Contaminated contact lenses have caused rates of Acanthamoeba keratitis to almost triple since 2011 in southeast England, researchers say.
The surge is likely due to multiple factors, including changes in disinfecting products and the materials used to make the lenses, according to Nicole Carnt, from the University of New South Wales in Sydney, Australia and colleagues from Moorfields Eye Hospital in London, England. They published their findings in the British Journal of Ophthalmology.
Using Moorfields data, they documented an increase in average of 50.3 cases per year between 2011 and 2016, up from 18.5 in prior years, in South East England. The data is likely to be relevant to the rest of the United Kingdom, since Moorfieds treats more than 35% of all Acanthamoeba keratitis cases in the country, the researchers say.
Acanthamoeba is a vegetative trophozoite. In the wild it usually feeds on other microorganisms, but in the cornea it is suspected of feeding on keratocytes. When they encounter harsh conditions, such as nutrient deficiency or toxins, trophozoites encyst. The cysts can resist many of the treatments meant to kill them. As a result Acanthamoeba keratitis often recurs.
Acanthamoeba ketatitis can cause severe damage, with only 70% of patients cured in a year. The most severely affected quartile end up with less than 6/24 vision after an average of 31 hospital visits and corneal transplants, the researchers report.
The incidence of Acanthamoeba keratitis among contact lens users in the United Kingdom has historically been 5 to 15 times higher compared to other countries. The researchers attributed this discrepancy to the greater use of domestic tank stored tap water in the United Kingdom.
In the United Kingdom, infections occur most often in hard-water areas where lime scale creates optimal conditions for the organism to grow.
To explore reasons for the outbreak, the researchers compared contact lens wearers who came to the hospital with Acanthamoeba keratitis infections to reusable daily wear contact lens wearers who came to the hospital for other emergencies.
They determined that risk factors included:
Lens disinfection solution failures have led to outbreaks of Acanthamoeba keratitis in both the United States and the United Kingdom in the past. Oxipol has largely been phased out by the manufacturer.
There is no standard methodology for reproducible in vitro testing of disinfectant solutions for use against Acanthamoeba, and independent testing has shown that most multipurpose disinfection solutions are not effective against the organism.
The role of lens materials is a complex one, the researchers say. Etafilcon A is one of the most widely used materials in group IV lenses. But a previous study found it to be less associated with all types of keratitis than other lens materials.
Organisms adhere differently to different lens materials. Most keratitis is bacterial, and the researchers speculated that reducing the risk of bacterial keratitis could have led to an increase in the risk of Acanthamoeba keratitis.
"Therefore, we suggest that group IV lens material users, rather than changing lens material, should optimize their lens hygiene and avoid wearing [the lenses] when exposed to water to minimise their risk," they wrote.
The researchers acknowledged weaknesses in their study. First Moorfields used different methods of data collection throughout these periods. Second, the controls were recruited in a different period from the cases to which they were compared.
They concluded that publicity for the following preventive measures should reduce the incidence of Acanthamoeba keratitis: