N. fowleri occurs worldwide and has been isolated from fresh water, thermal discharge of power plants, improperly disinfected heated swimming pools, hot springs, hydrotherapy pools, aquaria, sewage, and even nasal passages and throats of healthy individuals. N. fowleri can tolerate high temperatures, even up to 45° C. Therefore these amebae proliferate during summer months when the ambient temperature is likely to be high. Environmental sampling done in warm water lakes suggests that the amebae likely inhabit most warm fresh water habitats in southern-tier states in the United States. Typically, cases of PAM occur in the hot summer months when large numbers of people engage in aquatic activities in lakes, ponds, swimming pools, and other warm freshwater bodies that may harbor these amebae (Figure 73-2). Persons participating in aquatic activities, including swimming, diving, and water skiing, might come in contact with the amebae in water, resulting in infection. The portal of entry into the CNS is the olfactory neuroepithelium. The amebae that enter the nasal passages pass through the sievelike cribriform plate of the ethmoid bone, penetrate into the subarachnoid space, and enter the brain parenchyma. The incubation period from exposure to the ameba to disease may vary from 1 to 7 days (average 5 days), depending on the inoculum size. The disease progresses rapidly and leads to death within a week of symptom onset. PAM has also been diagnosed in the South American tapir and domestic cattle.

From 1962 to 2008, 111 PAM cases were reported in the United States (Figure 73-3). An additional 11 cases dating back to 1937 were identified through a retrospective examination of autopsy samples in Virginia. The median age of the 111 cases was 12 years (range 8 months to 66 years). Among the 111 cases, 88 (79.3%) involved male patients. Exposure occurred primarily in untreated, warm freshwater lakes or rivers in 15 warm-weather southern-tier states (Figure 73-4). Among the 100 cases for which the month of exposure was known, 87 (87%) occurred during July through September. Case patients were

Trophozoite on agar

Trophozoite, differential interference contrast (N = Nucleus; CV = Contractile vacuole)

Trophozoite, scanning electron microscopy

Cyst, phase contrast

Flagellate, phase contrast

Flagellate, transmission electron microscopy

Figure 73-1 Morphology of the three life stages of Naegleria fowleri. (Courtesy Centers for Disease Control and Prevention.)

Described as engaging in water-related activities including diving or jumping into the water, swimming, or other water sports before illness onset.

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