脳炎L:はじめに

Encephalitis is inflammation of the brain parenchyma caused primarily by viral infections or autoimmune mechanisms. Herpes simplex virus type 1 is the leading cause of sporadic viral encephalitis, while arboviruses like West Nile virus cause seasonal outbreaks. Autoimmune encephalitis develops through postinfectious or idiopathic pathways, often involving antibodies targeting neuronal proteins.

Immunocompromised patients are susceptible to opportunistic pathogens including fungi such as Cryptococcus neoformans and Histoplasma capsulatum, as well as parasites like Toxoplasma gondii. Cytomegalovirus and other viruses also commonly affect this population. These infections often present as meningoencephalitis, involving both brain and meningeal inflammation.

Encephalitis presents with fever, headache, neck stiffness, cognitive decline, and seizures. Neuropsychiatric features include altered mental status, behavioral changes, and sometimes autonomic instability. Specific signs may indicate particular causes, such as vesicular skin lesions in herpes zoster encephalitis or lymphadenopathy in Epstein-Barr virus infection.

Arboviruses are transmitted by mosquitoes or ticks and include West Nile virus and Japanese encephalitis virus. These viruses cause arboviral encephalitis that often emerges in outbreaks or seasonally, with higher incidence during warmer months. Arboviral infections represent an important public health concern in endemic regions.

Autoimmune encephalitis is mediated by autoantibodies against central nervous system components. Anti-NMDAR encephalitis, often linked to ovarian teratomas, is common in younger patients, while older adults frequently develop LGI1-associated limbic encephalitis. Antibodies can be detected in cerebrospinal fluid or serum for diagnostic confirmation.

The highest risk groups include children, older adults, and immunocompromised individuals such as those receiving chemotherapy, undergoing organ transplantation, or living with HIV/AIDS. Although viral encephalitis incidence is relatively low overall, these vulnerable populations experience significantly elevated rates of infection and severe disease.

Bacterial encephalitis results from pathogens like Neisseria meningitidis, Streptococcus pneumoniae, and Mycobacterium tuberculosis, often via direct invasion or immune-mediated responses. While viral encephalitis is more common overall, bacterial infections can cause severe complications including increased intracranial pressure and require different treatment approaches than viral forms.