and Acanthamoeba spp., commonly found in lakes, swimming pools,
tap water, and heating and air conditioning units. While only one
species of Naegleria is known to infect humans, several species
of Acanthamoeba are implicated, including A. culbertsoni,
A. polyphaga, A. castellanii, A. astronyxis, A.
hatchetti, and A. rhysodes. An additional agent of human
disease, Balamuthia mandrillaris, is a related leptomyxid ameba
that is morphologically similar in light microscopy to Acanthamoeba.
belonging to the genera Acanthamoeba, Balamuthia, and
Naegleria are important causes of disease in humans and animals.
Naegleria fowleri produces an acute, and usually lethal, central
nervous system (CNS) disease called primary amebic meingoencephalitis
(PAM). N. fowleri has three stages, cysts
and flagellated forms
in its life cycle. The trophozoites replicate by promitosis (nuclear
membrane remains intact)
Naegleria fowleri is found in fresh water, soil, thermal
discharges of power plants, heated swimming pools, hydrotherapy and
medicinal pools, aquariums, and sewage. Trophozoites can turn into
temporary flagellated forms which usually revert back to the trophozoite
stage. Trophozoites infect humans or animals by entering the olfactory
reaching the brain. N. fowleri trophozoites are found in
cerebrospinal fluid (CSF) and tissue, while flagellated forms are found
spp. and Balamuthia mandrillaris are opportunistic free-living
amebae capable of causing granulomatous amebic encephalitis (GAE) in
individuals with compromised immune systems. Acanthamoeba spp.
have been found in soil; fresh, brackish, and sea water; sewage;
swimming pools; contact lens equipment; medicinal pools; dental
treatment units; dialysis machines; heating, ventilating, and air
conditioning systems; mammalian cell cultures; vegetables; human
nostrils and throats; and human and animal brain, skin, and lung
tissues. B. mandrillaris however, has not been isolated from the
environment but has been isolated from autopsy specimens of infected
humans and animals. Unlike N. fowleri, Acanthamoeba and
Balamuthia have only two stages, cysts
in their life cycle. No flagellated stage exists as part of the life
cycle. The trophozoites replicate by mitosis (nuclear membrane does not
The trophozoites are the infective forms and are believed to gain entry
into the body through the lower respiratory tract, ulcerated or broken
skin and invade the central nervous system by hematogenous dissemination
Acanthamoeba spp. and Balamuthia mandrillaris cysts and
trophozoites are found in tissue.
infections appear to occur worldwide.
amebic meningoencephalitis (PAM) is caused by Naegleria fowleri.
It presents with severe headache and other meningeal signs, fever,
vomiting, and focal neurologic deficits, and progresses rapidly (<10
days) and frequently to coma and death. Acanthamoeba spp. causes
mostly subacute or chronic granulomatous amebic encephalitis (GAE), with
a clinical picture of headaches, altered mental status, and focal
neurologic deficit, which progresses over several weeks to death. In
addition, Acanthamoeba spp. can cause granulomatous skin lesions
and, more seriously, keratitis and corneal ulcers following corneal
trauma or in association with contact lenses.
infections, the diagnosis can be made by microscopic examination of
cerebrospinal fluid (CSF). A wet mount may detect motile trophozoites,
and a Giemsa-stained smear will show trophozoites with typical
morphology. In Acanthamoeba infections, the diagnosis can be
made from microscopic examination of stained smears of biopsy specimens
(brain tissue, skin, cornea) or of corneal scrapings, which may detect
trophozoites and cysts. Cultivation of the causal organism, and its
identification by direct immunofluorescent antibody, may also prove
Eye and skin
infections caused by Acanthamoeba spp. are generally treatable.
Topical use of 0.1% propamidine isethionate (Brolene) plus neomycin-polymyxin
B-gramicidin ophthalmic solution has been a successful approach;
keratoplasty is often necessary in severe infections. Although most
cases of brain (CNS) infection with Acanthamoeba have resulted in
death, patients have recovered from the infection with proper
treatment. Amphotericin B* has been successfully used to treat PAM
caused by Naegleria fowleri.
*This drug is
approved by the FDA, but considered investigational for this purpose.