is even less sensitive, detecting only about 32% of truly infected dogs while being 100% specific when it does return a positive result.
Because no single test is reliable enough on its own, veterinarians typically recommend serial testing: running multiple tests over several weeks. A single negative result doesn’t rule out infection, particularly if the dog was exposed recently and hasn’t yet developed detectable levels of antibodies in the blood. Blood cultures can confirm the diagnosis but take time and aren’t always available at general practices.
Treatment and What to Expect
This is where brucellosis gets difficult for dog owners. Antibiotic treatment exists, but it frequently fails. The most studied protocol, a four-week course of antibiotics with an additional drug during the first week, clears the infection in about 79% of dogs when success is measured by eliminating bacteria from the blood, lymph nodes, spleen, and reproductive organs. That sounds reasonable until you factor in the high relapse rate. Dogs that appear cured can begin shedding bacteria again weeks or months later.
Because of this, veterinary guidelines generally discourage antibiotic treatment except in carefully selected cases. Even under strict treatment conditions, a cure is never guaranteed, and a second round of treatment may be needed. Throughout and after treatment, the dog remains a potential source of infection for other dogs and for humans in the household. Spaying or neutering infected dogs reduces (but doesn’t eliminate) bacterial shedding.
For breeding kennels, the standard recommendation is to remove infected dogs from the breeding program permanently. Some veterinary authorities have historically recommended euthanasia for confirmed cases due to the ongoing public health risk, though individual pet owners and their veterinarians may choose long-term management with monitoring instead. This typically involves keeping the dog isolated from other dogs and retesting periodically.
Risk to People
Brucella canis can infect humans, though it tends to cause milder illness than the species of Brucella associated with livestock. People most at risk are those handling reproductive fluids, cleaning up after an aborting dog, or working closely with infected animals in veterinary or kennel settings. The bacteria enter through mucous membranes, broken skin, or accidental ingestion. In people, the infection can cause recurring fevers, fatigue, joint pain, and headaches. Immunocompromised individuals face a higher risk of serious illness. If your dog tests positive, your physician should be informed so appropriate monitoring or testing can be arranged for household members.