What is the most common cause of viral encephalitis?

Herpes Simplex Virus is the Most Common Cause of Viral Encephalitis

Herpes simplex virus (HSV) is definitively the most common cause of viral encephalitis in industrialized nations, not varicella zoster virus. 1

Epidemiological Evidence

In western countries, the epidemiological data clearly demonstrates that HSV is the predominant etiological agent in viral encephalitis:

• HSV is the most commonly diagnosed viral cause of encephalitis in industrialized nations 1
• The annual incidence of HSV encephalitis is approximately 1 in 250,000 to 500,000 people 1
• The age-specific incidence is bimodal, with peaks in childhood and the elderly 1
• Most HSV encephalitis is caused by HSV type 1 (approximately 90%), while HSV type 2 accounts for about 10% of cases 1

Comparison with Varicella Zoster Virus

While varicella zoster virus (VZV) is indeed a cause of viral encephalitis, it is not the most common:

• VZV is described as "a relatively common cause" of viral encephalitis, especially in immunocompromised patients 1
• VZV ranks below HSV in frequency of diagnosis in immunocompetent patients 1
• VZV encephalitis often presents as a multifocal leukoencephalopathy, particularly in immunocompromised patients 1

Clinical Significance

Understanding that HSV is the most common cause of viral encephalitis has important clinical implications:

• Early diagnosis and treatment of HSV encephalitis with acyclovir dramatically decreases morbidity and mortality 2
• Delays in treatment, particularly beyond 48 hours after hospital admission, are associated with worse outcomes 1
• Without antiviral treatment, HSV encephalitis has a mortality rate greater than 70% 3
• Even with appropriate acyclovir treatment, approximately 40% of patients will survive with normal development or minor impairment, but more than half will die or suffer significant impairment 3

Diagnostic Approach

When viral encephalitis is suspected:

1. Perform neuroimaging (preferably MRI) - more than 90% of patients with HSV encephalitis will have MRI abnormalities involving the temporal lobes 2
2. Conduct lumbar puncture for CSF analysis
3. Test CSF with polymerase chain reaction (PCR) for HSV, which has become the diagnostic procedure of choice 2
4. Consider that false-negative CSF HSV PCR results can occur within the first 72 hours of illness 2

Treatment Considerations

Immediate antiviral therapy with acyclovir is essential when HSV encephalitis is suspected:

• For adults with HSV encephalitis, acyclovir 10 mg/kg every 8 hours for 10 days is the standard treatment 4
• Patients who are young and have not reached coma or impaired consciousness may show 65% recovery and return to normal function 3
• Acyclovir has been shown to be superior to vidarabine in controlled trials 4

In conclusion, while both HSV and VZV can cause viral encephalitis, the evidence clearly establishes HSV as the most common viral cause of encephalitis in industrialized nations.