Brucellosis: antibodies to the causative agent of brucellosis in blood

Normally, there are no antibodies to the causative agent of brucellosis in the blood. The diagnostic titer in the agglutination reaction is 1:160 and higher.

The causative agents of brucellosis are brucellae, small non-motile gram-negative bacteria. When diagnosing brucellosis, the obtained clinical and epidemiological data must be confirmed in the laboratory. For this purpose, bacteriological and serological research methods are used. In acute brucellosis, a positive result of a blood culture test is obtained in 10-30% of cases (in 62-90% if the causative agent is Brucella melitensis, in 5-15% if it is Brucella abortus ). A cerebrospinal fluid culture is positive in 45% of patients with meningitis. When sowing blood, bone marrow, and urine, a brucellae culture can be obtained after 5-10 days, and in some cases - after 20-30 days. In this regard, serological methods have become widespread for diagnosing brucellosis.

The most reliable serological test for determining antibodies to the causative agent of brucellosis in blood serum is the standard test tube agglutination test (Wright reaction), which determines the content of antibodies that react mainly with lipopolysaccharide antigens of Brucella. An increase in antibody titers by 4 times or more in blood serum samples obtained at intervals of 1-4 weeks makes it possible to identify the etiologic factor of the disease. In most patients, titers of specific antibodies increase on the 3-5th day from the onset of the disease. An antibody titer of at least 1:160 with its subsequent increase is considered reliable. An increased antibody titer is detected in 97% of patients in the first 3 weeks of the disease. The highest antibody titer is usually noted 1-2 months after the onset of the disease, subsequently it begins to rapidly decrease. The standard test tube agglutination test detects antibodies to B.abortus, B.suis, B.melitensis, but not to B. canis. An elevated antibody titer may persist in 5-7% of patients for 2 years after the infection. Therefore, the Wright reaction cannot be used for differential diagnosis of brucellosis with other infectious diseases if there is a history of brucellosis within the last 2 years. False-positive results may be caused by a skin test for brucellosis, vaccination against cholera, as well as infections caused by the cholera vibrio, yersinia, Francisella tularensis. In some cases, false-negative results of the agglutination reaction are possible in patients with brucellosis, which is explained by the prozone effect, or the so-called blocking of antibodies. In chronic localized forms of brucellosis, titers may be negative or below 1:160. Against the background of the treatment, the IgG antibody titers rapidly decrease and approach zero within a year. In case of relapses, the IgG antibody level increases again. The presence of a single increase in the IgG antibody titer of more than 1:160 is a reliable objective indication of a current or recently suffered infection. After treatment and discharge of the patient from the hospital, serological tests are recommended during the first year at 1, 2, 3, 6, 9 and 12 months, and during the second year - quarterly.

RPGA is more sensitive and specific for detecting brucellosis antibodies in blood serum. Hemagglutinins are often detected in cases where the agglutination reaction gives a negative or questionable result.

The CFT allows to detect complement-fixing antibodies to brucellae, which appear in the blood later than agglutinins. Maximum antibody titers in the CFT are recorded by the 4th month of the disease, then their titer decreases, but they are detected in small quantities for 1 year. The CFT has no significant advantages over the agglutination reaction.

[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ]