Pathogenicity and epidemiology

The pathogenic features of Acanthamoeba spp. have been discovered relatively recently. Environmental strains pathogenic to laboratory animals represent potential human pathogens. Acanthamoeba has been isolated from stools, throats, and nasal cavities of asymptomatic individuals. The discrepancy observed between the low number of identified cases and the common presence of Acanthamoeba in the environment can be a result of either the low virulence of the pathogen or the resistance developed by humans under frequent contacts with parasites. Commonly, the genus Acanthamoeba represents facultative, opportunistic parasites. Both cysts and trophozoites are considered invasive. The skin and upper respiratory tract are recognised as a portal of entry in immunodeficient and chronically ill patients. Infection may present as a skin ulceration, respiratory tract, ear or nose infection, or gastrointestinal symptoms. The parasites which spread by a haematogenous route may invade the

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central nervous system producing GAE, a chronic and fatal disease (Martinez 1985). GAE is a rare condition - 103 cases have been reported around the world, the majority of them in the United States. In recent years, an increased number of Acanthamoeba invasions have been reported in HIV and AIDS patients (Martinez and Visvesvara 1997).

Ocular acanthamoebiasis - AK is a disease occurring among immunocompetent and otherwise healthy people. Several species have been involved in human ocular infections. Parasites enter their hosts through damaged cornea. The disease is painful and devastating. Cellular infiltration in the cornea may result in its perforation and loss of visual acuity (Auran et al. 1987). Although antibody response during AK is low, a successful immunization against Acanthamoeba infection has been obtained in an animal model by combined subconjunctival and intramuscular administration of the parasite antigen (Alizadeh et al. 1995). The incidence of AK is low. Approximately 1 in 250000 contact lens wearers develop infection (Seal 1994). More than 700 cases have been reported since the first AK case was detected in a Texas rancher (Martinez and Visvesvara 1997). Few cases have been reported from Sweden, Norway, and Denmark, which may suggest that the condition is very rare in Scandinavian countries (Brincker et al. 1988; Aasly and Bergh 1992; Stenevi et al. 1992; Nilsson and Montan 1994; Skarin et al. 1996). The exact prevalence, however, is unknown since reporting of identified cases is often not obligatory. Initially, corneal trauma and contact with contaminated water were considered to be the main risk factors. The recent epidemiological data however, indicate that the disease is strongly correlated with soft contact lens wearer (Moore et al. 1987). Inappropriate hygiene and disinfection of lenses and lens cases using tap water or home-made washing solution may result in contamination with Acanthamoeba. High water content and gel-like structure of modern soft lenses promotes adherence of amoebae which are then easily transported to the eye. Cysts of Acanthamoeba are resistant to several commercial lens disinfectants (Ahearn and Gabriel 1997). Moist heat disinfection is effective if the recommended procedure is respected (Seal 1994). The increasing use of contact lenses results in a growing population at risk. Compliance with lens care instructions and education and information about risk factors are essential for preventing AK.

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