Brucella

Brucella Background

Brucella species are small (0.5-0.7 by 0.6-1.5 µm), Gram-negative, facultative coccobacilli, mostly lacking a capsule, endospores, or native plasmids. They are intracellular within the host organism, but can persist in the environment, outside of the host. The Brucella genome typically encodes 3,200 to 3,500 ORFs and includes two chromosomes; the first chromosome mostly encodes genes related to metabolism, while the second, smaller chromosome includes genes related to pathogenicity. Brucella are able to evade the host immune response, allowing them to persist in vivo. They are found in association with phagocytic cells, most prominently macrophages, in which a subset of bacteria is able to evade killing in phagolysosomes and replicate successively with an endoplasmic reticulum–associated compartment and a modified autophagosome (reviewed in Byndloss & Tsoli, 2016).

The different species of Brucella are genetically very similar, although each has a slightly different host specificity and some are more dangerous for humans including, B. melitensis (goats and sheep), B. abortus (cattle), B. canis (dogs), B. suis (pigs) and B. ovis (sheep), which can trigger unspecific flu-like symptoms. Transmission from human to human is rare, but possible, and minimum infectious exposure is between 10 and 100 organisms. The disease is transmitted by ingesting contaminated food (such as unpasteurized milk products), direct contact with an infected animal, or inhalation of aerosols. Brucellosis can affect any organ or organ system and in humans the disease is characterized by a long incubation period that leads to a chronic, sometimes lifelong, debilitating infection. After exposure there is generally a 2-4-week latency period before exhibiting symptoms, which include acute undulating fever, headache, arthralgia, night sweats, fatigue, and anorexia. Foul-smelling perspiration is considered a classical sign. Later complications may include arthritis or epididymo-orchitis, spondylitis, neurobrucellosis, and liver abscess formation. Neurological symptoms include depression and mental fatigue. Cardiovascular involvement can include endocarditis resulting in death. At present, there is no available vaccine to prevent Brucella infection in humans. The treatment of choice consists of combination antibiotic therapy for at least 6 weeks; however, significant treatment failures and relapses of infection have been reported (reviewed in Byndloss & Tsoli, 2016). There is also a direct link between the disease and the economic status of a country (Pappas et al., 2006).

In animals, brucellosis symptoms can be varied from severe acute to sub-acute or chronic, depending upon the organ of infection and the type of animal. In livestock Brucella spp. cause an acute severe inflammatory response, which can lead to abortion, infertility and marked reduction in milk yield (reviewed in Byndloss & Tsoli, 2016). It infects almost all domestic species except cats, which are generally more resistant to Brucella infection. Bovine brucellosis is a serious disease of cattle that has significant animal health, public health, and international trade consequences. Large numbers of Brucella abortus bacteria are excreted in the foetus, placenta and uterine fluids of infected animals which can then infect other cattle on contact. The organism is also excreted in milk and disease can be spread through infected semen. Vaccination in cattle is practiced worldwide as a strategy to prevent the spread of brucellosis. However, there is demand for the development of new vaccines that are safer and more effective (Khan & Zahoor, 2018).

References

• Byndloss MX, Tsolis RM. Brucella spp. Virulence Factors and Immunity. Annu Rev Anim Biosci. 2016;4:111–127.
• Khan MZ, Zahoor M. An Overview of Brucellosis in Cattle and Humans, and its Serological and Molecular Diagnosis in Control Strategies. Trop Med Infect Dis. 2018;3(2):65.
• Pappas G, Papadimitriou P, Akritidis N, Christou L, Tsianos EV. The new global map of human brucellosis. Lancet Infect Dis. 2006;6(2):91‐99.