Antibodies Interpreted after Scrub Typhus Infection

Scrub typhus is a zoonotic infection caused by the bacterium Orientia tsutsugamushi. It is transmitted to humans by the bite of the larval stage of trombiculid mites. With an annual incidence of about one million cases, it is an important challenge to public health in Southeast Asia.

The clinical presentation may vary from a mild febrile illness to a life-threatening disseminated infection with serious complications such as acute respiratory distress syndrome, hepatitis, renal failure, meningitis, and multiple organ failure, making the diagnosis difficult. The serological detection of IgM antibodies is the most widely used test to diagnose scrub typhus infection.

Scientists at the Christian Medical College (Vellore, India) included in this cross-sectional study, 203 patients over 18 years of age with previously confirmed scrub typhus. Scrub typhus was confirmed by positive immunoglobulin M (IgM) enzyme-linked immunosorbent assay (ELISA) (optical density (OD) >0.8) and/or a positive polymerase chain reaction (PCR) for O. tsutsugamushi at a tertiary care teaching hospital in South India between December 2011 and March 2015.

The team performed ELISAs for IgM and IgG antibodies using the Scrub Typhus Detect kit (InBios International, Inc, Seattle, WA, USA). All patients had a positive IgM ELISA at diagnosis, of which 80 were confirmed by PCR. All patients had a blood sampling done during the study. Among them, eight patients had two blood samples taken at different time points. The time interval between confirmation of the original infection and study sample collection ranged from one month to 46 months. The overall trend showed a gradual fall in mean IgM levels over time, remaining above the threshold of positivity (OD >0.8) until 12 months after infection. The mean IgG levels showed a gradual rise, reaching a peak at 10 months post-infection, followed by a gradual decline, remaining above the cut-off threshold for more than 36 months.

The authors concluded that clinicians needed to be cautious in using a single serum sample for the detection of IgM to diagnose scrub typhus, as it remains elevated for up to 12 months after the infection, whereas the serum IgG level could be used as an indicator of past infection. The study was published in the June 2018 issue of the International Journal of Infectious Diseases.

Related Links:
Christian Medical College
InBios International