**4. Predisposing/ risk factors sufficient to contract AK**

In several studies it is underlined that the initiating of AK is a multi-factorial process, in which both host and environmental determinants are likely involved, apart connected with *Acantha‐ moeba* pathogenic strains [4,12].

It is considered that *Acanthamoeba* keratitis is mainly related to contact lens wear, although, *Acanthamoeba* corneal infections are also detected in persons not using contact lenses [7,12-14, 46]. After the first case of AK associated with contact lenses in Central Europe was reported from Germany, more than 85% of all incidences of the disease have been recognized in different countries in wearers of contact lenses [12,17,18,47-50].

The estimation of AK findings in several countries showed various, generally relatively low, but constantly increasing number of the cornel disease incidents during the last few decades i.e. 1.36 cases per million contact lens wears in the United States, 17 to 21 cases per million in England, 1 per 30,000 contact lens wears in France, 0.05 per 10,000 in Holland [4,12,13]. However, "it is noteworthy", as Khan [4] concluded "that variations in the incidence rate of *Acanthamoeba* keratitis do not reflect the geographical distribution of *Acanthamoeba*, and are most likely due to variations in the extended wear of soft contact lenses, varied awareness of the potential risk associated with the contact lens wear, enhanced detection, and/or local conditions that promote growth of pathogenic *Acanthamoeba* only e.g.. water hardness or salinity, or conditions that suppress growth of non-pathogenic *Acanthamoeba*." Interestingly, in Austria, women and men were affected almost equally; the highest AK incidences occurred in the 21-30-years-old patients; simultaneously, poor contact lens hygiene is indicated as the most important risk factor of AK in this country [46].

Some micro-traumas occurring earlier or appearing in connection with the use of the lenses predispose to contract AK; a human organism's susceptibility, tissue specificity, tear factors, and secretory immunoglobulin A (sIgA), important in the specific immune defense mecha‐ nism, are among other host factors influencing development of this corneal disease. Environ‐ mental conditions such as temperature, osmolarity, and pH may be important in initiating AK.

Simultaneously, the amoebae were found in contact lens and in storage cases that may be potential sources and reservoirs of the facultative parasites [12,18,47-53]. In spite of this, the incidence rate of AK in wearers of contact lenses is remarkably low in comparison with the contact lens storage cases contaminated with *Acanthamoeba*.

Additionally, it has been confirmed for various *Acanthamoeba* strains that a swimming in recreational pools while contact lenses wearing promotes the infection; it is because of some human corneal micro-defects caused by lenses and extremely high resistance of the amoebae to chlorine disinfectants [12,17, 49-53].

In persons not using contact lenses, other circumstances influence as important for contract AK [4,47]. The different *Acanthamoeba* strains are ubiquitous in natural and man-made environments thus, an exposure of the eye especially to dust, water or moist soil, as well as to any foreign particle, on which trophozoites and cyst of the amoebae can occur, is considered as an AK predisposing factor. Also, if corneal epithelial injuries appear, and also during eye surgery, circumstances promoting the infection may occur.

There have been no reports of *Acanthamoeba* keratitis being spread from one person to another.
