Encephalitis Etiology and Epidemiology

Introduction to Encephalitis Causes

The primary cause of encephalitis is viral infections (69% of confirmed cases), followed by bacterial (20%), prion diseases (7%), parasitic (3%), and fungal (1%) infections, as well as autoimmune causes which are increasingly recognized as frequent 1, 2

Viruses are the most frequently identified pathogens, particularly herpes simplex virus (HSV), West Nile virus, and enteroviruses 1, 2
HSV is the most commonly diagnosed viral cause in industrialized countries, with an annual incidence of 1 in 250,000 to 500,000 3, 4
Bacterial infections, especially by intracellular bacteria like Mycoplasma pneumoniae, play a role, although their direct involvement is sometimes disputed 1, 2
Parasites and fungi are less common but important to consider, especially in immunocompromised patients 3, 5

The reported global incidence of encephalitis varies by geographic location, study population, and differences in case definitions and research methods 3
In Western countries, the reported incidence ranges from 0.7 to 13.8 per 100,000 for all ages, approximately 0.7 to 12.6 per 100,000 in children, and 10.5 to 13.8 per 100,000 in adults 3, 4
The age-specific incidence of HSV encephalitis is bimodal, with peaks in young children and the elderly 3, 4

Identifying the specific etiologic agent is crucial for prognosis, potential prophylaxis, patient and family counseling, and public health interventions 1, 2
Aciclovir treatment has significantly improved outcomes for patients with HSV encephalitis, but delays in starting treatment, particularly beyond 48 hours after hospital admission, are associated with a poorer prognosis 3, 6
New technologies like next-generation sequencing (NGS) now enable the identification of organisms that would not have been suspected in the differential diagnosis 7
Autoimmune causes, such as anti-NMDA receptor encephalitis, are increasingly recognized, especially after viral infections like HSV encephalitis, with 41% of encephalitis cases in patients under 30 years old attributed to anti-NMDA encephalitis, surpassing the combined incidence of HSV, West Nile virus, and varicella-zoster virus encephalitis in this population 7
Autoimmune encephalitis can occur after herpetic encephalitis, with 24.5% of patients developing detectable anti-NMDA antibodies in the cerebrospinal fluid within 3 months following HSV infection 7

HSV ensefalitinin yaşa özgü insidansı özellikle bimodal dağılımı gösterir, küçük çocuklarda ve yaşlı popülasyonda görülür 8

ABD'de Powassan Virüs (PWV) ensefaliti insidansında artış gözlenmektedir, vaka fatalite oranı %10'dur 8
Chikungunya Virüs (ChikV) nörolojik tutulum nadir ancak ciddidir, nörolojik prezentasyonu olan hastalarda vaka fatalite oranı %10'dur 8
Primer Amebik Meningoensefalit (PAM) fulminan, neredeyse her zaman fatal bir enfeksiyondur, mortalite %95'i aşmaktadır 8

Vakaların %50'sinden fazlasında patojen tanımlanamadığı bildirilmektedir, standart vaka tanımlarının geliştirilmesi epidemiyolojik sürveyans için kritiktir 9, 10