Primary amoebic meningoencephalitis

Last revised by Dr Daniel J Bell on 13 Jun 2022

Primary amoebic meningoencephalitis, also known as naegleriasis, is a rare and usually fatal acute CNS infection caused by the free-living amoeba Naegleria fowleri.

This is a distinct clinical syndrome to granulomatous amoebic encephalitis, which is a subacute-chronic illness in immunocompromised patients with a highly variable presentation that is caused by Acanthamoeba spp. (main cause), Balamuthia mandrillaris and Sappinia pedata.

Primary amoebic meningoencephalitis has a very acute presentation, with fatality usually occurring within a week of symptom onset regardless of treatment status 1,2. It presents similarly to a severe and highly aggressive bacterial meningoencephalitis, with common clinical features including severe headache, high fever, nuchal rigidity, nausea, vomiting, photophobia, cranial nerve palsies, change in mental status, seizures, clinical signs of meningeal irritation and coma 1,5.

Characteristic lumbar puncture findings include 1:

  • neutrophilia
  • presence of red blood cells (especially as the disease progresses)
  • markedly low glucose levels
  • moderately high protein levels

Naegleria fowleri is most commonly found in fresh and warm water sources such as lakes and hot springs, and transmission occurs due to inhalation of infested water 3. Once in the brain, PAM results in extensive damage, characterized by a purulent exudate and necrosis of the brain parenchyma, ependyma, and meninges 2. The exudate usually contains a mixture of neutrophils and the amoeba itself 2

Imaging findings of primary amoebic meningoencephalitis are non-specific and rarely described in the literature, but are in keeping with a hemorrhagic meningoencephalitis and its associated complications 2,4,5:

The approach to treatment of naegleriasis is uncertain and based on case report-level evidence 2,3. A variety of antibacterial, antifungal and antiparasitic agents may be trialled in combination 2,3,5. Prognosis is often very poor even with treatment 1-3

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Cases and figures

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