Infections Caused by Free-Living Amebas

Etiology

The 4 genera of amebas that cause disease in humans are Acanthamoeba spp., N fowleri, B mandrillaris, and S pedata. All of these species cause CNS infections, but Acanthamoeba spp. may cause localized extra-CNS infections in immunocompetent hosts or disseminated infections in immunocompromised hosts.[1] All these pathogenic free-living amebas exist in 2 primary forms: an active trophozoite stage responsible for tissue invasion and a dormant cyst stage that is highly resistant to environmental stressors and antimicrobial therapy. N fowleri additionally exhibits a transient flagellated form.[2][3][4]

Epidemiology

Free-living amebas are distributed globally and have been isolated from natural bodies of water, soil, air, and artificial environments such as swimming pools, plumbing systems, contact lens solutions, and hospital water supplies.[5] Identifying the exact source of infection is often difficult, as exposure may precede clinical disease by weeks or months.[2] Certain populations appear to be at increased risk. For example, invasive infections involving the CNS or skin caused by Acanthamoeba species and B mandrillaris are more frequently observed in individuals with diabetes mellitus, HIV infection, or a history of solid organ transplant. Epidemiological data further suggest that Balamuthia infection occurs disproportionately among individuals of Hispanic ethnicity, although the reasons for this increased susceptibility remain unclear.[6][2]

Acanthamoeba Species

Acanthamoeba species are widely distributed in natural and artificial environments, including tap water, soil, air, contact lens solutions, and hospital water systems. This microorganism has been found in fresh, brackish, and seawater as well as sewage, swimming pools, and contact lens supplies.[5][7] Predisposing conditions include diabetes mellitus, alcoholism, cirrhosis, HIV infection, chronic renal failure, systemic lupus erythematosus, malignant neoplasm, chemotherapy, and organ transplant, including hematopoietic cell and solid organ transplant.[2]

Naegleria fowleri

N fowleri is a thermophilic ameboflagellate found in warm freshwater such as lakes, hot springs, and poorly chlorinated pools. The infection primarily affects children and young adults in the summer months. The organism can tolerate temperatures up to 45 °C and has therefore been identified in the thermal discharges of power plants and geothermal wells. Infection is typically acquired during recreational water exposure when contaminated water enters the nasal passages, generally during swimming or sinus irrigation, and migrates to the brain. The infection causes PAM—a rapidly progressive and often fatal disease.[8]

Balamuthia mandrillaris

B mandrillaris is a soil-associated ameba that can cause severe CNS and disseminated disease. Unlike Naegleria and Acanthamoeba infections, hematogenous dissemination occurs through skin wounds or inhalation of contaminated dust. Cases have also been reported following solid organ transplant from infected donors. Both immunocompetent and immunocompromised individuals may be affected.[4][6]

Sappinia pedata

S pedata is an exceptionally rare cause of human infection, with only a single confirmed case of GAE reported in the literature. The infection occurred in an immunocompetent farmer aged 38 with regular exposure to livestock and no underlying immunosuppression. Neuroimaging revealed a solitary focal brain lesion, and histopathological examination demonstrated large, binucleate trophozoites consistent with Sappinia species. Although the precise route of infection could not be definitively established, inhalation was considered the most likely mechanism, as the organism has been isolated from soil contaminated with animal feces.[9]

Pathophysiology

Trophozoites adhere to host tissues through specific surface-binding proteins and produce a variety of cytotoxic enzymes, including proteases, phospholipases, and metalloproteinases, that facilitate tissue destruction and invasion. Acanthamoeba spp. and B mandrillaris are capable of hematogenous dissemination after entry through the skin or respiratory tract, ultimately crossing the blood–brain barrier and causing CNS disease. In contrast, N fowleri organisms gain access to the CNS by migrating along the olfactory nerves following nasal exposure, resulting in rapid cerebral invasion.[10] 

Host immune responses vary depending on the organism and disease course. Granulomatous inflammation and cell-mediated immunity are characteristic of Acanthamoeba and Balamuthia infections. In contrast, the fulminant nature of Naegleria infection precludes the development of an effective adaptive immune response.[11][12]

History and Physical

Primary Amebic Meningoencephalitis

PAM is caused by N fowleri and is an acute, rapidly progressive disease that primarily affects children and young adults. Symptoms typically develop within days of exposure and include fever, severe headache, nausea, vomiting, neck stiffness, photophobia, and altered mental status. Neurological deterioration occurs rapidly, progressing to seizures, coma, cerebral edema, and death, often within one week of symptom onset.[10]

Granulomatous Amebic Encephalitis

GAE is caused by Acanthamoeba spp. or B mandrillaris and typically follows an indolent yet progressive course, with symptoms evolving over several weeks to months. Patients commonly develop persistent headaches, intermittent low-grade fevers, visual complaints, changes in behavior or cognition, and focal neurological deficits that correspond to the anatomic distribution of cerebral lesions.[13] The latency between exposure and symptom onset is often unclear, as the initial acquisition frequently goes unrecognized. In individuals with HIV infection, GAE may present with focal brain lesions that resemble toxoplasmic encephalitis on imaging; however, in contrast to toxoplasmosis, these lesions do not improve with standard antitoxoplasma therapy. With continued disease progression, patients may develop seizures, increasing intracranial pressure, diminished consciousness, and ultimately, coma and death.[2]

In cases associated with B mandrillaris, patients may describe a preceding cutaneous lesion, often weeks or months before neurological symptoms develop. These skin lesions may be painless and slow growing, contributing to underrecognition of their clinical significance. Respiratory symptoms or sinus complaints are occasionally reported and may represent an initial portal of entry before hematogenous dissemination.[14]

Acanthamoeba Keratitis

Acanthamoeba keratitis is a painful, vision-threatening corneal infection most commonly affecting patients using contact lenses, particularly those with poor lens hygiene or exposure to nonsterile water. Patients typically present with redness, excessive tearing, a sensation of a foreign body in the eye, ocular pain, and sensitivity to light. The infection is usually unilateral, although bilateral involvement has been reported. Epithelial surface irregularities and pseudodendritic lesions may mark early disease, while advanced infection is classically associated with the development of a ring-shaped stromal infiltrate. Delayed diagnosis may result in corneal scarring or blindness.[7]

Cutaneous and Disseminated Disease

Both Acanthamoeba and Balamuthia infections can cause cutaneous lesions and may present as nodules, plaques, ulcers, or necrotic lesions, particularly in immunocompromised patients. Disseminated disease may involve the lungs, sinuses, kidneys, adrenal glands, or bones and may precede or accompany CNS involvement.[1][6]

Evaluation

Diagnosis of infections caused by free-living amebas requires a high index of suspicion. Initial evaluation often includes neuroimaging, which may reveal nonspecific findings such as edema, hemorrhage, or ring-enhancing lesions. The temporal and parietal lobes are most commonly affected. Definitive diagnosis relies on microscopic examination, histopathological evaluation, and molecular testing. Trophozoites may be visualized in cerebrospinal fluid in PAM but are rarely detected in GAE. The diagnosis of GAE is established by brain tissue biopsy, which demonstrates necrotizing or granulomatous inflammation with trophozoites and cysts on hematoxylin-eosin staining. For patients with skin or pulmonary lesions, tissue samples should be obtained to evaluate for trophozoites.[1] Polymerase chain reaction (PCR) assays provide high sensitivity and specificity and are available through specialized reference laboratories. Confocal microscopy is particularly useful in the evaluation of suspected Acanthamoeba keratitis.

Treatment / Management

Randomized controlled trial data to guide therapy are not available, and treatment recommendations are based on case series and expert opinion. Management typically involves early initiation of combination antimicrobial therapy.[15][16](B3)

• PAM is treated with aggressive multidrug regimens centered on conventional amphotericin B, often combined with miltefosine, fluconazole, rifampin, and azithromycin.[10]
• GAE requires prolonged combination therapy, frequently including pentamidine, fluconazole, and miltefosine. Trimethoprim-sulfamethoxazole, metronidazole, and azithromycin may also be added to the regimen. Acanthamoeba spp. are generally resistant to amphotericin B.
• Acanthamoeba keratitis is managed with topical polyhexamethylene biguanide (0.02%) or biguanide-chlorhexidine in combination with propamidine (0.1%) or hexamidine (0.1%). Surgical intervention may be necessary in refractory cases.[7][17]
• Optimal therapy for B mandrillaris infection has not been clearly established, as the available evidence is largely derived from case reports and small series. In the United States, survivors have most commonly received combination regimens centered on pentamidine, flucytosine, fluconazole, and a macrolide (either azithromycin or clarithromycin), often with the addition of sulfadiazine, miltefosine, thioridazine, or liposomal amphotericin B. Nitroxoline has been recently reported to be successful in the treatment of B mandrillaris encephalitis.[18]
• A patient with GAE due to Sappinia spp. was treated with a combination of azithromycin, intravenous pentamidine, itraconazole, and flucytosine and survived.[9]
(B3)

Consultation with infectious disease specialists and the Centers for Disease Control and Prevention (CDC) is strongly recommended for any suspected case of infection due to free-living amebas to assist with diagnostic confirmation and therapeutic guidance.[16](B3)

Differential Diagnosis

PAM caused by N fowleri may initially mimic acute bacterial meningitis due to overlapping clinical features and cerebrospinal fluid findings. However, PAM is characterized by rapid progression and a diffuse meningoencephalitic process, whereas GAE follows a more indolent course and presents with focal neurological deficits and granulomatous inflammation. As a result, patients with PAM are frequently treated empirically for bacterial meningitis early in the disease course, while GAE is more often mistaken for conditions that cause space-occupying CNS lesions.[19][20]

The differential diagnosis of GAE is extensive because the disease typically presents with subacute or chronic focal neurological deficits and mass-like brain lesions, particularly in immunocompromised patients. Because symptoms are nonspecific and progression is gradual, GAE is frequently misdiagnosed as malignant neoplasms, vasculitis, or other infectious or inflammatory neurological conditions.[13] Important diagnostic considerations include bacterial brain abscess, CNS tuberculosis, nocardiosis, invasive aspergillosis, cryptococcal meningoencephalitis, and CNS histoplasmosis. Parasitic and neoplastic processes such as toxoplasmosis, neurocysticercosis, and primary CNS lymphoma should also be considered, particularly in patients with compatible risk factors and unexplained focal neurological findings.[14]

Prognosis

GAE carries an extremely poor prognosis, with reported mortality rates exceeding 90%, regardless of the causative organism (Acanthamoeba spp. or B mandrillaris). Outcomes are similarly devastating for PAM, which is associated with case-fatality rates approaching 99% and a rapidly progressive course, with death typically occurring within days of symptom onset.[14][19] Unfavorable outcomes are largely attributable to the nonspecific clinical presentation and frequent delays in diagnosis, which often occur only after advanced CNS involvement. Many cases are identified postmortem, further highlighting the diagnostic challenges associated with this disease.[20]

Survival has been reported in a small number of patients and is most often associated with early recognition, prompt initiation of combination antimicrobial therapy, and, in select cases, surgical resection of localized lesions. Even among survivors, long-term neurological sequelae are common, including cognitive impairment, motor deficits, and seizure disorders, underscoring the need for early interdisciplinary care and prolonged follow-up.[20]

Complications

PAM results in rapid cerebral edema, increased intracranial pressure, brain herniation, and profound neurological deterioration, frequently leading to death within days of symptom onset.[19] GAE may cause a more subacute course but is complicated by focal neurological deficits, seizures, altered mental status, and progressive brain necrosis, particularly in immunocompromised hosts. Acanthamoeba keratitis can cause corneal perforation and blindness.[7] Disseminated infection can involve the skin, lungs, kidneys, and sinuses, leading to ulcerative skin lesions, respiratory failure, and multiorgan dysfunction.