Measles IgG Levels During True SSPE Latency
No, measles IgG is NOT highly elevated during the true latency period of SSPE—it only becomes elevated once the disease becomes clinically active, at which point the "latency" has ended. 1
Understanding the Immunologic Timeline
The critical distinction lies in understanding what "true latency" actually means in SSPE:
During Acute Measles Infection
- Measles IgM becomes detectable 1-2 days after rash onset, peaks at 7-10 days, and becomes completely undetectable within 30-60 days after the acute infection 1
- IgG antibodies develop normally during acute measles and persist at typical post-infection levels 2
During True Latency (2-10 Years Post-Infection)
- There is no systemic viremia and no active immune stimulation during this period 1
- The virus establishes persistent infection in CNS neurons, spreading trans-synaptically, but remains immunologically "silent" 1
- Measles IgG levels during this phase would be at normal post-infection or post-vaccination levels—not elevated 1
- IgM is completely absent during true latency, as it disappeared within 30-60 days of the original measles infection 1
Once SSPE Becomes Clinically Active (No Longer "Latent")
- Persistent measles-specific IgM reappears in both serum and CSF, often higher in CSF than serum 1, 3
- Measles IgG becomes markedly elevated in both serum and CSF 2
- The CSF/serum measles antibody index becomes ≥1.5, confirming intrathecal synthesis 1, 4
- This combination (persistent IgM + elevated IgG + elevated CSF/serum index ≥1.5) has 100% sensitivity and 93.3% specificity for SSPE diagnosis 1
Key Diagnostic Insight
The presence of persistently elevated measles IgM (which should have disappeared decades earlier) combined with extremely high IgG titers indicates ongoing immune stimulation from continuous CNS viral replication—this defines clinically active SSPE, not latency 1, 3
The persistently elevated IgG and reappearance of IgM reflect the fact that the virus is now actively replicating and releasing antigen in the CNS, preventing the normal shut-off of IgM synthesis that should have occurred years earlier 3
Clinical Implication
If you detect elevated measles IgG with persistent IgM in a patient years after measles infection, the disease is no longer in true latency—it has progressed to active SSPE 1, 3. True latency, by definition, is immunologically silent with no detectable serologic abnormalities beyond normal post-infection antibody levels.