Can Borrelia infection activate microglia and nervous-system inflammation that may contribute to cognitive symptoms?

Neurologic Lyme borreliosis can involve intrathecal inflammation, making a biologically credible link between Borrelia exposure, glial immune activation, and neurologic symptoms. The evidence is strongest for inflammatory mechanisms; the link to measurable cognitive impairment is less direct.

Mechanistic and inflammatory evidence

• Experimental studies in primary murine, rhesus macaque, and human microglia show that Borrelia burgdorferi or its antigens activate innate immune programs. Reported responses include increased TLR2/CD14 signaling, NF-κB activation, and release of IL-6, TNF-α, PGE₂, IL-8, CCL2, CCL3, CCL4, and CCL5.
• TLR1/2 and TLR5-associated pathways have also been implicated. In human microglia, exposure increases pattern-recognition and phagocytosis-related programs; killed or disrupted organisms may elicit stronger inflammatory responses than live bacteria.
• In Lyme neuroborreliosis, cerebrospinal fluid (CSF) shows compartmentalized inflammatory mediator responses, including CXCL13, CXCL10, CXCL12, and CXCL8. These mediators generally decline following antibiotic treatment.

Cognitive implications

• In confirmed CNS involvement, glial activation, cytokine/chemokine signaling, immune-cell recruitment, and potential axonal injury provide a plausible pathway for subjective cognitive symptoms such as memory difficulty or “brain fog.”
• This pathway does not establish that neuroinflammation reliably causes objective cognitive impairment. A mouse model of prolonged intrathecal OspC exposure produced microglial activation and axonal injury without demonstrable cognitive impairment.
• Human findings are mixed: some prospective treated-neuroborreliosis cohorts found no reduction in attention or processing speed, while a smaller longer-term study found deficits in a subset.

Clinical interpretation

• Cognitive symptoms alone are not evidence of active neurologic borreliosis. Their attribution is most credible with compatible neurologic features and inflammatory CSF findings; otherwise, alternative neurologic, systemic, sleep-related, psychiatric, or medication-related causes remain important.

Bottom line

• Borrelia can activate microglia and nervous-system inflammatory signaling, and this offers a plausible pathway to cognitive symptoms in confirmed neurologic disease, but it does not by itself demonstrate a consistent causal basis for objective cognitive impairment.