Encephalitis l: Introduction

Encephalitis is inflammation of the brain parenchyma, most often due to infections or autoimmune processes. It presents with neuropsychiatric features such as fever, altered mental status, behavioral changes, cognitive dysfunction, seizures, focal deficits, and sometimes autonomic instability. In some cases, the meninges are also involved, resulting in meningoencephalitis.

Infectious Causes

Infectious encephalitis is most commonly viral but can also result from bacterial, fungal, or parasitic infections.

The most common sporadic cause is herpes simplex virus type 1. Other viruses include Japanese encephalitis virus, West Nile virus, enteroviruses, varicella-zoster virus, and cytomegalovirus, especially in immunocompromised individuals.

Bacterial causes include Neisseria meningitidis, Streptococcus pneumoniae, Mycoplasma pneumoniae, and Mycobacterium tuberculosis, often via direct invasion or immune-mediated responses.

Fungal pathogens such as Cryptococcus neoformans and Histoplasma capsulatum, as well as parasitic infections such as Toxoplasma gondii, are important in immunocompromised hosts. Interestingly, Plasmodium falciparum causes cerebral malaria, which mimics encephalitis. Additionally, arboviruses, transmitted by mosquitoes or ticks, such as West Nile and Japanese encephalitis virus, cause arboviral encephalitides. Arboviral encephalitis often emerges in outbreaks or seasonally, with higher incidence during warmer months.

Autoimmune Causes

Autoimmune encephalitis may be paraneoplastic, postinfectious, or idiopathic, mediated by autoantibodies against CNS components. In younger patients, anti-NMDAR encephalitis, often linked to ovarian teratomas, is common, while older adults more frequently have LGI1-associated limbic encephalitis. Antibodies can be detected in CSF or serum.

Epidemiologic Considerations

The incidence of viral encephalitis is relatively low. However, the risk is highest among children, older adults, and immunocompromised individuals—such as those receiving chemotherapy, undergoing organ transplantation, or living with conditions like HIV/AIDS.