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Tularemia: antibodies to the causative agent of tularemia in the blood
Medical expert of the article
Last reviewed: 18.10.2021
Antibodies to the causative agent of tularemia in the blood serum are normally absent.
Tularemia - the primary disease of animals (rodents), in humans occurs as an acute infectious disease with a diverse clinical picture. Pathogen - Francisella tularensis, coccoid or ellipsoidal polymorphic rods, Gram-negative. The causative agent of tularemia is an intracellular parasite, in the S-form it has two antigens - O and Vi (capsular antigen). In connection with the polymorphic clinical picture of tularemia, serological responses are crucial in its diagnosis (excretion from a sick person is carried out only in specialized laboratories for especially dangerous infections).
For the diagnosis of tularemia, the agglutination test (in test tubes and microagglutination) and ELISA are used. When using the agglutination reaction, antibodies are detected from the 2nd week after the onset of the clinical picture of the disease. The diagnostic titer is 1: 160 and higher with agglutination in test tubes, 1: 128 and above - with microagglutination, in case of anamnesis and clinical picture of the disease. Elevated antibody titer 2 weeks after the onset of infection can be detected in 89-95,4% of patients. The agglutination reaction can give a cross reaction with brucellosis antibodies, however, the titer is usually not more than 1:20.
On the 3-5th day of the disease, an intradermal allergic test with tularin can be used for diagnosis (0.1 ml is injected intradermally into the middle third of the forearm). The reaction is monitored after 24-48 hours. The cutaneous test is considered positive in the presence of hyperemia and infiltration.
ELISA is a more sensitive and specific method of diagnosing tularemia, it allows detecting antibodies of classes IgA, IgM and IgG. Detection of IgM antibodies or a 4-fold increase in the IgG titer confirms an acute infection or reinfection with an appropriate clinical picture of the disease. Evaluation of the results of the detection of IgM antibodies in endemic areas by tularemia should be carried out more cautiously. IgM antibodies disappear within a few months after successful treatment (they last no more than 1 year), IgG persist for life. The ELISA method does not allow the differentiation of the serotype A and B of Francisella tularensis, since it uses a recombinant antigen for both serotypes. However, the ELISA method does not respond to antibodies to other Francisella species .