2. Epidemiology and Pathogenesis
Acanthamoeba is one of three amoebic parasites that are thought to be significant to human disease, amongst Entamoeba (responsible for amoebic dysentery) and Naegleria (responsible for amoebic meningoencephalitis). Acanthamoeba is known for causing keratitis or granulomatous amoebic encephalitis (GAE). Found naturally in soil and fresh water, it can exist in the pathogenic trophozoite form, or in times of physiological stress, it will encyst and become metabolically dormant. The cystic form is very resistant to chemical injury or desiccation, which makes it harder to treat than other microbes. Acanthamoeba has been found to colonise the nasal mucosa in up to 24% of environmentally exposed populations [8], although its pathogenic activity is much more rare. Acanthamoeba keratitis has a large variation in reported rates between countries, which is thought to be largely due to differences in diagnostic criteria, rather than differences in populations. Schaumberg et al. found a US incidence of 2 cases per million contact lens wearers in the late 1980s [9] as compared to over 21 cases per million contact lens wearers in the UK in 1998 [10]. Whether this growth represents an increased accuracy in diagnosis, a shift in the habits of contact lens wearers, or indeed a combination of the two, we have to accept that Acanthamoeba keratitis is becoming an increasingly significant problem.