The genus Brucella and clinical manifestations of brucellosis/O genero Brucella e as manifestacoes clinicas de brucelose.
Brucella spp. are Gram-negative, facultative intracellular pathogens. In nature, Brucella spp. are pathogens that do not multiply in the environment, but usually are transmitted directly from host to host. (GORVEL & MORENO, 2002). These organisms belong to the genus Brucella and cause brucellosis, which has variable clinical features that are strongly dependent on the bacterium and host species. Some of these organism can potentially cause human infections, resulting in one of the most important and widespread bacterial zoonosis in the world (SANTOS et al., 2005).
The genus Brucella was named after David Bruce, who first isolated the organism (then named Micrococcus melitensis) in 1887 from the spleen of a soldier suffering from a disease that was called Malta Fever (NICOLETTI, 2002; SANTOS et al., 2005). The zoonotic nature of brucellosis was demonstrated in 1905 by isolating Brucella from goat milk (NICOLLETI, 2002). Importantly, even as late as 1955, over 200 cases of brucellosis were caused by ingestion of a special cheese from Maltese goats (WYATT, 1999). In 1895, 1914, and 1966, Brucella species were isolated from aborted bovine, swine, and canine fetuses, respectively. In 1953, Brucella ovis was identified as a cause of epididymitis in rams (NICOLLETI, 2002). In the last 15 years 3 new non-classical species of Brucella has been identified (ROSS et al., 1994; FOSTER et al., 1996; SCHOLZ et al 2008a). The recent isolation and characterization of non-classical species of Brucella demonstrates that in spite of brucellosis being an old disease, there is still several aspects of these organisms and their associated diseases that remain unknown.
Taxonomy is important to identify and classify in logical order the great diversity of living beings (MORENO et al., 2002). The genus Brucella belongs phylogenetically to the a-proteobacteria, a group that contains bacterial species with a wide variety of lifestyles, including symbionts of animals and plants (Wolbachia, Sinorhizobium), as well as obligate or facultative intracellular and extracellular pathogens, such as Rickettsia, Brucella and Agrobacterium (TSOLIS, 2002).
Six species are currently recognized within the genus Brucella: B. melitensis, B. abortus, B. suis, B. ovis, B. canis, and B. neotomae. This classification is mainly based on differences in biochemical characteristics, pathogenicity and host preferences (Table 1). Each of these species of Brucella is adapted to a specific host, but not exclusively (ALTON et al, 1975; ALTON 1990).
These marine mammal Brucella isolates have phenotypic and molecular features as well as host preferences that are clearly distinct from the six previously recognized species (JAHANS et al., 1997; CLAVAREAU et al., 1998; VERGER et al., 2000; WATSON et al., 2003; JACQUES et al., 2007). In 2000, a new species of the genus, with different features when compared to other known species, was isolated from the rodent Microtus arvalis, and named of B. microti (SCHOLZ et al., 2008a). Recently, a novel Brucella species has been isolated from post partum uteruses of pregnant baboons with history of stillbirth (SCHLABRITZ-LOUTSEVITCH et al., 2009).
In recent years, an important controversy has developed concerning the taxonomy of the genus Brucella. Although Brucella species can be differentiated by conventional phenotypic tests, theses species display a high degree of DNA homology in DNA-DNA hybridization assays (>90% identity), including the recently recognized marine mammal strains (VERGER et al., 1987, 1998, 2000). Therefore, it has been proposed that the genus Brucella should be a monospecific genus, with B. melitensis as the sole species and the other species should be considered as biovars (VERGER et al., 1985, 1987). Conversely, several molecular genotyping methods have been developed and applied to characterize Brucella species, indicating that significant DNA polymorphisms occur between species, which favor the current multi-species classification of Brucella (HALLING et al., 2005). Importantly, comparison of genome sequences of B. suis and B. melitensis demonstrated that exist clusters of genes that are unique in both species (designated genetic islands). It is reasonable to hypothesize that these unique genes may contribute to the differences in host specificity between Brucella species (TSOLIS, 2002). Furthermore, recent studies based on comparative whole genome analysis of several Brucella species indicate that there is limited divergence with a large number of pseudogenes. Interestingly, these genomic analyses do not clearly explain the host preferences of Brucella spp. (WATTAM et al., 2009; FOSTER et al., 2009). One of these studies indicates that at the B. ovis is the basal lineage to the rest of the Brucella spp., and that apparently most Brucella species diverged from their common B. ovis ancestor in the past 86,000 to 296,000 years (FOSTER et al., 2009).
It is noteworthy that the International Committee on Systematics of Prokaryotes, Subcommittee on the Taxonomy of Brucella has taken a clear position recommending a taxonomic classification that includes different species within the genus, either classical or new, which are still considered as individual species. Therefore, the genus currently group nine species, namely B. melitensis, B. abortus, B. suis, B. ovis, B. canis, B. neotomae, B. ceti, B. pinnipedialis, and B. microti (http://www.the-icsp.org/ subcoms/Brucella.htm, Last modified 15 February 2008). The newly isolated Brucella species from baboons (SCHLABRITZ-LOUTSEVITCH et al., 2009) has not yet been classified nor included in the above mentioned list.
B. melitensis is the most important etiologic agent of brucellosis in small ruminants, although cattle and other ruminants may also be infected. This species has three different biovars (BRICKER & HALLING, 1994) and it has the higher zoonotic potential within the genus, and thus it is recognized as the most important agent of human brucellosis (ALTON, 1990). This pathogen is widespread in several parts of the world, particularly the biovar 3 in Mediterranean and Middle Eastern countries (BANAI, 2002). Parts of Latin America are also seriously affected with biovar 1, especially Mexico, Peru and Northern Argentina (LUCERO et al., 2008). Importantly, B. melitensis have never been isolated in Brazil, where it is considered a foreign disease (POESTER et al., 2002).
In goats and sheep, B. melitensis infection causes abortion, reduced milk yield, and orchitis. Both sexually mature genders are equally susceptible. The predominant sign of acute infection is reproductive failure with abortion and birth of weak offspring. Abortions occur mostly during the last two months of gestation. Generally, transmission in sheep and goats occurs through materials excreted from the female genital tract (ALTON, 1990).
In goats, approximately two thirds of acute natural infections during pregnancy lead to infection of the udder and milk excretion of the bacteria during the subsequent lactation. Persistent infection of the udder is accompanied by intermittent shedding of the agent in milk. Inflammation of the mammary gland reduces milk production. However, clinical signs of mastitis are seldom detectable in naturally infected goats (ALTON, 1990).
B. melitensis is the most virulent Brucella for humans with a few organisms (10 to 100) being sufficient to cause a debilitating chronic infection (FUGIER et al., 2007). Humans acquire brucellosis mainly through ingestion of contaminated milk and unpasteurized dairy products. Contact of mucosas and skin abrasions with fluids and tissues from aborted fetuses of infected animals are also important sources of Brucella transmission (HARTIGAN 1997; FUGIER et al., 2007). Furthermore, people may be infected by inhalation of contaminated dust or aerosols. Thus, Brucella is one of the most common laboratoryacquired pathogens worldwide and is included in the potential biological weapon list (SANTOS et al., 2005).
Human infections with B. melitensis may have variable clinical manifestations and can become life threatening (COLMENERO et al., 2002). Although the majority of patients present with general symptoms, such as fever, malaise, sweats and lymphadenopathy and/or hepatosplenomegaly, a more severe form of the disease can be accompanied with osteo-articular signs (spondylitis, arthritis and osteomyelitis) or genitourinary tract changes (orchitis, epididymitis, glomerulonephritis and kidney abscesses) (HARTIGAN, 1997; COLMENERO et al., 2002). More severe complications comprise, in descending order of frequency, neurobrucellosis, liver abscesses, and endocarditis (FUGIER et al., 2007).
B. abortus has seven different biovars, namely biovars 1-6 and 9. Cattle is the preferential host for B. abortus, but the organism can be transmitted to buffaloes, camels, deer, dogs, horses, goats, sheep, and man (KUDI et al., 1997). In Brazil, bovine brucellosis due to B. abortus is the most prevalent Brucella infection (POESTER et al., 2002).
B. abortus causes primarily a disease in cows, being isolated from the udder, uterus, and lymphoid organs (POESTER et al., 2006). Outbreaks of brucellosis in dairy herds result in decreased milk production, increase somatic cell count in milk, occurrence of abortions and post-partum metritis (MEADOR & DEYOE, 1989). Late abortion is associated with necro-hemorrhagic placentitis (Figure 1) and fetal lesions, particularly fibrinous pleuritis and pericarditis and pneumonia (XAVIER et al., 2009). Infected cows usually abort only once, and subsequent gestations may generate calves that may be born weak or healthy. Some infected cows will not exhibit any clinical symptoms of the disease and give birth to normal calves. Transmission occurs mainly after abortion or parturition of infected cows via contaminated fetus, fetal membranes, and uterine secretions (SILVA et al., 2005).
[FIGURE 1 OMITTED]
Bulls can be infected but they do not readily spread the disease. B. abortus is a common cause of orchitis that is often associated with a vesiculitis and epididymitis. Infection in males may result in either temporary or permanent infertility, depending on the intensity of the lesions (EAGLESOME & GARCIA, 1992).
Porcine brucellosis is an emerging disease caused by B. suis biovars 1, 2 and 3. It is mainly a disease of domestic and wild pigs but it can also affects other species such as cattle, horses, rabbits, dogs, and humans (EWALT et al., 1997, FRETIN et al., 2008). Biovars 1 and 3, which have pathogenic potential for humans, occur in Europe, North, South and Central America, Southern Asia and Pacific islands (FRYE et al., 1991). In Brazil, only the biovar 1 has been isolated, and there are just a few reports of B. suis in the country, with a seroprevalence of 0.34% in recent surveys (BRAZIL, 2000). Prevalence is very low in industrial swine production systems (CARVALHO NETA et al., 2005), but it may be quite high among backyard pigs slaughtered without sanitary inspection in Brazil (FREITAS et al., 2001).
Porcine brucellosis is a herd problem. Pigs of all ages can acquire the infection, but the disease primarily occurs in adults. B. suis is excreted in large numbers, for long periods in the semen and urine as well as in uterine discharges and milk being transmitted by both venereal and oral routes (ALTON, 1990).
B. suis infection in pigs often does not result in clinical signs, and therefore clinical diagnosis is very difficult. B. suis causes primarily a genital disease with abortions, but it also affects other organs, especially bones and joints (FELDMAN & OLSON, 1933). Brucellosis is the only disease in which reproductive failure in sows is accompanied by orchitis in boars and osteo-articular disorders such as arthritis, osteomyelitis, spondylitis and paralysis. The most important clinical signs in sows are infertility, irregular estrus, abortion in any stage of gestation and birth of weak piglets with a high neonatal mortality rate (DEYOE, 1967). Although orchitis and epididimitis are the most common lesions in boars, in some cases the infection is restricted to sexual glands and may not result in impaired fertility but can be an important source for shedding the organism in the semen (VANDEPLASSCHE et al 1967).
B. ovis infection has been reported in Australia, New Zealand, North and South America, South Africa, and many countries in Europe. It occurs in most sheep-raising regions of the world (BURGUESS, 1982). In Brazil, serological surveys in the state of Rio Grande do Sul demonstrated an average seroprevalence in positive sheep flocks of 13.4% (ranging from 6.9 to 50%) (MAGALHAES NETO & GIL-TURNES, 1996), whereas a seroprevalence of 5.57% has been reported in the Northeastern part of the country (CLEMENTINO et al., 2007).
Poor semen quality associated with decreased sperm motility and concentration as well as sperm abnormalities is often associated with early infection (CAMERON & LAUERMAN, 1976). B. ovis causes primarily epididymitis in sexually mature rams, and occasionally abortion in ewes (LAWRENCE, 1961). Later on, palpable lesions may develop in the epididymis, which may be unilaterally (Figure 2) or, occasionally, bilaterally affected (LAWRENCE, 1961). Conversely, some infected rams not develop palpable lesions (CARDOSO et al., 1989). In addition, a considerable number of infected rams may shed B. ovis in the semen for long periods, without any clinical sign of infection. Asymptomatic rams may develop only a mild subfertility or retain normal fertility, thus increasing the risk of spreading the infection in the herd. The transmission can occur by direct contact between rams kept in the same premises for prolonged periods of time (HUGHES et al., 1972; BROWN et al., 1973).
[FIGURE 2 OMITTED]
In ewes, B. ovis can uncommonly cause abortion associated with placentitis beginning at 30 days of gestation. Infected ewes may give birth to weak lambs with a high neonatal mortality rate (MEINERSHAGEN et al., 1974).
Canine brucellosis is caused by B. canis that infects domestic dogs, wild carnivores and rarely other domestic animals (CARMICHAEL, 1990). It is especially common in Central and South America (MIRANDA et al., 2005). Infection of dogs with B. canis is widespread in Brazil, with prevalence ranging between 0.84 to 58.3% and it is concentrated mostly in the Southeast and South regions of the country (AZEVEDO et al., 2003; KEID et al., 2004; MIRANDA et al., 2005). Humans are susceptible to B. canis, but infections are uncommon and they are usually mild. Most natural human infections have been acquired through close contact with infected dogs. Laboratory infections have also been reported (CARMICHAEL, 1990).
Natural infections occur most commonly after ingestion of contaminated placental tissues or aborted fetuses, vaginal secretions from infected bitches, and during breeding. The organism may be shed for long periods in vaginal secretion after abortion and semen (CARMICHAEL & KENNEY, 1970; GEORGE et al., 1979).
In females, the most prominent clinical sign is abortion after 45-55 days of gestation in about 75% of the cases. Early embryonic death and reabsorption, or abortion 10-20 days after mating, may occur in some cases (CARMICHAEL, 1990). In males, the main sign is epididymitis and orquitis, which may be unilateral or bilateral, and often results in infertility. Semen from infected males usually contains large numbers of abnormal sperm and inflammatory cells, especially during the first three months after infection. Chronically infected males may have no sperm, or reduced numbers of immature sperm (CARMICHAEL & JOUBERT, 1988).
A particularity of B. canis infection is a prolonged bacteremia. Therefore, blood culture is a valuable diagnostic approach in this case (CARMICHAEL & KENNEY, 1970).
Brucella ceti and Brucella pinnipedialis
Since 1990, Brucella strains have been isolate from a variety of marine mammal species, including seal (Phoca vitulina), dolphins (Tursiops truncates; Delphinus delphis; Lagenorhynchus acutus; Stenella coeruleoalba), whale (Balaenoptera acutorostrata), and other species (EWALT et al 1994.; ROSS et al., 1994; FOSTER et el., 1996; CLAVAREAU et al., 1998; WYATT, 1999). These isolates have been classified as B. ceti and B. pinnipedialis, referring to isolates from cetaceans and seals, respectively (FOSTER et al., 2007). Transmission may occurs by direct contact trough mucosas and injured skin, oral route due to ingestion of other infected marine mammals (FOSTER et al., 2002), or by vertical or horizontal transmission to fetus since Brucella has been isolated in fetal tissues and in milk from dolphins (HERNANDEZ-MORA et al., 2008).
Pathological changes include skin abscesses, hepatic and splenic necrosis or/and histiocytic inflammation, meningitis, discospondilitis and abortion (FOSTER et al., 1996). Non suppurated meningoencephalitis has been described as the most consistent histological change in dolphins with neurological signs and positive serology and immumohistochemistry to Brucella sp. (GONZALEZ et al., 2002; HERNANDEZ-MORA et al., 2008). These marine Brucella species are capable of infecting terrestrial mammal species as demonstrated by experimental infection of cattle (RHYAN et al., 2001).
Marine Brucella species are capable of infecting humans causing neurological disorders (SOHN et al., 2003; HERNANDEZ-MORA et al., 2008). Transmission to human occurs probably through direct contact with marine mammals, although there reports of human brucellosis caused by marine isolates in which there was no evidences of contact of the patient with marine animals (SOHN et al., 2003; McDONALD et al. 2006; HERNANDEZ-MORA et al., 2008).
B. neotomae was discovered by STOENNER & LACKMAN (1957) and approved as a new species of Brucella in 1980. B. neotomae is known to infect only the desert wood rat under natural conditions in the USA, and no other cases in addition to the original isolation have been reported.
B. microti has been isolated from systemically infected common voles (Microtus arvalis) in South Moravia, Czech Republic in 2000 (SCHOLZ et al., 2008a). Later on, B. microti was isolated from mandibular lymph nodes of wild red foxes (Vulpes vulpes) hunted in Austria. SCHOLZ et al., 2008b). Furthermore, specific B. microti DNA sequences were recently detected in soil, but whether soil is the primary habitat of B. microti remains to be investigated (SCHOLZ et al., 2008c).
The genus Brucella includes several organisms, some of which with a very significant zoonotic potential, whereas some species poses significant risk for animal health and production. In the recent past years the there has been an increasing number of newly recognized species of Brucella.
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Mariana Noyma Xavier (1) Erica Azevedo Costa (1) Tatiane Alves Paixao (1) Renato Lima Santos (1) *
(1) Departamento de Clinica e Cirurgia Veterinarias, Escola de Veterinaria, Universidade Federal de Minas Gerais (UFMG). Av. Antonio Carlos, 6627, 31270-901, Belo Horizonte, MG, Brasil. E-mail: firstname.lastname@example.org. * Autor para correspondencia.
Table 1--Preferential hosts and zoonotic potential of Brucella species. Zoonotic Species Preferential host potential Brucella melitensis Sheep, goat +++ Brucella abortus Cattle ++ Brucella suis Pig ++ Brucella canis Dog + Brucella ovis Sheep - Brucella neotomae Desert wood rat (Neotomae lepida) - Brucella ceti Cetaceans + Brucella pinnipedialis Seals + Brucella microti common voles (Microtus arvalis) -