Infection is also more commonly seen in tropical or sub-tropical climates.Beyond the route of inoculation into the eye and external risk factors, host factors are also likely to play a significant role in the development of Acanthamoeba keratitis. In fact, studies of contact lens users in the United Kingdom, Japan, and New Zealand found that 400 to 800 per 10,000 asymptomatic contact lens users had lens storage cases contaminated with Acanthamoeba spp. However, the rate of Acanthamoeba keratitis among these patients was only 0.01 to 1.49 per 10,000 contact lens users. Although the exact host factors have not been fully described, it is likely that corneal epithelial defects, tear film composition, eye surface pH, and the level of anti-Acanthamoeba IgA antibodies in the tear film play a role in the development of Acanthamoeba keratitis. Life cycle
Species within the genus, Acanthamoeba, are generally free-living trophozoites. These trophozoites are relatively ubiquitous and can live in, but are not restricted to, tap water, freshwater lakes, rivers and soil. In addition to the trophozoite stage, the organism can also form a double-walled cyst which may also be present in the environment, and can be very difficult to eradicate through medical treatment. Both of these stages are usually non-nucleated and reproduce by the means of binary fission. Diagnosis
Due to the relative rarity of Acanthamoeba keratitis (AK) compared to other causes of keratitis (bacterial, viral, etc. ), it is often misdiagnosed, especially in the early stages of the disease.
Biguanides have also been found to act synergistically when used in combination with diamidines, with propamidine isethionate and hexamidine being the most commonly used. A limitation of diamidine use is relative corneal toxicity with long term use. A combined regimen of propamidine, miconazole nitrate, and neomycin has also been suggested. Due to the potential for negative longterm visual outcomes with AK, therapy is usually started with a combination of a biguanide and a diamidine. Early use of high dose dual therapy helps to eliminate both trophozoite and cyst forms of the organism, while also preventing deep penetration of cysts into the corneal stroma. Cysts that are not eradicated from the cornea will cause recurrence. The treatment is often initiated by instilling drops onto the surface of the eye every hour, 24 hours a day, for at least the first 48–72 hours. If an appropriate response to therapy, this may be reduced to hourly administrations during the day only, which is continued for several weeks to months.Beyond anti-amoebic therapies, there is also a role for topical steroids of anti-inflammatory medications in the treatment of Acanthamoeba keratitis. During infection, severe inflammation in the cornea and anterior chamber can cause more severe symptoms including pain and visual disturbance. Topical steroids may be used to reduce this inflammation and thereby alleviate symptoms. However, the role of steroids is typically very limited, because their dampening of the immune response may lead to worsening of the infection. Additionally, steroids can increase the number of trophozoites in the cornea by inducing excystation.