Undulant Fever

Undulant fever in dogs, more commonly known as canine brucellosis, is a chronic bacterial infection caused primarily by the organism Brucella canis. The name "undulant fever" derives from the characteristic wave-like pattern of recurring fevers observed in affected individuals, including humans who contract the disease from infected animals. This condition holds significant importance in veterinary medicine due to its impact on canine reproductive health and its potential for transmission to humans, making it a notable zoonotic concern.

Brucella canis is a small, gram-negative, intracellular bacterium that has evolved sophisticated mechanisms to survive within the host's immune cells, particularly macrophages. This intracellular lifestyle makes the organism remarkably difficult to eliminate with antibiotic therapy and allows it to persist in the body for months or even years. The bacterium's ability to evade the immune system is central to the chronic and relapsing nature of the disease, which can frustrate treatment efforts and lead to long-term health consequences.

The disease was first identified in dogs in the 1960s and has since been recognized as a worldwide problem, particularly in regions where stray dog populations are large and breeding practices are poorly regulated. While Brucella canis is the primary species affecting dogs, other Brucella species such as B. abortus, B. melitensis, and B. suis can occasionally infect dogs that come into contact with infected livestock. Each species carries its own epidemiological significance and zoonotic risk profile.

Understanding undulant fever is essential for dog owners, breeders, and veterinary professionals alike. The disease can cause devastating reproductive losses in breeding operations, and its zoonotic potential means that infected dogs pose a health risk to their human families. Early recognition, proper diagnostic testing, and informed decision-making regarding treatment or management are critical components of addressing this challenging infection.

The primary causative agent of undulant fever in dogs is Brucella canis, although dogs can occasionally be infected by other Brucella species through contact with infected livestock or wildlife. B. canis is uniquely adapted to canine hosts and is the most common species isolated from infected dogs worldwide. The organism is shed in high concentrations through reproductive fluids, making sexual contact and birthing events the most efficient routes of transmission between dogs.

Venereal transmission is considered the most significant route of spread. During mating, infected males shed enormous quantities of bacteria in their semen, directly exposing the female's reproductive tract. Infected females shed the organism in vaginal discharges, particularly during estrus, abortion, or whelping. The bacteria can be present in reproductive secretions for weeks to months following an infectious event, creating prolonged windows of potential transmission to other dogs or to humans handling these animals.

Oronasal transmission also plays an important role in the spread of brucellosis. Dogs can become infected by licking or sniffing contaminated reproductive discharges, aborted materials, or urine from infected individuals. The bacteria can enter through intact mucous membranes of the mouth, nose, and conjunctiva. This route of transmission is particularly relevant in kennels, shelters, and multi-dog households where dogs have close social contact and may investigate each other's discharges.

Environmental contamination contributes to indirect transmission, though B. canis is relatively fragile outside the host. The organism can survive in moist environments for several weeks but is readily killed by sunlight, desiccation, and common disinfectants. Contaminated bedding, whelping areas, and equipment can serve as fomites for short periods. Humans can contract the infection through direct contact with infected reproductive fluids, blood, or urine, typically through mucous membranes or breaks in the skin, making proper hygiene and protective equipment essential when handling potentially infected animals.

The clinical presentation of undulant fever in dogs is highly variable and often subtle, which contributes to delayed diagnosis. Many infected dogs appear outwardly healthy for extended periods, making the disease particularly insidious in breeding populations where it can spread silently before reproductive failures prompt investigation. The classic undulating fever pattern that gives the disease its name may be present but is often mild and easily overlooked without careful monitoring.

Reproductive signs are the most dramatic and commonly recognized manifestations of the disease. In female dogs, late-term abortion occurring between days 45 and 59 of gestation is the hallmark clinical sign. Affected bitches may also experience early embryonic death and resorption, stillbirths, conception failure, and prolonged vaginal discharge following abortion or whelping. Puppies born alive to infected mothers are often weak, may fail to thrive, and frequently die within days to weeks of birth. Some infected females may carry one or two litters to term before reproductive failure becomes apparent.

In male dogs, brucellosis targets the reproductive tract with particular affinity. Epididymitis, or inflammation of the epididymis, is a common finding and may initially present as scrotal swelling. Over time, chronic infection leads to testicular atrophy as the germinal epithelium is progressively destroyed. Infected males typically develop poor semen quality with abnormal sperm morphology, decreased motility, and the presence of inflammatory cells. Infertility may become permanent even with treatment due to irreversible damage to the reproductive tissues.

Non-reproductive signs of brucellosis include lethargy, exercise intolerance, lymph node enlargement (lymphadenopathy), and intermittent lameness. Discospondylitis, an infection of the intervertebral discs and adjacent vertebral bodies, is a recognized complication that can cause significant back pain, reluctance to move, and neurological deficits if spinal cord compression occurs. Ocular manifestations such as uveitis, retinal detachment, and endophthalmitis may develop in some cases, potentially leading to vision loss. Splenic enlargement and chronic skin infections have also been reported in chronically infected individuals.

Diagnosing undulant fever in dogs requires a combination of clinical suspicion, serological testing, and ideally bacterial culture, as no single test provides definitive results in all cases. The intracellular nature of Brucella and the variable antibody responses in infected dogs make diagnosis challenging, and false-negative results are a well-recognized problem. A systematic approach using multiple diagnostic modalities significantly improves the accuracy of diagnosis.

Serological testing is the most widely used initial screening method. The rapid slide agglutination test (RSAT) is a common point-of-care screening test that detects antibodies against Brucella canis. While the RSAT is sensitive and relatively inexpensive, it has a notable rate of false-positive results because it can cross-react with antibodies against other bacterial species such as Pseudomonas and Bordetella. Positive RSAT results should always be confirmed with more specific tests. The agar gel immunodiffusion (AGID) test using cytoplasmic protein antigens is considered the standard confirmatory serological test and offers greater specificity than the RSAT.

Blood culture remains the gold standard for definitive diagnosis of brucellosis. Brucella canis can be isolated from the blood of infected dogs, particularly during episodes of bacteremia, which are most common in the first few weeks to months following infection. However, bacteremia can be intermittent in chronically infected dogs, and cultures may be falsely negative if blood is collected during periods of low bacterial circulation. Multiple blood cultures collected over several days increase the likelihood of successful isolation. The organism can also be cultured from semen, vaginal discharges, aborted materials, and tissue samples obtained at necropsy.

Polymerase chain reaction (PCR) testing has emerged as a valuable diagnostic tool that can detect Brucella DNA in blood and other body fluids with high sensitivity and specificity. PCR can identify the organism even when bacterial numbers are low and can provide results more quickly than traditional culture methods. Advanced imaging studies such as radiography and MRI may be employed to evaluate for complications such as discospondylitis. A complete blood count may reveal mild anemia and leukocytosis, while serum chemistry panels can identify elevated liver enzymes or kidney values in cases with systemic involvement.

Treatment of undulant fever in dogs is notoriously difficult, and complete eradication of the organism from the body is rarely achieved with antibiotic therapy alone. The intracellular lifestyle of Brucella canis provides the bacterium with a protected niche within macrophages where many antibiotics cannot reach effective concentrations. This biological reality forms the foundation for the generally guarded prognosis associated with treatment and the controversial nature of management decisions for infected dogs.

The most commonly recommended antibiotic protocol involves a combination of a tetracycline-class antibiotic, typically doxycycline or minocycline, administered for a minimum of four to eight weeks in combination with an aminoglycoside such as dihydrostreptomycin or gentamicin given during the initial one to two weeks of treatment. The tetracycline provides sustained intracellular activity against the organism, while the aminoglycoside delivers potent bactericidal action during the early treatment phase. Some protocols also incorporate fluoroquinolones such as enrofloxacin as an alternative or additional agent.

Surgical neutering is strongly recommended as an adjunctive measure for both male and female infected dogs. In males, castration removes the primary reservoir of bacteria in the epididymides and testes, significantly reducing bacterial shedding in semen and decreasing the risk of transmission. In females, ovariohysterectomy eliminates the uterine reservoir and prevents future pregnancy losses. Surgical sterilization combined with antibiotic therapy offers the best chance of reducing bacterial load and managing the infection long-term, though it does not guarantee bacteriological cure.

Despite aggressive treatment, relapse is common and infected dogs may continue to shed bacteria intermittently. Post-treatment monitoring with serial serological tests and cultures is essential to assess treatment response. Many veterinary authorities and some regulatory bodies recommend euthanasia of confirmed positive dogs, particularly in breeding facilities, due to the persistent zoonotic risk and the difficulty of achieving true cure. This recommendation remains controversial among pet owners whose dogs are companions rather than breeding animals. In cases where treatment is pursued, owners must be thoroughly informed about the limitations of therapy, the need for long-term monitoring, and the ongoing zoonotic risk.

The prognosis for dogs diagnosed with undulant fever depends on numerous factors including the stage of infection at diagnosis, the extent of organ involvement, and the treatment approach chosen. For the infection itself, the prognosis for complete bacteriological cure is generally poor. Studies have shown that even with prolonged combination antibiotic therapy and surgical sterilization, a significant percentage of treated dogs remain bacteremic or experience relapses within months of completing treatment. This reality shapes much of the clinical decision-making surrounding the disease.

From a quality-of-life perspective, many dogs with brucellosis can live comfortable lives with appropriate management. Dogs without serious complications such as discospondylitis or severe uveitis may show minimal outward signs of illness. However, these apparently healthy carriers still pose a transmission risk to other dogs and to humans, which must be factored into management decisions. The ongoing responsibility of preventing exposure to other animals and maintaining strict hygiene practices represents a significant long-term commitment for owners.

Reproductive prognosis is particularly poor. Infected females should never be bred due to the near-certainty of reproductive failure and the high risk of transmitting the infection to puppies and breeding partners. Males with established testicular damage will have permanently compromised fertility regardless of treatment outcome. For breeding operations, a diagnosis of brucellosis in any dog necessitates immediate quarantine, testing of all animals in the facility, and suspension of breeding activities until the situation is fully resolved.

Long-term complications can significantly affect quality of life in some individuals. Discospondylitis may require prolonged pain management and activity restriction. Ocular complications can progress to permanent vision impairment. The emotional toll on owners should not be underestimated, as the combination of a difficult-to-treat infection, zoonotic concerns, and controversial management recommendations can be deeply stressful. Regular veterinary follow-up at three to six month intervals with repeat serology and clinical evaluation is recommended for the remainder of the dog's life.

Undulant fever holds particular significance in veterinary medicine because Brucella canis is a zoonotic pathogen capable of causing disease in humans. While human infections with B. canis are generally considered less severe than those caused by B. melitensis or B. abortus, they can still produce significant illness characterized by recurrent fevers, fatigue, joint pain, headaches, and sweating. Immunocompromised individuals, pregnant women, young children, and elderly persons are at heightened risk for serious complications from human brucellosis.

Transmission to humans most commonly occurs through direct contact with infected reproductive fluids, blood, or urine. Veterinarians, veterinary staff, dog breeders, and kennel workers are at highest occupational risk. Activities that generate aerosols from infected materials, such as cleaning contaminated areas or assisting with whelping of infected bitches, can lead to infection through inhalation. Laboratory personnel handling Brucella cultures are also at significant risk, and the organism is classified as a biosafety level 3 agent in many countries.

The zoonotic risk associated with pet dogs living in households is a subject of ongoing concern and debate. While the risk is lower than in professional breeding or shelter settings, documented cases of household transmission exist. Owners of infected dogs should be counseled about the importance of strict hygiene measures including hand washing after handling the dog, avoiding contact with the dog's body fluids, wearing gloves when cleaning up urine or discharges, and preventing immunocompromised family members from having close contact with the animal. Any household member who develops unexplained febrile illness should inform their physician about the dog's brucellosis status.

Physicians should be aware that human brucellosis caused by B. canis can be diagnostically challenging because standard human brucellosis serological tests are designed to detect antibodies against smooth Brucella species and may not reliably detect antibodies against the rough-type B. canis. Specialized testing through reference laboratories may be required. Treatment of human brucellosis typically involves combination antibiotic therapy for six weeks or longer, and relapse can occur. The public health implications of canine brucellosis underscore the importance of testing, prevention, and responsible management of infected animals.

Undulant fever does not show a strong breed predisposition in the traditional genetic sense, as susceptibility to Brucella canis infection is determined primarily by exposure rather than hereditary factors. However, certain breeds and populations of dogs face elevated risk due to their management circumstances and the environments in which they are maintained. Understanding these risk factors is essential for targeted screening and prevention efforts.

Breeding dogs of all breeds represent the highest-risk population simply because the primary transmission route is venereal. Dogs used in commercial breeding operations, particularly those in high-volume facilities where multiple dogs are bred frequently and biosecurity practices may be inconsistent, face substantially elevated risk. The practice of using popular stud dogs that breed with many females from different sources creates an efficient mechanism for widespread dissemination of the organism throughout a breeding community. Testing protocols before each breeding event are the most effective way to interrupt this transmission chain.

Stray and feral dog populations serve as significant reservoirs of Brucella canis in many regions. Dogs adopted from shelters or rescue organizations, while typically wonderful companions, may carry the infection if they were previously living as strays or in uncontrolled breeding situations. Shelter dogs that arrive pregnant or with recent reproductive histories should be considered candidates for brucellosis screening, particularly in areas with known endemic disease. Dogs imported from countries with high prevalence of canine brucellosis also represent an elevated risk group.

Age and reproductive status influence clinical presentation more than susceptibility to infection. Sexually mature, intact dogs are most likely to develop classic reproductive signs, while prepubertal and neutered dogs may carry the infection with minimal clinical evidence. Environmental risk factors include high-density housing, poor sanitation, shared breeding facilities, and lack of quarantine protocols for new arrivals. Geographic location plays a role as well, with higher prevalence reported in the southern United States, Central and South America, and parts of Asia compared to regions with more aggressive surveillance and control programs.

Prevention of undulant fever in dogs relies on a combination of testing, biosecurity measures, and responsible breeding practices, as no commercial vaccine is currently available for canine brucellosis. The cornerstone of prevention in breeding populations is pre-breeding serological testing of both the male and female within 30 days prior to mating. This practice, while adding cost and logistical complexity to breeding programs, is the single most effective measure for preventing the introduction and spread of the disease.

Kennel and breeding facility biosecurity should include quarantine of all new arrivals for a minimum of 30 days with serological testing upon entry and again before release from quarantine. This two-test protocol accounts for the possibility that a recently infected dog may not yet have developed detectable antibody levels at the time of the first test. Facilities should maintain closed populations whenever possible, limiting the introduction of outside dogs. When outside breeding is necessary, requiring current negative brucellosis test results from the breeding partner's owner should be mandatory.

Environmental management plays a supporting role in prevention. Regular cleaning and disinfection of kennel areas, whelping rooms, and common spaces reduces the risk of indirect transmission. Brucella canis is susceptible to most common disinfectants including quaternary ammonium compounds, bleach solutions, and iodine-based products. Prompt removal and proper disposal of reproductive materials, aborted fetuses, and contaminated bedding minimizes environmental contamination. Facilities should maintain good drainage and ventilation to prevent the accumulation of moisture that can prolong bacterial survival.

Public health prevention measures are equally important. Veterinary professionals should use personal protective equipment including gloves, eye protection, and masks when handling suspected cases or performing procedures that may generate aerosols from infected materials. Dog owners should be educated about the zoonotic risk at the time of diagnosis and provided with specific guidance on hygiene practices. Reporting requirements vary by jurisdiction, and veterinarians should be familiar with local regulations regarding brucellosis notification. International movement of dogs should include brucellosis testing requirements to prevent cross-border spread of the disease.

For owners who choose to manage a dog diagnosed with undulant fever rather than pursue euthanasia, a comprehensive and sustained management plan is essential to protect both the dog's welfare and the health of humans and other animals in the household. This decision should be made in close consultation with a veterinarian who can provide realistic expectations about treatment outcomes, monitoring requirements, and the ongoing responsibilities that accompany living with a brucellosis-positive dog.

Surgical sterilization should be performed as soon as the dog is stable enough for anesthesia if it has not already been spayed or neutered. This step is non-negotiable in the management of brucellosis-positive dogs, as it removes the primary reproductive reservoirs of the bacteria and eliminates the risk of reproductive transmission. Following sterilization, the prescribed antibiotic protocol should be followed precisely, with the full course completed even if the dog appears clinically improved. Premature discontinuation of antibiotics increases the likelihood of relapse and development of antibiotic resistance.

Household management requires ongoing vigilance. The infected dog should not have contact with other intact dogs, and ideally should be the only dog in the household or should be separated from other dogs during any episodes of illness or discharge. All areas where the dog urinates or produces bodily fluids should be cleaned and disinfected regularly. Family members should wash hands thoroughly after handling the dog, especially before eating or touching their faces. Children should be supervised during interactions with the dog, and immunocompromised household members should discuss the risk with their physicians.

Regular veterinary monitoring is a lifelong commitment for owners of brucellosis-positive dogs. Follow-up serological testing should be performed every three to six months to track antibody levels, which can provide indirect evidence of ongoing or resolved infection. Any recurrence of clinical signs such as lethargy, fever, lameness, or lymph node enlargement should prompt immediate veterinary evaluation and repeat testing. Owners should maintain detailed records of test results and clinical observations to help their veterinary team assess trends over time and make informed management decisions. The emotional burden of managing this chronic condition should be acknowledged, and owners should not hesitate to seek support from their veterinary team when feeling overwhelmed.