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Rev. sci. tech. Off. int. Epiz., 2013, 32 (1), 53-60
Epidemiology of brucellosis in domestic animals
caused by Brucella melitensis, Brucella suis and
Brucella abortus
E. Díaz Aparicio
Centro Nacional de Investigación Disciplinaria en Microbiología Animal (CENID Microbiología), Instituto
Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Secretaría de Agricultura, Ganadería,
Desarrollo Rural, Pesca y Alimentación (SAGARPA), Km 15.5 Carretera Federal México–Toluca, C.P. 05110,
Mexico, DF, Mexico
E-mail: [email protected]
Brucellosis is a disease that causes severe economic losses for livestock farms
worldwide. Brucella melitensis, B. abortus and B. suis, which are transmitted
between animals both vertically and horizontally, cause abortion and infertility in
their primary natural hosts – goats and sheep (B. melitensis), cows (B. abortus)
and sows (B. suis). Brucella spp. infect not only their preferred hosts but also
other domestic and wild animal species, which in turn can act as reservoirs of the
disease for other animal species and humans. Brucellosis is therefore considered
to be a major zoonosis transmitted by direct contact with animals and/or their
secretions, or by consuming milk and dairy products.
Brucella abortus – Brucella melitensis – Brucella suis – Brucellosis – Epidemiology.
The main characteristic of the Brucella genus is its ability to
survive within phagocytic and non-phagocytic cells. While
a wide variety of factors explain the capacity of the Brucella
genus to multiply and spread to new cells, so far no single
factor has been shown to be responsible for its virulence (19).
Brucellae usually enter the body via the oral route and
lodge in the mucosa, where the bacteria are ingested by
professional phagocytes beneath the sub-mucosa. Once
internalised, Brucella is localised in a vacuole that matures
from an early to a late endosome and, unless destroyed,
goes on to multiply in the endoplasmic reticulum of
macrophages. However, not all brucellae survive: where
bacteria are not sufficiently numerous and the animal has
a competent immune system, they are directed towards
the lysosomes where they are destroyed and the major
histocompatibility complex on the cell surface presents the
peptides to Th1 and Th2 lymphocytes to elicit an immune
response (6, 43).
These bacteria multiply abundantly in the placental
cotyledons, chorion and fetal fluids, where they cause
lesions in the organ wall, inducing endometriosis ulcerosa
in the intercotyledonary spaces and destruction of the
villi, leading to death and expulsion of the fetus (75).
Three species of Brucella affect humans: B. melitensis,
B. abortus and B. suis (other species can cause infection in
humans, but only rarely). Of these three species, infections
by B. melitensis are the most common in humans and are
also the most serious (55).
of Brucella abortus
Bovine brucellosis is usually caused by B. abortus. Nonbovine animals, including humans, can also contract the
disease and play a role in its persistence and transmission.
Brucella abortus has seven recognised biovars, the most
reported of which are biovars 1, 2, 3, 4 and 9, with
biovar 1 being the most prevalent in Latin America. The
distribution of biovars could be important in ascertaining
the source of some infections (22, 31, 33, 46, 76). Bovine
brucellosis is reported in virtually all countries where cattle
are farmed, with some northern and central European
countries, Australia, Canada, Japan and New Zealand
considered free. In 2008, 12 European Union countries
were considered officially free from bovine brucellosis, as
well as caprine and ovine brucellosis. In 2008, 15 countries
that were not free reported cases of bovine brucellosis (herd
prevalence of 0.12%). The situation is less favourable in
southern Europe but, even there, prevalence is less than
1% (28). Although all states of the United States (USA) are
ranked free from B. abortus in cattle, the infection remains in
wildlife in and around the Yellowstone area, with occasional
spread to cattle. Brucellosis-infected cattle herds have been
detected in the states of Montana, Wyoming and Idaho
since 2007. The primary source of infection for cattle is
believed to be elk (Cervus elaphus) (67, 73).
Ruminants are generally susceptible to B. abortus, which is of
particular relevance in areas where eradication programmes
are in operation. Buffaloes, camels, deer, goats and sheep
are highly susceptible to infection (20, 23). Brucellosis
has been reported in the one-humped camel (Camelus
dromedarius) and two-humped camel (C. bactrianus), as well
as in a number of South American camelids – llama (Lama
glama), alpaca (Lama pacos), guanaco (Lama guinicoe) and
vicuña (Vicugne vicugne) – following contact with ruminants
infected with B. abortus or B. melitensis. It has also been
observed in the water buffalo (Bubalus bubalis), American
bison (Bison bison), European bison (Bison bonasus), yak (Bos
grunniens) and elk (Cervus elaphus), as well as in the African
buffalo (Syncerus caffer) and several species of African
antelope. The manifestations of brucellosis in these animals
are similar to those of bovine brucellosis and can become
epidemiologically important in sustaining the infection in
cattle where they share pasture and water holes (20, 23,
30, 35, 71). The infection is prevalent in horses cohabiting
with cattle, it presents with characteristic swelling of the
supraspinous bursa, known as fistulous withers. The
infection is usually transmitted to pigs by feeding them
whey as a by-product from cheese-making (52, 54).
Brucella abortus infection of dogs has been demonstrated
under experimental and field conditions. In addition, dogs
play an important role in the distribution of the disease
by feeding on aborted fetuses, dragging them along and
spreading the bacteria (8, 15).
A herd becomes infected with Brucella when animals that
are infected but not yet diagnosed are introduced into
it. Livestock fairs and shows are a risk, since animals
can become infected and infect other animals when they
return to their herd of origin. It is therefore advisable not
to purchase animals that are infected or do not come from
brucellosis-free herds (61, 62).
Cows aborting in stables and farmyards are the main factor
for the spread of brucellosis. It has been determined that
cows infected with Brucella are three to four times more
likely to abort than unexposed cows (51, 63).
The main route of entry for Brucella is oral, by the ingestion
of food or water contaminated with secretions or aborted
fetal remains from infected cows, or by licking the
Rev. sci. tech. Off. int. Epiz., 32 (1)
vaginal secretions, genitals, aborted fetuses or newborn
calves of infected cows. While the venereal route is not
generally considered to be epidemiologically important
in transmitting brucellosis in cattle, infected semen used
in artificial insemination could be important. Infected
cows shed Brucella in their milk and this is key in its
transmission to calves. In dairies, milking is another mode
of transmission that must be taken into account because the
bacteria are highly likely to be transmitted from cow to cow
if the same teat cups are used for milking. For this reason,
it is recommended that healthy cows be milked first and
infected cows last (61, 62, 71).
The disease is usually asymptomatic in non-pregnant
females, but pregnant adult females infected with B. abortus
develop placentitis, which normally causes abortion
between the fifth and ninth month of pregnancy. Even in
the absence of abortion, there is heavy shedding of bacteria
through the placenta, fetal fluids and vaginal exudates. The
mammary gland and regional lymph nodes can also be
infected and bacteria can be excreted in milk (56, 71).
Vertical transmission was proved by Plommet, who states
that between 60% and 70% of the fetuses born to infected
mothers carry the infection. Female calves can also be
infected during birth when passing through the birth canal,
or by suckling colostrum or milk from infected cows.
While most of these calves rid themselves of Brucella,
a small percentage may continue to be infected until
adulthood, remaining negative to diagnostic serological
tests but aborting during their first pregnancy. Such animals
pose a serious threat to brucellosis control and eradication
(57, 66, 70).
After contracting the infection, cows commonly abort
during their next pregnancy, but 80% of infected cows
abort only this once. In most cases, subsequent pregnancies
will come to term with no apparent signs, or with the birth
of weak calves, but infected cows suffer repeated uterine
and mammary infections, with microorganisms evident in
birth products and milk. This means that such cows will
continue shedding large numbers of brucellae at delivery
that will go on to become sources of infection for other
animals. Adult males can develop orchitis, and brucellosis
can cause sterility in both sexes. Neutered males used for
fattening are not important in the distribution of the disease
(17, 61, 71).
There have been reports of cows infected with B. abortus
that test seronegative. Brucella abortus was isolated in
119 milk samples from seronegative cows and the isolates
were biovars 2, 3 and 9 (76). In Mexico, the field strain
of B. abortus with a smooth phenotype is reported to have
been isolated from the vaginal exudate of two primiparous
cows, vaccinated with reduced-dose RB51, that had calved
normally and were seronegative (7).
Rev. sci. tech. Off. int. Epiz., 32 (1)
In stables with a high prevalence of brucellosis, a major
risk factor for spread is when infected cows contaminate
the herd’s pasture, where Brucella can remain viable for
long periods assisted by adequate moisture levels. The
microorganism becomes even more resistant in the presence
of organic material (1).
As an example, animals infected during critical periods
of the pregnancy (the first third to half of pregnancy) will
abort approximately 30 to 45 days after infection. However,
animals infected at full term will not abort, and animals
infected out of the pregnancy period will not abort during
their next pregnancies (26).
Even though cattle have been shown to exhibit considerable
variation in their immune response to disease challenges,
and much of this difference is attributable to genetics,
cattle breeds are not genetically susceptible or resistant to
brucellosis. The higher incidence of brucellosis in dairy
cows is the result of the close cohabitation that results from
this form of farming (50, 69).
In a primary infection with B. suis in pig farms, the bacteria
can spread within a few months from one infected pig to
more than 50% of animals on the farm. The infection can
often reach 70% to 80% of infected animals at the start
of the outbreak (11, 56). As there tend to be few or mild
clinical signs, the disease can go unnoticed in infected
herds. However, recently infected herds may manifest major
signs of infection, such as a high percentage of abortions,
increased neonatal mortality and infertility, causing adverse
economic consequences. Although the minimum infective
dose of the various B. suis biovars has yet to be established,
it has been demonstrated that doses as low as 104 to 105
colony-forming units are sufficient to infect most pigs
challenged by the conjunctival route, but the severity of the
infection is not correlated with the dose nor with the route
of challenge (18).
Epidemiology of Brucella suis
While domestic pigs are infected mainly by B. suis, less
frequently they may become infected with B. abortus or
B. melitensis in regions where brucellosis is endemic in
cattle or small ruminants. There are five biovars of B. suis,
with 1, 2 and 3 being responsible for porcine brucellosis
worldwide (27).
Brucella suis biovars 1 and 3 are distributed worldwide in
most areas where there are pigs. They affect both sexes,
causing infertility, abortion, orchitis and bone and joint
lesions. Biovar 1 is present in the Americas and Asia, while
biovar 3 has been reported in China, the USA, and Europe
(72). Prevalence is generally low, except in parts of South
America and South-East Asia. Within the European Union,
the epidemiological status of porcine brucellosis varies.
Some countries are free from the disease, while others
report sporadic outbreaks and some report infections as
an emerging problem. Available epidemiological evidence
shows that, in Europe, B. suis biovar 2 is the most common
source of infection in pigs but biovars 1 and 3 are also
present (27, 34).
Porcine brucellosis is believed to affect both sexes equally
and age is no determinant of susceptibility, although this
is not proven (4). It has also been reported that some pig
breeds, such as Duroc and Jersey Red crosses, may be less
susceptible to experimental challenge with B. suis, which
suggests the existence of genetic resistance to infection
(16). The main risks associated with the entry of porcine
brucellosis into pig farms are: the introduction of infected
animals, contact with wildlife reservoirs, and artificial
insemination with semen from infected boars. Infected
boars can shed 104 to 105 colony-forming units of B. suis per
millilitre of semen (45), with semen being one of the routes
of spread in artificial insemination. Many combinations of
antibiotics are used to preserve semen in the mistaken belief
that this inactivates the pathogens; in fact, no combination of
commercial antibiotics is capable of completely inactivating
B. suis in semen (27).
Brucella suis is moderately influenced by environmental
factors, with the microorganism’s survival time in the
environment decreasing as temperatures rise. However, the
bacteria often survive dessication and can survive freezing
temperatures for over two years (58). Facilities and pasture
can remain contaminated for long periods but direct
sunlight reduces the bacteria’s survival significantly (38).
While there is no hard evidence to prove it, as with other
animal species, infected sows can also transmit infection to
their piglets, either transplacentally (resulting in the birth
of seronegative latent carriers) or when piglets ingest B. suis
in their mother’s milk, as the bacterium is shed in the milk
of infected sows (4).
The B. suis entry sites are similar to those identified for
other types of Brucella infection, being essentially the oral,
nasopharyngeal, conjunctival and vaginal mucosa. There is
generally a relatively long incubation period before clinical
signs appear. These are not usually visible in young animals,
and their occurrence will depend mainly on the age, sex and
physiological state of animals at the time they are infected.
Other domestic species apart from pigs may be susceptible
to B. suis infection. Horses kept in close contact with infected
pigs can become infected, as evidenced by fistulous withers
(23). The infection has also been reported in dogs, causing
lameness and granulomatous lesions in genital organs (59).
While all states in the USA, with the exception of Texas, are
classified as free from porcine brucellosis (20), B. suis was
isolated from the testicles of two dogs on small farms in the
state of Georgia (54). Similarly, B. suis biovars 1 and 3 have
been isolated as the cause of brucellosis cases in cattle (29).
Brucella suis biovar 4 causes fever, depression, abortion,
retained placentas, mastitis, bursitis and orchitis in reindeer
and caribou (Rangifer tarandus and its various subspecies)
throughout the Arctic region, including Alaska, Canada
and Siberia (72). Arctic foxes and wolves may contract
B. suis biovar 4 from reindeer and dogs, while rodents,
such as rats and mice, may acquire other B. suis biovars by
cohabitation with infected hosts. In Australia, Kenya and
the former Union of Soviet Socialist Republics there have
been reports of small rodents becoming infected with B. suis
biovar 5 (72).
Brucellosis infection caused by B. suis biovar 2 differs from
that caused by biovars 1 and 3 in terms of geographical
distribution, host and virulence, and is considered less
pathogenic for humans than the highly infectious biovars
1 and 3; humans must be immunocompromised to become
infected with biovar 2 (33, 44, 47).
Although the epidemiological cycle of B. suis biovar 2
has not been described fully, there is evidence of a close
relationship between infection in domestic animals and
infection in wildlife. Biovar 2 has been isolated in wild boar
(Sus scrofa) in Belgium, Croatia, France, Germany, Spain and
Switzerland (23, 36, 42, 52, 74), as well as in the European
hare (Lepus europaeus) (65). Brucella suis biovar 2 can be
transmitted to cattle (29, 32), dogs (10) and horses (23).
of Brucella melitensis
Brucella melitensis is the most virulent species of the Brucella
genus and has three biovars, with biovars 1 and 3 being
the ones isolated most frequently in small ruminants in
the Mediterranean, the Middle East and Latin America (14,
46). Brucellosis is a barrier to trade in animals and animal
products and causes significant losses from abortion, as well
as being a serious zoonosis (9, 12, 64).
Goats are the classic and natural host of B. melitensis and,
together with sheep, are its preferred hosts. In pathological
and epidemiological terms, B. melitensis infection in small
ruminants is similar to B. abortus infection in cattle: the
main clinical manifestations of brucellosis in ruminants
are abortion and stillbirths, which usually occur in the last
third of the pregnancy following infection and usually only
once in the animal’s lifetime (14, 25).
Healthy animals can be exposed to Brucella infection in many
ways, as a large number of bacteria are shed in the birth
Rev. sci. tech. Off. int. Epiz., 32 (1)
fluids or fetus, placenta and abortion secretions of infected
females. The bacteria have the ability to survive several
months outdoors, especially in cold, wet conditions, where
they remain infectious to other animals, mainly through
ingestion. Brucellae also colonise the udder and contaminate
milk (9, 13). Although females calve apparently normally in
pregnancies following the first abortion, they continue to
shed large numbers of bacteria into the environment.
As with B. abortus infection in cows, B. melitensis can be
transmitted congenitally in utero but only a small proportion
of lambs and kids are infected in this way and most latent
infections of B. melitensis are probably acquired by ingesting
colostrum or milk (37). Despite the low transmission rate,
the existence of such latent infections makes it even more
difficult to eradicate the disease because, as the bacteria
persist without inducing detectable immune response,
infected animals are silent carriers of the disease. It is
therefore recommended that infected females and their
offspring be culled as part of an eradication programme in
infected herds (9). The exact mechanism enabling latent
Brucella infection to develop is unknown (14).
Some female hoggets testing seropositive to brucellosis have
been found to shed B. melitensis in milk postpartum, whereas
others do not shed brucellae despite being infected. While
lambs sampled for seven months showed seropositivity,
some tested seronegative for brucellosis in routine tests even
though a post-mortem study later revealed them to have
been infected with B. melitensis. This was also observed in
lambs from the same herd that had been born from mothers
seronegative for brucellosis (34). A previously unreported
fact is that B. melitensis was successfully isolated from the
vaginal discharge of a goat that had aborted but tested
seronegative for brucellosis, making the animal a potential
risk for spread undetectable by serological diagnosis (39).
While orchitis and epididymitis are uncommon in rams
and billy goats, they do occur (21). Brucella melitensis
biovar 3 has been isolated from a testicular hygroma of a ram
(53). Brucella melitensis can infect not only cattle but also
calves, through the ingestion of infected milk (5, 41, 68).
The isolation of B. melitensis in dogs has been demonstrated
and this has been observed to favour incidence of the
disease, as dogs can drag placentas or aborted fetuses to
uninfected areas (40, 48). There is one report of B. melitensis
biovar 3 having been isolated from a black bullhead catfish
(Ameiurus melas) (24), but clarification is needed as to
whether such animals can act as disease carriers or whether
the finding was a result of water pollution.
In extensive goat and sheep farms, it is common practice
for herds to share pasture and water holes before returning
to their pens. Such mixing of animals is a factor of risk for
spreading the disease from infected to free herds and makes
it harder to control. In this case, all goats sharing such sites
Rev. sci. tech. Off. int. Epiz., 32 (1)
must be considered as a single, large herd, and all goat
farmers must carry out control activities, e.g. vaccination
and the separation of positive and negative animals. Failure
by goat farmers to act will negate the efforts of those that do
take action to reduce the incidence of brucellosis (2, 3, 60).
Any strategy for the control or eradication of brucellosis
should begin by establishing the different epidemiological
contexts within a country or even a region or district, and
must have the support and collaboration of farmers. Above
all, the effectiveness of any such strategy will rely heavily
on the quality of the Veterinary Services and administrative
organisations involved, because the requisite diagnostic
and prophylactic tools are already fully validated and
standardised (14, 49).
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