Brucella spp. infections

Brucella spp. infections

Bacteria of the genus Brucella are Gram-negative cocci, coccobacilli or short rods. They are transmissible to a wide range of animal species. Early bacteraemia is followed by localization of the infection in the genital organs and cells of the monocyte-macrophage series. They are the causative agents of brucellosis, an important disease of humans and domestic and wild animals worldwide. In humans the disease caused by Brucella melitensisis also known as Malta fever, Mediterranean fever or Bang’s disease, or as undulant fever where Brucella abortus is implicated. In domestic animals, cattle, sheep, goats and pigs are mainly affected (Table 1). A number of wildlife species, such as African buffalo, antelope, bison, camels, elk, reindeer, caribou and other wild ruminants, are also susceptible to infection. In animals the most important clinical manifestations are reproductive failure, i.e. abortion and the birth of weak offspring, or orchitis and epididymitis. Signs of arthritis may be evident in chronic infections. The primary sources of contamination of the environment are foetal membranes and fluids, and vaginal discharges which are expelled by infected females when they abort or at parturition. Brucellae are also commonly shed in milk and semen. The rare occurrence of brucellosis in domestic animals in many countries these days is directly linked to the success of brucellosis control/eradication programmes.

Table 1 Brucella spp. in livestock

B. abortus Brucellosis (contagious abortion) Cattle
B. ovis Epididymitis/orchitis Sheep
B. melitensis Abortion and orchitis Sheep and goats
B. suis Abortion, stillbirth, sterility in sows, and orchitis Pigs


In cattle, biovars of B. abortus are usually the cause of brucellosis but in some countries, particularly in southern Europe and in the Middle East, B. melitensis has also been implicated as a cause of abortion in cattle where they are kept in close association with infected sheep or goats. Occasionally, Brucella suis may infect the mammary gland of cattle but it has not been reported to cause abortion in this species.

Caprine and ovine brucellosis (excluding Brucella ovis infections) are most commonly caused by B. melitensis but sporadic cases are caused by B. abortus. In sheep and goats the disease caused by B. melitensis and its epidemiology are very similar to those of B. abortus infection in cattle.

Brucellosis in pigs is caused by B. suis and is characterized by an initial bacteraemia followed by the production of chronic lesions in the bones and reproductive organs of both sexes. Brucella suis consists of five biovars. Pigs are infected by B. suis biovars 1, 2 or 3: disease caused by biovars 1 and 3 is similar, while that caused by biovar 2 differs in its pathology, host range, and geographical distribution which is limited. Brucella suis biovar 2 is rarely pathogenic to humans, whereas biovars 1 and 3 are highly pathogenic and cause severe disease.

Brucella ovis is one of the most common causes of epididymitis in rams but is a rare cause of abortion in ewes and neonatal mortality in lambs. Low reproductive rates may occur in affected flocks. It is not a zoonosis.

Brucella canis causes epididymitis and orchitis in male dogs and metritis in bitches, and is a rare infection in humans. It does not infect other animal species.

Brucella neomatae has been isolated from a rodent species in western USA.

Brucellae are Gram-negative coccobacilli or short rods with straight or slightly convex sides and rounded ends. They do not ferment carbohydrates in conventional media. The genus Brucella contains six terrestrial: B. abortus, B. melitensis, B. suis, B. ovis, B. canis and B. neotomae. Seven biovars are recognized for B. abortus, three for B. melitensis and five for B. suis. However, the degree of genetic relatedness, as shown by DNA hybridization studies, is consistent with the existence of a single species within the genus Brucella.

Brucella abortus, B. melitensis, B. suis and B. neotomae generally occur in the smooth form, while B. ovis and B. canis are invariably rough species. Differentiation of the biovars of the species of the genus Brucella is shown in Table 2.

Three Brucella species have been described from a wide variety of cetacean and pinnipeds (marine mammals). They are Brucella pinnipedialis, B. delphini and B. ceti.

Most of the serological tests for the diagnosis of smooth Brucella spp. infections (i.e. B. melitensis, B. abortus and B. suis) have been developed to detect antibodies directed against antigens (mainly A and/or M epitopes) associated with the smooth lipopolysaccharide (S-LPS) and are shared by all the naturally occurring biovars of B. abortus, B. melitensis and B. suis. Diagnostic antigens must be prepared from smooth strains of B. abortus (strain 1119-3 or strain 99) and comply with minimum standards for purity, sensitivity and specificity. Such antigens are suitable for the detection of antibodies directed against B. melitensis, B. abortus and B. suis, in different animal species infected with smooth brucellae. Because the S-LPS is absent in the naturally rough B. ovis and B. canis, specific B. ovis and B. canis antigens (mainly R epitopes) associated with the rough lipopolysaccharide (R-LPS) have to be used for the diagnosis of infections caused by these organisms.

Table 2 Biovar differentiation of the species of the genus Brucella

        Thionin* Fuchsin* Safranin O** A M R Tb°
B. abortus 1 (+) + + + + -   L
  2 (+) + +   L
  3 (+) + + + + +   L
  4 (+) + +*** + +   L
  5 + + + +   L
  6 (–) + + + +   L
  9 + + + + +   L
B. suis 1 + + –**** +   NL
  2 + +   NL
  3 + –**** +   NL
  4 + (–) + +   NL
  5 + +   NL
B. melitensis 1 + + + +   NL
  2 + + + +   NL
  3 + + + + +   NL
B. ovis   + + (+)   NL
B. canis   +   NL
B. neotomae   + +     NL/PL

* Concentration = 1/50000 p/v

** Concentration = 1/10000 p/v

*** Certain strains are fuchsin sensitive

**** Certain strains are fuchsin and safranin O resistant

Tb° = Tbilisi phage; L = confluent lysis; PL = partial lysis; NL = no lysis

A = monospecific antiserum; M = monospecific antiserum; R = monospecific antiserum

(+) = most of the strains are positive; (–) = most of the strains are negative

CO2 = CO2 requirement

H2S=H2S production

The brucellin allergic skin test can be used as a screening or complementary test in unvaccinated animals, provided that a purified lipolysaccharide (LPS)-free, standardized antigen preparation (consisting mainly of brucellae cytoplasmic proteins) is used.


It is recommended by the Office International des Epizooties (OIE) that sheep and goats should be immunized with B. melitensis strain Rev.1 live vaccine against both B. melitensis and B. ovis infections, and that B. abortus strain 19 live vaccine and B. abortus strain RB51 live vaccine (produced from a laboratory-derived rough mutant of smooth B. abortus strain 2308) should be used in cattle. No vaccine is recommended by the OIE for use in pigs. Vaccines for the prevention of brucellosis have to fulfil minimal requirements and their production should be based on a seed-lot system: seed cultures to be used for vaccines should originate from reference centres and they must conform to minimum standards for viability, smoothness, residual infectivity and immunogenicity.

The World Health Organization (WHO) laboratory biosafety manual classifies Brucella in Risk group III. Brucellosis in humans is an acute influenza-like clinical disease characterized by a febrile illness — undulant fever — that may progress to a chronic form with serious complications in which the musculo-skeletal, cardiovascular and central nervous systems are affected. Infection is acquired mainly via the oral, respiratory or conjunctival routes. Ingestion of infected dairy products is the main public health risk but veterinarians, farmers, and abattoir and laboratory workers are also at risk of becoming infected by handling infected animals, carcasses and aborted material.

Brucellosis is one of the most easily acquired laboratory infections and strict safety precautions should always be observed when handling cultures and infected or potentially infected samples, such as aborted material. The examination of infected material should be carried out in class III safety cabinets by trained staff. Protective clothing should be worn and decontamination of laboratory equipment and sterilization of cultures, and hazardous material should be implemented. Specific safety recommendations have been made and must be followed when dealing with Brucella infected material. The handling of serum samples represents a minimal hazard if good laboratory practices are implemented.


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