Herpetic keratitis is an infection of the cornea, the transparent front part of the eye, caused by the herpes simplex virus. While the herpes simplex virus that affects the genitals is type 2, the type that affects the eys and mouth is type 1. 

This infection may affect the different layers of the cornea and cause an inflammatory reaction. It may also affect the most profound structures of the eye like the iris and cause inflammation inside the eye.

The cause of herpetic queratitis may also be the herpes zoster virus, commonly known as "shingles". The symptoms, affectations and treatment are different to herpetic simplex keratitis.

The symptoms of an initial eye infection caused by the herpes simplex virus are caused by conjunctivitis which may pass for conjunctivitis caused by any other virus. Red eye, watering eyes, discomfort and stinging are the main symptoms. 

When the virus infects the cornea for the first time, it generally affects the epithelium, the most external layer. Pain, red eye, discomfort caused by light, watering eyes and blurred vision are the most common manifestations. 

When the virus reactivates, it can once again infect the epithelium or the most profound layers of the cornea like the stroma, the thickest layer of the cornea, or the endothelium, the inner most layer of the cornea, In addition, it may cause an inflammatory reaction in the corneal stroma or inside the eye itself. The main symptoms of this affectation are a loss of vision, pain, red eye and discomfort caused by light. 

The herpes simplex virus is transmitted through mucosa to mucosa contact or by secretions from infected mucosas, like mouth to mouth or saliva or tears containing the virus coming into contact with to the eye. Once the virus infects the eye, it can never be eliminated. When the manifestations or symptoms of keratitis disappear, the virus remains dormant in the nervous system of the eye. 

When the virus activates, the keratitis recurs. There are different hypotheses on the factors that may "awaken" the dormant virus and cause recurrent keratitis. Exposure to the sun, stress or a low immunity system are just a few of these, although studies have not been able to confirm this. 

The types of herpetic keratitis depend on the layer of the cornea affected and if the keratitis is caused mainly by a direct infection of the virus or by an inflammatory reaction caused by the virus.

1. Epithelial keratitis. The virus invades the corneal epithelium causing linear lesions with branches called dentritics. If not treated, these lesions widen and grow to look like maps called geographic lesions. The longer it takes to treat and cure, the more scarring is left, causing a permanent loss of vision especially if the centre of the cornea is affected. 
2. Immune stromal keratitis. It appears if the patient has already suffered an episode of epithelial keratitis and the virus has become dormant. This type of keratitis is due to inflammation caused by particles of the virus inside the stromal layer of the cornea. There is no active virus. The cornea becomes oedematised or "swollen", causing a lack of vision. The corneal epithelium remains intact. 
3. Stromal necrotic keratitis. It appears if the patient has already suffered an episode of epithelial keratitis and the virus has become dormant. This type of keratitis affects the corneal stroma, but unlike the type above, stromal necrotic keratitis is caused by a reactivation and a direct invasion of the virus inside the cornea. In addition, it usually causes more virulent inflammation of the cornea and the symptoms are more severe. The cornea becomes oedematised but an invasion of white blood cells also appears in that area, making it look similar to a corneal infection caused by other germs such as bacteria or fungus. It is very important to rule out these causes of infection by getting a culture of the corneal sample affected.
4. Endotheliitis. It is a direct infection of the virus with secondary inflammation in the most profound layer of the cornea, the endothelium. In addition, it causes an inflammatory reacion inside the eye. The cornea becomes oedematised, there are white blood cells adhering to the endothelium and floating inside the aqueous humour, the liquid found inside the eye.

Prevention is not touching your eyes with hands that may be infected by secretions containing the virus. Do not use eyedrops, tissues or towels of patients with keratitis in whom the disease is active. 

Treatment involves a topic antiviral medication for keratitis caused by the active virus.

If the main cause of the keratitis is inflammation, a topic corticosteriod should be used as directed by your ophthalmologist and always along with adjuvant antiviral treatment as the use of a corticosteroid without preventive antiviral medication may reactivate the virus and cause another type of keratitis. In some cases, the use of systemic antiviral or corticosteroid treatment is indicated.

In cases where herpetic keratitis has caused corneal cicatrisation that compromises the eyesight, a cornea transplant is the most suitable option for restoring vision.