Brain-eating amoebae: is killing the parasite our only option to prevent death?

KEYWORDS:

• Brain-eating amoebae
• Naegleria fowleri
• Cysts: Primary amoebic meningoencephalitis
• treatment
• encystation

Naegleria fowleri is a free-living protist pathogen with the ability to produce fatal infection of the central nervous system with over 95% mortality rate [1–3]. It is considered to be one of the deadliest parasites known to humans, i.e., if contracted, it almost always results in death. Being free-living, it is widely distributed in the environment, particularly in warm freshwater [1–3]. Humans and animals contract this parasite when exposed to contaminated water. Parasites enter via the nose and then traverse the olfactory neuroepithelial route to invade the central nervous system (CNS) via porous cribriform. Once there is involvement of the CNS, it almost always warrants mortality. This is owing to the inefficacy of drugs to kill the parasite effectively [1]. Although advances in antimicrobial therapy over the past several decades have been made, it is distressing that the mortality rate has remained significant, suggesting the need to find effective therapies. A complete understanding of the biology of the parasite and its pathogenetic mechanisms will elucidate targets for the rational development of therapeutic interventions.

Depending on the environmental conditions, it exhibits three phenotypic stages: (i) under favorable conditions it transforms into a trophozoite/amoeboid form during which it feeds, propagates, and expands in numbers; (ii) in the absence of food, it transforms into a flagellate form that allows it to travel long distance in search of food; and (iii) under harsh conditions, and it transforms into a cyst form [2]. During the cyst stage, Naegleria becomes dormant with little metabolic activity, but it can remain viable for decades. Basically, the parasite builds a shell around itself and goes into hibernation until the return of favorable conditions. In this regard, cyst formation is an integral part of the life cycle of Naegleria. This process is known as ‘encystation’. Upon return of favorable conditions, the parasite leaves the shell and emerges as a viable trophozoite (a process known as excystation), multiplies, and establishes an infection, if contracted by a host upon accidental encounter with humans and animals.

Notably, during the cyst form, Naegleria is harmless as it does not feed or attack human cells. It is only a requirement to survive stressful conditions such as famine, extreme changes in pH, temperatures, irradiation, osmolarity, drugs, etc. [1–3]. In the cyst form, parasite neither binds nor produces damage to the human cells. In this regard, the cyst form of the parasite can possibly be considered as ‘avirulent’. As long as the parasite remains in the cyst form, it will not produce damage to the host tissue. This is consistent with findings that N. fowleri is always found in the host brain tissue in the trophozoite form and never in the cyst form. In addition to finding drugs that can effectively and selectively kill the parasite, here we propose that identifying drugs that can induce encystation will be an effective strategy against primary amoebic meningoencephalitis (PAM). Once encysted, N. fowleri presents no threat; hence, identification of compounds that can induce dormancy can lead to the rational development of novel therapeutic strategies. Killing the parasite is far harder than inducing its phenotypic transformation. In contrast to the high concentration of drugs needed to kill the parasite that can have associated host tissue damage, simple exposure to physiological changes, chemicals, environmental and radiological changes, etc. can induce encystation. For example, Cordingley et al. showed that osmolarity can induce encystation in the related parasite Acanthamoeba [4]. At the molecular level, Yang and Villemez identified monoclonal antibodies that specifically bind to the membrane protein (40 kDa) of trophozoites and induce encystation indicating receptor–ligand interactions, leading to phenotypic transformation [5]. Other external stimuli may also involve cell surface receptor changes, leading to structural and conformational changes in the receptor and inducing encystation process. Accordingly, any circumstance(s) that encourages receptor conformational changes will initiate encystation. Additional studies, by Cordingley et al., revealed that both monoclonal antibodies and increased osmolarity stimulate encystation by utilizing similar pathways [4], yet the receptor(s) and the underlying molecular mechanisms in Naegleria are unexplored. Once encysted, Naegleria do not present a threat; hence, identification of compounds that can induce encystation in Naegleria can be of tremendous value in developing novel therapeutic strategies. Given the fulminant nature of PAM that can lead to death within a few days, the proposed strategy has the potential to allow alternative therapeutic measure and/or immunity-driven eradication of the parasite.

Overall, the induction of encystation in N. fowleri will lead to transformation of virulent amoebae into the ‘avirulent’ form and delay the infection process. As the cell surface receptors leading to conformational changes induce cyst formation, any ligands (molecules in addition to antibody or osmolarity) that can act as a trigger to induce encystation can be used to turn virulent amoebae into the dormant form. Given the large molecular mass, antibodies are unlikely to cross the blood–brain barrier, there is a need to identify permeable compounds to reach the infection site in the CNS to trigger encystation. If small molecules that are blood–brain barrier permeable are identified, this approach can be of potential value in designing combinatorial chemotherapies against this devastating infection. Additionally, differential gene expression analysis using RNA-seq in response to encystation media could be of value in the identification of specific receptors involved in the cellular differentiation process. Bioassay-guided testing of a range of chemical libraries, together with a thorough understanding of the structure, composition, and knowledge of the permeability of the outer surface membrane of Naegleria, would be valuable in determining appropriate treatment regimen against brain-eating amoebae; however, intensive research is needed to realize these expectations.

Author contributions

RS and NAK envisioned the concept amid critical discussions with JSM. RS reviewed literature and NAK prepared the first draft of the article. RS and JSM finalized the manuscript. All authors contributed equally to the manuscript and will act as guarantors.

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 materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

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

Acknowledgments

We are grateful to the University of Sharjah, UAE, for supporting this work.

Additional information

Funding

This paper was not funded.