Human Herpesvirus Type 1 (HSV-1) is the Most Common Cause of Sporadic Infectious Encephalitis
The answer is A) Human herpesvirus type 1 (HSV-1), which accounts for 25%–10% of all identified cases and represents the most common cause of sporadic infectious encephalitis worldwide. 1
Why HSV-1 is the Answer
HSV-1 is explicitly identified in authoritative guidelines as "one of the most common causes of identified sporadic encephalitis worldwide" 1. This designation is critical because the question specifically asks about sporadic encephalitis—meaning non-epidemic, individual cases occurring without geographic or temporal clustering.
Key Distinguishing Features:
- HSV-1 affects all age groups and occurs year-round without seasonal variation 1
- It is more common in adults, whereas HSV-2 primarily causes neonatal encephalitis 1
- Recent evidence continues to confirm HSV as "the most common cause of sporadic viral encephalitis" 2, 3
Why the Other Options Are Incorrect
HSV-2 (Option B): While HSV-2 can cause encephalitis, it predominantly affects neonates rather than causing sporadic cases across all age groups 1
West Nile Virus (Option C): This is an arbovirus that causes epidemic rather than sporadic encephalitis, with geographic and seasonal patterns tied to mosquito vectors 4. In endemic regions it contributes significantly, but it does not represent the most common sporadic cause globally 3
Yellow Fever Virus (Option D): This is geographically restricted to endemic areas and is vaccine-preventable, not a common cause of sporadic encephalitis
Rabies Virus (Option E): While universally fatal if untreated, rabies is rare due to post-exposure prophylaxis and does not represent a common cause of encephalitis cases
Clinical Significance for Mortality and Morbidity
HSV-1 encephalitis carries severe consequences if untreated, making early recognition critical:
- Even with optimal acyclovir treatment, approximately 50% of patients experience significant neurological sequelae 5
- Nearly 30% of patients develop neurological relapses after the first episode 2
- The condition is associated with high mortality and severe morbidity 2, 6
Diagnostic Approach:
The gold standard is CSF PCR for HSV DNA with sensitivity and specificity of 95% and 99%, respectively 1. However, a critical pitfall exists: PCR can be falsely negative early in disease or in children 4. If the first lumbar puncture is negative but HSE remains suspected (particularly with temporal lobe involvement on MRI), repeat lumbar puncture within 3–7 days 4.
Characteristic MRI Findings:
MRI reveals temporal and/or inferior frontal lobe edema with high signal intensity on FLAIR and T2-weighted images; bilateral temporal lobe involvement is nearly pathognomonic 1
Treatment Priority:
Immediate empiric intravenous acyclovir should be initiated when HSE is suspected, as treatment delays directly impact mortality and quality of life outcomes 2, 7