Acanthamoeba Keratitis – A Growing Menace For Contact Lens Users?
The name Acanthamoeba comes from the Greek where ‘acantho’ means curled referring to the spear shaped pseudopodia of the trophozoite. Infection of humans with free-living amoebae is uncommon. CNS invasion by Naegleria, Acanthamoeba, and amoebae of the order Leptomyxida has been reported in fewer than 200 people worldwide, while Acanthamoeba keratitis has been diagnosed in more than 200 patients.
Free living amoebae belonging to the genus Acanthamoeba have been found world wide in soil, dust, air and water and are relatively resistant to normal level of chlorine in tap water. Recent interest in Acanthamoeba spp. has focussed on their causative role in a painful, vision threatening keratitis that occurs mainly in contact lens users. Although disseminated Acanthamoeba infection is increasingly described in immunocompromised host, Acanthamoeba keratitis is usually seen in the healthy individuals. Till date a total of 1350 cases have been reported worldwide. Of the first 100 cases of Acanthamoeba keratitis reported to the Centre for Disease Control and Prevention, 83% occurred in people who were contact lens wearers and corneal infection was associated with the use of homemade saline to clean the lenses and wearing the lens while swimming. In non contact lens wearers, it is generally accepted that eye infection ensues subsequent to minor corneal trauma with introduction of amoebae from environmental sources.
At this moment there are more than 35 species known (based on cyst morphology, immunofluorescence and on isoenzyme structure), among which possible causative agents for Acanthamoeba Keratitis include: A.castellani, A. polyphaga, A. hatchetii, A. culbertsoni, A.rhysodes and A. griffini.
From a historical perspective Acanthamoeba Keratitis has been described as a recent epidemic. It was extremely rare before the widespread use of contact lenses. The first case of Acanthamoeba Keratitis, that involved Acanthamoeba polyphaga, was reported in 1974 when 隱形眼鏡網店 a Texas rancher splashed tap water from a contaminated river source into his eye. Very little is known about incidence of Acanthamoeba Keratitis before 1970s. The number of cases started to increase dramatically beginning in 1984, and by 1985, an association with the use of contact lenses was established. It is interesting to note that thiomersol, a mercury-based preservative used in contact lens solutions, was increasingly withdrawn from use at the same time owing to reports of thiomersol-related superior limbic keratoconjunctivitis and other allergic reactions.
Risk factors for infection in contact lens wearers are:
o Use of tap water during lens care (to rinse lenses or the storage case).
o Wearing lenses while swimming (without goggles), showering or in hot tubs.
o Use of ineffective lens care solutions.
o Failure to follow lens care instructions.
Many contact lens users ignore the advice of their contact lens practitioner and lens care instructions and rinse their lenses or storage case in tap water, which may introduce Acanthamoeba to the storage case. Once inside the case, Acanthamoeba can survive and grow, feeding on bacteria that may also contaminate the case. Organisms are then transferred from the case to the cornea on the contact lens. The lens holds the organisms in place on the eye, which may ultimately lead to infection.
The use of contact lenses while in contact with water sources that may be contaminated – i.e. swimming in fresh, salt or chlorinated water, water sports, showering and use of hot tubs – is also a risk factor for Acanthamoeba keratitis. The infecting organisms can reach the eye directly from the water source, or indirectly by introduction of Acanthamoeba into the storage case when a lens contaminated by the water source is not cleaned properly.