Brucellosis in humans derives from exposure to infected animals through the ingestion of unpasteurised dairy products, inhalation of aerosolised bacteria, or from direct contact with infected animals through contaminated skin or mucosal surfaces.3
Infection is initiated by rapid replication of the organism within regional lymph nodes followed by haematogenous dissemination, seeding the reticulo- endothelial system including the liver, spleen, and bone marrow with bacteria.4
This feature of the disease is important because a biopsy from any of these organs can often permit diagnosis in suspected cases with persistently negative blood cultures.5
Clinical features of brucellosis are variable and frequently non-specific, hampering early diagnosis and treatment. Gastrointestinal and hepato-biliary involvement can afflict up to 70% of patients.6
Endocarditis is encountered in less than 2% of cases, but accounts for the majority of brucellosis-related deaths.7
Interestingly, Andriopoulos et al8
describe such diverse manifestations as splenomegaly (51%), osteo-articular involvement (42%), cervical lymphadenitis (31%), hepatomegaly (25%), genitourinary involvement (13% of male cases), cholecystitis (2%) and breast abscess (0.7%) as occurring in cases of brucellosis. Infected patients had a relevant occupational history in fewer than 20% of cases. Mantur et al,9
reported urinary tract infections and Stevens-Johnson syndrome as presentations of the disease. Respiratory involvement can also occur in brucellosis.10
Kocher et al11
in a study of neuro- brucellosis, reported cases of meningoencephalitis, myelitis leading to spastic paraparesis, polyradiculoneuropathy and polyneuroradiculomyelo-encephalopathy.
Laboratory tests often reveal only subtle abnormalities such as mild elevation in inflammatory markers, with occasionally elevated liver enzymes.12
Radiographic changes can be non- specific, often mimicking slow growing neoplasms such as giant-cell tumours and multiple myeloma6
Such a wide range of clinical manifestations coupled with non-specific results on routine laboratory parameters can pose a significant diagnostic challenge to physicians. In such situations, evaluation can proceed by either attempted isolation of the organism in culture, or serological evidence of infection, or as in our case, by a combination of both methods.
While isolation from tissue or blood culture can yield a definitive diagnosis, there are a number of pitfalls to this approach. Brucellosis is an important cause of laboratory acquired infection among health-workers to the extent that the CDC now recommends biosafety level 3 (BSL-3) practices, equipment and facilities in all laboratories handling specimens from suspected cases of Brucella.
Unfortunately, such facilities are often unavailable in the developing world; the risk of infection can be a deterrent to attempts to isolate the organism in culture. Partial treatment with empirical broad-spectrum antibiotics can suppress bacteremia14
without eradicating the infection, rendering blood cultures sterile. In such situations, bone marrow cultures can still detect the organism, and are therefore considered the gold standard of diagnosis.15
Nonetheless, bone marrow aspiration and biopsy is a technically cumbersome, invasive and painful procedure, and is often relegated in favour of other easier techniques, principally serological. Moreover, isolation in culture is possible in 50 to 80% of patients with acute brucellosis, with the yield rate falling to less than 5% for individuals with chronic brucellosis.16
Finally, even in the presence of bacteraemia, conventional culture in broth media can take up to six weeks, which is an extremely long period of time in regions such as India where durable patient follow-up is difficult to achieve. To some extent, this problem can be overcome by utilising automated blood culture systems such as the one utilised in this case. This can accelerate growth producing positive results from blood cultures within seven days and bone marrow cultures within four days, thus providing a relatively quick diagnosis17
within the constraints of a limited yield rate as
This combination of drawbacks to culturing the organism has spurred the development of alternative serology-based tests for brucellosis, with the aim of achieving rapid diagnosis and cost-effectiveness.
Amongst these tests, the first-generation standard tube agglutination (STA) test and the indirect fluorescent antibody (IFA) test utilise whole cell preparations of B. melitensis and B. abortus containing A and B antigenic epitopes shared by the various species of Brucella . In contrast, newer ELISA-based tests employ purified lipopolysaccharide extracts of B. melitensis and B. abortus. While the older STA and IFA tests are comparable in reliability to ELISA with regard to acute brucellosis, they are of lesser value in cases of chronic brucellosis.16 Nevertheless, their low cost and simplicity have ensured their continued application in developing countries, where the burden of brucellosis is the greatest.
In contrast, the IgM and IgG ELISA tests are considered extremely reliable with a sensitivity of 100% and a specificity of 96%.18
Moreover, of the two, the IgM ELISA is considered superior being positive in both acute as well as chronic cases of brucellosis.19
In our case, an IgM ELISA test was performed and was negative. Furthermore, an STA test was also negative; a negative result in a combination of two different serological tests is usually considered sufficient to rule out false-negative results.20