Clinical manifestations

Presentation and course

CNS infection with free-living amebae can be broadly categorized under two clinical syndromes: primary amebic meningoencephalitis or granulomatous amebic encephalitis. These presentations differ with respect to causative agents, clinical presentation, and host risk factors. Primary amebic meningoencephalitis is caused by infection with Naegleria fowleri. This organism is abundant in fresh water and thrives in warm temperatures. Not surprisingly, most cases of primary amebic encephalitis occur in the summer months, with a history of swimming in fresh water within the week prior to the onset of symptoms (19). The clinical presentation of primary amebic meningoencephalitis mimics bacterial meningitis and is characterized by rapid progression, leading to the popular description of Naegleria as a “brain-eating parasite” in the lay press. The disease course for primary amebic meningoencephalitis is usually fulminant with the abrupt onset of headache, nausea, vomiting, and fever, with rapid evolution to coma. Death typically ensues within a median of five days of symptom onset, usually due to overwhelming encephalitis and cerebral edema (04). The CSF shows a neutrophilic pleocytosis, an elevation in protein, and, not infrequently, hypoglycorrhachia.

In contrast, granulomatous amebic encephalitis caused by Acanthamoeba species and Balamuthia presents as a subacute or chronic process, progressing over weeks to months. Some cases, however, may be more acute with coma and death within 7 to 9 days. Patients may manifest a prodromal flu-like illness with headache and fever. Subsequently, patients with granulomatous amebic encephalitis often develop vomiting, altered mental status, cranial nerve palsies, and seizures, suggesting the possibility of a mass lesion (18). The CSF usually shows a lymphocytic pleocytosis with a normal or near-normal protein and glucose concentration. Patients with immune compromise such as AIDS, or hematopoietic stem cell/organ transplants are more susceptible to granulomatous amebic encephalitis due to Acanthamoeba species (25; 10). However, Acanthamoeba species are increasingly recognized as pathogens in immunocompetent patients as well (11; 01).

Granulomatous amebic encephalitis due to Balamuthia mandrillaris occurs in both immunocompromised and immunocompetent hosts, particularly in children. A summary of patients reported to the CDC identified 109 laboratory-confirmed cases of Balamuthia between 1974 and 2016, 99% of whom had encephalitis, with a disproportionately high number of cases among people of Hispanic ethnicity (07). The clinical picture for Balamuthia is similar to that seen with Acanthamoeba encephalitis, with progressive neurologic decline variably associated with focal signs (eg, hemiparesis), cranial nerve palsies, and a CSF mononuclear pleocytosis; the time from symptom onset to death averages 24 days (07).

Concurrent cutaneous disease, identified in 6% of Balamuthia encephalitis cases and 10% of Acanthamoeba cases, presents as an erythematous indurated plaque, often involving the face and may present a more easily accessible site for diagnostic biopsy (07; 23).

The significance of Sappinia species in causing human disease is controversial. To date there has been a single patient identified—a 38-year-old immunocompetent man with visual disturbances, headache, and a seizure. Brain images showed a solitary mass in the posterior left temporal lobe. Pathologic evaluation revealed necrotizing hemorrhagic inflammation that contained free-living amoebae. This was ultimately identified as Sappinia pedatadiploidea, an organism usually found in soil and tree bark (32). This patient was treated successfully with a sequential regimen of antiamebic drugs (azithromycin, pentamidine, itraconazole, and flucytosine) after surgical excision (17; 16).

Prognosis and complications

Primary amebic meningoencephalitis and granulomatous amebic encephalitis are extremely lethal diseases, and often the diagnosis is only recognized postmortem. Early recognition and initiation of combination therapy, including miltefosine, has led to a slight improvement in outcomes. To date, there have been five patients in North America (one in Mexico, four in the United States) with primary amebic meningoencephalitis who have survived. Three of these cases occurred since 2013, all of whom were 16 years or less in age with favorable neurologic recovery. More information can be accessed at:https://www.cdc.gov/parasites/naegleria/health_professionals.html). Among patients with granulomatous amebic encephalitis, mortality is 90% to 97%% (07; 23).

Clinical vignette

A 13-year-old boy was admitted with a 2- to 3-day history of headache, lethargy, and vomiting. His parents recalled a trip to a lake one week prior to presentation, where he engaged in water sports and went cliff diving. On physical examination he was febrile to 39.5ºC with meningeal signs, increased reflexes, and lethargy. Brain CT and MRI scan were normal. The CSF showed increased protein of 300 mg/dL, glucose of 45 mg/dL, and 160 white blood cells/mm3 (65% polymorphonuclear cells). Motile amoebae were observed in a wet mount of the CSF. Despite treatment, he died within three days of admission.

Biological basis

Etiology and pathogenesis

Naegleria are frequently present in fresh water, especially warm, stagnant water. These organisms have been identified in 46% of hot springs (13). Naegleria exist in three forms, which are all nucleated: trophozoite, flagellate, and cyst (26). The trophozoite is a motile form that resembles a eukaryotic cell, and it is the stage that causes human infection. After placement into distilled water, trophozoites can transform into a distinctive flagellate form; the latter stage appears pear shaped, with two to four flagellae protruding from the anterior end, providing the capacity for rapid motility. Under conditions of stress, such as nutritional deprivation or desiccation, Naegleria form resistant cysts. Clinical specimens typically contain the trophozoite form.

Acanthamoeba are common soil inhabitants that only exist as trophozoites or double-walled cysts, with no flagellate form. The trophozoite form has fine pseudopodia at the leading edge as it moves; these thorn-like spikes give the genus its name, from the Greek word akantha for “thorn.” The cyst form of Acanthamoeba has a more wrinkled appearance than the cyst of Naegleria; this dormant phase can survive in the environment for decades.

Balamuthia mandrillaris is indistinguishable from Acanthamoeba in formalin-fixed tissue examined at the light microscope level; however, there are distinctive differences with respect to ultrastructure, physiology, and antigenicity (20).

Pathophysiology. Naegleria thrive in warm, stagnant water. Raised temperatures during the hot summer months, water in hot spring recreational facilities, or warm water from power plants facilitate the growth of N fowleri (38). Classically, primary amebic meningoencephalitis occurs through inoculation of infected water through the cribriform plate. The classic scenario is cliff diving, where water under pressure enters the nasal cavity. Infection through tap water exposure is also a risk factor, and cases have been reported after neti pot nasal irrigation, ritual nasal ablution, use of a slip-n-slide supplied with tap water, and rarely, ingestion of contaminated tap water (05). The agent then spreads along the extensions of the olfactory nerve or blood vessels, spreading throughout the meninges and within the brain (26). The presence of severe amebic involvement of the olfactory regions and frontal regions of the cerebral hemispheres further supports the postulated route of spread. Preclinical evidence demonstrated that N fowleri strongly adhered to host extracellular matrix (24) and disrupted epithelium by changes in tight junction proteins such as claudin-1 (35). The microscopic pathology is characterized by intense polymorphonuclear (neutrophil and eosinophil) infiltration resulting in necrotizing encephalitis.

The pathogenesis of Acanthamoeba encephalitis is less well understood. Acanthamoeba colonizes the nares in up to 24% of cases (39), suggesting intranasal spread through inhalation of airborne dust as a route. Cutaneous inoculation leading to hematogenous spread is another postulated mechanism of infection. An experimental model demonstrated that Acanthamoeba disrupt the blood-brain barrier by secretion of proteases, allowing invasion of the brain (12). Organisms then multiply intracranially, with resulting edema, hemorrhage, and necrosis; perivascular cuffing by neutrophils, trophozoites, and cysts is often present (26). Granulomatous inflammation is frequently present, although may be atypical or absent in the profoundly immunocompromised.

Balamuthia mandrillaris has been isolated from soil, dust, and water. The pathogenesis of CNS disease caused by this organism is unclear; however, hematogenous spread is suggested based on clustering of amebae around blood vessels. Pathology of brains from patients with Balamuthia infection showed a spectrum of inflammation that ranged from neutrophilic infiltrates to necrotizing granulomas. Diffuse meningitis with massive coagulative necrosis and perivascular infiltration by lymphocytes and histiocytes has been reported on autopsy (21; 02). Trophozoites and cysts can be found in subarachnoid and Virchow-Robin spaces (26).

Epidemiology

Naegleria have been found in thermal discharges of power plants, heated swimming pools, hot springs, hydrotherapy pools, sewage, and even tap water. Among 120 cases of primary amebic encephalitis reported to the CDC, 85 (71%) had recreational water exposure, including lake or pond (69 patients), river or stream (14 patients), or recreational aquatic venue (2 patients) (19). Cases have been associated with use of contaminated tap water for nasal irrigation (43) or following ritual nasal ablution (34). Occasional clusters have occurred, usually because of swimming in an inadequately chlorinated swimming pool (31; 37).

As of 2022, a total of 157 cases of primary amebic meningoencephalitis have been reported in the United States, with most cases occurring in Southern states during summer months. However, increasingly cases are identified in Northern states such as Minnesota and Indiana (04). In 2022, cases were reported in Iowa, Nebraska, and Arizona. Primary amebic meningoencephalitis is primarily a pediatric illness, with a median age 11 years. Infections are most common in males (76%), likely related to recreational exposure. More information can be accessed at the following site:CDC.gov.

Despite the ubiquity of Acanthaemoeba in the environment, CNS infection with this agent is reported primarily in immunocompromised patients (15; 09; 10; 23). As this is not a reportable disease, scant data exist regarding the epidemiology of Acanthamoeba granulomatous amebic encephalitis, with most information coming from case reports or small case series. There is no clear geographic localization for this infection.

Balamuthia granulomatous amebic encephalitis cases are disproportionately male (68%), and recreational or occupational soil exposure is present in 85% of cases (07). Although previously believed to be a disease of immunocompetent hosts, this study revealed 39% of cases were immunocompromised. Transmission through transplantation of organs from an infected donor has also been described (14).

Prevention

Risk for infection can be reduced by measures that minimize exposure when engaging in water-related activities in warm freshwater lakes or rivers in warmer months. Strategies to minimize risk include avoiding jumping or diving into bodies of warm fresh water, occluding nostrils during water exposure, and avoiding submerging in hot springs or geothermal waters. Use of boiled or filtered water for nasal rinsing and ablation also decreases the risk of infection. Currently, no vaccines are available for prevention of these infections.

Differential diagnosis

Confusing conditions

Primary amebic meningoencephalitis is frequently mistaken for acute bacterial meningitis. Diagnostic confusion between these two entities is compounded because the CSF formula of primary amebic meningoencephalitis is similar to that seen with bacterial meningitis. Failure to identify a bacterial pathogen on culture or molecular studies might suggest the diagnosis of primary amebic meningoencephalitis, particularly if there is a history of fresh water exposure in the previous seven days. Visualization of mobile amebic trophozoites in fresh, warm CSF specimens is diagnostic. More information can be accessed at the following site:https://www.cdc.gov/parasites/naegleria/diagnosis-hcp.html.

In granulomatous amebic encephalitis, focal CNS signs and imaging is often suggestive of other causes of mass lesions, such as bacterial brain abscess, tuberculosis, fungal infection, toxoplasmosis, or tumor. The absence of amebae in the CSF in cases of granulomatous amebic encephalitis makes the diagnosis even more elusive. Cerebral biopsy (or autopsy) is often necessary to confirm the diagnosis.

Diagnostic workup

Patients with primary amebic meningoencephalitis caused by Naegleria have a clinical syndrome resembling that of acute bacterial meningitis. The CSF shows features characteristic of acute bacterial meningitis with a neutrophilic pleocytosis, elevated protein, and decreased glucose levels. Visualization of motile trophozoites in fresh, nonrefrigerated CSF specimens is considered diagnostic. Giemsa-Wright stain or modified trichrome stain of CSF may also demonstrate Naegleria trophozoites. Culture of the organism and detection by polymerase chain reaction are possible but these tests are not uniformly available outside of research settings. Identification of Naegleria in tissue by immunohistochemistry or immunofluorescence can be performed by the Centers for Disease Control and Prevention, with diagnostic assistance available (CDC.gov).

In patients with granulomatous amebic encephalitis, neuroimaging may suggest the diagnosis, although findings are often nonspecific and include single and multifocal parenchymal lesions, ring-enhancing lesions, hydrocephalus, nonspecific white matter lesions, meningeal exudates, hemorrhagic infarcts, or necrosis (33; 22; 03; 28). In contrast to primary amebic meningoencephalitis, organisms are rarely seen in the CSF. Serologic testing is of limited value given the ubiquity of these organisms in nature (26). Diagnostic testing is limited through commercial laboratories, and when granulomatous amebic encephalitis is suspected the CDC free-living and intestinal ameba laboratory can provide assistance through telediagnosis or testing of tissues (CDC.gov). PCR testing of formalin-fixed tissue may be useful for detecting free-living amebae in brain tissue (30). CSF evaluation through metagenomic deep sequencing may be diagnostic (41; 42).

Management

Early recognition and prompt aggressive management of primary amebic meningoencephalitis and granulomatous amebic encephalitis is critical given the poor prognosis of these infections. There have been no clinical trials to guide optimal treatment, and most recommendations are based on anecdotal reports of success in survivors.

Treatment of primary amebic meningoencephalitis requires combination therapy with a miltefosine-containing regimen. This medication was previously only available through the CDC but is now available commercially, with consignment programs allowing on-site stocking of medication (www.impavido.com). Studies show variable penetration of this medication into the cerebrospinal fluid (29). Other agents active against Naegleria include amphotericin B, rifampin, fluconazole, and azithromycin (06). The CDC recommends use of both intravenous and intrathecal amphotericin B deoxycholate for this infection, which is significantly more active than liposomal preparations (26). Dexamethasone is often indicated for treatment of cerebral edema. Specific regimens and dosing should be determined in consultation with experts at the CDC (770-488-7100).

Most cases of granulomatous amebic encephalitis are fatal, with combination therapy offering the best chance of survival for CNS infections with Acanthamoeba and Balamuthia. The use of drug combinations addresses resistance patterns that may exist or develop during treatment, ensuring that at least one of the drugs may be effective against the amoebae. Because of the complexity of treatment, consultation with an expert at the CDC is strongly recommended (770-488-7100). Based on reports of survivorship, the recommended regimen for treatment includes pentamidine IV, sulfadiazine, flucytosine, fluconazole, azithromycin, and miltefosine (26). The specific regimen and dosing recommendations are available online (CDC.gov). Miltefosine can be obtained commercially (www.impavido.com), although is not FDA-approved for this indication. An article describes successful treatment of B mandrillaris encephalitis with a regimen that included nitroxoline, a quinolone antibiotic (36). This agent was chosen based on screening of more than 2000 clinically approved agents to assess for in vitro activity against B mandrillaris (27).