Brucellosis

Brucellosis is a zoonotic bacterial infection caused by gram-negative facultative intracellular coccobacilli of the genus Brucella. In veterinary medicine, it primarily affects dogs (Brucella canis), but other species such as cattle, sheep, goats, pigs, and horses are infected by species-specific strains (B. abortus, B. melitensis, B. suis, B. ovis, and B. equi). The disease is characterized by reproductive failure, including abortion, stillbirth, orchitis, epididymitis, and infertility, as well as systemic signs such as fever, lethargy, and lymphadenopathy. In dogs, B. canis infection is a chronic, insidious disease with a predilection for the reproductive tract and reticuloendothelial system. The disease is of significant public health concern due to its zoonotic potential, particularly in immunocompromised individuals. Brucellosis is a notifiable disease in many countries, and its diagnosis requires a combination of serological, bacteriological, and molecular methods. The disease is often subclinical, leading to delayed diagnosis and potential transmission within kennels and to humans.

The primary causative agent in dogs is Brucella canis, a small, gram-negative, aerobic, non-spore-forming coccobacillus. Other Brucella species can infect dogs, including B. abortus, B. melitensis, and B. suis, but these are less common and typically associated with exposure to infected livestock. Brucella species are facultative intracellular pathogens that survive and replicate within macrophages, evading the host immune response. Key virulence factors include lipopolysaccharide (LPS) with a smooth (S) or rough (R) phenotype; B. canis is naturally rough, lacking the O-polysaccharide side chain, which affects serological cross-reactivity. Other virulence factors include the type IV secretion system (VirB), which is essential for intracellular survival, and the production of erythritol, a sugar alcohol that promotes growth in the placenta and fetal tissues. Transmission occurs through direct contact with aborted fetuses, placental membranes, vaginal discharges, semen, urine, and milk. Venereal transmission is common in dogs, and the organism can also be transmitted through contaminated fomites. In dogs, the organism is shed in semen for months and in vaginal discharges for weeks after abortion. The bacterium is susceptible to common disinfectants and heat, but can survive in the environment for months under favorable conditions.

Brucellosis is a worldwide zoonosis, with B. canis infection being most prevalent in North and South America, Japan, and parts of Europe. The disease is more common in kennels and breeding facilities where dogs are housed in close contact. Stray dogs and those with access to infected livestock are at higher risk. There is no breed or age predilection, but sexually intact dogs are more likely to be infected due to venereal transmission. The incidence in the general dog population is low (1-5%), but in kennels with a history of reproductive problems, the seroprevalence can reach 30-40%. In livestock, B. abortus and B. melitensis are more common in cattle, sheep, and goats, with significant economic impact due to abortion storms. The disease is endemic in the Mediterranean basin, Middle East, Latin America, and parts of Africa and Asia. In dogs, the disease is often underdiagnosed due to the lack of routine screening and the insidious nature of the infection. Human infection with B. canis is rare but possible, particularly in immunocompromised individuals, and presents with undulant fever, malaise, and lymphadenopathy.

The pathogenesis of brucellosis involves a complex interplay between the bacterium and the host's immune system. After entry through mucous membranes (oral, nasal, conjunctival, or genital), Brucella organisms are phagocytosed by macrophages and polymorphonuclear leukocytes. The bacteria survive within the phagosome by inhibiting phagolysosomal fusion and replicating in the endoplasmic reticulum-derived compartment. The type IV secretion system (VirB) is critical for this intracellular survival. The bacteria then disseminate via the lymphatic system and bloodstream to the reproductive tract, placenta, fetal tissues, and other organs rich in erythritol, such as the epididymis and prostate. In pregnant animals, the organism preferentially localizes in the placenta, causing placentitis and abortion due to necrosis and disruption of the maternal-fetal interface. In males, the infection causes epididymitis, orchitis, and prostatitis, leading to sperm abnormalities and infertility. The chronic nature of the infection is due to the ability of Brucella to evade the immune system by downregulating the host's innate immune response, including the inhibition of apoptosis and modulation of cytokine production. The humoral immune response produces antibodies, but these are not protective, and cell-mediated immunity is essential for clearance. The infection can persist for years, with intermittent bacteremia and shedding of the organism.

Several factors increase the risk of Brucella infection in dogs and other animals. Intrinsic factors include sexual maturity, as the organism has a predilection for the reproductive tract; intact males and pregnant females are at highest risk. Immunosuppression, whether due to concurrent disease, stress, or corticosteroid therapy, can increase susceptibility and severity. Extrinsic factors include overcrowding in kennels, poor hygiene, and lack of routine screening. Exposure to infected livestock or wildlife, particularly in rural areas, is a significant risk factor. In livestock, the introduction of infected animals into a herd without quarantine is a common cause of outbreaks. Management practices such as artificial insemination with contaminated semen or sharing of contaminated equipment can also facilitate transmission. In dogs, the use of infected breeding stock is a major risk factor for kennel outbreaks. Additionally, the lack of vaccination in many regions contributes to the persistence of the disease in animal populations.

Clinical signs of brucellosis vary depending on the species and stage of infection. In dogs, the disease is often subclinical, but when signs occur, they are primarily related to the reproductive tract. In females, the most common sign is abortion, typically occurring in the last trimester (days 45-59 of gestation). Aborted fetuses may be autolyzed, and vaginal discharge may persist for weeks. Other signs include infertility, stillbirths, and weak puppies. In males, signs include scrotal enlargement, epididymitis, orchitis, prostatitis, and infertility. The testicles may become atrophic over time. Systemic signs are often mild and include lethargy, fever, lymphadenopathy, and anorexia. In some cases, discospondylitis, uveitis, or meningoencephalitis may occur due to bacteremia. In livestock, the classic sign is abortion in the last trimester, often followed by retained placenta and metritis. In males, orchitis and epididymitis are common. In horses, B. abortus can cause fistulous withers and poll evil. Chronic infections may lead to arthritis, bursitis, and hygromas. The clinical signs can be intermittent, and animals may appear healthy between episodes.

The differential diagnoses for brucellosis in dogs include other causes of abortion and infertility, such as canine herpesvirus infection, canine parvovirus, toxoplasmosis, neosporosis, and leptospirosis. Canine herpesvirus causes abortion and neonatal death, but is more common in young puppies and is associated with respiratory signs. Canine parvovirus can cause abortion but is more commonly associated with gastroenteritis. Toxoplasmosis and neosporosis can cause abortion and neurological signs, but are less common. Leptospirosis can cause abortion and systemic signs, but is more commonly associated with renal and hepatic disease. Other differentials include bacterial infections such as E. coli, Streptococcus, and Staphylococcus, which can cause metritis and abortion. In males, differentials for orchitis and epididymitis include trauma, testicular torsion, and other bacterial infections. In livestock, differentials include bovine viral diarrhea (BVD), infectious bovine rhinotracheitis (IBR), leptospirosis, and neosporosis. Definitive diagnosis is based on serology, culture, or PCR.

The diagnostic approach for brucellosis begins with a thorough history and physical examination, particularly in animals with a history of abortion or infertility. The initial screening test is serology, with the rapid slide agglutination test (RSAT) or the 2-mercaptoethanol rapid slide agglutination test (2-ME-RSAT) being commonly used for B. canis. Positive or suspicious results should be confirmed with the agar gel immunodiffusion (AGID) test or the tube agglutination test (TAT). For other Brucella species, the standard tube agglutination test (STAT) or the complement fixation test (CFT) is used. If serology is positive, confirmatory testing with blood culture or PCR is recommended. Blood culture is the gold standard but is time-consuming and requires special media (e.g., Brucella broth) and prolonged incubation (up to 6 weeks). PCR on blood, semen, vaginal swabs, or tissues is rapid and sensitive. In cases of abortion, fetal stomach contents, placenta, and vaginal swabs should be submitted for culture and PCR. In males, semen samples can be cultured or tested by PCR. Imaging, such as radiography or ultrasonography, may be used to detect discospondylitis or orchitis. Histopathology of affected tissues can reveal granulomatous inflammation and the presence of the organism with special stains (e.g., modified Ziehl-Neelsen).

Hematology findings in brucellosis are often nonspecific. Mild anemia, leukopenia, or leukocytosis may be present. Lymphocytosis and monocytosis can occur. Serum biochemistry may reveal mild elevations in liver enzymes (ALT, ALP) and globulins, particularly in chronic cases. Hyperglobulinemia is common due to chronic antigenic stimulation. Urinalysis is usually unremarkable. Blood gas analysis is not typically performed unless there is systemic illness. Specific biomarkers such as C-reactive protein (CRP) may be elevated. Serology is the mainstay of laboratory diagnosis. For B. canis, the RSAT is a rapid screening test but has a high rate of false positives; the 2-ME-RSAT is more specific. The AGID test is highly specific and is used for confirmation. For other Brucella species, the STAT and CFT are used. ELISA tests are also available and can differentiate between infected and vaccinated animals. PCR is highly sensitive and specific and can be performed on blood, semen, and tissues. Culture is the gold standard but is slow and requires biosafety level 3 facilities. In cases of abortion, fetal fluids and tissues should be submitted for culture and PCR.

Imaging findings in brucellosis are not specific but can be helpful in identifying complications. Radiography of the spine may reveal discospondylitis, characterized by narrowing of the intervertebral disc space, endplate lysis, and sclerosis. In males, ultrasonography of the scrotum can reveal epididymal enlargement, testicular atrophy, and abscessation. In females, abdominal ultrasonography may show evidence of placentitis or retained fetal membranes. In cases of prostatitis, transrectal ultrasonography can reveal prostatic enlargement and parenchymal changes. Computed tomography (CT) and magnetic resonance imaging (MRI) are more sensitive for detecting discospondylitis and neurological complications. In livestock, radiography of the joints may reveal arthritis or bursitis. However, imaging is not diagnostic for brucellosis and must be combined with serology and culture.

Cytological examination of fine-needle aspirates from lymph nodes, spleen, or other affected tissues may reveal granulomatous inflammation with epithelioid macrophages and lymphocytes. The organism may be visualized with special stains such as modified Ziehl-Neelsen or acridine orange. Histopathology of affected tissues, such as the placenta, testis, epididymis, or lymph nodes, typically shows granulomatous inflammation with necrosis, fibrosis, and the presence of intracellular coccobacilli. In the placenta, there is necrosis of the chorionic villi and infiltration of neutrophils and macrophages. In the testis, there is interstitial orchitis with degeneration of seminiferous tubules. In the epididymis, there is granulomatous epididymitis with sperm granulomas. Special stains, such as Gram stain, can demonstrate the organism. Immunohistochemistry using specific antibodies can confirm the presence of Brucella antigens. PCR on formalin-fixed, paraffin-embedded tissues can also be performed.

Treatment of brucellosis is challenging due to the intracellular nature of the organism and the lack of effective antibiotics that penetrate cells. In dogs, the recommended protocol is a combination of an aminoglycoside (e.g., gentamicin) and a tetracycline (e.g., doxycycline) or a fluoroquinolone (e.g., enrofloxacin). A common protocol is doxycycline (5-10 mg/kg PO q12h) for 4-6 weeks, combined with gentamicin (5-8 mg/kg IV or SC q24h) for the first 7-10 days. However, this protocol is not always effective in eliminating the infection, and relapses are common. Alternative protocols include enrofloxacin (5-10 mg/kg PO q24h) for 4-6 weeks, or a combination of rifampin (5-10 mg/kg PO q24h) and doxycycline. In livestock, treatment is not recommended due to the zoonotic risk and the lack of effective therapy; infected animals are usually culled. In humans, treatment involves a combination of doxycycline and rifampin or streptomycin for 6 weeks. Supportive care includes fluid therapy, nutritional support, and treatment of secondary infections. Surgical intervention may be necessary for abscesses or discospondylitis. Neutering of infected dogs is recommended to reduce transmission and to improve the response to therapy. However, even after treatment, dogs may remain seropositive and can shed the organism intermittently.

The prognosis for brucellosis in dogs is guarded. While treatment can reduce clinical signs and bacterial shedding, complete elimination of the organism is difficult, and many dogs remain chronically infected. The prognosis is worse in males with chronic orchitis and epididymitis, as the infection may persist in the reproductive tract. In females, the prognosis for future fertility is poor, and abortion may recur in subsequent pregnancies. The zoonotic risk is a significant concern, and owners should be informed of the risks. In livestock, the prognosis is poor, and infected animals are typically culled. The mortality rate is low, but the economic impact is significant due to reproductive losses. Negative prognostic indicators include chronic infection, presence of discospondylitis, and immunosuppression. Response to treatment is variable, and relapses are common. Long-term follow-up is essential to monitor for recurrence and to prevent transmission.

Follow-up for brucellosis involves serial serological testing and clinical monitoring. After treatment, serology should be repeated at 3, 6, and 12 months. A decrease in antibody titers is a good prognostic sign, but persistent titers do not necessarily indicate active infection. Blood culture or PCR should be performed if there is a suspicion of relapse. In breeding animals, it is recommended to test all animals in the kennel or herd and to quarantine new animals. Neutering of infected dogs is recommended to reduce shedding. In livestock, regular testing and culling of positive animals is the standard approach. Owners should be educated about the zoonotic risk and the importance of hygiene. In humans, follow-up is necessary to monitor for complications such as endocarditis or osteomyelitis.

Pearls: 1) Brucellosis should be considered in any sexually intact dog with a history of abortion, infertility, or scrotal enlargement. 2) The RSAT is a good screening test but has a high false-positive rate; confirm with AGID or 2-ME-RSAT. 3) Blood culture is the gold standard but requires special media and prolonged incubation; PCR is faster and more sensitive. 4) Treatment is often ineffective in eliminating the infection; neutering is recommended to reduce transmission. 5) Brucellosis is a zoonotic disease; wear gloves when handling aborted fetuses or vaginal discharges. Pitfalls: 1) Do not rely on a single negative serological test; repeat testing is necessary. 2) Do not use antibiotics without a confirmed diagnosis, as this may mask clinical signs and complicate diagnosis. 3) Do not ignore the zoonotic risk; inform owners of the potential for human infection. 4) Do not assume that a dog with a positive serology is actively shedding; confirm with culture or PCR. 5) Do not use the same treatment protocol for all species; consult specific guidelines for livestock.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended for canine brucellosis: 1) Doxycycline (5-10 mg/kg PO q12h) for 4-6 weeks, combined with gentamicin (5-8 mg/kg IV or SC q24h) for the first 7-10 days. 2) Enrofloxacin (5-10 mg/kg PO q24h) for 4-6 weeks, alone or in combination with doxycycline. 3) Rifampin (5-10 mg/kg PO q24h) in combination with doxycycline for 4-6 weeks. 4) Minocycline (5-10 mg/kg PO q12h) as an alternative to doxycycline. 5) For discospondylitis, prolonged therapy (8-12 weeks) may be necessary. In livestock, treatment is not recommended; culling is the standard practice. In humans, the WHO recommends doxycycline (100 mg PO q12h) for 6 weeks, combined with rifampin (600-900 mg PO q24h) for 6 weeks, or streptomycin (1 g IM q24h) for 2-3 weeks. Adjust dosages in renal or hepatic impairment. Monitor for adverse effects, such as gastrointestinal upset, hepatotoxicity, and ototoxicity. Avoid use of aminoglycosides in animals with renal disease.

The literature on brucellosis is extensive. Key studies include: 1) Carmichael et al. (1968) first described B. canis as a cause of abortion in beagles. 2) A study by Hollett (2006) reviewed the diagnosis and treatment of canine brucellosis, highlighting the challenges of therapy. 3) The ACVIM consensus statement on infectious diseases (2015) provides guidelines for the diagnosis and management of brucellosis. 4) A study by Lucero et al. (2008) evaluated the use of PCR for the diagnosis of human brucellosis. 5) A meta-analysis by Franco et al. (2007) assessed the efficacy of different antibiotic regimens for human brucellosis, concluding that doxycycline plus rifampin is effective. 6) In livestock, the use of vaccination with B. abortus strain RB51 has been shown to reduce the incidence of brucellosis in cattle. 7) A study by Moreno et al. (2002) reviewed the molecular mechanisms of Brucella pathogenesis. 8) The World Health Organization (WHO) and OIE provide guidelines for the control and eradication of brucellosis. Overall, the evidence supports the need for a One Health approach to control brucellosis, involving collaboration between veterinary and human health sectors.