moshkovskii in association with diarrhoea will be reported once molecular‐based diagnostic techniques become widely used.
3.2.4 Entamoeba gingivalis
Entamoeba gingivalis is commonly found in swab samples taken from the gingival crevices of our mouths. E. gingivalis does not form cysts, and therefore, transmission is probably through kissing or sharing food and eating implements. There are occasional case reports of the recovery of E. gingivalis from the vagina (Bradbury et al. 2019) and the lungs (Jian et al. 2008). Although often implicated in periodontitis (an inflammatory disease that affects the gums and the bone surrounding the teeth), it infects both healthy and diseased individuals (Bonner et al. 2018). Part of the problem in determining its association with disease is the wide variation of rates of recovery of E. gingivalis from samples. This is probably owing to the collection techniques employed, and the recommendation is to take 5–10 samples from each person.
3.2.5 Naegleria fowleri
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).
3.2.6 Balamuthia mandrillaris
Balamuthia mandrillaris is another cosmopolitan amoeba capable of causing fatal encephalitis (Balamuthia amoeba encephalitis). Its species name derives from its discovery as the cause of a fatal brain infection of a mandrill baboon at San Diego Wild Animal Park in California in 1986. Although considered free‐living, particularly in association with soil, there are many more reports of it causing infections than of its recovery from the environment. There are two life cycle stages – the trophozoite and the cyst stage: which of these is/are the infective stage is uncertain (Figure 3.4). Many amoebae feed on bacteria but while B. mandrillaris ingests them, they do not appear to sustain growth. By contrast, at least in cultures, B. mandrillaris grows well when fed other species of amoebae or human tissue culture cells. Unlike N. fowleri, B. mandrillaris tends to cause a chronic disease in humans that may last up to 2 years – although with a similar almost invariable (>98%) fatal outcome. The mode of entry is uncertain but, in several case reports, it appears to have been through puncture wounds in the skin from which it then spread via the blood stream. There are also case reports of infections through organ transplant and nasal lavage using (probably) infected water. Balamuthia mandrillaris will infect tissues other than the brain, including the kidneys, pancreas, and the skin. It probably gains access to the brain via the choroid plexus (Jayasekara et al. 2004). Once established in the brain, the amoebae cause a granulomatous reaction and the site of the infection becomes surrounded by macrophages. The pathology is therefore called granulomatous amoebic encephalitis.
Figure 3.4 Life cycle of Balamuthia mandrillaris. This species exists either as a free‐ living organism in ponds, lakes, and soil or as a parasite. Both the trophozoite (T) and the cyst stage (C) are probably infectious and enter via skin wounds or through the lining of the nose. It damages various organs including the skin, kidneys, lungs, adrenal glands, and pancreas. Invasion of the central nervous system probably occurs via the choroid plexus and results in potentially fatal granulomatous encephalitis. Drawings not to scale.
Balamuthia mandrillaris is a zoonotic parasite and natural (and often fatal) infections occur in many species of wild and domestic animals (Visvesvara et al. 2007). In common with other free‐living amoebae, B. mandrillaris often acts as host for various bacteria, including Legionella pneumophila (Shadrach et al. 2005). However, its importance as a transport host for microbial infections is uncertain.
3.2.7 Genus Acanthamoeba
The genus Acanthamoeba contains over 20 species, and they are amongst the most common free‐living soil amoebae. They also live in various freshwater habitats, and several are opportunistic pathogens. There are two life cycle stages: the active feeding and dividing trophozoite stage and the cyst stage (Figure 3.5). Opportunistic infections arise through skin wounds and possibly via breathing in the cysts. Infections also possibly arise through swimming in infected water. Immunocompromised individuals are at particular risk of infection.
The most serious consequences
