Contact Lenses and Fusarium Keratitis
Contact Lenses and Fusarium Keratitis
Fusarium is a type of fungus that is commonly found in the soil and on plants. In a 2005 outbreak of disease caused by Fusarium, the fungus was identified in contact lens cleaning solution, where it was transferred to the inner surface of a contact lens during the cleaning process. When the lens was worn, fungal growth caused an inflammation of the part of the eye called the cornea. Corneal inflammation is generally termed keratitis; in the case of this fungal infection, the inflammation is called Fusarium keratitis. Until recently, Fusarium keratitis was more common in agriculture-intensive regions, such as Florida, rather than in the general population.
The symptoms of Fusarium keratitis in contact lens wearers include blurred vision and a red and/or swollen eye. These symptoms do not improve when the contact lens is removed, since fungal growth is taking place in or on the cornea. Treatment typically involves antifungal drugs, such as natamycin and amphotericin B, which can be irritating and even toxic in high doses. In extreme cases, removal of the cornea and transplantation of another cornea is performed.
Until 2005, Fusarium keratitis was a rare disease. This is because sources of the fungus, such as soil and plants, rarely come in contact with the solution used to clean contact lenses. However, in 2005, a case of Fusarium keratitis was diagnosed in the United States in a person who did not have a history of recent corneal damage. The infection was subsequently traced to contaminated contact lens cleaning solution. A wider investigation undertaken by the U.S. Centers for Disease Control and Prevention (CDC) uncovered 164 cases of Fusarium keratitis in 33 U.S. states and one U.S. territory by mid-2006.
Analysis of the data implicated a particular brand of contact lens solution (ReNu with MoistureLoc). The source of the fungal contamination was not determined, since the fungus was not isolated from the production factory, storage warehouse, filtered samples of cleaning solutions, or unopened solution bottles from the same production runs. Nonetheless, sales of the product were stopped by Bausch & Lomb, who subsequently issued a recall of the product.
WORDS TO KNOW
ANTIFUNGAL: Antifungals (also called antifungal drugs) are medicines used to fight fungal infections. They are of two kinds, systemic and topical. Systemic antifungal drugs are medicines taken by mouth or by injection to treat infections caused by a fungus. Topical antifungal drugs are medicines applied to the skin to treat skin infections caused by a fungus.
KERATITIS: Keratitis, sometimes called cornea ulcers, is an inflammation of the cornea, the transparent membrane that covers the colored part of the eye (iris) and pupil of the eye.
RESISTANT BACTERIA: Resistant bacteria are microbes that have lost their sensitivity to one or more antibiotic drugs through mutation.
Fusarium keratitis research is focused on understanding the scope of the problem. Whether Fusarium keratitis is mainly due to an infrequent contamination of lens cleaning solution during manufacture, or is a more widespread problem involving improper hygiene on the part of the user is not clear. In addition, improved lens cleaners are being investigated, with the goal of developing a cleaner that is lethal to microbes but is safe for the user if cleaner residue remains on the lens.
IN CONTEXT: THE HUMAN EYE
The eye is the organ of sight in humans and animals. It transforms light waves into visual images and provides about 80% of all information received by the human brain. In humans, light enters the eye through the cornea (the transparent layer at the front of the eye), passes through the pupil (the opening in the center of the iris, the colored portion of the eye), and then through a clear lens behind the iris. The lens focuses light onto the retina, which functions like the film in a camera. Photoreceptor neurons in retinas, called rods and cones, convert light energy into electrical impulses, which are then carried to the brain via the optic nerves. At the visual cortex in the occipital lobe of the cerebrum of the brain, the electrical impulses are interpreted as images.
IN CONTEXT: REAL-WORLD RISKS
Eye infections can be caused by viral, bacterial, and fungal microorganisms. These organisms do not cause infections solely in the eye. In reality, eye infections tend to occur as infections disseminate, or spread, in the body. The cornea, the clear front part of the eye through which light passes, is subject to many infections and to injury from exposure and from foreign objects. Infection and injury cause inflammation of the cornea—a condition called keratitis. Tissue loss because of inflammation produces an ulcer. The ulcer can either be centrally located, thus greatly affecting vision, or peripherally located. There are about 30,000 cases of bacterial corneal ulcers in the United States each year.
The advent of molecular techniques of microorganism detection has aided the diagnosis of Fusarium keratitis. Advances in a technique that can quickly obtain many copies of a gene(s) of interest and the use of anti-bodies to Fusarium are being exploited to develop a rapid detection test for the fungus. Presently, the fungus is identified by culturing a scraping of cells from the cornea, but this process can take up to a week to yield a result.
In the United States, about one out of every 20 contact lens wearers develops a lens-related eye complication every year. Some of these complications can threaten vision permanently. As of early 2007, the source of the 2005–2006 fungal contamination outbreak is still being investigated by the CDC in collaboration with the Food and Drug Administration and Bausch & Lomb.
One factor that may play a role in the survival of the Fusarium fungus in the lens cleaning solution is the surface growth of the organism. It is now well established that some microorganisms, including fungi and bacteria, become very resistant to a variety of agents when the organisms grow attached to a nonliving or living surface. When unattached, the organisms are usually readily killed by antibiotic agents. The increased hardiness of the attached organisms involves changes in their growth following attachment. These changes can be the result of genetic adaptation, with the activity of some genes enhanced by attachment, while other genes becoming less active.
Until recently, contact lens keratitis usually involved bacterial infections, predominantly caused by bacteria common in the environment or on the surface of the skin. Fungal keratitis due to organisms such as Fusarium has typically been due to accidental contact of plant material with the eye, especially in people whose immune systems are functioning inefficiently as a consequence of illness or drug therapy. Common routes of transmission include rubbing an eye with soil-laden fingers, injury to the eye by a thorn, or contact of plant material with the eye during harvesting. The association of Fusarium keratitis with contact lenses is new, and may reflect the growing popularity of these lenses, particularly lenses that are non-disposable and repeatedly cleaned.
Fusarium keratitis is an example of how improper hygiene or contaminated lens cleaners can cause illness. Even a properly cleaned lens can become contaminated if, after handling soil or plant materials, hands have not been washed off. Also, repeated use of lens cleaning solution can cause contamination. Fresh solution should be used for each cleaning. According to the American Optometric Association, other useful precautions are wiping the lenses before storing them in the lens case and replacing the lens case every few months.
The problem of fungal keratitis is a growing concern, since extended wear contact lenses are becoming increasingly popular. These extended wear lenses remain in contact with the cornea for a longer period of time than conventional non-disposable lenses and they are cleaned less often. With the convenience of extended wear can come a relaxed vigilance concerning lens hygiene.
Black, Jacquelyn. Microbiology: Principles and Explorations. New York: John Wiley & Sons, 2004.
Richardson, Malcolm, and Elizabeth Johnson. Pocket Guide to Fungal Infection. Boston: Blackwell, 2006.
Chang, Douglas C., et al. “Multistate Outbreak of Fusarium Keratitis Associated with Use of a Contact Lens Solution.” Journal of the American Medical Association 296 (2006): 953–963.
Margolis, Todd P., and J. P. Whitcher. “Fusarium—A New Culprit in the Contact Lens Case.” Journal of the American Medical Association 296 (2006): 985–987.