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Encephalitis is inflammation of the brain, with variable severity. Symptoms often include reduction or alteration in consciousness, headache, fever, confusion, stiff neck, and vomiting. Complications may involve seizures, hallucinations, trouble speaking, memory issues, and hearing problems.

MRI scan image shows high signal in the temporal lobes and right inferior frontal gyrus in someone with herpes simplex encephalitis.
MRI scan image shows high signal in the temporal lobes and right inferior frontal gyrus in someone with herpes simplex encephalitis.


The aetiology of encephalitis includes infections by viruses such as herpes simplex virus, rabies virus, and bacteria, fungi, or parasites. Autoimmune diseases and certain medications can also be causes. Notably, 30%-40% of encephalitis cases have an unknown cause.

Viral Causes

Viral infections are the usual culprits, either as a direct effect of an acute infection or as sequelae of a latent infection. Common identifiable viral causes include herpes simplex, rabies, poliovirus, and measles virus. Arboviral flavivirus (e.g., West Nile virus), bunyavirus (La Crosse strain), reovirus (Colorado tick virus), and henipavirus infections are additional possibilities. The Powassan virus is a rare cause.

Bacterial and Other Causes

Bacterial infections such as bacterial meningitis or complications from syphilis can lead to encephalitis. Parasitic infestations like toxoplasmosis, malaria, or primary amoebic meningoencephalitis can cause encephalitis in immunocompromised individuals. Lyme disease and Bartonella henselae are other potential bacterial pathogens.

Limbic and Autoimmune Encephalitis

Limbic encephalitis affects the limbic system, presenting with disorientation, memory loss, and behavioural anomalies. Autoimmune encephalitis, such as anti-NMDA receptor encephalitis, can include catatonia, psychosis, and abnormal movements.

Encephalitis Lethargica

Characterised by high fever, headache, and lethargy, encephalitis lethargica's cause is unknown. An epidemic from 1917 to 1928 affected people globally.

Rabies virus
Rabies virus

Signs and Symptoms

Adults typically present with fever, headache, confusion, and sometimes seizures. Children may show irritability and poor appetite. Neurological exams often reveal drowsiness or confusion and a stiff neck, indicative of meningitis or meningoencephalitis.


Diagnosis involves identifying decreased or altered consciousness, lethargy, or personality change lasting at least 24 hours without another explainable cause. Diagnostic methods include:

  • Brain Scan (MRI): Determines inflammation and differentiates from other causes.
  • EEG: Monitors brain activity, showing abnormal signals in encephalitis.
  • Lumbar Puncture (Spinal Tap): Tests cerebrospinal fluid for signs of infection.
  • Blood and Urine Tests
  • PCR Testing of Cerebrospinal Fluid: Detects viral DNA.
Spinal tap on a newborn
Spinal tap on a newborn


Treatment is based on supportive care and includes:

  • Antiviral Medications: For viral causes.
  • Antibiotics: For bacterial causes.
  • Steroids: To reduce brain swelling.
  • Sedatives: To manage restlessness.
  • Acetaminophen: For fever.
  • Occupational and Physical Therapy: Post-infection rehabilitation.

Pyrimethamine-based maintenance therapy is used for toxoplasmic encephalitis, particularly in immunocompromised individuals. HAART therapy decreases the relapse rate in HIV patients. The effectiveness of intravenous immunoglobulin for childhood encephalitis remains unclear, though it may benefit certain forms.


Poor prognostic factors include cerebral oedema, status epilepticus, and thrombocytopenia. Conversely, a normal encephalogram early in diagnosis suggests a higher survival rate.


The incidence is about 7.4 cases per 100,000 people per year in Western countries and 6.34 per 100,000 in tropical regions. Approximately 250,000 cases occur annually in the US. In 2015, encephalitis affected 4.3 million people and resulted in 150,000 deaths globally. Herpes simplex encephalitis has an incidence of 2–4 per million per year.

Encephalitis deaths per million persons in 2012
Encephalitis deaths per million persons in 2012

Self-assessment MCQs (single best answer)

What is the primary characteristic of encephalitis?

Which of the following is NOT a common symptom of encephalitis?

What type of infection is most commonly associated with encephalitis?

Of these, which virus is the rarest cause of encephalitis?

Limbic encephalitis primarily affects which part of the brain?

What diagnostic method is used to test cerebrospinal fluid for signs of infection in encephalitis?

What is the mainstay of treatment for viral encephalitis?

What is a poor prognostic factor for encephalitis?

Approximately how many cases of encephalitis occur annually in the United States?

Which demographic is more likely to show irritability and poor appetite as symptoms of encephalitis?


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