Do persistent IgG antibodies to Borrelia, Ehrlichia, or Rickettsia indicate active infection?

In an 83-year-old man, positive serology for tick-borne pathogens should be interpreted in the context of symptoms, timing, exposure history, and confirmatory testing rather than treated as evidence of ongoing infection alone.

Persistence and clinical interpretation

• Borrelia burgdorferi IgG can remain detectable for 10–20 years or longer after treated Lyme borreliosis, including in people without evidence of viable organisms.
• Ehrlichia chaffeensis IgG may persist for months to years, often declining substantially during the first year.
• For Rickettsia typhi, longitudinal IFA data estimated a median IgG half-life of 177 days, with marked individual variation; CDC notes antibodies may remain elevated for months to years.
• Thus, an isolated IgG-positive result primarily documents prior antigen exposure and durable humoral immune memory. It does not determine when exposure occurred or, without other evidence, establish that infection is active.

Diagnostic implications

• A single rickettsial IgG titer—even a reciprocal titer ≥1:64—is supportive rather than confirmatory. In a clinically compatible illness, paired acute and convalescent specimens obtained about 2–4 weeks apart showing a ≥4-fold IgG rise provide stronger confirmation of recent infection.
• The same principle applies to ehrlichiosis: a single acute IgG result is presumptive, whereas seroconversion or a fourfold rise on paired testing is more informative.
• Direct pathogen detection can address active infection more directly: detection of R. typhi DNA by PCR supports active disease, although a negative PCR does not exclude it, particularly depending on specimen timing and doxycycline exposure. Rickettsial serology may also cross-react across antigen groups.

Bottom line

• Persistent IgG to Borrelia, Ehrlichia, or Rickettsia is compatible with remote exposure and immune memory; management should rest on compatible current illness plus temporal serology and/or appropriate direct detection, not isolated IgG reactivity.