Brucellosis is an infectious, contagious, and worldwide spread form of an important zoonotic disease caused by bacteria of the genus Brucella. Brucellae are facultative intracellular parasites of the reticuloendothelial system .

The disease primarily affects cattle, sheep, goats, swine, and dogs. Among the members of the Brucella group Brucella abortus, B. melitensis, and B. suis species are not host-specific, and may transmit to other animal species; hence, from epidemiological evidence, the three species (B. abortus, B. melitensis, and B. suis) have distinct host preferences and the organisms are capable to cause an infection in a wide range of host species,

including humans. The remaining three members of the species have much greater host specificity. Cross transmission of brucellosis can occur between cattle, swine, sheep and goats and other species including dogs  horses, feral swine, bison, rein deer and camels,

It is a public health problem in developing countries with adverse health implications both for animals and human beings as well as economic implications for individuals and communities. It is an occupational hazard with those particularly at risk such as laboratory workers, veterinarians, abattoir workers, farmers and animal keepers either living in close proximity with animals or handling aborted fetus and animal products that contaminated by Brucella agents. (Radostits et al., 2000, FAO et al., 2006 & Jim et al., 2012).

Millions of individuals are at risk worldwide, especially in countries where infection in animals has not been brought under control, procedures for heat treatment of milk such as pasteurization are not routinely applied, and standards of hygiene in animal husbandry are low. It has a considerable impact on animals and humans health, as well as wide socio-economic impacts especially in countries in which rural income relies largely on livestock

The risk of disease and its severity is determined by the species of Brucella to which an individual is exposed.

Etiology and types of Brucella

The genus Brucella resides within the family Brucellaceae order Rhizobiales, class Alphaproteobacteria and phylum Proteobacteria. The Proteobacteria are a major phylum of bacteria, which include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobactelr. All proteobacteria are Gram-negative, with an outer membrane mainly composed of lipopolysaccharides).

The genus of Brucella are subdivided into six species categorized by antigenic variation and primary preferred host and these include B. abortus, B. melitensis, B. suis, B. ovis, B. canis and B. neotomae,The ability of genus Brucella to replicate and persist in host cells is directly associated with its capacity to cause persistent disease and to circumvent innate and adaptive immunity. There are different species of Brucella organism that cause disease in different animal species and humans. A single species can cause disease in different animal species and humans, which means it has a range  of hosts

Hosts affected by Brucella species

B. canis B. ovis B. suis B. melitensis B. abortus  
­- (+) + + Cattle
+ + + (+) Sheep
+ (+) Goats
+ (+) (+) Swine
+ (+) + + Dogs
+ + Camels
+ + + + Humans


Source: FAO et al. (2006)

Key: +: can be affected, – : can’t be affected, (+): rarely affected


The species of Brucella and their major hosts are B. abortus (cattle), B. Melitensis (goats), B. suis (pigs), B. canis­(dogs),  B. ovis (sheep) and B. neotomae (desert wood rats) as indicated in Table 1 above. Some Brucella species like B. abortus, B. melitensis, B. suis and B. canis can affect a ranges of hosts in addition to their natural hosts resulting hazards on the health of animals including humans; due to this, infected countries are challenged and have been under difficulties to overcome or control brucellosis effectively. In addition to cattle, B. abortus can affect other animals like sheep, goats, horses, camels, swine, dogs and humans. Brucella melitensis also affects other animals like sheep, horses, swine, camels, dogs and humans. Brucella suis also affects different animal species such as cattle, sheep, goats, dogs, camels, horses and humans. Brucella ovis affects only ovine while B.canis affects dogs and human


Source of Infection and Mode of Transmission Both vertical and horizontal transmissions of brucellosis exist in animals. Horizontal transmission occurs through ingestion of contaminated feed, skin penetration, via conjunctiva, inhalation and udder contamination during milking or by licking the discharge of an animal, newborn calf or retained fetal membrane. Fetus can be infected in uterus or suckling of infected dams. Congenital infection that happens during parturition is frequently cleared and only few animals remained infected as adult (Radostits et al., 2000). Venereal infections can also occur and mainly seen with B. suis infections. The importance of venereal transmission varies with the species; it is the primary route of transmission for B. ovis. Brucella suis and B. canis are also spread frequently by this route. Brucella abortus and B. melitensis can be found in semen, but venereal transmission of these organisms is uncommon. Some Brucella species have also been detected in other secretions and excretions including urine,feces, hygroma fluids, saliva, and nasal and ocular secretions. In most cases, these sources seem to be relatively

unimportant in transmission; however, some could help account for direct non-venereal transmission of B. ovis between rams Of the transmission ways of brucellosis to human, ingestion of unpasteurized dairy foods produced from unlicensed family owned flocks whose products are sold door-to-door at low prices is one of the known ways.

Dairy products are the main source of infection for people who do not have direct contact with animals.Transmission of infection to humans occurs through breaks in the skin, following direct contact with tissues blood, urine, vaginal discharges, aborted fetuses or placentas. Occupational aerosol infection in laboratories and abattoirs has also been documented. Accidental inoculation of live vaccines (such as B. abortus Strain 19 and

B,melitensis Rev.1) can also occur, resulting in human infections. There are also case reports of venereal and congenital infection; and it can be transmitted through transplacental transfer and breast feeding even though rarely


Clinically, the disease is characterized by one or more of the following signs in animal species and these are abortion, retained placenta, orchitis, epididymitis and, rarely, arthritis, with excretion of the organisms in uterine discharges and in milk .Infertility is a common sequel of animal brucellosis and this is one of the factors that bring negative impacts on the development of economy of the infected countries. In  horses, B. abortus and occasionally B. suis can cause inflammation of the supraspinous or supraatlantal bursa; these syndromes are known, respectively, as fistulous withers or poll evil. The bursal sac becomes distended by clear, viscous, straw colored exudates and develops a thickened wall. Fistulous withers are most common clinical sign of equine brucellosis and some horses appear to suffer a generalized infection with clinical signs including, general stiffness, lameness, fluctuating temperature and lethargy


Brucellosis is a major veterinary and human health importance in economy of affected countries. Among the genus Brucella, B. melitensis, B. abortus, B. suis, and B. ovis which preferentially infect sheep and goats, cattle,

pigs and sheep, respectively are the most important from a socioeconomic standpoint. In addition to decreasing productivity in animals, the first three species are the main ones responsible for brucellosis in human beings Costs include production loss associated with infection in animals, preventive program, and in human disease cost of treatment and absenteeism from work brings many economical impacts. Losses in animal production due to brucellosis disease can be of major important, primarily because of the decreased milk production by aborting dairy animals; the common sequel of infertility increases the period between lactation, and in an infected herd the average inter calving period may be prolonged by several months. This is of greatest importance in beef herds where the calves represent the sole source of income. A high incidence of temporary and permanent infertility results in heavy culling of valuable and some deaths occur as the result of acute metritis following retention of the placenta. The effect of the disease on ram’s fertility can influence the number of rams that are required in a flock; the required ram to ewe ratio is

significantly reduced in B. ovis-free flocks. The percentage of lambs born early and within the first three weeks of the lambing period is also markedly increased


Expansion of animal industries, the lack of hygienic measures in animal husbandry and poor food handling partly account for brucellosis to remain a public health hazard. International travel and the importation of different dairy products into Brucella free regions contribute to the ever-increasing concern over human brucellosis. Brucellosis is a zoonotic disease occurring in humans and various species of domesticated and feral animals. Human brucellosis can be a very debilitating disease, although the case fatality rate is generally low; it often becomes sub-clinical or chronic, especially if not recognized early and treated promptly. All ages of human beings are susceptible, and even congenital cases have been recorded . High risk groups include those exposed through occupation in contexts where animal infection occurs, such as slaughterhouse  workers, hunters, farmers and veterinarians             The three species of Brucella of major concern here are B. abortus

affecting primarily cattle and other bovidae, B. suis , affecting primarily swine;

and B. melitensis, affecting primarily sheep and goats. The persistent infection of the mammary glands and supramammary lymph nodes leads to a constant or intermittent shedding of the organisms in the milk in succeeding lactations. It provides an important source of infection for man and young animals. Of the three species, B. melitensis is highly pathogenic for human beings (FAO, 2003 & SCAHAW, 2001). In addition to the above three Brucella species, B. canis also has zoonotic importance and its infections in humans resemble brucellosis caused by other Brucella species


The purpose is to search for brucella infection, to reveal prevalence and distributio

and (in countries where eradication has been achieved) to monitor freedom

from reinfection. Techniques employed are serological and allergic tests, and isolation

of the agent by bacteriological methods.

  • Serological diagnosis

This utilises basic tests suitable for large-scale testing, coupled with additional

tests for clarifying doubtful results. The samples tested are blood, taken from live

or slaughtered animals (e.g. Canada), and milk. The tests detect antibodies, which

are evidence of infection, and make their appearance after a variable incubation period, after which they tend to persist. The relative proportion of antibodies in the

immunoglobulin classes IgGl, IgG2 and IgM (32) differs according to the stage of

evolution of the disease. In addition, vaccination (which is often resorted to) is also

responsible for inducing antibodies of the same classes. An ideal serological test

would establish an early diagnosis, identify chronic infections, and distinguish between the antibodies of vaccination and those of infection. A test also needs to be

economical, simple and rapid, and possible to repeat on numerous occasions. No

serological test possesses all these qualities.

The slow tube agglutination test on serum (SAT), the ancestor of all serological tests, is still widely used, for it is the basic test in twelve countries, and supplementary test in seventeen others. Evidence of its efficacy comes from the countries

from which the disease has been eradicated. It is often used in conjunction with the

complement fixation and ring tests. SAT detects antibodies of the classes IgG2 and

Igm. Shortcomings are its inability to detect recently infected animals and an

inability to detect chronic infection, which may be accompanied by low titres which

are difficult to assess). The last-named fault is particularly serious, bearing

in mind the habitual chronic nature of the disease.

Complement fixation (CF) is the basic test in two countries (Malaysia and New

Zealand — automated in the latter country), and a complementary test in practically

all the others. It detects IgGl and IgM antibody classes, and is considered to

be the most sensitive and the most accurate, enabling a distinction to be made between antibodies of vaccination and those of infection Disadvantages of CF

are its delicate and long procedure, requiring the services of a trained technician,

which unfortunately often rules out its use as a basic test.

The Rose Bengal test (RBT) is a rapid plate agglutination test on pure serum

using a stained antigen at pH 3.6. This is the basic test for twenty-three of the thirty

countries where it is used. There is therefore a large measure of agreement on the

use of this test, which is justified to the extent that the RBT is economical, simple

and rapid, and gives few false negative or false positive results, requiring verification

by CF (with or without SAT) The immunoglobulins responsible for the

reaction are IgG1 as for CF, and sometimes IgM, depending on the type of

antigen preparation (D. Levieux, personal communication). The test can detect

infection at an earlier stage than SAT and at the same time (or perhaps sooner than) CF

The ring test (RT) is also widely used. It detects immunoglobulins present in

milk, whether originating from the blood by filtration (IgM) or locally within the

mammary gland (IgA), an organ which is among those most frequently infected. It

is efficient, easy to perform and economical The RT can be performed frequently(monthly) both to detect infected dairy herds (being the basic diagnostic

test, particularly in Switzerland), and to provide continuous surveillance of

brucellosis-free herds. Its success seems to be due more to the frequency of testing

than to its sensitivity, which diminishes with increasing herd size. It can be used as an early alerting test, because it is not possible to repeat serological testing so frequently. Addition of formaldehyde (to a final concentration of 0.2)preserves milk samples for up to 14 days, and seems to augment the sensitivity of the test rather than reduce it (Great Britain). Finally, the RT can be used on individual cows.

The detection of serologically positive herds is easy at the commencement of a

control scheme. With the progress of eradication, or when eradication is almost

complete, new problems arise which have to be solved in a more elaborate way,

often at the level of individual cases. In addition to tests which have become established, such as the Rivanol, mercaptoethanol and Coombs’ antiglobulin

tests, new tests are under investigation .Among those still being tried

out is ELISA, performed on blood serum or milk whey (Great Britain), which is

very promising. Other difficulties involving SAT and CF are due to cross-reactions between Brucella and other bacteria, such as Yersinia enterocolitica serogroup 09, various salmonellas and Escherichia coli, etc. Here the RBT seems to be more specific because of its pH of 3.6Addition of EDTA seems to overcome this snag in the case of SAT

  • Allergic diagnosis

Brucella infection creates a state of sensitisation which can be detected by

hypersensitivity reactions of the delayed type, provoked by the injection of allergen extracted from Brucella. Among a large number of allergens, those prepared by the method of Bhongbhibhat . have the advantage of not provoking the formation of antibodies detectable by serological tests, and of not inducing sensitisation. Therefore this test can be repeated without upsetting subsequent serological or allergic tests. This test has been proposed for routine diagnosis and as a supplementary

test in problem herds, and it is used in four countries (New Zealand,

Czechoslovakia, FRG and the USSR). New Zealand claims a specificity of 100% and a sensitivity of 60-68%, and the intention is to use this type of test for the simultaneous diagnosis of tuberculosis and brucellosis

  • Bacteriological diagnosis

Most countries employ bacteriological techniques to detect Brucella, mainly to

investigate abortions, the notification of which is often compulsory. Vaginal discharge,colostrum, the aborted fetus and the placenta from an infected cow contain

large numbers of Brucella. Excretion of the bacteria in milk is tested either on individual cows (those which are serologically negative but RT positive), or on a large

scale in combination with other tests (USA). Lymph nodes and other organs taken

at slaughter may also be tested for Brucella (Canada) or fluids from hygromas ,

which occur frequently in infected African animals (Burkina Faso). This testing has

the advantage of providing direct evidence of the disease, if the bacteria are isolated.

It requires good laboratories and well-trained technicians. Most laboratories

use selective media. However, it must be borne in mind that biovar 2 of B. abortus

requires the addition of serum to the medium. Many reports mention the use of the

medium of Kuzdas and Morse, and the WE medium of Renoux, but these will not

detect this biovar. It would be better to use the medium of Farrell , which contains

serum. The bacteriological identification of Brucella, in addition to the abovementioned methods, is indispensable to obtain an accurate evaluation of the epidemiological status of herds or of communities of people. Identification of species

and biovars of Brucella has made considerable progress, thanks to the use of a set

of bacteriophages, and is done in many countries. Correct identification can be made by non-specialised laboratories. Information obtained from the typing or recognition of rare markers, makes it possible to identify and trace sources of infection

Treatment and Prevention of Brucellosis

Antibiotic treatment of known infected animals, or of those which are potentially exposed to Brucellae agents, has not been commonly used and it should be ruled out as an option in the control of brucellosis. A limited number of studies have shown rapid reductions in the incidence of brucellosis when the herd of flock was treated

but this procedure is considered to be restricted in practice. Treatment has been used in animals of special breeding value, but because of the uncertain outcome it is not generally recommended (Radostits et al., 2000 &FAO, 2003).

It is nearly always more economical and practical to prevent diseases than to attempt to control or eliminate them. For brucellosis, the measures of prevention include Careful selection of replacement animals. Replacement animals, whether purchased or produced from existing stock, should originate from Brucella-free herds or flocks.

Pre-purchase tests are necessary unless the replacements are from populations in geographically circumscribed areas that are known to be free of the disease. In addition, a serological test prior to commingling different animal species is necessary; preventing contacts and commingling with herds of flocks of unknown status or

those with brucellosis. If possible, laboratory assistance should be utilized to diagnose causation of abortions, premature births, or other clinical signs.

Suspect animals should be isolated until a diagnosis can be made. Herds

and flocks should be included in surveillance measures such as periodic milk ring tests in cattle (at least four times per year), and testing of slaughtered animals with simple screening serological procedures such as the jRBT. Proper disposal (burial or burning) of placentas, non-viable fetuses and disinfection of contaminated areas

should be performed thoroughly (Radostits et al., 2000 & FAO et al., 2006).

Control of Brucellosis

The aim of animal brucellosis control programme is to reduce the impact of a disease on human health and the economic consequences. The elimination of the disease from the population is not the objective of a control programme, and it is implicit that some “acceptable level” of infection will remain in the population. Control programmes have an indefinite duration and will need to be maintained even after the “acceptable level” of infection has been reached, so that the disease does not re-emerge.

  • Methods of control
  1. a) Strict application of disease control measures, including eradication by

slaughtering infected animals. This is the most radical method and there is no doubt that it is the most economical when prevalence is low and conditions favourable. It should be carried out rigorously and rapidly in order to diminish the risk of reinfection as much as possible. The reports submitted provide a partial answer to the question often asked, about the threshold of prevalence below

which this sort of control is technically and economically feasible. Three countries achieved eradication after starting with 10.5-11% of herds infected (Finland), 20% (Czechoslovakia) and 5% infected animals (Switzerland). Unfortunately this information does not include data on the size of herds, their dispersion, or geographical obstacles to the spread of the disease, all of which have considerable influence on the progress of eradication.

  1. b) Medical control by general vaccination leads to a decrease in prevalence of

this disease and its maintenance at a very low level. In fact, none of the reporting countries use this method alone, without at least some compulsory or recommended slaughtering, even though it is the principal method adopted by many countries of Central and South America.

  1. c) Mixed control is a simultaneous or successive combination of the two above methods, utilising the slaughter of reactors and the protection of healthy herds by vaccination. Compulsory or voluntary vaccination is chiefly aimed at heifers, by inoculating strain B19 vaccine at 3-8 months of age, except in Saudi Arabia where strain H38 vaccine is used.

vaccination of Brucellosis

Vaccination of animals usually results in elimination of clinical disease and the reduction in numbers of organisms excreted by animals which become infected. In many countries, vaccination is the only practical and economical means of control of animal brucellosis. The most successful method for prevention and control of brucellosis in animals is through vaccination. While the ideal vaccine does not exist, the attenuated strains of B.melitensis strain Rev.1 for sheep and goats and B. abortus strain 19 have proven to be superior to all others. It is often recommended that vaccination with strains 19 and Rev.1 should be limited to sexually immature female animals. This is to minimize stimulation of post vaccinal antibodies which may confuse the interpretation of diagnostic tests and also to prevent possible abortions induced by the vaccines. Positive serological reactors and secretors must be removed from the herd on detection (Radostits et al., 2000 & FAO et al.2006).

Three vaccines are available: inactivated 45/20  and H38  vaccines and the live strain Brucella abortus B19 . Inactivated vaccines, in principle, have the advantage of keeping better than live vaccines. The 45/20 vaccine used in addition to the B19

vaccine in Ethiopia, France, Ireland, Malawi and Portugal is nonagglutinogenic, but raises problems of control and efficacy, which varies with the method of preparation. H38 vaccine used in Saudi Arabia and Ethiopia is very effective but agglutinogenic, which is a major drawback posing insoluble detection problems among vaccinated animals.

Consequently it is not surprising that strain B19 is employed in every country

which practises vaccination, with the exception of Saudi Arabia. This large consensus is justified by the extensive research conducted on this vaccine since the 1930’s, which has given rise to large numbers of publications concerning its efficacy  method of use  and testing The resistance induced by strain B19 is not complete, but it remains constant throughout the economic life of a cow although some doubt has been cast on this opinion.

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