Primary amoebic meningoencephalitis

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Primary amoebic meningoencephalitis (PAM, or PAME) is a disease of the central nervous system caused by infection from the Percolozoid amoeba Naegleria fowleri. The organism propagates in warm, stagnant bodies of freshwater (typically during the summer months), and enters the central nervous system after insufflation of infected water by attaching itself to the olfactory nerve. It then migrates through the cribiform plate and into the olfactory bulbs of the forebrain, where it multiplies itself greatly by feeding on nerve tissue. During this stage, occurring approximately 3-7 days post-infection, the typical symptoms are parosmia, rapidly progressing to anosmia (with resultant ageusia) as the nerve cells of the olfactory bulbs are consumed and replaced with necrotic lesions.

After the organisms have multiplied and largely consumed the olfactory bulbs, the infection rapidly spreads through the mitral cell axons to the rest of the cerebrum, resulting in onset of frank encephalitic symptoms, including cephalgia (headache), nausea, and rigidity of the neck muscles, progressing to vomiting, delirium, seizures, and eventually irreversible coma. Death usually occurs within 14 days of exposure as a result of respiratory failure.

The disease is both exceptionally rare and exceptionally lethal: there have been less than 200 confirmed cases in recorded medical history, with an in-hospital case fatality rate of ~97% (3% patient survival rate).

This extreme morbidity is largely blamed on the unusually non-suggestive symptomology of the early-stage disease compounded by the necessity of microbial culture of the cerebrospinal fluid to effect a positive diagnosis. The amoeba also demonstrates a particularly rapid late-stage propagation through the nerves of the olfactory system to many parts of the brain simultaneously (including the vulnerable medulla).

Cause

Naegleria fowleri is an amoeba that is ubiquitous in soils and warm waters. Infection typically occurs during the summer months and patients typically have a history of exposure to a natural body of water. The organism specifically prefers temperatures above 32°C. The organism is extremely sensitive to chlorine (<0.5ppm). Exposure to the organism is extremely common due to its wide distribution in nature, but thus far lacks the ability to infect the body through any method other than direct contact with the olfactory nerve; the contaminated water must actually be fully insufflated into the sinus cavities for transmission to occur.

Michael Beach, a recreational waterborne illness specialist for the Centers for Disease Control and Prevention, stated in remarks to the Associated Press that the wearing of nose-clips to prevent insufflation of contaminated water would be an effective protection against contracting PAM, noting that "You'd have to have water going way up in your nose to begin with".^[1]

Treatment

The current standard treatment is prompt intravenous administration of heroic doses of Amphotericin B, a systemic antifungal which is one of the few effective treatments for systemic infections of Protozoan parasitic diseases (such as leishmaniasis and toxoplasmosis). The success rate in treating PAM is usually quite poor, since by the time of definitive diagnosis most patients have already manifested signs of terminal cerebral necrosis. Even if definitive diagnosis is effected early enough to allow for a course of medication, Amphotericin B also causes significant and permanent nephrotoxicity in the heroic doses necessary to quickly halt the progress of the amoebae through the brain.

See also

• Amoebic meningitis
• Free-living amebic infection
• Meningoencephalitis
• Waterborne diseases