Gram Negative Coccobacilli
All are true about brucella except (AIIMS May 2011)
|A||B. abortus is capnophilic|
|B||Transmission by aerosol can occur occasionally|
|C||Pasteurisation destroys it|
|D||2 ME is used to detect IgA|
2 ME is used to detect IgA
a. Brucellae are destroyed by heat at 60C in 10 min. They are killed by pasteurization. They resist drying and may survive in soil for long periods. They survive for many weeks in meat.
b. Brucella is primarily an intracellular pathogen affecting the reticuloendothelial system. They spread from the initial site of infection to the local lymph nodes, where they multiply, spill over to the blood stream and disseminate throughout the body. Tissue reaction to brucella consists of granuloma formation.
c. Diagnosis depends either on isolation of the organism or on serology. Blood culture is the most definitive method for diagnosis. The Castaneda’s method (biphasic) has several advantages and is recommended. Blood cultures are positive only in 30-50% cases. Bone marrow, lymph nodes, CSF, urine and abscesses can also be used for culture.
d. Cultures are often unsuccessful. Several serological tests are used in diagnosis. In brucellosis, both IgM and IgG antibodies appear in 7-10 days after onset. As disease progresses, IgM antibodies decline, while IgG antibodies persist or increase in titre.
e. Standard agglutination test (SAT) is a tube agglutination test in which a titre more than 160 or more is considered significant. It detects IgM antibodies effectively, which tend to decline after the acute phase of illness. As the prozone phenomenon is very frequent in brucellosis, it is essential to test several serum dilutions.
f. 2-mercapto ethanol selectively destroys IgM by breaking the disulfide bonds and hence 2-ME agglutination test is used to detect IgG antibodies, as they are the best indicators of active infection especially in patients with a long history of symptoms and also prognostic indicator following therapy. It can also be used for diagnosis of relapse. This test also is used to diagnose cross-reacting antibodies produced in cholera infections and Y. enterocolitica infections.
g. IgG antibodies may act as blocking or incomplete antibodies. These can be detected by prior heating of serum at 55C for 30 min or by using 4% saline as diluent or by the direct Coomb’s test.
h. ELISA and complement fixation tests can detect IgM and IgG antibodies and therefore useful for differentiation between acute and chronic brucellosis.