Dogs get brucellosis primarily through contact with infected reproductive fluids, especially during mating or exposure to material from an abortion or stillbirth. The bacterium Brucella canis enters the body through mucous membranes in the mouth, nose, eyes, or genitals, and it can also pass through broken skin. While sexual transmission gets the most attention, dogs can also pick up the infection through everyday social behaviors like sniffing and licking.
Sexual Transmission Is the Primary Route
Brucellosis spreads most efficiently through breeding. An infected male dog sheds high concentrations of the bacterium in his semen, particularly during the first 6 to 8 weeks after infection. After that initial window, shedding becomes intermittent but can continue for up to two years, meaning an apparently healthy dog can still pass the infection during mating long after the initial exposure.
Females transmit the bacteria too. Vaginal discharge during estrus and especially after an abortion or stillbirth carries roughly one million organisms per milliliter, a dose well above what’s needed to cause infection. An infected female sheds the bacterium in vulvar secretions for up to six weeks after losing a pregnancy. A male that mates with her during this period is at high risk.
Aborted Tissue and Birth Fluids
The single largest burst of bacteria into the environment happens when an infected dog aborts a litter or delivers stillborn puppies. The fetal tissue, placenta, and surrounding fluids are loaded with Brucella. Any dog that investigates, licks, or ingests this material can become infected through the oral or nasal route. The minimum infectious dose through the mouth is about one million organisms per milliliter, which aborted material easily exceeds. Through the eyes (conjunctival route), the threshold is even lower: roughly 10,000 to 100,000 organisms.
This is why outbreaks tend to cluster around breeding events. A single abortion in a kennel can expose every dog that comes near the whelping area.
Urine, Saliva, and Nasal Secretions
Both male and female dogs can shed Brucella canis in their urine, and the bacterium has also been found in saliva and nasal secretions. These are considered minor transmission routes compared to reproductive fluids, and the exact role of urine in spreading infection isn’t fully understood. Still, normal dog behaviors like communal urination spots, mutual grooming, and face-to-face sniffing create opportunities for the bacteria to reach mucous membranes. In close quarters, these small exposures add up.
Other Less Common Routes
Several additional pathways have been documented, though they account for a smaller share of infections:
- In utero transmission: An infected mother can pass the bacteria to her puppies before birth, meaning some puppies are born already carrying the infection.
- Broken skin: Open wounds or skin abrasions that come into contact with contaminated fluids can serve as an entry point.
- Blood transfusions: A transfusion from an infected donor dog can transmit the bacterium directly into the bloodstream.
- Contaminated equipment: Shared veterinary tools like vaginoscopes, artificial insemination equipment, and even syringes can carry the bacteria between animals if not properly sterilized.
How Quickly an Infected Dog Becomes Contagious
After a dog is first exposed, the bacteria enter the bloodstream within 1 to 4 weeks. This initial bacteremia typically peaks and then wanes over 3 to 4 months. But the picture doesn’t end there. The infection can persist in the bloodstream for as long as 18 months, and bacterial levels may rise and fall unpredictably for years. This waxing and waning pattern is one of the things that makes brucellosis so difficult to contain: a dog that tests negative on one occasion may be shedding bacteria a few weeks later.
Which Dogs Are Most at Risk
Sexually intact dogs face the highest risk, for the simple reason that the primary transmission route is sexual. In the United States, brucellosis is more common in stray and free-roaming dogs and in animals from the southern states. Studies of stray dogs in Florida, Georgia, and Tennessee have found infection rates between 3.6% and 9.4%, while owned pet dogs in similar regions test positive at much lower rates, typically under 2%.
Breeding kennels are particularly vulnerable environments. Dogs are housed together, mated regularly, and exposed to birth and abortion fluids. A single unscreened dog introduced into a breeding program can spark an outbreak that affects the entire facility. Shelters and rescue operations face similar risks, especially when they take in dogs of unknown origin or intact strays from high-prevalence areas.
Risk to Humans
Brucella canis is zoonotic, meaning it can spread from dogs to people. Humans become infected through direct contact with reproductive secretions, aborted material, or urine from infected dogs. Pet owners, breeders, veterinary staff, and kennel workers are the most commonly affected groups. One documented outbreak involved an entire family of six, including three children, who were infected after caring for a sick dog and her puppies. The risk for the average pet owner with a spayed or neutered household dog is low, but anyone involved in breeding or handling dogs with unknown health histories should take hygiene precautions seriously.
How Screening Helps Prevent Spread
Because brucellosis can circulate silently, with infected dogs looking perfectly healthy for months or years, testing is the main line of defense. Any dog being used for breeding should be tested before each mating. This is especially important because intermittent shedding means a dog that was negative several months ago may now be positive. Blood tests detect antibodies to the bacteria, and semen cultures in males are most reliable between 3 and 11 weeks after infection, when bacterial concentrations are highest. After that window, semen cultures often come back negative even in dogs that are still infected, so a negative culture alone isn’t enough to clear a dog.
For kennels and shelters, testing all new arrivals before they have contact with resident animals is the most effective way to prevent an outbreak from taking hold. Dogs imported from regions with higher prevalence, or any intact dog with an unknown history, should be considered higher risk and tested accordingly.