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Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996.

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Medical Microbiology. 4th edition.

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Chapter 81Free-Living Amebas: Naegleria, Acanthamoeba and Balamuthia


General Concepts

Naegleria Fowleri

Clinical Manifestation

Naegleria fowleri causes primary amebic meningoencephalitis, a rare, rapidly fatal disease with sudden onset of headache, fever, stiff neck, lethargy, and coma in otherwise healthy people.


The trophozoites are 10 to 15 μm in diameter and produce broadly rounded lobopodia. Cysts are single-walled, spherical and 8 to 12 μm in diameter. The trophozoites can also transform to a flagellated form.

Multiplication and Life Cycle

The trophozoites are free-living inhabitants of soil and warm fresh water. They reproduce by binary fission.


Amebas splashed or inhaled onto the olfactory epithelium migrate up the olfactory nerve to the brain and spread via the subarachnoid space.

Host Defenses

None are known.


The organism is found worldwide in soil and warm fresh water. Infectious cysts may be carried in dust.


Diagnosis relies on identifying trophozoites by microscopic examination of fresh cerebrospinal fluid specimens or histologic sections of CNS tissue, and on culturing, if necessary.


Early, aggressive treatment with amphotericin B and miconazole may be effective, but almost all patients die.

Acanthamoeba Species

Clinical Manifestation

Acanthamoeba species and Balamuthia mandrillaris usually act as opportunistic pathogens in immunocompromised or debilitated individuals in whom they cause pneumonitis or dermal ulcerations. From these lesions the amebas may spread to the brain to cause an insidious, slowly progressive, and usually fatal encephalitis called granulomatous amebic encephalitis. In healthy individuals, Acanthamoeba spp can cause an ulcerating keratitis, which is often associated with the use of improperly sterilized contact lenses.


Trophozoites are 25 to 40 μm in diameter with characteristic spine-like pseudopodia. Cysts are double-walled, usually polygonal and spherical, and 15 to 20 μm in diameter.

Multiplication and Life Cycle

The trophozoites are free-living inhabitants of soil and of fresh and salt water. They reproduce by binary fission.


Encephalitis is caused by the hematogenous spread from superficial or pulmonary lesions to the brain. Keratitis results from contamination of superficial corneal abrasions.

Host Defenses

Except in the case of keratitis, the defenses of a healthy host seem sufficient to prevent infection.


Acanthamoeba and B mandrillaris organisms live worldwide in soil and fresh and salt water. They may contaminate contact lens solution, physiotherapy pools, air-conditioning units, etc.


Diagnosis is usually by microscopic examination of biopsy specimens from lesions; both trophozoites and cysts may be seen. Amebas may also be cultured.


No effective treatment is known for opportunistic Acanthamoeba and B mandrillaris infections in debilitated and immunosuppressed individuals. The incidence of keratitis may be reduced by properly cleaning and sterilizing contact lenses. Keratitis may respond to treatment with propamidine (often combined with neomycin), followed if necessary, by keratoplasty.


Three genera of free-living amebas, Naegleria, Acanthamoeba, and B mandrillaris are known to infect humans. Naegleria fowleri causes an acute and almost invariably fatal encephalitis, which, fortunately, is rare. Several species of Acanthamoeba and B mandrillaris can cause lung and skin infections, as well as an insidious encephalitis, in immunocompromised patients.In addition, Acanthamoeba may cause an ulcerative keratitis, which is usually associated with improper sterilization of soft contact lenses. These amebas live freely in soil and in fresh and coastal waters. The resistant cysts can be transported in dust.

Naegleria Fowleri

Clinical Manifestation

Naegleria fowleri is the agent of primary amebic meningoencephalitis, a fulminating, rapidly fatal disease. More than 150 cases of this disease have been recorded worldwide (Table 81-1). The disease usually affects children and young adults. In almost all cases, the victims contact the amebas by swimming in infected fresh water. The amebas enter the brain via the olfactory tract after being inhaled or splashed onto the olfactory epithelium. The incubation period ranges from 2 to 3 days to as long as 7 to 15 days, depending partly on the size of the inoculum and the virulence of the strain. The incubation period in animals infected experimentally with N australiensis or with a mildly virulent strain of N. fowleri has been as long as 3 or 4 weeks.

Table 81-1. Comparison of diseases caused by free-living amebas.

Table 81-1

Comparison of diseases caused by free-living amebas.

The disease appears with the sudden onset of bifrontal or bitemporal headache, fever, nausea, vomiting, and stiff neck (Table 81-2). Symptoms progress rapidly to lethargy, confusion, and coma. In all of the recorded cases, the patient died within 48 to 72 hours.

Table 81-2. Signs and Symptoms in Primary Amebic Meningoencephalitis (PAM) and Granulomatous Amebic Encephalitis (GAE).

Table 81-2

Signs and Symptoms in Primary Amebic Meningoencephalitis (PAM) and Granulomatous Amebic Encephalitis (GAE).


Naegleria fowleri isolated from humans is morphologically identical to the common, nonpathogenic amebas N gruberi and N australiensis. The trophozoites are active and constantly change size and shape (Fig. 81-1). When rounded, they measure about 10 to 15μm in diameter. The cytoplasm is finely granular and contains a conspicuous clear nuclear halo and a dense central nucleolus. Numerous vacuoles are usually visible in the cytoplasm. The trophozoites travel by producing broadly rounded processes (lobopodia), which are clear initially but fill with granular cytoplasm. Under adverse conditions, the trophozoites encyst. The cysts are spherical, 8 to 12 μm in diameter, with a smooth, single-layered wall. The wall is pierced by one or two flat, mucus-plugged pores through which the regenerated trophozoite will emerge. The cytoplasm of the cyst is finely granular, with a characteristic central nucleus. When exposed to distilled water, trophozoites can convert within a few minutes to a flagellated form.

Figure 81-1. Comparative morphology of free-living amebas.

Figure 81-1

Comparative morphology of free-living amebas.

The pathogenic species of Naegleria was named N fowleri after Malcolm Fowler, who first isolated the organism from a patient with primary amebic meningoencephalitis. Previously, these amebas were also called N aerobia and N invadens.Naegleria australienses and N australiensis italica are species that show low virulence in animal experiments. Immunoelectrophoresis is important in identifying free-living amebas. Even though N fowleri and other free-living amebas may reliably be differentiated by morphology and immunoperoxidase methods, the use of various zymodemes helps in distinguishing pathogenic from nonpathogenic amebas.

Multiplication and Life Cycle

Naegleria fowleri is a free-living inhabitant of fresh water and soil. The ameboid trophozoite form reproduces by binary fission and also gives rise to the encysted and flagellated forms, which do not reproduce. Naegleria fowleri is thermophilic, preferring warm water and reproducing successfully at temperatures up to 46°C. In temperate climates, the amebas overwinter as cysts in bottom sediments of lakes, swimming pools and rivers.


In almost all cases, N fowleri enters the body by being inhaled or splashed onto the olfactory epithelium (Fig. 81-2). In some cases, however, the patients had no recent contact with fresh water, and apparently contracted the disease by inhaling cyst-laden dust. The sustentacular cells of the olfactory neuroepithelium are capable of active phagocytosis, and this appears to be the mechanism by which the amebas invade the body. The amebas then travel up the mesaxonal spaces of the unmyelinated olfactory nerve to the brain. The olfactory nerve terminates in the olfactory bulb, which is located in the richly vascularized subarachnoid space and is bathed by cerebrospinal fluid. The subarachnoid space is the route of dissemination to the rest of the central nervous system (CNS). Respiratory symptoms in some patients may be the result of hypersensitivity or allergic reactions, or may represent a subclinical infection.

Figure 81-2. Pathogenesis of Naegleria infection.

Figure 81-2

Pathogenesis of Naegleria infection.

The brain of a patient with primary amebic meningoencephalitis usually shows swollen, edematous, congested cerebral hemispheres and evidence of increased intracranial pressure. Uncal and cerebellar tonsillar hernias may be present. The arachnoid is severely congested, and a scant purulent exudate may be found along the sulci. The olfactory bulbs and orbitofrontal cortices are usually necrotic and hemorrhagic. The leptomeninges show a fibrinous-purulent exudate composed of polymorphonuclear leukocytes, eosinophils, a few monocytes, and some lymphocytes. These changes may be present throughout the cerebral hemispheres, brain stem, cerebellum, and upper portion of the spinal cord. Necrotizing angiitis is occasionally seen. A few amebas can be found in the purulent exudate, some in the process of being phagocytosed by polymorphonuclear leukocytes and macrophages.

Host Defenses

It is not yet clear whether N fowleri can elicit a protective cellular or humoral immune response.


N fowleri is ubiquitous in warm fresh waters. It is clear that the number of infections represents only a minute fraction of the number of exposures. Nevertheless, clusters of cases associated with a given source occur. It is not clear why primary amebic meningoencephalitis is not found predominantly in the tropics, where the ameba flourishes. Most patients with primary amebic meningoencephalitis have been in a swimming pool, freshwater lake, or pond a few days before the onset of symptoms. However, as mentioned above, the disease may also be transmitted by cyst-laden dust. Chlorination of water does not entirely eliminate pathogenic strains. Naegleria fowleri has also been isolated from air conditioning units.


Primary amebic meningoencephalitis cannot be distinguished clinically from acute pyogenic or bacterial meningoencephalitides. The disease usually occurs in children and young adults in good health who have recently swum in warm water. Computed tomography of the brain shows obliteration of the cisternae surrounding the midbrain and of the subarachnoid space over the cerebral hemispheres. The disease may be diagnosed rapidly by examining one or two drops of fresh cerebrospinal fluid under a light microscope for N fowleri. The organism may also be cultured from cerebrospinal fluid or brain tissue for a definitive diagnosis. Retrospective diagnoses have been made by examining paraffin-embedded brain tissue sections stained with hematoxylin and eosin.


Only three patients have survived primary amebic meningoencephalitis. In these patients, the disease was diagnosed early and treated aggressively with high doses of amphotericin B. Amphotericin B and miconazole appear to be the drugs of choice. The chance of catching the disease can presumably be reduced by properly chlorinating swimming pools, whirlpools, and Jacuzzis, and by not diving or splashing in warm water ponds.

Acanthamoeba/B Mandrillaris (Family Leptomyxidae)

Acanthamoeba castellanii, A culbertsoni, and other Acanthamoeba species as well as the recently described B mandrillaris, can cause opportunistic lung and skin infections in immunocompromised or otherwise debilitated individuals. The amebas may spread hematogenously from such lesions to the brain, where they cause a subacute, slowly progressive, and usually fatal encephalitis. In addition, Acanthamoeba can cause an ulcerating keratitis in healthy individuals, usually in association with improperly sterilized contact lenses.

Clinical Manifestations

Granulomatous Amebic Encephalitis

Granulomatous amebic encephalitis is a multifocal, hemorrhagic and necrotizing encephalitis caused by opportunistic free-living Acanthamoeba species, principally A castellanii and A culbertsoni or by B mandrillaris. The disease usually afflicts debilitated or immunosuppressed individuals. It sets in with insidious, focal neurologic changes that mimic the clinical picture of single or multiple space-occupying brain lesions (Table 81-2). Focal neurologic changes, hemiparesis, drowsiness, personality changes, and seizures are common early symptoms. Headache sets in early and is insidious. Nausea and vomiting may also be early symptoms. Fever is sporadic and generally low. Signs and symptoms of brain parenchymal inflammation develop, such as altered mental status, diplopia, paresis, lethargy, and cerebellar ataxia. The disease progresses over a period of one to several weeks and usually ends in coma and death.

The incubation period of the disease is difficult to determine, as pulmonary and skin lesions containing the organisms may be present for months before encephalitis appears. Acanthamoeba species apparently multiply more slowly than Naegleria fowleri.

Acanthamoeba and B mandrillaris Infections of the Lungs and Skin

Acanthamoebic pneumonitis and dermatitis, characterized by the presence of cysts and trophozoites in alveoli or in multiple nodules or ulcerations of the skin, are opportunistic diseases that usually affect immunosuppressed or debilitated individuals. In acanthamoebic pneumonitis, chest radiographs may show areas of consolidation. Granulomatous amebic encephalitis usually develops as a result of hematogenous spread from lesions in the lungs, upper respiratory tract, or skin. Multiple skin nodules may represent “terminal” dissemination in cases of granulomatous amebic encephalitis.

Acanthamoeba Keratitis

Painful corneal ulcerations that fail to respond to the usual antibacterial, antiviral, and antifungal treatments may be caused by Acanthamoeba. The disease is a nonsuppurative keratitis that characteristically follows a waxing and waning clinical course. The damaged corneal tissue may show a characteristic annular infiltrate and congested conjunctivae or there may be a dendriform epitheliopathy and patchy stromal infiltrate with lacunar areas. If not successfully treated, the disease progresses to corneal perforation and loss of the eye or to a vascularized scar over thinned cornea, with impaired vision. The disease is quite rare. It is usually associated with contaminated contact lenses.


Acanthamoeba and B mandrillaris trophozoites may be recognized by the presence of slender, spine-like processes (Fig. 81-1). When rounded, the cells measure 25 to 40 μm in diameter. The finely granular cytoplasm, as a rule, contains a single nucleus with a large, dense central nucleolus surrounded by a nuclear clear zone. Water and digestive vacuoles are usually visible in the cytoplasm. The double-walled cysts are generally polygonal, spherical, or star-shaped, 15 to 20 μm in diameter, with a nucleus containing a large dense central nucleolus surrounded by a clear nuclear halo. The smooth inner wall of the cyst contacts the wrinkled outer wall at a number of points, forming pores, opercula, or ostioles.

Multiplication and Life Cycle

Acanthamoeba species and B mandrillaris are free-living amebas of soil and of fresh and salt water. Reproduction is by binary fission of the trophozoites. Infective cysts can be transmitted in dust and aerosols.


Free-living amebas have been isolated from human throats, suggesting that they are generally harmless in healthy individuals. Acanthamoeba spp usually act as opportunistic pathogens, taking advantage of a loss of metabolic, physiologic, or immunologic integrity by the host. Among the most common factors predisposing an individual to Acanthamoeba infection are immunosuppressive therapy, treatment with broad-spectrum antibiotics, diabetes mellitus, various cancers, malnutrition, pregnancy, acquired immune deficiency syndrome (AIDS), and chronic alcoholism. Surgical trauma, burns, wounds, and radiation therapy can also promote infection.

The primary focus of infection for opportunistic Acanthamoeba is usually the lower respiratory tract or skin. The amebas may enter the respiratory tract by the inhalation of aerosols or dust containing cysts (Fig. 81-3). Spread to the CNS is apparently hematogenous. The cerebral hemispheres in granulomatous amebic encephalitis may be edematous, with focal cortical softening, hemorrhage and abscesses. Uncal notching and cerebellar herniation may be present. Foci of hemorrhagic necrosis may be seen in the basal ganglia, midbrain, brainstem, and cerebellum. The histopathologic changes consist of a chronic granulomatous encephalitis with multinucleated giant cells, mainly in the posterior fossa structures, basal ganglia, and cerebellum. Trophozoites and cysts may be found in the lesions.

Figure 81-3. Pathogenesis of Acanthamoeba infection.

Figure 81-3

Pathogenesis of Acanthamoeba infection.

Acanthamoeba keratitis usually results from direct invasion of ocular tissue by the amebas through a break in the corneal epithelium. In most cases, the portal of entry is a minor corneal lesion, such as those caused by a previous episodes of herpes simplex or by abrasion from hard or soft contact lenses. The amebas are often introduced in the eye by an individual's use of contaminated contact-lens cleaning solutions or by swimming in contaminated water. The incubation period is unknown. Amebic trophozoites and cysts are usually located deep in the corneal stroma, with moderated granulomatous inflammation and negligible acute inflammatory response.

Host Defenses

Except in the case of amebic keratitis, the defenses of a healthy host seem sufficient to prevent Acanthamoeba or B mandrillaris infection. Patients who contract granulomatous amebic encephalitis usually have impaired humoral and/or cell-mediated immunity. However, there are reports of patients with no demonstrable underlying disease or predisposing factor.


Pathogenic Acanthamoeba species and B mandrillaris are ubiquitous in fresh and salt water and in soil. Infective cysts can be carried by water or dust.


In many cases, granulomatous amebic encephalitis is not diagnosed until after or, at best, shortly before death. Immunosuppression or other predisposing factors may provide important clues. The differential diagnosis includes space-occupying lesions such as tumors, abscesses, and even infarcts, as well as tuberculoma or fungal infection (Table 81-1). Computed tomography and magnetic resonance imaging of the brain are important diagnostic tests, as is examination of cerebrospinal fluid and brain biopsy specimens. The diagnosis usually is made after examination of brain tissue with light a microscope. Amebic “dermatitis” is often diagnosed by microscopic examination of a skin biopsy. Both trophozoites and cysts are usually visible.

In the case of amebic keratitis, scrapings of the corneal ulceration and biopsy specimens may contain amebic trophozoites and cysts. Both light and electron microscopy may be useful. Amebic cysts in the corneal stroma may be demonstrated by staining with hematoxylin and eosin, trichrome, calcofluor-white, or immunofluorescence techniques. Alternatively, amebas may be cultured at 37°C on non-nutrient agar with Page's saline containing Escherichia coli, Enterobacter aerogenes, or other Gram-negative bacteria. Cysts and trophozoites may be identified on the basis of morphology and locomotion; isoenzyme electrophoresis may be used to further classify species.


There is no effective treatment for lung, skin, or brain Acanthamoeba or B mandrillaris infections. Sulfadiazine is apparently ineffective, perhaps as a result of the host's impaired immune system. Since the amebas are ubiquitous, preventive measures to reduce contact are difficult. The incidence of Acanthamoeba keratitis, on the other hand, can be greatly reduced by correct sterilization of contact lenses. Lenses should be cleaned properly, using commercial rather than home-made saline, and should be disinfected with a chemical or (preferably) a thermal system. Lenses should be removed before swimming.

Drug treatment of Acanthamoeba keratitis is sometimes successful. Brolene (propamidine isethionate) and dibromopropamidine have been reported to be effective. Polymyxin B, miconazole, and neomycin also appear to be useful in combination with propamidine. Ketoconazole gives evidence of being effective both in vitro and in vivo. Acridine derivatives and paromomycin are effective in vitro but are still in the experimental stage. Cases that do not respond to drug therapy have been treated by penetrating keratoplasty and corneal grafting. Cysts apparently may occupy the deeper layers of the cornea. These cysts are probably responsible for the resistance of the infection to drug and for recurrence after corneal transplantation. For this reason, the infection should be controlled before corneal transplantation is performed. Steroids should be used with caution in ophthalmic infections, even though they may reduce or inhibit inflammation and tissue damage and prevent an immune response.


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Copyright © 1996, The University of Texas Medical Branch at Galveston.
Bookshelf ID: NBK7960PMID: 21413280


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