More than 100 users of a recalled contact lens solution have come down with a rare and potentially blinding eye infection, and scientists don't yet know why.
At least 138 contact lens wearers, most of them users of AMO Complete Moisture Plus Multi-Purpose Solution, are believed to have contracted acanthamoeba keratitis, an infection associated with a parasite found in soil and fresh water but that rarely targets humans.
The company, which insists there is no evidence that its product is contaminated, voluntarily recalled the solution over the weekend, and the Centers for Disease Control and Prevention has urged consumers to throw away any bottles they have.
Dr. M. Bowes Hamill, a professor of ophthalmology at Baylor College of Medicine, told ABC News that acanthamoeba infections were generally rare, and that while the recent outbreak has been linked to the AMO solution, "nobody knows why."
Though the microbe rarely targets humans, it has in the past generally presented itself in lens wearers who made their own saline solution at home or who swam in contaminated bodies of fresh water, such as quarries, he said.
Of the 138 cases, the CDC has confirmed 46 have been confirmed through cultures. The figures, Hamill said, should be put in perspective before people panic. There are some 35 million soft lens wearers in the United States.
Nevertheless, "there is no question that there's been an epidemic," said Dr. Richard Yee, a professor of ophthalmology at the University of Texas Health Science Center.
Yee said the upswing in recent cases over the past several years could perhaps be linked to "a change in the solutions, a change in people's habits or a change in water supply and water cleaning plants. Those scenarios have all been suggested."
Last year, at least 164 people came down with a fungal infection while using ReNu with MoistureLoc, a lens solution made by Bausch & Lomb. The infection behind that outbreak was fusarium keratitis; just how it got into the carefully composed bottles remains a mystery.
"It seems that the fungal disease was encapsulated in solution and kept away from the antibiotic. Nobody knows why, but something similar may have occurred this time," Hamill said.
The symptoms of acanthamoeba keratitis are similar to those of other, less-dangerous eye diseases. However, if untreated, the infection could necessitate a corneal transplant or the loss of an eye.
"A lot of the time, the diagnosis isn't made until later. It is much rarer than bacterial and viral infections of the cornea," Yee said. "At first, it's treated with eye drops. If the ulcer is severe enough, then the cornea could become clouded, necrosing and thinning and could eventually need a transplant."
Doctors have reported seeing a rise in infections since 2003, but the CDC began its investigation in March 2006, when doctors at the University of Illinois in Chicago reported a cluster in that city.
Since January of this year, the CDC has found that 13 of 22 eye-care facilities it surveyed have seen an upturn in infections since 2004.
Despite issuing a recall over the weekend, AMO said there was no conclusive evidence linking its products to the spate of infections.
"There is no evidence to suggest that today's voluntary recall is related to a product contamination issue, and this does not impact any of AMO's other contact lens care products," the company said in a statement.