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Although there are over 30 species of Naegleria, only one of these, Naegleria fowleri, is pathogenic. Like the other members of the genus, N. fowleri is a free‐living amoeboflagellate. That is, an amoeba that in one of its life cycle stages possesses flagellae. It is a cosmopolitan species normally found in freshwater ponds and lakes, but it lives in numerous wet or moist environments such as swimming pools, humidifier systems, and damp soil (Siddiqui et al. 2016). There are three life cycle stages: the active amoeboid trophozoite, the non‐feeding flagellate stage that is produced when the food supply runs low and acts as a dispersal stage, and a cyst stage that forms in response to adverse environmental conditions (Figure 3.3). Free‐living N. fowleri feed on bacteria that they ingest using special ‘feeding cups’ on the outer cell membrane, and they are often packed with vacuoles containing microbes. If the flagellate stage enters the nasal cavity, it can transform to the trophozoite stage and become invasive.

Figure 3.3 Life cycle of Naegleria fowleri. This is normally a free‐living species found in warm ponds, lakes, and soil. The trophozoite stage (T) has 1–12 suckers that it uses to feed on bacteria. The flagellate stage (F) develops in response to adverse conditions and is the dispersal stage; it forms in response to adverse conditions. The cyst stage (C) develops in response to dehydration and is not infective. Human infections arise when the trophozoite stage enters the nasal cavity. The amoebae penetrate the lining of the nose and reach the brain via the cribiform plate where they cause primary amoebic meningoencephalitis. Drawings not to scale.

The trophozoite is the infective stage, and it gains entry via our nose. We usually become infected by swimming in infected water. However, Mahmood (2015) suggested that a rise in the number of cases in Pakistan might be linked to the practice of ‘wudu’ (also referred to as ‘wuzu’ and ‘ablution’) by muslims before they pray. Wudu involves irrigating the nose with water, and if contaminated water is used, the practitioner could become infected. After entering our nose, the trophozoite migrates from the nasal mucosa along the olfactory nerves through the cribiform plate and thence into the brain where it causes primary amoebic meningoencephalitis. The use of the term ‘primary’ distinguishes it from encephalitis caused by E. histolytica in which invasion of the brain is a secondary consequence of infection in the gut. The trophozoite of N. fowleri moves remarkably quickly: the time between initial exposure and first symptoms takes as little as 24 hours and death commonly occurs after 4–10 days. The trophozoites ingest host tissues and red blood cells and cause a serious inflammatory reaction that contributes to the pathology. Highly pathogenic strains of N. fowleri kill cells on contact – presumably by secreting toxic substances. The mortality rate is more than 90%. The symptoms of infection are non‐specific and often start with neck stiffness followed by headaches, photophobia, confusion, seizures, and the patient then enters a coma from which he/she seldom recovers. There are isolated case reports of it causing meningoencephalitis in various domestic and wild animals including cattle (Pimentel et al. 2012) and rhinoceros (Yaw et al. 2019).