An outbreak of a rare type of eye infection that can lead to blindness has sparked concern among people in the US who wear contact lenses.
Preliminary investigations by the country's health officials have suggested a potential link between the infections - caused by a parasitic amoeba - and use of a moisture-locking contact solution. But some experts believe that the outbreak simply signifies a larger problem with multipurpose contact solutions that simultaneously aim to disinfect and lubricate lenses.
On 25 May, Advanced Medical Optics announced a voluntary recall of its Complete MoisturePlus contact solution.
The US Centers for Disease Control and Prevention (CDC) issued a health advisory the same day tentatively linking this lens solution to the outbreak of Acanthamoeba keratitis - damage to the cornea cased by a toxin released by the parasite. About one quarter of the people with the infection require corneal transplants.
The CDC said it had received reports of 138 recent cases of the infection across 35 states. Of this group, the agency had full clinical data for 46 patients, 39 of whom wore soft contact lenses. And of those who wore such lenses, 58% used Complete MoisturePlus contact solution.
Still, Advanced Medical Optics stresses that there is currently no direct evidence showing the solution was contaminated with the Acanthamoeba parasite.
In 2006, health officials connected the MoistureLoc contact solution made by Bausch & Lomb to a similar corneal infection called Fusarium keratitis, caused by a fungus. Exactly how the solution contributed to this infection remains unclear, however.
The reasons for the more recent Acanthamoeba keratitis outbreak has similarly puzzled experts.
"I don't think there is a single consensus about the reason for the increased reporting of infections," says ophthalmologist Steven Verity at the University of Texas Southwestern Medical Center in Dallas, Texas.
He points out that the Acanthamoeba microbe is ubiquitous: "It's basically found everywhere. You can't run away from it." The waterborne parasite is present in tap water, hot tubs and lakes, for example.
"We're exposed to this stuff all day, every day," explains Kathryn Colby of the Massachusetts Eye and Ear Infirmary in Boston.
Colby has treated four patients with Acanthamoeba keratitis in the past year, and says the number of such cases seems to be on the rise. There are few treatments for the infection, and doctors sometime must resort to applying baquacil - a swimming pool cleaner - to patients' eyes.
Normally, a protective layer about six cells thick on the outside of the cornea prevents the parasite from establishing itself in the eye. Colby believes that the combination of newer, silicone hydrogel contact lenses, and lens solution that locks in moisture, might create a "perfect storm" that dismantles this protective layer outside the cornea. This, in turn, would make the eye susceptible to infection.
There has been some suggestion that changes in chlorination of water supplies has increased the prevalence of Acanthamoeba, notes Dan Jones, chairman of ophthalmology at Baylor College of Medicine in Houston, Texas, US.
But Jones agrees that some chemical component in the moisture-locking solutions probably plays a large role in promoting eye infection. According Jones, the US Food and Drug Administration "must step in" and "look critically at multipurpose contact lens solution". He suggests that it might be best to revert to using separate solutions to disinfect and lubricate contact lenses.
Cornea specialists say that health officials should quickly address the recent outbreak of Acanthamoeba keratitis. "It's really a wake-up call that some current contact solutions are not doing the job," says Colby.
Experts say that people should avoid "risky behaviours" such as swimming in lakes and rivers while wearing contact lenses. And even washing the face in tap water can be a cause of infection. People are also advised to change their eye solution every day, not merely top up the solution sitting in the lens cases.
Colby also urges people to consider wearing disposable daily lenses instead of those that are replaced every month.
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