Many of the viruses, bacteria, parasites, and fungi that can invade the human body are also capable of attacking the surface or interior of the eye. Infectious eye diseases can be categorized in two ways.
Firstly, doctors normally talk about the part of the eye that's infected or inflamed. Conjunctivitis, for example, is an inflammation of the conjunctiva, the membrane that covers the front surface of the eye and the inner surface of the eyelid. Other possible locations of inflammation include the eyelid (blepharitis), the cornea (keratitis), the liquid inside the eye (vitritis), the retina and the blood vessels that feed it (chorioretinitis), or the optic nerve (neuroretinitis). These are just a few examples – the eye is a complex organ of many parts.
Secondly, eye infections are also classified according to what's causing them. Ocular histoplasmosis syndrome (OHS), for example, is caused by a fungus (the condition is a type of chorioretinitis). It generally attacks the blood supply of the retina, on the inner rear surface of the eye.
The most common eye infection is conjunctivitis caused by an adenovirus (a type of common cold virus). This type of infectious conjunctivitis is sometimes called pinkeye and is most common in children. Viral conjunctivitis is very contagious because the virus can be spread from the eye to hands that then touch doorknobs and other surfaces that other people use.
There are other causes of infectious conjunctivitis, such as bacteria like Staphylococcus aureus. Bacterial infections occur most commonly in children and tend to result in longer-lasting cases of pinkeye.
Infectious conjunctivitis is the most common cause of pinkeye around the world. Causes of infectious conjunctivitis are numerous and can usually be classified as viral, bacterial, or fungal.
Some of the most common causes of serious eye infection include:
Ocular histoplasmosis syndrome (OHS): Histoplasmosis is a fungal infection of the lungs, which is caught by inhaling spores. It's common in river valleys around the world. It's far more common in the US than in Canada, and is concentrated in the Midwestern states located in the Ohio and Mississippi River valleys. Approximately 60% to 90% of people living in these areas have been exposed to the fungus during their lifetime. Most infections do not cause symptoms and resolve on their own. In a small fraction of cases, the fungus migrates to the retina many years or decades later. Once there, it damages the retina, particularly the macula (the vital centre part where the vision cells are most concentrated). It causes symptoms and retinal decay very similar to macular degeneration, and can destroy the central part of the field of vision. Although only a tiny minority of people with histoplasmosis go on to suffer OHS, the fungus is so common that OHS is a significant infectious cause of legal blindness in Americans between the ages of 20 to 40 years. Anyone with a history of histoplasmosis should see a doctor if they notice any changes in their vision.
Chlamydia and gonorrhea: These extremely common sexually transmitted infections (STIs) can also cause conjunctivitis. The infection gets into the eye either directly through genital fluids such as semen, or when infected people rub their eyes after touching infected genital areas. Babies born to genitally infected women are at especially high risk of eye infection. Neisseria gonorrheae is one of the few bacteria capable of penetrating the protective layers of the eye, causing inner-eye infection.
Herpes simplex: This widely prevalent virus can be caught as a skin disease (cold sores) or as an STI. Herpes viruses can infect the eye by touching an active lesion (cold sore or blister) then touching the eye. Herpes infections in the eye tend to infect the top layer but it can cause pitting and ulceration of the cornea. Chronic herpes infection, which is uncommon, can cause acute retinal necrosis (ARN), particularly in men. This causes a major destruction of retinal tissue, and causes dramatic damage to vision. Herpes simplex keratitis is a major cause of blindness worldwide.
Shingles (herpes zoster, varicella zoster): Shingles are a reactivation of the virus that initially causes chickenpox. The sores known as shingles are infectious and can cause chickenpox in others. They can also cause ocular infection if you touch the eyes after touching a sore. While herpes simplex is the leading cause of acute retinal necrosis in the young, varicella zoster is the leading cause in people over 50 years of age because shingles is more common in this age group.
Bacterial keratitis: This is an infection of the cornea by common bacteria found on the skin and in the mouth and nose. Normally, these bacteria can't penetrate the outer layer of the eye, and cause only conjunctivitis. However, eye injury, lack of oxygen due to contact lenses, infection from using contact lenses too long, or a weak immune system can all facilitate entry into the cornea, the clear layer in the front of the eye. Fungi can cause fungal keratitis under similar circumstances.
Infections that can cause conjunctivitis or keratitis (inflammation of the cornea) include:
- the STIs syphilis, gonorrhea, chlamydia, herpes simplex, and hepatitis B
- Lyme disease
- acanthamoeba - a common parasite
- crab lice - these tiny animals can live and breed in your eyelashes and are invisible to the naked eye
- Epstein-Barr virus or infectious mononucleosis
- mumps, measles, influenza, or shingles
- onchocerciasis (river blindness)
- sarcoidosis - the cause of this condition is not clear, but it may be due to an infection
- mycosis (general name for fungal infections) - Candida, the fungus that causes thrush, is one of more than 60 types of fungus that can infect the eye
Infections that can damage the retina and the inner eye include:
- herpes simplex
- varicella zoster (shingles and chickenpox)
- cytomegalovirus, which doesn't affect healthy people but is the leading cause of blindness in people with HIV/AIDS
Symptoms and Complications
Obviously, with so many possible causes, the symptoms of eye infection can vary a lot. What a person feels generally depends more on where the infection is rather than on what's causing it.
Common symptoms of conjunctivitis include:
- redness and itching
- viral conjunctivitis: discharge is usually watery or mucous-like
- bacterial conjunctivitis: discharge is thick and can be white, yellow, or green
- feels like there is sand in the eye
- crusting over of the eyelid
Common symptoms of keratitis and other frontal eye infections are:
- pain, itching, or sensation of a foreign body in the eye which is often sudden in onset
- photosensitivity (aversion to bright light)
- redness or small red lines in the white of the eye
- discharge of yellow or green pus that may make the eyes crusty upon waking up – a possible sign of bacterial infection
- swollen eyelids
- constant involuntary blinking (blepharospasm)
- reduced vision
Unfortunately, diseases that damage the retina, the optic nerve, or the blood vessels that feed them often cause no pain at all. The primary symptom is deteriorating vision, which is usually stoppable but not reversible. That's why it's vital to get your eyes checked regularly. One possible symptom of internal eye damage is floaters, tiny fragments in the liquid inside the eye. You see tiny bubbles or dark spots slowly falling through your line of vision. Everyone has a few floaters – you should only worry if you notice a sudden increase in them.
Almost all eye infections accompany disease in some other part of the body, even if it's just a cold. Some but not all of these diseases have clear symptoms. Be on the lookout for eye pain or visual symptoms if you have any of the diseases listed in the "Causes" section of this article.
Serious complications of eye infection include damage to the retina and the formation of scars and ulcers in the cornea that can obstruct vision. Some infections, like syphilis, can also provoke glaucoma. Moreover, eye problems are often the only visible symptom of wider infections. Chlamydia, for example, often causes no genital symptoms, but can cause infertility and heart damage if left untreated.
Making the Diagnosis
Ophthalmologists and optometrists are trained to recognize various eye infections by the appearance of the surface of the eye and the retina, the progress of the disease, whether it's in one eye or both, and your medical history. There's a wide range of lighted devices for looking at the cornea and retina.
If there's pus or discharge from the eye, it can be cultured to identify the organism. Quite possibly, you'll also be tested for common diseases like chlamydia, gonorrhea, and herpes simplex.
You should see your doctor or eye care professional if you have:
- eye pain, altered vision, severe redness of the eye, or continuous discharge from the eye
- recurring eye problems
- eye problems along with a chronic condition such as diabetes
- changes in pupil size
- recent injury to the eye
- sensitivity to light
You should also see your doctor or eye care professional if:
- you have been treating symptoms yourself for 48 hours and no improvement is noted
- the condition worsens with treatment
- the condition has lasted longer than 48 hours without treatment
Treatment and Prevention
Viral conjunctivitis usually improves in a few days without treatment. Broad-spectrum antibiotic eye drops will deal with most cases of bacterial conjunctivitis or keratitis, while particular antibiotics are used to treat gonorrhea and chlamydia. All of these diseases can be cured.
Most fungal and parasitic infections are also treatable by various medications. The exception is histoplasma, which can't even be detected in the retina, though we know it's there. The only current treatment is laser cauterization of the affected area, which dramatically slows the destruction of the macula (the centre of the retina). This operation is done in hopes of saving the existing vision, though in some cases it may cause some vision loss of its own. It often has to be repeated several times. There's still no way to repair the damage already done, though new surgical techniques are under study.
Herpes simplex can't be eradicated from the body, but flare-ups in the eye can often be fought off with topical (surface-applied) or oral (taken by mouth) antiviral medications. Corneal transplant may be necessary if there is severe scarring or vision loss from the infection. Serious diseases like tuberculosis, syphilis, and toxoplasmosis need to be treated for the body as a whole before eye problems will clear up.
There's not much you can do to avoid a disease like histoplasmosis, unless you avoid endemic areas (areas where the fungus is found, such as river valleys). You'll notice, however, that a great many eye infections are actually complications of sexually transmitted or genital diseases such as syphilis, chlamydia, gonorrhea, crab lice, herpes simplex, thrush, and hepatitis B. If you have herpes simplex, avoid touching your eyes if you have an active cold sore or blister.
Hand-washing is extremely important in preventing the spread of organisms that can cause infection. Sharing of towels, pillow cases, wash clothes, and makeup should always be avoided to prevent spread of an eye infection.
You can cut your risk of eye infection by observing safe sexual practices. That means using condoms, and, ideally, limiting your number of sexual partners. It’s also important to get checked for STIs regularly, since many of these diseases can go unnoticed until damage has been done, and you can infect other people. If you do have one of these diseases, keep your hands clean and away from your eyes.
The same is true of cold sores, shingles, chickenpox, measles, mumps, flu, and the common cold. All of these diseases produce spots, rashes, or mucus that carry the organism. Wash your hands frequently and don't touch your eyes. Watch children carefully and never let them wipe their nose with an upward stroke of the hand.
If you or a member of your family has an eye infection you should use separate linens and a fresh facecloth and towel for each cleaning. Cosmetics may be a source of recurrent infection, so avoid them if possible if you have eye problems. Be sure to clean the eye area, especially before applying any medication and when there is any type of discharge from the eye.
If you wear contact lenses, care for them and throw them away according to the manufacturer's instructions. Be sure to wash your hands before putting them in.
Watch for tree branches and twigs when walking in the woods. Getting scratched on the eye by organic matter is another way of getting a fungal eye infection.
Finally, avoid excessive sun exposure by wearing sunglasses, as this can weaken the protective layer of the eye.