Kinetics of the serological response up to one year after tularemia

Abstract

Serological analysis is the predominant method used to diagnose tularemia, a zoonotic disease caused by the highly virulent bacterium F. tularensis. We determined F. tularensis-specific IgM and IgG antibody titers by an LPS-based ELISA assay on five occasions one to twelve months after onset of ulceroglandular tularemia in 19 individuals. Peak IgM antibody titers were observed at the one-month time point and peak IgG antibody titers at the two-month time point. Both IgG and IgM antibody levels declined linearly thereafter with rather similar kinetics. Compared to the average one-month antibody titers, average IgG titers were not significantly lower before the 12-month time point and IgM titers before the 4-month time point. All, but one average titer, were significantly increased compared to the cut-off of the assay. Average IgG and IgM titers were significantly lower for the group = 69 years old compared to the group < 69 years. Collectively, the data demonstrate a persistence of F. tularensis-specific IgM and IgG antibody titers for at least 12 months after ulceroglandular tularemia. Thus, low, but significantly elevated F. tularensis-specific antibody titers are of limited diagnostic value since they are not indicative of ongoing tularemia.

Keywords: elderly; kinetics; one year; serological response; tularemia.

Figure 1

IgM (A) and IgG (B) serum titers from 19 individuals diagnosed with ulceroglandular tularemia. The dotted line indicates the cut-off value of 200. Stars indicate significant differences compared to the one-month value (*P < 0.05, ***P < 0.001).

Figure 2

Age-dependent IgM and IgG serum titers from 19 individuals diagnosed with ulceroglandular tularemia. Diagram (A, B) show individual IgM and IgG titers respectively. Diagram (C) and (D) show mean values ± SEM of IgM and IgG, respectively, in individuals < 69 years (n = 10) and ≥ 69 years (n = 9) at a given time point. Stars indicate significant differences between the two age groups at a given time point (*p < 0.05, ***p < 0.001).