Pathogenesis, pathology and immunity
From studies performed in livestock, the infectious routes are mainly oral (e.g. licking of aborted fetuses or fetal membranes, ingestion of contaminated feedstuffs) via the conjunctival mucosa, and venereal: up to 104-105 colonyforming units (CFU) of B.
suis biovar 1 per ml semen has been reported in pigs. In cattle, brucellosis can also be transmitted in utero or to the newborn calf immediately after birth.Our knowledge of the pathogenesis of brucellosis is based on in vivo studies, performed mainly in mice and cattle as well as in vitro studies in murine macrophages and human macrophage cell lines.
Brucella spp. enters the organism mainly through oral, nasopharynx, conjunctival and genital mucosae but also through cutaneous lesions. Bacteria rapidly drain to lymph nodes and can persist there for a long time. If Brucella spp. are not eliminated at this stage, bacteraemia (which is usually transient in cattle but more persistent in pigs) and colonization of a broad range of tissues takes place. Brucella spp. are frequently isolated from lymphoid tissues associated with the mammary glands and reproductive organs but can also be found in other organs (e.g. bones, joints, nervous tissue). Abortion is associated with the extensive replication of Brucella spp. within the chorioallantoic trophoblasts of the placenta. This massive intracellular replication ruptures the infected trophoblasts and permits direct access to the fetus. The loss of placental integrity and fetal infection lead to abortion or to premature birth of a weak and infected calf.
Brucella spp. have a predilection for the gravid uterus, udder, testicle and accessory male sex glands, lymph nodes, joint capsules and bursae. At necropsy, purulent lesions and granulomas may be seen. In the cow, B. abortus localizes initially in lymph nodes, infects the gravid uterus during bacteraemia, and multiplies to enormous numbers in chorioallantoic trophoblasts.
Retained placenta and residual infection of the uterus are often indicators of the disease. Characteristic placental lesions may vary in severity and are not pathognomonic. Acute or chronic placentitis is sometimes seen. Placentae may show degenerative and necrotic changes in the chorioallantoic trophoblasts at the edges of the cotyledons. The cotyledons may be normal or necrotic. In affected cotyledons, the trophoblastic epithelium lining the chorionic villi is necrotic. In some villi, epithelial necrosis and infiltration of mononuclear cells and neutrophils are observed. Bacterial colonies are found in the necrotic areas of villi, surrounded by necrotic cell debris and neutrophils. In infected mammary glands, diffuse inflammation has been observed and can lead to atrophy of the glandular tissue and fibrosis when the disease progresses.I n males, the predilection sites for infection are the reproductive organs and the associated lymph nodes. Seminal vesiculitis and/or orchitis and infection of the accessory sex glands are commonly observed. During the acute phase of infection the semen contains large numbers of Brucella spp., but as the infection becomes more chronic the number of excreted Brucella spp. decreases.
Fetuses infected with Brucella spp. may show pneumonia, serohaemorrhagic exudates in body cavities and lesions in the muscles. Other lesions such as abnormal abomasal content, fibrinous pleuritis, vasculitis and meningitis occur less frequently. Fetal bacteraemia occurs after replication of Brucella spp. in trophoblasts. Subsequently, fetal viscera and placental cotyledons become heavily infected with Brucella spp. Histological lesions observed in aborted fetuses are not specific enough to be able to incriminate B. abortus as the cause of abortion. In chronic cases, uni- and bilateral hygromas, mainly localized at the carpal joint, are observed in more than 60% of chronically infected animals.
Specific pathologies have been described in different wildlife species.
In chamois and ibex, these include blindness and neurological signs as well as thick-walled carpal joints and enlargement of the testicle characterized by necrosis and fibrosis(12’13). In wild boar one of the most striking features is that a very high percentage of infected animals can be detected in all categories of age by bacteriology, often in the absence of gross lesions(17). There is no definitive explanation for this. In the brown hare, the infection is either latent or involves the development of granulomatous nodules or abscesses in the testes, liver, spleen, lung and other tissues1-18). In reindeer when clinical signs are present, abortions and metritis are seen in females and orchitis in males. In both sexes, abscesses in joints and often bursitis and lameness are observed)34). In moose, carpal pathology and osteomyelitis of subjacent bone was observed)21). In common vole (Microtus arvalis) acute infections were characterized by oedema of extremities, occasionally with colliquating abscesses, arthritis, lymphadenitis, perforations of the skin resulting from colli- quated abscesses, orchitis and peritoneal granulomas)31). In marine mammals it is important to note that pathology induced by Brucella spp. is different in cetaceans compared with seals. As a general rule, no gross pathology has been associated with B. pinnipedialis infections in seals, whereas different acute and chronic pathological changes have been associated with B. ceti infection both in Odontoceti and Mysticeti. A range of Brucella spp. associated pathology has been found in cetaceans, which includes sub-blubber abscessation, hepatic and splenic necrosis, macrophage infiltration in liver and spleen, possible abortion, epididymitis, spinal discospondylitis, meningitis, lymphadenitis and mastitis. There is currently no information on the levels of abortion in seals or cetaceans)6). A chronic, nonsuppurative meningoencephalitis was found in three young striped dolphins (Stenella coeruleoalba))35).IMMUNITY
The first role of the innate immune system in Brucella infection is to reduce the initial number of bacteria. Activation of the classical pathway mediated by IgM and low concentrations of IgG has been considered to be the dominant bactericidal mechanism of serum against B. abortus during the early stage of infection. Neutrophils are probably the first immune-associated cells in human infection to encounter Brucella spp. Rapid phagocytosis of virulent and attenuated Brucella strains by neutrophils occurs only following opsonization with normal serum, but the role of these cells in controlling the infection is questionable, as survival of B rucella in neutrophils during early infection has been observed)36). The macrophage is the main cellular host of Brucella spp. The bactericidal functions of these cells are mainly mediated by reactive oxygen intermediates )ROI). Reactive nitrogen intermediates )RNI) also play a limited role in the killing of Brucella in macrophages)37). Both ROI and RNI are induced by interferon gamma )IFN-γ) and tumour necrosis factor alpha )TNF-α). Antigen processing and recognition is a key feature of antibacterial immune responses to intracellular bacteria. In contrast to other pathogens, which are primarily controlled by conventional major histocompatibility complex )MHC) II- and MHC I-restricted CD4+ or CD8 + T cells, respectively, unconventional T cells participate additionally in antibacterial protection. Both natural killer )NK) cells and γδ T-cells play a role in the control of Brucella infection by killing Brucella infected cells and activating macrophages via activating-cytokine production)38).
The innate response against lipopolysaccharide )LPS) typically involves the release of a range of pro -inflammatory mediators, which locally promote inflammation.
Brucella abortus induces interleukin )IL)- 12 produced by human monocytes via the CD14 LPS preceptor)39). Additionally, the ability of Brucella spp. to elicit IL- 12 secretion will drive ThO cells to differentiate into Th1 effector and memory cells.Passive transfer of serum containing anti-LPS antibodies protects mice against virulent B. abortus challenge. Passive transfer of monoclonal antibodies directed against outer membrane proteins induced only a limited protective effect. Opsonization, probably coupled with enhancement of intracellular killing, is regarded as the principal protective role of antibody against Brucella infection.
I mmunity against Brucella spp. requires cell-mediated immunity. In mice, antigen-presenting cells stimulated by B. abortus produce IL-12 that will induce a Th1 response mediated by CD4+ T cells. This Th1 response will stimulate CD8+ T cells to induce their direct cytotoxicity activity against Brucella infected cells, and to produce IFN-γ which will enhance the bactericidal functions of macrophages. The development of a Th1 response, under the control of TNF- α, IFN-γ and IL-12, promotes the clearance of the infection.