Primary amebic encephalitis is an acute hemorrhagic meningoencephalitis caused by the free-living ameba Naegleria fowleri. Infection is more common in children and males, with a mean age of 12 years at presentation, and is more common in the summer months. Patients typically present with acute symptoms 1-12 days (median 5 days) after swimming or having another nasal exposure to water containing Naegleria.

Clinical symptoms at the time of presentation include high fever, severe headache, photophobia, nausea, vomiting, altered mental status, behavioral changes, and seizures. Physical examination reveals meningeal signs and cranial nerve palsies. Symptoms progress rapidly over days, leading to depressed mental status, intracranial hypertension, coma, herniation, and death.

Naegleria fowleri is found in warm freshwater sources (lakes, rivers, streams, hot springs, and ponds) but is not found in seawater. Very rarely, it may be found in swimming pools that are not adequately chlorinated. Infection has also very rarely occurred after nasal rinsing / use of neti pots with contaminated tap water (thus use of boiled or distilled water is recommended). Human transmission occurs through inhalation of infected water.

Amebae cause central nervous system (CNS) infection by penetrating the olfactory mucosa, crossing the cribriform plate, and entering the olfactory bulb. This disease is uniformly fatal, with a mortality rate of 99%. Death typically follows 1-18 days (median 5 days) after symptoms begin.

For detailed guidance from the Centers for Disease Control and Prevention (CDC), see Information for Public Health & Medical Professionals.