Kinetics of IgM and IgG antibodies after scrub typhus infection

Background: Scrub typhus, a mite borne zoonotic bacterial infection endemic in Asia, is diagnosed commonly by serological tests using immunoglobulin (Ig) M & IgG antibodies against the causative agent. Significant antibody levels in the blood after an acute infection contributes to false positive results. Hence, we studied the kinetic profiles of IgM and IgG antibodies over the course of time by using ELISAs.

Methods: A cohort of confirmed scrub typhus patients using IgM ELISA with or without positive PCR in the past was followed up for repeat serum IgM & IgG levels. Serum samples taken at various times up to 3 years after infection were tested for IgG and IgM antibodies; dynamic serological changes in these antibodies are described. 

Results: Serum samples were analyzed from 210 scrub typhus patients (34% male and 66% female) with a mean age of 46 years. Blood samples were collected 2 – 40 months after confirmation of acute scrub typhus infection. 57% of all examined individuals with confirmed scrub typhus in the past had increased IgM antibody levels for 12 months after the infection. Increased levels of IgG antibody remained elevated in 66% until 40 months. IgG antibody in convalescent serum of patients revealed an increasing trend, peaking 6-12 months after onset of illness followed by a slow decline. IgG remained at a high level for a much longer period, serving as a good indicator for follow-up and for assessing past infection. 

Conclusion: Increased levels of IgM antibody for scrub typhus are serologic indicators of acute scrub typhus infection which remained elevated up to 12 months after the infection in majority. Elevated IgG levels are a good measure of past infection which remained elevated for more than 40 months.