Fighting with brain-eating amoeba: challenges and new insights to open a road for the treatment of Naegleria fowleri infection

KEYWORDS:

• Naegleria fowleri
• Brain-Eating Amoeba
• Primary amoebic meningoencephalitis
• Antimicrobial therapy
• Immunological pathways

1.

Naegleria fowleri, often called Brain-Eating Amoeba, is the deadliest free-living thermophilic pathogen affecting the human central nervous system [1]. The epidemiological data states that its existence was rare but was first recognized in the United States in 1937 [2]. It generally grows in warm fresh waters during the months of July, August, and September in the United States but not in salt waters. However, several cases of this infection have been reported in Pakistan, and two of the documented cases of primary amoebic meningoencephalitis (PAM) were in people who had previously participated in recreational water activities. As a result, it can be concluded that N. fowleri is present in Karachi’s residential water supply and that ablution is a major cause of infection [3]. This is surprising because Karachi’s water is often salty, and amoebas cannot thrive in salt water. This data shows that either the N. fowleri strain found in Pakistan is distinct from strains reported in the rest of the world or that it has evolved a resistance to saline surroundings.

N. Fowleri had a wide distribution range worldwide (around 15 countries) except Antarctica and was most prevalent in warmer nations [4]. The pathogen even thrives and adapts to warmer or higher temperatures (50–65°C) and salinity around 1.4–2.0% NaCl and remains viable by feeding on bacteria, yeast, and algae. Though the number of cases with PAM remained stable (0–8) throughout the year, a recent breakout in South Korea is of great concern. On NaN Invalid Date , a recent breakdown of brain-eating amoeba was first reported in South Korea, according to the Korea Disease Control and Prevention Agency (KDCA). It was confirmed that the person in his 50s returned from Thailand after his 4 months stay. After admitting to the hospital the next day, he suffered from vomiting, nausea, neck stiffness, and slurred speech. On NaN Invalid Date , the man died due to an infection with N. fowleri, as suggested by the reports. Though it was reported earlier, the unavailability of effective treatment options makes it a great concern.

Over 90% of N. fowleri infections arise from activities like swimming or splashing in warm water, permitting trophozoites to enter through the nasal cavity [5]. This infection route extends to practices like religious ablutions and using hygiene devices such as neti-pots [6]. Furthermore, there’s a ‘dry-infection’ concept wherein trophozoites can form from cyst-laden dust, though this accounts for a smaller portion of PAM cases, roughly 6.5% [7]. Despite its limited occurrence, this mode is concerning due to its potential prevention challenges. Once inside the nasal cavity, the amoeba attaches, penetrates the nasal mucosa, migrates through olfactory nerves, and reaches the brain via olfactory bulb-associated nerve bundles. There, it reproduces, leading to cerebral edema, herniation, and ultimately death [6]. The virulence of N. fowleri stems from its pathogenicity and the ensuing robust immune response [6]. Although insights into N. fowleri’s virulence factors are scarce, and a range of models has been developed to explore the molecular mechanisms underlying PAM’s pathogenesis [6]. Notably, two mechanisms stand out: contact-dependent processes involving adhesion and phagocytic food-cups and contact-independent mechanisms driven by cytolytic molecules released by the amoeba [8].

According to US Centers for Disease Control and Prevention (CDC), the recent developments in global technology favor the rise in environmental temperatures and may favor the infection spread. Most PAM cases in the U.S.A. were recorded in children under the age of 14 [9], but in Pakistan, most cases are documented in adults between the ages of 26 and 45 [10], suggesting that Pakistan may have a genetically distinct strain. Before the monsoon season and during the summer, PAM cases are most frequently reported. Scientists’ focus is now on climate change because of the spread of N. fowleri in Pakistan. Due to climate change, summers are getting longer, and the humidity conditions around longer make water bodies the perfect environment for amoebas. Most recently, on July 2023, an 18 years boy was infected and dead due to N. fowleri in the Alappuzha district of Kerala state, India.

The etiology of N. fowleri is uncertain and may involve multiple factors. A thorough understanding of its genetic origin can provide light on the cause of this severe and quickly lethal disease. The precise identification of novel paralogues of well-known genes or protein families, singletons, novel genes, and genes obtained by horizontal gene transfer would be essential for understanding the mechanism of disease caused by N. fowleri. Examples of these unique contents could be found using a genomic technique that examines the entire genome of N. fowleri. With this method, an interesting research opportunity exists to clarify the genome sequence of Pakistan’s recently discovered resistant strain, aiding in early disease detection and prevention. Additionally, there is a critical need to inform people about the value of boiled water for nose-rinsing.

The treatment of PAM involves a combination of medications, which commonly include amphotericin B, azithromycin, fluconazole, rifampin, miltefosine, and dexamethasone [11]. These drugs are chosen due to their perceived effectiveness against Naegleria fowleri and their previous use in treating individuals who have recovered from the infection. However, the success rate is limited, especially when the infection is diagnosed in later stages. Among these medications, miltefosine is the most recent addition and has demonstrated the ability to eliminate free-living amoebae, including Naegleria fowleri, in laboratory settings [12]. Targets for the logical creation of therapeutic interventions will become clear with a thorough grasp of the parasite’s biology and pathogenetic mechanisms. The virulent amoebae in N. fowleri will change into the ‘avirulent’ form due to the induction of encystation, which will slow the spread of infection. Any ligand (something other than an antibody or an increase in osmolarity) that can act as a trigger to induce encystation can be used to transform pathogenic amoebae into the dormant form, just as cell surface receptors leading to conformational changes cause cyst formation [13]. A blood-brain barrier permeable compound is indeed required to reach the infection site in the CNS and induce encystation, transforming pathogenic amoebae into dormant form. The RNA-seq analysis data on differential gene expression in other protozoa upon exposure to encystment media will be instrumental in bolstering the credibility of this therapeutic regimen. Developing an effective treatment regimen against brain-eating amoebae would benefit from bioassay-guided testing of various chemical libraries, along with a thorough understanding of the structure, composition, and permeability of the outer surface membrane of Naegleria. However, extensive research is required to realize these expectations.

2. Expert opinion

Understanding the mechanisms underlying the early immune failure and the triggers for the ensuing fulminant inflammation may point to advances in clinical care as there are still no proven clinical treatments for PAM. The rapidity of PAM and the rarity of surviving individuals make it extremely difficult to decipher these immunological pathways and retroactively comprehend the human immune response. Fortunately, animal models of PAM show striking similarities to human infections and provide an effective method for describing how the immune system views and reacts to N. fowleri. Although many of the pathogenic pathways used by N. fowleri have been identified through in vitro investigations [14], a lack of mechanistic in vivo studies on the immune response to N. fowleri has left many fundamental concerns unexplained. Various research studies showed a comprehensive exploration of the immune responses and defense mechanisms against Naegleria fowleri infection, the causative agent of PAM.

The findings underscore the significance of both humoral and cellular immune responses in protecting against N. fowleri. Intranasal administration of N. fowleri lysates, in combination with cholera toxin, proves effective in inducing robust immune reactions, as evidenced by increased antibody titers and altered migration molecules in immune compartments [15]. Notably, the involvement of the Signal Transducer and Activator of Transcription 6 (STAT6)-induced T helper type 2 (Th2) response is highlighted, where Th2-deficient mice exhibit reduced protection. Furthermore, the researchers delve into the intriguing role of neutrophil extracellular traps (NETs) in responding to N. fowleri, showcasing their potential to counteract the amoeba’s evasion tactics [16]. Neutrophils release NETs to capture and neutralize pathogens, but N. fowleri was observed to evade killing by NETs unless opsonized (bound to antibodies) [17]. Identification of immunogenic antigens offers promising avenues for vaccine development, with specific antigens conferring substantial protection against infection. Altogether, the research investigations provide valuable insights into the complexities of N. fowleri immunity, offering a foundation for future strategies in preventing and managing PAM.

Despite the apparent protective potential of antibody responses, developing vaccines or immunotherapeutic strategies still needs a better understanding of the relevant antibody isotypes and Fc receptors that confer protective immunity. In addition, identifying specific antigens for recognition is crucial in developing immunotherapeutic strategies. Although it is simple to assess antibody titers, in-depth mechanistic in vivo studies are necessary to address the functions of other lymphocytes, including gamma/delta T cells, natural killer (NK) cells, NKT cells, or even possibly CD8+ T cells. This will necessitate rectifying a historical underfunding of basic research on the host response to N. fowleri and other free-living amoebic viruses. Still, it will trigger revolutionary improvements in the clinical options for preventing and treating a fatal disease. Proteomic studies have identified various proteins in Naegleria fowleri, including those involved in metabolic pathways, cell structure, and virulence. These analyses have provided a snapshot of the molecular machinery that enables the amoeba to survive and interact with its environment, contributing to potential therapeutic strategies and preventive measures against infections caused by this amoeba.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or material discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or mending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

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Funding

This paper was not funded.