Salmonellosis: antibodies to salmonella in the blood

The diagnostic titer of antibodies to salmonella in the serum at RPGA is 1: 200 (1: 100 in children under 1 year) and above; during the agglutination reaction (Vidal's reaction) - 1:40 (1:20 in children under 1 year) and above.

Described more than 2,200 serologic variants of Salmonella, including the person - more than 700. The most frequently encountered following salmonella : of Salmonella typhimurium, of Salmonella heidelberg, of Salmonella enteritidis, of Salmonella anatum, of Salmonella derby, of Salmonella london, of Salmonella panama, of Salmonella newport. Each year, 20-35% of the isolates are in Salmonella typhimurium.

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 if there is a relapse in 90% of patients, less than 30% after 3 weeks of the disease. Positive culture during the sowing of stool is obtained during 10 days to 4-5 weeks in less than 50% of cases. The detection of salmonella in stool after 4 months after the disease and later (found in 3% of patients), indicates a bacteriocarrier. In urine cultures, positive results are obtained for 2-3 weeks in 25% of patients, even if the culture of the blood 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, it is heat-stable, one of its components is Vi-antigen;
• H-antigen has flagellar apparatus, thermolabile.

Differences in the structure of O-antigen allowed us to distinguish the serological groups of Salmonellae: A, B, C, D, E, etc. Based on the differences in the structure of the H-antigen, serological variants are established within each group. Among the serological diagnostic methods, until recently Vidal's reaction was widely used, and in recent years it has gradually lost its importance.

Based on the antigenic structure inherent in various types of salmonella, O- and H-monodiagnosticums have been developed, which allow to establish a serological version of salmonella. Initially, the serum was examined in the RPHA with a complex preparation of erythrocyte Salmonella Diagnosticum containing O-antigen. Further, in the presence of agglutination with complex diagnosticum, RPGA is given with preparations of groups A (1,2,3), B (1, 4, 12), C1 (6, 7), C2 (6, 8), D (1, 9, 12) and E (3, 10). In Table. 8-5 presents the antigenic characteristics of Salmonella, on the basis of which the serological variants of Salmonella are diagnosed.

Antigenic characteristics of salmonella

Group	Salmonella	Antigens
		Somatic - About	Flagellar - 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 choleraesuis	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, can give a nonspecific reaction with other infections; so its definition is not very useful for diagnosing salmonellosis.

Vi-antibodies in the infectious process do not impart diagnostic and prognostic value. The situation is different with the detection of Vi-antibodies in bacterial carriers. The large resistance of the salmonella-containing Vi-antigen to the human defense mechanisms results in a longer carrier of these forms (Vi-forms) of salmonellae, resulting in the detection of Vi-antibodies in the blood of such patients. Vi-antibodies are direct evidence of carriage.

Currently, to identify antibodies to Salmonella (to O-antigen), the most widely used are RPGA and ELISA, they are more sensitive than Vidal's and give positive results from the 5th day of the disease (Vidal's reaction on the 7th-8th day ). Antibodies in patients with typhoid fever, paratyphoid or other serological types of salmonella appear in the blood as early as the 4th day of the disease and increase sharply by the 8th-10th day. Their number is even more increased at the 2-3 th week of the disease. In adults and older children, RPHA provides confirmation of the diagnosis of salmonellosis in 80-95% of cases 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 salmonella diagnosis is negative throughout the disease. In the first months after recovery, the study of antibodies to salmonella can serve for the purposes of retrospective diagnosis. However, it is necessary to take into account individual deviations from the normal cycle of immunogenesis and the described dynamics of antibody titer. 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 may lead to a decrease in antibody titer or absence. Therefore, an antibody titer of less than 1: 200 does not allow the elimination of the disease, it is extremely important to investigate the antibody titer in dynamics - at the onset of the disease and 10-14 days later. Increasing the antibody titer 10-14 days at least 4 times when examining paired sera indicates an infectious process.

When using the Vidal's reaction, a titer of ≥ 1: 40 to ≥ 1: 160, depending on the geographical area and the laboratory, is considered diagnostic meaningful. When using the 1: 160 separation point for the diagnosis of infection, the sensitivity of the method is 46%, the specificity is 98%; 1:80 gives a sensitivity of 66%, specificity is 94%; at 1:40 the sensitivity is 90%, the specificity is 85%.

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