All iLive content is medically reviewed or fact checked to ensure as much factual accuracy as possible.
We have strict sourcing guidelines and only link to reputable media sites, academic research institutions and, whenever possible, medically peer reviewed studies. Note that the numbers in parentheses ([1], [2], etc.) are clickable links to these studies.
If you feel that any of our content is inaccurate, out-of-date, or otherwise questionable, please select it and press Ctrl + Enter.
Tularemia: antibodies to the tularemia pathogen in the blood
Medical expert of the article
Last reviewed: 04.07.2025
Antibodies to the causative agent of tularemia are normally absent in the blood serum.
Tularemia is a primary disease of animals (rodents), in humans it occurs as an acute infectious disease with a variety of clinical pictures. The causative agent is Francisella tularensis, coccoid or ellipsoid 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). Due to the polymorphic clinical picture of tularemia, serological reactions are of decisive importance in its diagnosis (isolation of the pathogen from a sick person is carried out only in specialized laboratories for especially dangerous infections).
To diagnose tularemia, an agglutination reaction (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. A titer of 1:160 or higher for agglutination in test tubes, 1:128 or higher for microagglutination, in the presence of anamnesis and a clinical picture of the disease, is considered diagnostic. An increased 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, but the titer is usually no more than 1:20.
On the 3rd-5th day of the disease, an intradermal allergy test with tularin can be used for diagnosis (0.1 ml is injected intradermally into the middle third of the forearm). The reaction is recorded after 24-48 hours. The skin test is considered positive in the presence of hyperemia and infiltrate.
ELISA is a more sensitive and specific method for diagnosing tularemia, allowing detection of IgA, IgM and IgG antibodies. Detection of IgM antibodies or a 4-fold increase in IgG titer confirms acute infection or reinfection in the presence of the corresponding clinical picture of the disease. Evaluation of the results of determining IgM antibodies in endemic areas for tularemia should be carried out more carefully. IgM antibodies disappear within a few months after successful treatment (they persist for no more than 1 year), IgG persist for life. ELISA does not allow differentiation of serotypes A and B of Francisella tularensis, since it uses a recombinant antigen for both serotypes. However, ELISA does not give a reaction with antibodies to other species of Francisella.