A 24-year-old man is brought to the emergency department due to seizures. He has had 2 days of worsening fever, headache, and vomiting. Physical examination shows signs of meningeal irritation. The patient rapidly becomes comatose and dies 48 hours later despite aggressive medical care. Autopsy examination shows congested leptomeninges with fibrinopurulent exudate (not shown). Microscopy reveals numerous ameba in the exudate and brain tissue. Which of the following is the most likely portal of entry of this pathogen into the CNS?
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Primary amebic meningoencephalitis is a rare, highly fatal condition caused by the free-living, flagellated protozoan Naegleria fowleri. This pathogen resides in warm fresh water (30-45 C) and moist soil. Humans are primarily exposed when the organism comes into contact with nasal mucosa during recreational water activities (eg, swimming, diving, water skiing, unchlorinated hot tub bathing). Although serologic evidence of exposure is widespread, only a small number of individuals develop invasive disease (~300 known cases).
Patients typically have rapidly progressive meningeal signs, cranial nerve deficits, and confusion. Cerebrospinal fluid analysis with wet mount can usually identify motile trophozoites. Although treatment should be attempted, almost all patients die within 2-3 days.
(Choice A) Rabies migrates centrally through peripheral nerves to the spinal cord and then rapidly ascends to the brain. Patients with rabies usually have preceding history of animal bite; furthermore, rabies is a virus, not an amoeba, and it generally presents with several days of constitutional symptoms followed by fever, hydrophobia, and pharyngeal spasm.
(Choice B) Most bacterial and fungal causes of meningitis enter the brain via the hematologic dissemination (eg, basilar artery). Although meningeal signs, vomiting, and headache frequently occur, amoebas would not be seen in the cerebrospinal fluid.
(Choices C and D) The oculomotor and optic nerve do not generally transmit infections into the brain; the herpes virus can travel via the trigeminal nerve (not labeled) into the brain, leading to temporal lobe encephalitis.
Educational objective:
Primary amebic encephalitis occurs when the free-living, motile protozoan Naegleria fowleri penetrates the olfactory mucosa and migrates in a retrograde fashion through the olfactory nerve to the brain. This rare, deadly infection primarily occurs after exposure to warm, fresh water during recreational activities.