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[Ann Emerg Med. 2009;54:471-472.]

This MMWR report provides an important alert to emergency physicians about a rare but fatal waterborne illness¹ affecting the central nervous system. PAM was first described in 1965 by Fowler and Carter,² and since then nearly 300 cases have been reported worldwide. PAM presents a diagnostic challenge; presenting symptoms range from a mild febrile illness to frank meningitis. Victims are typically young, previously healthy, and immunocompetent. Diagnosing this rare infection early enough to give effective amebicidal therapy has proven difficult.

Infection tends to occur in warm summer months after individuals are exposed to contaminated water in which they submerge their heads or inhale the water. The latent period between exposure to contaminated water and the onset of symptoms is not clearly defined, but data from case reports suggest an incubation time of 2 to 7 days before the onset of symptoms.

Patients seeking medical attention are often initially thought to have a viral illness.³ However, the progression of PAM is swift and unrelenting, and patients initially discharged from medical care returned in extremis. More advanced presentations of PAM include high fever, tachycardia, meningismus, seizure, lethargy, or agitation.

There are no prospective trials to validate treatment for PAM, mainly because of its very low incidence and difficulty of diagnosis. However, a number of in vitro and animal studies suggest that amphotericin B may be effective in the treatment of PAM. Other drugs that may be effective include azithromycin, rifampin, and azole antifungal drugs. Sadly, it is very difficult to implement treatment early in the course of PAM because of its rapidly fatal course, which leaves clinicians little time to make the diagnosis.

Because of the rarity of this illness, clinicians generally consider alternative diagnoses. Early diagnosis is possible with a high index of suspicion and early examination of cerebral spinal fluid wet mount. PAM should be considered in patients presenting with signs of meningitis, who have a history of recent fresh water exposure, and in whom no organisms are visualized on CSF Gram's stain. In such patients, clinicians should request that the laboratory examine a wet mount of freshly centrifuged CSF for motile amebae. Although other diagnostic tests such as fluorescent antibody staining or PCR are available, they will not be done in the period of an ED evaluation. Amphotericin B can be initiated if amebae are visualized. Because of PAM's rarity, treatment for other forms of meningitis or encephalitis that are far more common should not be delayed.

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References 

1. Jain R, Prabhakar S, Modi M, et al. Naegleria meningitis: a rare survival. Neurol India. 2002;50;:470–472
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2. Fowler M, Carter RF. Acute pyogenic meningitis probably due to Acanthamoeba sp: a preliminary report. Br Med J. 1965;25:740–742734-732
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3. Bond B. Primary amoebic meningoencephalitis presenting without fever. N Zealand Med J. 2004;117:U783
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