DIAGNOSIS

Since infection with N. Fowleri often results in death in a matter of days, early diagnosis is crucial. Most cases are not detected until autopsy. Clinical diagnosis of primary amebic meningoencephalitis (PAM) is difficult because the disease is so rare, many physicians are unfamiliar with it. In addition, clinical signs are difficult to distinguish from other forms of bacterial and viral meningitis. It has often been misdiagnosed as acute bacterial meningitis.

When should you suspect primary amebic meningoencephalitis (PAM)?

PAM should be suspected if the individual has:

• clinical signs of bacterial meningitis

  • fever
  • headache
  • nausea
  • vomiting
  • stiff neck due to inflammation of membranes covering brain and spinal cord
• recent history of swimming in warm, fresh water

How can PAM be diagnosed?

The primary method of diagnosis is observation of active or stained trophozites in the cerebral spinal fluid (CSF). If infection is suspected, the physician should order a spinal tap and request a laboratory examination of the CSF. In addition to a routine CSF workup, the physician should request a wet mount exam of a sample of the patient's CSF. This is to detect active trophozites, 8-20 micrometers in size with a central nucleus and a large karyosome that stains red with trichrome. Ocassionally, the pear-shaped flagellate form can also be detected. Cysts are generally not observed because the patient dies before the trophozites can encyst. CSF examination is characterized by the following:

• mobile trophozites in wet mount exam (most important sign)
• CSF Gram stain is negative for bacteria
• normal or slightly reduced glucose levels
• CSF protein levels increased
• CSF white blood cell levels increased (400-26,000)
• CSF red blood cell levels increased

N. fowleri can be difficult to distinguish from Acanthamoeba. A distinctive characteristic of N. fowleri is that it can convert into a pear-shaped flagellate form. This can be seen by suspending the trophozites in distilled water, which causes them to change into the flagellate form.

Alternative methods of diagnosis

Other methods of diagnosis include indirect hemagglutination, ELISA, and indirect /direct immunofluorescence. Serologic tests are generally not a good method of diagnosis due to the widespread existence of N. fowleri antibodies without manifestation of disease.