Naegleria Infection and Primary Amebic Meningoencephalitis

Etiology

N fowleri is the causative agent of PAM, which is a rare but rapidly progressive and usually fatal infection of the CNS.[4] Although Acanthamoeba spp and B mandrillaris are opportunistic pathogens that cause granulomatous amebic encephalitis in immunocompetent and immunocompromised individuals, N fowleri primarily affects healthy, immunocompetent children and young adults who have been exposed to contaminated freshwater.[10]

Widely distributed worldwide, N fowleri thrives in warm freshwater environments as a thermophilic ameboflagellate, growing at temperatures up to 45 °C. This amoeba is commonly found in soil, lakes, and rivers and can also persist in inadequately chlorinated water sources, such as swimming pools, tap water, and industrial wastewater.[7][8]

The life cycle of N fowleri consists of 3 stages—trophozoites, flagellates, and cysts. The trophozoite stage is the reproductive and invasive form responsible for human infection. As a thermophilic organism, it is most active during the warm summer when human exposure is more likely. Trophozoites are long and slender, approximately 22 µm long, equipped with pseudopodia for movement and bacterial ingestion, and capable of forming large colonies.[11] 

Under poor nutritive conditions, trophozoites transition into flagellates by developing flagella.[11] If unfavorable conditions persist, they further transform into metabolically inactive cysts, measuring 7 to 12 μm. These cysts are highly resistant, capable of withstanding low temperatures, and can survive during winter.[11] Trophozoites feed on bacteria and fungi in warm waters and can encyst and settle into sediments when water temperatures drop during winter.[12]

Of the 47 known species of Naegleria, only N fowleri causes PAM, which is a rare but aggressive infection that typically leads to death within 3 to 7 days.[4][5] Infection occurs when contaminated water enters the nasal passages, usually through freshwater exposure or nasal rinsing with nonsterile water.[13] Once inside, N fowleri migrates to the CNS via the olfactory nerves and cribriform plate, leading to severe and nearly always fatal infection.[4] Notably, ingestion of N fowleri does not cause infection.

Epidemiology

N fowleri is ubiquitous and has been found worldwide in soil and freshwater environments, including lakes, hot water springs, poorly chlorinated pools, and thermally polluted water bodies.[7][14][15] Although cases of N fowleri PAM have been reported on all continents, the true incidence is likely underdiagnosed or underreported.[16] The disease has an incubation period of 1 to 14 days and progresses rapidly, presenting with nonspecific signs and symptoms. A high index of suspicion is essential for diagnosis, as mortality reaches 98%, with a median time from symptom onset to death of 5 days.[17] In many cases, death occurs before a diagnosis is made, and the disease is confirmed postmortem if an autopsy is performed.[16][18] 

Diagnosing PAM requires specialized laboratory tests on cerebrospinal fluid (CSF) or brain tissue, which must be conducted promptly. However, many hospitals lack the necessary diagnostic resources. Establishing surveillance systems and classifying PAM as a notifiable disease would improve data collection and facilitate early recognition.

Globally, N fowleri infections are more prevalent in healthy, immunocompetent young males during the summer when water temperatures are high and people engage in outdoor water activities.[11] A recent study analyzing global epidemiological data from published and unpublished reports and the Centers for Disease Control and Prevention (CDC) Free-Living Ameba Surveillance System (1962–2018) identified 381 cases of PAM caused by N fowleri—157 from the United States and 223 from 32 other countries.[4] Of these, 182 cases were confirmed, 89 were probable, and 110 were suspect. The highest number of cases worldwide occurred in the United States, Pakistan, and Mexico.

In the United States, N fowleri has been responsible for 157 known cases from 1962 to 2022, with only 4 survivors.[17] Annually, 0 to 8 laboratory-confirmed cases are reported, and PAM accounts for fewer than 0.5% of diagnosed encephalitis-related deaths.[16] Historically, most N fowleri infections have been reported in southern states, particularly Florida, Texas, and southern California. However, recent cases have been detected in northern states, such as Maryland, Indiana, northern California, Connecticut, and Minnesota, potentially due to rising temperatures and increased waterborne exposure.[7][17][19]

Most N fowleri infections occur through recreational freshwater exposure, such as swimming or diving.[14] However, rare cases have been linked to household water use. In Arizona, 2 children contracted PAM while bathing at home, and N fowleri was traced to an untreated community well-water system.[20]

Pathophysiology

N fowleri is a thermophilic amoeba that thrives in temperatures up to 45 °C and is widely distributed in warm freshwater environments. This amoeba is commonly found in soil, lakes, and rivers but can also persist in inadequately chlorinated water sources, such as swimming pools and tap water.[7][8] 

N fowleri infection occurs when contaminated freshwater enters the nasal passages, typically during swimming in freshwater bodies, using poorly chlorinated pools, or performing nasal rinses with nonsterile water.[9] After entering through the nose, N fowleri migrates via the olfactory nerves, crosses the cribriform plate, and invades the CNS, thereby causing PAM. This infection is characterized by cerebral edema, hemorrhagic necrosis, herniation, and, in most cases, death.[4] The most severely affected regions of the brain include the olfactory bulbs, the basilar portion of the frontal cerebrum, and the cerebellum.

N fowleri triggers a strong innate immune response, and its virulence is influenced by multiple factors, including the protein Nfa1, nitric oxide production, and pore-forming proteins. The Nfa1 protein facilitates amebic attachment to target cells, while specialized feeding structures enable the amoeba to ingest bacteria and fungi in the environment and directly phagocytose brain cells. The organism further contributes to tissue destruction by secreting cytolytic molecules, including cysteine proteases, phospholipases, and phospholipolytic enzymes, which mediate extensive necrosis. This intense immune response and aggressive virulence of N fowleri result in significant destruction of brain parenchymal tissue, leading to the rapid progression of PAM.[21]

History and Physical

A detailed patient history and thorough physical examination are crucial for diagnosing PAM. The disease is challenging to identify clinically, as its early symptoms are often nonspecific and can resemble flu-like illnesses, bacterial meningitis, or viral meningoencephalitis. Clinicians must maintain a high index of suspicion and be aware of epidemiological risk factors and potential exposures, as failure to recognize PAM can result in delayed diagnosis and rapid fatality.[3] 

Given its severity, high fatality rate, and limited treatment options, obtaining an accurate epidemiological history is crucial. Clinician awareness is necessary for prompt diagnosis and the immediate initiation of available therapies.[3] 

The incubation period for PAM ranges from 1 to 14 days. Early symptoms, such as fever, headache, lethargy, nausea, and vomiting, may resemble bacterial meningitis. As the disease progresses, more severe manifestations develop, including confusion, neck stiffness, photophobia, seizures, and cranial nerve abnormalities. In most cases, PAM progresses rapidly, leading to coma and death within a matter of days.

Evaluation

Diagnosing PAM is challenging, as patients do not always present with the classic symptoms of meningitis. Early symptoms are often vague and nonspecific, resembling mild viral or bacterial infections. Patients may initially experience fever and headache, which can progress to meningismus and confusion, depending on the stage of the disease at the time of presentation.[22] Given its rapid progression and high fatality rate, PAM should be suspected in any patient with meningoencephalitis or meningitis who has a recent history of freshwater exposure.

Laboratory findings in PAM can be nonspecific. CSF analysis often mimics bacterial meningitis, showing low to normal glucose levels, elevated protein, and leukocytosis with a polymorphonuclear predominance. CSF pressures are typically elevated, with recorded values reaching up to 600 mm H₂O in some cases.[1][22] However, standard laboratory tests may not be sufficient for diagnosis.

Rapid diagnosis is critical due to the aggressive nature of PAM. Few laboratories in the United States and worldwide can test for N fowleri. Even when specimens are sent to reference laboratories, the disease often progresses too quickly for the results to be clinically useful. If CSF is obtained, several diagnostic tests can help identify N fowleri. A quick method involves visualizing trophozoites directly using a wet mount. Staining techniques, including Giemsa, Wright, hematoxylin and eosin (H&E), and periodic acid–Schiff (PAS), can also be utilized.

Polymerase chain reaction (PCR) testing can detect N fowleri DNA directly from CSF and is now considered the gold standard for PAM diagnosis. However, PCR limitations include limited availability and the necessity of clinical suspicion. Gram staining and standard bacterial cultures are unreliable, as the fixation process destroys the amoebae. Antigen detection via immunohistochemical staining can also be performed on CSF samples.[23][24]

Additional diagnostic techniques for tissue samples include immunohistochemical staining, indirect immunofluorescence, PCR, and culture. Serological testing is generally not useful, as PAM progresses too rapidly for results to be available before the patient’s death. Magnetic resonance imaging (MRI) may provide some insight, although findings are often nonspecific in the early stages. MRI may show diffuse hemorrhages, infarctions, or contrast enhancement in affected brain regions as the infection advances.

A brain biopsy, if feasible, can provide definitive evidence of PAM. Histopathological examination typically reveals hemorrhagic meningitis, with the base of the brain and olfactory bulbs being the most severely affected areas. Numerous trophozoites are often observed within the necrotic meninges.[3]

Metagenomic next-generation sequencing has recently gained attention as a promising diagnostic tool for free-living amoebae, including N fowleri.[17] This shotgun sequencing approach enables broad pathogen detection from CSF, tissue, and serum samples without requiring large specimen volumes. Microbial cell-free DNA analysis has also been explored for detecting free-living amoebae. A 2023 report identified cases of Acanthamoeba spp, B mandrillaris, and N fowleri between 2018 and 2021 using this technique.[25] These emerging sequencing technologies have the potential to reduce diagnostic delays and improve early detection, ultimately enhancing patient outcomes.

Treatment / Management

Treatment options for PAM remain limited due to the lack of controlled trials or clinical studies. Current therapeutic approaches are based on in-vitro studies and case reports.[26][27][28][29](B3)

The recommended treatment regimen typically involves a combination of 5 or 6 antimicrobial or antifungal agents that either demonstrate in-vitro activity against N fowleri or have been associated with survival in patients. Miltefosine is the most recent addition to this regimen, which commonly includes amphotericin B, azithromycin, fluconazole, rifampin, and dexamethasone. However, the efficacy of these drugs remains inconsistent, and their use is often linked to significant toxicity.[11][30]

• Amphotericin B is the primary drug of choice for PAN, exhibiting amebicidal activity at low concentrations. This drug is recommended to be used in combination with at least one other amebicidal agent. The CDC advises the use of conventional amphotericin B over the liposomal formulation for both intrathecal and intravenous administration, as in-vitro studies indicate a higher minimum inhibitory concentration (MIC) for the liposomal formulation compared to conventional amphotericin B.[30]

• Miltefosine, originally developed as an antineoplastic agent for breast cancer, has shown promise in treating N fowleri infections. Notably, 2 patients who received miltefosine in 2013 and another in 2016 survived, marking a significant milestone in PAM treatment. Although miltefosine was initially accessible only through the CDC, it is now commercially available.[31][32]

• Azoles, such as fluconazole and voriconazole, penetrate the CNS effectively and are commonly used as adjunctive therapy with amphotericin B.

• Azithromycin has demonstrated in-vitro and in-vivo activity against N fowleri, with some studies suggesting a synergistic effect when combined with amphotericin B.[11]

• Rifampin exhibits amebicidal activity against N fowleri in vitro and enhances the effectiveness of amphotericin B when administered intravenously. It is commonly included in standard PAM therapy; however, clinicians should be mindful of potential drug-drug interactions. Rifampin induces hepatic cytochrome P450 enzymes, which can affect the metabolism of azoles and amphotericin B.

• Controlled hypothermia has been used in recent cases of PAM, with 3 children surviving the infection in 2013 and 2016 after receiving it as part of their treatment. These patients achieved full neurological recovery and were able to return to school, suggesting that controlled hypothermia may have a role in improving outcomes.[33][34]
(B3)

Differential Diagnosis

Patients with PAM do not always exhibit the classic symptoms of meningitis, making diagnosis challenging. Early symptoms are often vague and nonspecific, resembling mild viral or bacterial infections. Patients may initially present with fever, headache, and general malaise, which can progress to meningismus and confusion depending on the disease's stage at presentation.[22] 

PAM should be suspected in any patient presenting with meningoencephalitis or meningitis, particularly if there is a recent history of freshwater exposure. A thorough patient history is vital for identifying potential risk factors, and clinician awareness is essential for ensuring timely recognition and intervention.

Prognosis

PAM is a rare but rapidly progressing disease with a poor prognosis, even when treatment is administered. N fowleri infection is associated with a mortality rate exceeding 98%, with a median time from symptom onset to death of just 5 days. Of the 157 reported cases in the United States between 1962 and 2022, only 4 patients survived.[17] 

Diagnosing PAM is challenging due to limited awareness among healthcare professionals. The disease is rarely considered in the differential diagnosis, as its early symptoms resemble viral or bacterial meningoencephalitis and flu-like syndromes. Moreover, laboratory resources to confirm the diagnosis are often unavailable. With no standardized, highly effective treatments, PAM remains nearly universally fatal.

Complications

N fowleri infection causes PAM, which is a rapidly progressive and often fatal disease. The infection leads to severe cerebral edema, hemorrhagic necrosis, and increased intracranial pressure, which can result in brain herniation and death within days of symptom onset. Other complications include seizures, cranial nerve dysfunction, hydrocephalus, and coma.

Due to its aggressive nature, PAM has a 98% mortality rate, with most patients succumbing despite treatment. Survivors may experience neurological deficits, although recovery is rare. Early diagnosis and prompt initiation of therapy are critical in preventing fatal outcomes and minimizing long-term complications.