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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 of infection arise when the amoebae disseminate from their initial entry site via the blood stream and reach the brain. This can result in granulomatous amoebic encephalitis that has a high fatality rate. More commonly, Acanthamoeba causes keratitis (inflammation of the cornea) (Roozbahani et al. 2018) when it invades the surface of our cornea. This usually occurs following trauma to the eye and/or if we use contaminated contact lenses. Soft contact lenses are particularly likely to harbour the parasite and transfer it to the wearer’s cornea. The infection is extremely painful and if not successfully treated may lead to the loss of the eye. Acanthamoebae are extremely common organisms in the environment, so it is surprising that infections are not reported more frequently than they are. Presumably, this is at least in part owing to an effective host immune response.

Acanthamoeba castellanii is the species most associated with keratitis. It harbours numerous other microorganisms including bacteria such as L. pneumophila, Chlamydia pneumoniiae and Pseudomonas spp., yeast such as Cryptococcus neoformans and various Mimiviridae. The extent to which the amoeba transmits these infections is uncertain, but the associations result in the acquisition and maintenance of microbial virulence factors (Guimaraes et al. 2016). Cryptococcus neoformans is not pathogenic in healthy individuals, but it causes potentially fatal meningitis in those who are immunocompromised. Many species of amoebae harbour mimiviridae although the effect they have on their hosts is uncertain. There are isolated case reports of mimiviruses in association with pneumonia (e.g., Saadi et al. 2013). However, the role of mimiviruses in human disease remains controversial (Abrahão et al. 2018).

Figure 3.5 Life cycle of Acanthamoeba spp. These amoebae are normally free‐living in ponds, lakes, and soil. Both the trophozoite (T) and the cyst stage (C) are probably infectious and enter via skin wounds or through the lining of the nose. Infections of the eye also occur. Fatal disease occurs when they invade the brain. Drawings not to scale.