Brucella melitensis in Small Ruminants: Malta Fever, Zoonosis, and Diagnostic Approach

Etiology and Taxonomy

Brucella melitensis is a Gram-negative, facultative intracellular coccobacillus belonging to the genus Brucella within the family Brucellaceae (order Rhizobiales, class Alphaproteobacteria). The organism is nonmotile, non-spore-forming, and exhibits a characteristic oxidative metabolism. B. melitensis is the primary etiologic agent of brucellosis in small ruminants (sheep and goats) and is the most frequent cause of human brucellosis globally, a disease historically termed Malta fever. The species is divided into three biovars (1, 2, and 3) based on phage lysis patterns, oxidative metabolic profiles, and agglutination with monospecific antisera. Biovar 3 is the most widely distributed and is associated with the highest virulence in both animal and human hosts.

The bacterium possesses a lipopolysaccharide (LPS) layer that is a key virulence factor. The O-polysaccharide side chain of the smooth LPS is immunodominant and is the target of most serological diagnostic tests. B. melitensis lacks classical exotoxins; its pathogenicity derives from its ability to survive and replicate within host macrophages, evading innate immune clearance through inhibition of phagolysosome fusion and modulation of host cell apoptosis.

Epidemiology and Transmission

Brucella melitensis infection in small ruminants is a notifiable disease in most countries and is classified as a List B disease by the World Organisation for Animal Health (WOAH). The disease is endemic in the Mediterranean basin, the Middle East, Central Asia, parts of Africa, and Latin America. Northern Europe, North America, Australia, and New Zealand have achieved eradication or official brucellosis-free status through sustained test-and-slaughter programs.

Transmission occurs primarily through direct contact with infected reproductive tissues, vaginal discharges, and milk. The most efficient route of transmission is ingestion of contaminated feed or water. Lambs and kids acquire infection by suckling infected milk. Venereal transmission is possible but epidemiologically less significant than oral and conjunctival routes. The bacterium can survive for weeks to months in the environment under cool, moist conditions, particularly in fetal membranes, aborted fetuses, and manure. Survival is prolonged at low temperatures and in neutral pH; desiccation and direct sunlight rapidly inactivate the organism.

The primary reservoir is the infected female. Following abortion or parturition, massive numbers of B. melitensis (up to 10^10 colony-forming units per gram of placental tissue) are shed into the environment. Latent infections in nonpregnant animals can reactivate during subsequent gestations, leading to intermittent shedding.

Pathogenesis

After ingestion or inhalation, B. melitensis penetrates the mucosal epithelium of the oropharynx, gastrointestinal tract, or conjunctiva. The bacteria are taken up by M cells and resident macrophages, then transported to regional lymph nodes (pharyngeal, retropharyngeal, or mesenteric). Within macrophages, the organism survives by inhibiting phagosome-lysosome fusion and replicating within the endoplasmic reticulum-derived compartment. This intracellular niche protects the bacterium from humoral immunity and many antibiotics.

A transient bacteremia ensues, typically lasting 2 to 4 weeks, during which the organism disseminates to the reticuloendothelial system (spleen, liver, bone marrow) and to the reproductive tract. In pregnant females, B. melitensis exhibits a marked tropism for the chorioallantoic trophoblasts. The bacterium replicates to high numbers within these cells, causing placentitis, necrosis, and disruption of fetal-maternal nutrient exchange. The result is abortion, typically in the last trimester (third to fifth month of gestation in sheep and goats). The organism also colonizes the mammary gland and supramammary lymph nodes, leading to chronic shedding in milk.

In males, the organism localizes to the epididymis, seminal vesicles, and testicular parenchyma, causing orchitis and epididymitis. The resulting inflammation can lead to permanent sterility.

Clinical Signs in Small Ruminants

The hallmark clinical sign of Brucella melitensis infection in small ruminants is abortion in the last trimester of gestation. Abortion storms can occur in naive flocks, with up to 30% to 50% of pregnant females aborting within a single lambing or kidding season. Subsequent pregnancies may result in full-term births, but the placenta may be retained, and the offspring are often weak or infected.

Other clinical manifestations include:

• Retained placenta and metritis.
• Reduced milk yield due to mastitis (often subclinical).
• Orchitis and epididymitis in rams and bucks, with palpable scrotal swelling and pain.
• Arthritis and hygromas, particularly in the carpal and tarsal joints, though less common than in B. abortus infection in cattle.
• Lameness secondary to joint involvement.
• Infertility in both sexes.

Systemic signs such as fever, depression, and anorexia are often absent or transient in adult animals. The disease is typically chronic and insidious, with abortion being the most overt indicator of infection.

Pathology

Gross pathological findings are most evident in the reproductive tract of pregnant females. The placenta shows diffuse, necrotizing placentitis with a thickened, edematous, and yellowish intercotyledonary area. The cotyledons are necrotic, hemorrhagic, and covered with a fibrinous exudate. The aborted fetus may be autolyzed and edematous, with serosanguinous fluid in the thoracic and abdominal cavities. Fibrinous pleuritis and peritonitis are common.

In males, the epididymis is enlarged and firm, with fibrous adhesions. The testicular parenchyma may contain necrotic foci or abscesses. Chronic cases show testicular atrophy and fibrosis.

Histologically, the placenta exhibits severe necrosis of trophoblasts with infiltration of neutrophils and macrophages. Intracellular bacteria are visible within trophoblasts and macrophages using modified Ziehl-Neelsen (Stamp stain) or immunohistochemical staining. The fetal lung and liver show interstitial pneumonia and multifocal necrosis, respectively.

Zoonosis: Malta Fever

Brucella melitensis is the most virulent Brucella species for humans and is the primary cause of Malta fever, also known as undulant fever or Mediterranean fever. The zoonotic link is direct: humans acquire infection through consumption of unpasteurized milk and dairy products (especially fresh cheese) from infected goats and sheep, or through direct contact with aborted tissues, placentas, or vaginal discharges. Occupational exposure is a major risk for shepherds, veterinarians, abattoir workers, and laboratory personnel.

In humans, the disease presents as an acute febrile illness with undulating fever, night sweats, malaise, arthralgia, and myalgia. Chronic complications include spondylitis, sacroiliitis, endocarditis, and neurobrucellosis. The incubation period ranges from 1 to 4 weeks. Human-to-human transmission is rare.

The control of Malta fever in humans is inextricably linked to the control of B. melitensis in small ruminant populations. Eradication of the infection in livestock is the most effective public health intervention.

Diagnostic Approach

The diagnosis of Brucella melitensis in small ruminants relies on a combination of serological, bacteriological, and molecular methods. No single test is perfect; a diagnostic algorithm using multiple tests is recommended for herd-level and individual animal diagnosis.

Serological Tests

Serology is the mainstay of surveillance and eradication programs. The target antigen is the smooth LPS O-polysaccharide. The following tests are WOAH-prescribed or recommended:

The RBT is the most widely used screening test due to its simplicity and low cost. A positive RBT result should be confirmed by CFT or cELISA, especially in nonendemic areas or for trade purposes. False positives can arise from cross-reacting antibodies induced by Yersinia enterocolitica O:9, Escherichia coli O:157, and Salmonella group N.

Bacteriological Culture

Isolation of B. melitensis is the definitive diagnostic method. Samples of choice include abomasal contents, lung, and spleen from aborted fetuses; placental cotyledons; vaginal swabs; and milk from infected females. In males, testicular or epididymal tissue is suitable.

Samples are plated onto selective media such as Farrell medium (containing antibiotics including bacitracin, polymyxin B, nalidixic acid, and cycloheximide) or modified Thayer-Martin medium. Plates are incubated at 37 degrees Celsius in 5% to 10% CO2 for up to 10 days. Colonies appear as small, smooth, translucent, and honey-colored. Identification is confirmed by Gram stain (small Gram-negative coccobacilli), oxidase and urease positivity, and agglutination with Brucella-specific antiserum. Biotyping requires phage lysis and oxidative metabolic profiling, which is performed only by reference laboratories.

Culture is highly specific but has limited sensitivity, particularly in chronic infections or when samples are contaminated. Biosafety level 3 (BSL-3) facilities are required for handling live cultures due to the high zoonotic risk.

Molecular Diagnostics

Polymerase chain reaction (PCR) assays have become essential for rapid and sensitive detection of B. melitensis DNA. Real-time PCR targeting the IS711 insertion element (present in multiple copies in the Brucella genome) or the bcsp31 gene (encoding a 31-kDa immunogenic protein) offers high sensitivity and specificity. Species-specific PCR can differentiate B. melitensis from B. abortus and B. suis by targeting the BMEI1162 or per genes.

PCR is particularly useful for:

• Detection of Brucella DNA in aborted fetal tissues and vaginal swabs when culture is negative due to antibiotic treatment or sample degradation.
• Confirmation of serological positives in eradication programs.
• Genotyping and molecular epidemiology using multilocus variable-number tandem-repeat analysis (MLVA) or whole-genome sequencing.

PCR does not distinguish between live and dead organisms, which is a limitation for post-treatment or post-exposure assessment.

Diagnostic Algorithm

The following Mermaid diagram illustrates a recommended diagnostic workflow for a suspect abortion case in a small ruminant flock.

flowchart TD
    A[Abortion in sheep or goat], > B[Collect samples: fetal tissues, placenta, vaginal swab, milk]
    B, > C{Serology on dam}
    C, >|RBT positive| D[Confirm with cELISA or CFT]
    C, >|RBT negative| E[Repeat serology in 30 days]
    D, >|Positive| F[Presumptive positive: initiate quarantine]
    D, >|Negative| G[Consider cross-reaction; retest]
    B, > H[Direct detection]
    H, > I[PCR on fetal abomasum, placenta, vaginal swab]
    I, >|Positive| J[Confirmed infection]
    I, >|Negative| K[Culture on selective media]
    K, >|Positive| J
    K, >|Negative| L[Consider other abortifacient agents]
    J, > M[Report to veterinary authority; implement control measures]

Treatment

Treatment of Brucella melitensis infection in small ruminants is not recommended for food-producing animals. Antibiotic therapy (typically a combination of oxytetracycline and streptomycin or rifampicin) can reduce bacterial shedding and clinical signs but rarely achieves bacteriological cure. Treated animals may become seronegative while remaining latently infected, posing a continued risk of shedding and zoonotic transmission. Furthermore, treatment selects for antimicrobial resistance and is incompatible with eradication programs.

The standard approach in endemic areas is vaccination combined with test-and-slaughter. In brucellosis-free regions, any seropositive animal is culled immediately.

Control and Prevention

Control of Brucella melitensis in small ruminants is based on three pillars: vaccination, biosecurity, and test-and-slaughter.

Vaccination

The live attenuated B. melitensis Rev.1 vaccine is the most effective vaccine for small ruminants. It is administered subcutaneously or conjunctivally at 3 to 5 months of age. The conjunctival route is preferred as it induces strong mucosal immunity with minimal serological interference. Rev.1 is pathogenic for humans (causing Malta fever) and must be handled with caution. It is also abortigenic in pregnant animals and should not be administered to pregnant females.

Vaccination reduces the incidence of abortion and the shedding of B. melitensis in milk and vaginal discharges. It does not provide sterile immunity but significantly decreases the force of infection within a flock. Vaccination is the cornerstone of control in endemic regions where test-and-slaughter is economically unfeasible.

Biosecurity

Biosecurity measures include:

• Quarantine of newly introduced animals for 30 to 60 days with serological testing.
• Separation of pregnant females from the main flock during lambing and kidding.
• Hygienic disposal of aborted fetuses, placentas, and contaminated bedding (incineration or deep burial with lime).
• Pasteurization of milk before feeding to lambs and kids or for human consumption.
• Disinfection of contaminated pens with 2% formaldehyde, 1% sodium hypochlorite, or 2% glutaraldehyde.
• Rodent and feral animal control, as wildlife can act as mechanical vectors.

Test-and-Slaughter

In regions with low prevalence or official brucellosis-free status, eradication is achieved through regular serological testing (RBT or iELISA) and immediate slaughter of reactors. This approach requires strong veterinary infrastructure, financial compensation for farmers, and sustained surveillance. It has been successfully implemented in Northern Europe, Australia, and New Zealand.

Differential Diagnosis

Abortion in small ruminants has multiple infectious causes that must be differentiated from Brucella melitensis. Key differentials include:

• Chlamydia abortus (enzootic abortion of ewes).
• Coxiella burnetii (Q fever).
• Campylobacter fetus subsp. fetus and Campylobacter jejuni.
• Salmonella enterica serovars (e.g., Salmonella abortusovis).
• Listeria monocytogenes.
• Toxoplasma gondii.
• Neospora caninum.
• Border disease virus (pestivirus).
• Bluetongue virus.

A thorough diagnostic workup incorporating serology, PCR, and culture for multiple abortifacient agents is essential for accurate diagnosis and effective flock management.

Conclusion

Brucella melitensis remains one of the most important bacterial pathogens of small ruminants globally, causing significant economic losses through abortion, infertility, and reduced milk production. Its status as the primary agent of Malta fever in humans underscores the critical need for effective veterinary control. Diagnosis requires a combination of serological screening, confirmatory testing, and molecular detection. Control strategies must be tailored to the epidemiological context, ranging from vaccination in endemic areas to test-and-slaughter in eradication zones. The elimination of B. melitensis from small ruminant populations is the most direct and sustainable means of preventing human Malta fever.

References

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