Salmonellosis: antibodies to salmonella in blood

The diagnostic titer of antibodies to salmonella in the blood serum with RPGA is 1:200 (1:100 in children under 1 year) and higher; with an agglutination reaction (Widal reaction) - 1:40 (1:20 in children under 1 year) and higher.

More than 2200 serological variants of salmonella have been described, of which more than 700 occur in humans. The most common salmonella are: Salmonella typhimurium, Salmonella heidelberg, Salmonella enteritidis, Salmonella anatum, Salmonella derby, Salmonella london, Salmonella panama, Salmonella newport.Salmonella typhimurium accounts for 20-35% of isolates annually.

Bacteriological examination of blood, feces and urine is the main method of diagnosing salmonella infection. Blood cultures give a positive result during the first 10 days of fever or in the presence of a relapse in 90% of patients, in less than 30% - after 3 weeks of the disease. A positive culture in stool culture is obtained within 10 days to 4-5 weeks in less than 50% of cases. Detection of salmonella in feces 4 months after the disease and later (found in 3% of patients) indicates carriage of the bacteria. Positive results in urine cultures are obtained within 2-3 weeks in 25% of patients, even if the blood culture is negative. The antigenic structure of salmonella is complex. It contains O- and H-antigens:

• O-antigen is associated with the somatic substance of the cell, is thermostable, one of its components is Vi-antigen;
• H-antigen has a flagellar apparatus and is thermolabile.

Differences in the structure of the O-antigen made it possible to identify serological groups of salmonella: A, B, C, D, E, etc. Based on differences in the structure of the H-antigen, serological variants were established within each group. Among serological diagnostic methods, the Widal reaction was widely used until recently; in recent years, it has gradually lost its significance.

Based on the antigen structure inherent in various types of salmonella, O- and H-monodiagnosticums have been developed that allow the serological variant of salmonella to be established. Initially, serum is tested in RPGA with a complex preparation of erythrocyte salmonellosis diagnosticum containing the O-antigen. Then, if agglutination is present with the complex diagnosticum, RPGA is administered with preparations of groups A (1, 2, 12), B (1, 4, 12), C1 (6, 7), C2 (6, 8), D (1, 9, 12), and E (3, 10). Table 8-5 presents the antigen characteristics of salmonella, based on which the serological variants of salmonella are diagnosed.

Antigenic characteristics of Salmonella

Group	Salmonella	Antigens
		Somatic - O	Flagellates - H (specific)
A	Salmonella paratyphi A	1, 2, 12	A
B	Salmonella paratyphi B	1, 4, 5, 12	B
	Salmonella typhimurium	1, 4, 5, 12	I
	Salmonella heidelberg	4, 5, 12	R
	Salmonella derby	1, 4, 12	F, g
C1	Salmonella paratyphi C	6, 7, Vi	C
	Salmonella choleraeus	6, 7,	C
	Salmonella newport	6, 8	E, h
D1	Salmonella typhi	9, 12, Vi	D
	Salmonella enteritidis	1, 9, 12	G, m
E1	Salmonella anatum	3, 10	E, h
	Salmonella London	3, 10	L, v

The titer of antibodies to H-antigen in the blood serum of patients with salmonellosis is very variable and can give a non-specific reaction with other infections; therefore, its determination is of little use for the diagnosis of salmonellosis.

Vi-antibodies do not provide diagnostic or prognostic value in the infectious process. The situation is different with the detection of Vi-antibodies in carriers of bacteria. The greater resistance of salmonella containing Vi-antigen to human defense mechanisms causes a longer carriage of these forms (Vi-forms) of salmonella, as a result of which Vi-antibodies are detected in the blood of such patients. Vi-antibodies are direct evidence of carriage.

Currently, the most widely used methods for detecting antibodies to salmonella (to the O-antigen) are RPGA and ELISA; they are more sensitive than the Widal reaction and give positive results from the 5th day of the disease (the Widal reaction - on the 7th-8th day). Antibodies in patients with typhoid fever, paratyphoid fever or other serological types of salmonella appear in the blood by the 4th day of the disease and increase sharply by the 8th-10th day. Their number increases even more in the 2nd-3rd week of the disease. In adults and older children, RPGA confirms the diagnosis of salmonellosis in 80-95% of cases already at the end of the first week of the disease. In children of the first year of life (especially up to 6 months), RPGA with salmonellosis diagnosticum is negative throughout the disease. In the first months after recovery, the study of antibodies to salmonella can be used for retrospective diagnostics. However, it is necessary to take into account individual deviations from the normal immunogenesis cycle and the described dynamics of antibody titer changes. In a weakened organism with reduced reactivity, antibodies are weakly and slowly synthesized. Intercurrent diseases can also delay their formation. Early treatment with chloramphenicol or ampicillin can lead to a decrease in antibody titer or their absence. Therefore, an antibody titer of less than 1:200 does not allow us to exclude the disease; it is extremely important to study the antibody titer in dynamics - at the onset of the disease and after 10-14 days. An increase in the antibody titer after 10-14 days by at least 4 times when studying paired sera indicates an infectious process.

When using the Widal reaction, a titer of ≥ 1:40 to ≥ 1:160 is considered diagnostically significant, depending on the geographic area and laboratory. When using a cut point of 1:160 for diagnosing infection, the sensitivity of the method is 46%, specificity is 98%; 1:80 gives a sensitivity of 66%, specificity is 94%; at 1:40, the sensitivity is 90%, specificity is 85%.

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