GENERAL CHARACTERISTICS
There are both trophozoite and cyst stages in the life cycle, with the stage present primarily dependent on environmental conditions. Trophozoites can be found in water or moist soil and can be maintained in tissue culture or other artificial media. The amebae may enter the nasal cavity by inhalation or aspiration of water, dust, or aerosols containing the trophozoites or cysts. N. fowleri is incapable of survival in clean, chlorinated water. Fol lowing inhalation or aspiration, the organisms then penetrate the nasal mucosa, probably through phagocytosis of the olfactory epithelium cells, and migrate via the olfactory nerves to the brain.
The trophozoites can occur in two forms: ameboid and flagellate (Table 1, Figure 1). The size ranges from 7 to 35 µm. The diameter of the rounded forms is usually 15 µm. There is a large, central karyosome and no peripheral nuclear chromatin. The cytoplasm is some what granular and contains vacuoles. The ameboid form organisms change to the transient, pear-shaped flagellate form when they are transferred from culture or teased from tissue into water and maintained at a temperature of 27° to 37° C. These flagellate forms do not divide, but when the flagella are lost, the ameboid forms resume reproduction. Cysts are generally round, measuring from 7 to 15 µm with a thick double wall.
Table1. Free-Living Amebae Causing Disease in Humans
Fig1. Naegleria fowleri, Acanthamoeba spp. Diagram of trophozoites and cysts (upper row). Flagellate and cyst forms of Naegleria fowleri; (lower row) trophozoite and cyst of Acanthamoeba spp. (Illustration by Sharon Belkin; from Garcia LS: Diagnostic medical parasitology, ed 5, Washington, DC, 2007, ASM Press.)
PATHOGENESIS AND SPECTRUM OF DISEASE
Primary amebic meningoencephalitis (PAM) caused by N. fowleri is an acute, suppurative infection of the brain and meninges (Figure 2). With extremely rare exceptions, the disease is rapidly fatal in humans. The period between organism contact and onset of symptoms such as fever, headache, and rhinitis varies from a few days to 2 weeks. Early symptoms include vague upper respiratory tract distress, headache, lethargy, and occasionally olfactory problems. The acute phase includes sore throat; a stuffy, blocked, or discharging nose; and severe head ache. Progressive symptoms include pyrexia, vomiting, and stiffness of the neck. Mental confusion and coma usually occur approximately 3 to 5 days before death, which is usually caused by cardiorespiratory arrest and pulmonary edema.
Fig2. Naegleria fowleri in brain tissue. Hematoxylin and eosin stain. Note the large karyosome.
PAM resembles acute bacterial meningitis, and these conditions may be difficult to differentiate. Unfortunately, if the CSF Gram stain is interpreted incorrectly as a false positive, the resulting antibacterial therapy has no impact on the amebae and the patient will usually die within a few days.
LABORATORY DIAGNOSIS
Routine Methods
Clinical and laboratory data usually cannot be used to differentiate pyogenic meningitis from PAM. A high index of suspicion is often critical for early diagnosis. Most cases are associated with exposure to contaminated water through swimming or bathing. There is normally an incubation period of 1 day to 2 weeks, and then a course of 3 to 6 days, most often ending in death.
Analysis of the CSF will show decreased glucose and increased protein concentrations. The leukocyte count will range from several hundred to >20,000 cells per mm3. Although Gram stains and bacterial cultures of CSF will be negative, serious patient complications can occur as the result of incorrect therapy if false-positive Gram stains are reported.
A confirmed diagnosis is made by the identification of amebae in the CSF or in biopsy specimens. CSF should be placed on a slide, under a cover slip, and observed for motile trophozoites; smears can be stained with any of the blood stains. It is important not to mistake leukocytes for actual organisms or vice versa. This type of misidentification often occurs when using a counting chamber and the amebae sink to the bottom and round up, hence the recommendation to use just a regular slide and cover slip. Depending on the temperature and lag time between specimen collection and examination, motility may vary. Slides may be warmed slightly to improve motility. The most important differential characteristic is the spherical nucleus with a large karyosome.
Specimens should never be refrigerated before examination, and CSF should be centrifuged at a slow speed (250× g). If N. fowleri is the causative agent, only trophozoites are normally seen, whereas cysts and trophozoites can be seen with Acanthamoeba spp.
Other Methods
Most cases are diagnosed at autopsy; confirmation of tissue findings must include culture and/or special staining with monoclonal reagents in indirect fluorescent antibody procedures. Organisms can be cultured on non-nutrient agar plated with Escherichia coli. In tissue, the amebae can be identified using indirect immunofluorescence and immunoperoxidase techniques.
THERAPY
Although many antimicrobial and antiparasitic drugs have been screened for activity against N. fowleri, only a few patients have recovered after receiving intrathecal and intravenous injections of amphotericin B or in com bination with miconazole. Unfortunately, delay in diagnosis and the fulminant nature of PAM result in few survivors.
