CONDITION

Brucellosis in Dogs

Brucellosis in dogs is a bacterial infection caused by Brucella canis, an organism that tends to settle in the reproductive organs and lymph nodes. The infection spreads between dogs through breeding, contact with infected reproductive fluids, or occasionally through contaminated environments. It can persist for months or years without obvious signs in some animals. Owners often encounter this condition in one of several ways: difficulty achieving pregnancy despite repeated breeding attempts, unexpected loss of a litter in mid- to late pregnancy, or discharge from the reproductive tract that prompts investigation. In other cases, the infection is identified through screening before breeding or during routine health checks. Some dogs may show subtle signs such as lethargy, slightly enlarged lymph nodes, or joint discomfort, though many appear entirely well. This page explores the signals that may prompt investigation, the mechanisms by which the infection affects the body, the tests used to identify and monitor it, and the approaches taken to manage it in affected dogs and breeding populations.

Why this matters now

Brucellosis tends to surface in dogs that are part of breeding programmes or have been used for breeding, though infection can occur at any age once an animal becomes sexually mature. The condition is often identified when reproductive performance declines or when pre-breeding screening is carried out in kennels or breeding establishments. Dogs acquired from environments with multiple animals or uncertain health histories may carry the infection without having shown any signs. Brucella canis is not currently endemic in the UK; almost all cases identified here have been in dogs imported or rescued from countries where the infection is established, particularly in Eastern Europe, so a heightened index of suspicion is warranted for any dog with an overseas import or rescue history, not only those used for breeding.

The infection typically establishes itself quietly, often persisting for months or years without prompting investigation. In some dogs, the first observable event is reproductive failure—either difficulty conceiving or loss of a pregnancy—whilst others may develop intermittent signs such as mild lethargy or joint discomfort that come and go. The course varies considerably between individuals; some remain largely asymptomatic whilst others show more persistent, though still subtle, patterns over time.

Signals & patterns

Early signals

Difficulty achieving pregnancy

Repeated breeding attempts that do not result in conception may be the first indication that something is affecting reproductive function. This pattern can be difficult to interpret without investigation, as many factors influence fertility.

Subtle lethargy or reduced stamina

Some dogs may seem less enthusiastic about activity or tire more easily than usual, though the change is often mild and can be mistaken for normal variation in energy. This sign is non-specific and easily overlooked.

Slightly enlarged lymph nodes

Lymph nodes, particularly those beneath the jaw or behind the knees, may feel a little larger than expected during routine handling. The change is often subtle and may not be accompanied by any other obvious sign.

Intermittent stiffness or joint discomfort

Occasional reluctance to jump, climb stairs, or move freely can occur, often affecting one or more joints. The discomfort may come and go, making it difficult to pin down a clear pattern without further observation.

Later signals

Loss of pregnancy in mid- to late gestation

Absorption of developing puppies or late-term miscarriage may occur, sometimes with little warning. Discharge from the reproductive tract may be noticed around the time of loss.

Persistent or recurring discharge

Fluid or mucus from the vulva or prepuce that continues beyond what would be expected after oestrus or mating can prompt investigation. The discharge may be intermittent and vary in character.

Testicular changes in male dogs

One or both testicles may appear swollen initially, then shrink over time, becoming smaller and firmer than usual. The change may develop gradually and can be painless, so may not prompt immediate concern.

Click to read about the biological mechanisms

How this is usually investigated

Investigation typically begins with a reproductive history, including breeding dates, pregnancy outcomes, and any patterns of infertility or foetal loss. Physical examination may reveal subtle findings such as enlarged lymph nodes, testicular changes, or vulvar discharge, though many infected dogs appear entirely normal. Blood tests that detect antibodies or bacterial DNA form the core of diagnosis, often performed in combination to improve accuracy, and findings are interpreted alongside the individual animal's history and clinical picture.

Serology (antibody testing)

Purpose: Blood tests detect antibodies the immune system has produced in response to Brucella canis. Several test formats exist, including rapid screening tests and more detailed laboratory assays that measure specific antibody types.
Considerations: Antibody levels can fluctuate over time, and some infected dogs may test negative early in infection or intermittently later. False positive results can occur, particularly in dogs that have been vaccinated against certain other bacteria, so positive screening tests are typically followed by confirmatory methods.

Blood culture

Purpose: A sample of blood is incubated in the laboratory to allow any bacteria present to grow and be identified. This method can confirm active infection by isolating the organism itself.
Considerations: Brucella bacteria circulate in the blood intermittently rather than continuously, so a negative culture does not rule out infection. The process can take several weeks, and handling the organism requires specialised laboratory precautions.

PCR testing

Purpose: Polymerase chain reaction (PCR) detects bacterial DNA in blood, semen, vaginal discharge, or tissue samples. It can identify infection even when bacteria are present in very small numbers.
Considerations: PCR is sensitive but may occasionally detect dead bacteria or DNA fragments rather than active infection. Results are most useful when interpreted alongside other tests and the clinical context.

Reproductive tract examination and imaging

Purpose: Ultrasound of the uterus, prostate, or testicles may reveal changes such as thickening, fluid accumulation, or tissue irregularities that can accompany chronic infection. Examination of vaginal or preputial discharge under a microscope can identify inflammatory cells.
Considerations: Many of these findings are non-specific and can occur with other infections or inflammatory conditions. Imaging is more useful for assessing the extent of tissue involvement than for confirming the presence of Brucella itself.

Joint radiography

Purpose: X-rays of affected joints or the spine may show changes such as narrowing of joint spaces, new bone formation, or erosion of vertebral endplates in dogs with joint or spinal discomfort.
Considerations: Joint changes develop slowly and may not be visible early in infection. Similar patterns can occur with other causes of arthritis or discospondylitis, so radiographic findings are considered alongside other evidence.

Options & trade-offs

Management of brucellosis in dogs is shaped by the persistent nature of the infection, the difficulty of eliminating the bacteria completely, and the potential for ongoing transmission. Approaches vary depending on whether the dog is part of a breeding programme, lives with other dogs, or is kept as a companion animal. Different combinations of monitoring, treatment, and environmental management suit different situations, and the goals of management often need to be tailored to the individual household or kennel.

Long-term antibiotic therapy

Prolonged courses of antibiotics, often combining two different drugs, are used to suppress bacterial replication and reduce the number of organisms in the body. Treatment typically continues for several weeks to months, with monitoring through repeat blood tests to assess response. Some protocols involve extended or repeated courses if antibody levels remain elevated or clinical signs persist.

Trade-offs: Antibiotics can reduce bacterial load and may resolve clinical signs in some dogs, but complete elimination of infection is uncommon. Treatment can be lengthy and may carry side effects such as gastrointestinal upset or, rarely, effects on liver or kidney function, and there is no guarantee of clearing the infection even after prolonged courses.

Neutering

Surgical removal of the reproductive organs (ovariohysterectomy in females, castration in males) reduces the bacterial load by removing tissues where Brucella organisms preferentially reside. Neutering is often discussed in conjunction with antibiotic therapy, though the sequence and timing vary. In breeding animals, this approach ends reproductive potential.

Trade-offs: Neutering can lower the concentration of bacteria and reduce the risk of transmission through reproductive fluids, but it does not eliminate infection from other tissues such as lymph nodes or bone. The decision involves weighing the value of future breeding against the challenges of managing ongoing infection.

Isolation and biosecurity measures

Infected dogs are kept separate from uninfected animals to prevent transmission through direct contact, breeding, or environmental contamination. Protocols may include isolating the dog within the household, restricting access to communal areas, and careful handling of bodily fluids, bedding, and waste. Regular cleaning and disinfection of surfaces and objects the dog contacts are part of this approach.

Trade-offs: Isolation can be effective in preventing spread to other dogs but requires sustained effort and space, which may not be practical in all households or kennels. It does not alter the infection status of the affected dog and can be socially isolating for animals that previously lived in groups.

Serial monitoring without treatment

Some owners and veterinary teams choose to monitor the dog through periodic blood tests and clinical examinations without pursuing antibiotic therapy, particularly if the dog shows no signs of illness. This approach accepts the infection as a chronic, managed condition rather than attempting eradication. The focus is on detecting any change in the dog's health or antibody levels over time.

Trade-offs: Monitoring avoids the side effects and cost of prolonged antibiotic use and may be appropriate for dogs that are neutered, isolated, and clinically well. However, the infection persists, the dog remains a potential source of transmission, and there is no reduction in bacterial load, so careful biosecurity is essential.

Depopulation or removal from breeding programmes

In kennels or breeding establishments, infected dogs may be removed from the breeding population, rehomed as neutered companion animals, or, in some contexts, euthanased to protect the health of the remaining group. This decision is influenced by the prevalence of infection within the group, the value of the individual dog, and the biosecurity risks posed by retaining infected animals.

Trade-offs: Removing infected individuals can halt transmission within a kennel and allow uninfected dogs to continue breeding safely. It is a difficult decision that involves ethical, emotional, and practical considerations, and may not be acceptable to all owners or in all contexts.

Common misconceptions

Misconception:

"A single negative blood test means the dog is definitely not infected."

Reality:

Antibody levels can fluctuate, and some infected dogs test negative at certain points in time, particularly early in infection or intermittently later. A single negative result is reassuring but does not rule out infection entirely, particularly in dogs with a history of exposure or reproductive failure. Repeat testing or the use of multiple test methods can improve confidence in the result.

Misconception:

"Antibiotics will cure brucellosis and make the dog safe to breed again."

Reality:

Antibiotic therapy can reduce bacterial numbers and may resolve clinical signs, but complete elimination of Brucella canis is uncommon even after prolonged treatment. Dogs that have been treated may still harbour bacteria in tissues and can remain infectious, so return to breeding is generally not considered appropriate. Monitoring after treatment often shows persistent or fluctuating antibody levels, indicating ongoing infection.

Misconception:

"Brucellosis only affects dogs used for breeding and poses no concern for household pets."

Reality:

Whilst brucellosis is most commonly identified in breeding dogs, any dog can become infected through contact with infected reproductive fluids, urine, or contaminated environments. Dogs that have never been bred but were exposed before acquisition or through contact with infected animals can carry the infection. The risk to household pets is generally lower, but it is not absent, particularly in animals with unknown histories or those acquired from multi-dog environments.

Related conditions

Pyometra

Pyometra is another bacterial infection of the reproductive tract that can present with similar signs—discharge, difficulty breeding, and subtle systemic illness—though it tends to occur after a season rather than spreading between dogs. Both conditions may prompt investigation when reproductive function is impaired or when discharge is observed.

Vaginitis

Vaginitis can produce vaginal discharge that may prompt initial investigation, and in some cases brucellosis can contribute to inflammation of the reproductive tract. Distinguishing between isolated vaginitis and infection involving deeper structures often requires laboratory testing.

Septic Arthritis

Brucella canis can occasionally spread through the bloodstream and settle in joints, causing infection and inflammation that resembles septic arthritis from other bacterial sources. Joint discomfort in a dog with reproductive signs or known brucellosis exposure may raise the possibility of this spread.

Endocarditis

In some cases, bacteria circulating in the bloodstream—including Brucella canis—can settle on heart valves and trigger endocarditis, though this is an uncommon consequence. Dogs with brucellosis who develop signs of systemic illness or new heart murmurs may be assessed for this possibility.

Discospondylitis

Brucella canis has an affinity for the discs and bones of the spine, and can occasionally cause discospondylitis, leading to back pain, stiffness, or reluctance to move. This may be the presenting sign in some dogs, particularly when reproductive signs are subtle or absent.

Understanding the long-term implications of infection, the options available, and the practical realities of managing a dog with brucellosis can help shape decisions about testing, treatment, and household biosecurity. The broader context of reproductive health in dogs, as well as conditions that affect the immune system or cause chronic inflammation, may offer useful background as the picture develops. Conversations about testing frequency, interpretation of results over time, and the goals of management—whether focused on the individual dog or a breeding group—can help clarify the path forward.