Primary infection most commonly manifests as blepharoconjunctivitis i.e. infection of lids and conjunctiva that heals without scarring. Lid vesicles and conjunctivitis are seen in primary infection. Corneal involvement is rarely seen in primary infection.

Recurrent herpes of the eye is caused by reactivation of the virus in a latently infected sensory ganglion, transport of the virus down the nerve axon to sensory nerve endings, and subsequent infection of ocular surface.

The following classification of herpes simplex keratitis is important for understanding this disease:

Any potential ocular involvement should be assessed by an ophthalmologist as complications such as episcleritis and uveitis may occur.

Herpes zoster ophthalmicus (HZO) and also known as ophthalmic zoster is a disease characterised by reactivation of dormant varicella zoster virus residing within the ophthalmic nerve (the first division of the trigeminal nerve). This condition is an important subtype of shingles, representing 15% of all cases.

Herpes zoster ophthalmicus is transmitted via direct contact or droplets. Varicella zoster virus is a DNA virus which produces acidophilic intranuclear inclusion bodies. The virus is neurotrophic in nature.

The frontal nerve is more commonly affected than the nasociliary nerve or lacrimal nerve.

Several aetiologies are suggested, and any combination of these may be present in any given case.

- Vitamin deficiency (A, B or C)

- Viral infection

- Bacterial infection

- "Leptospira

- "Streptococcus

- "Brucella

- Parasitic infection

- Strongyle

- "Onchocerca cervicalis"

- Autoimmune disease

The disease has been suggested to be primarily autoimmune in nature, being a delayed hypersensitivity reaction to any of the above agents.

Prevention of trauma with vegetable / organic matter, particularly in agricultural workers while harvesting can reduce the incidence of fungal keratitis. Wearing of broad protective glasses with side shields is recommended for people at risk for such injuries.

Conjunctivitis due to chemicals is treated via irrigation with Ringer's lactate or saline solution. Chemical injuries (particularly alkali burns) are medical emergencies, as they can lead to severe scarring and intraocular damage. People with chemically induced conjunctivitis should not touch their eyes, regardless of whether or not their hands are clean, as they run the risk of spreading the condition to another eye.

Conjunctivitis may also be caused by allergens such as pollen, perfumes, cosmetics, smoke, dust mites, Balsam of Peru, and eye drops.

This disease is quite common in the tropics and with large agrarian population. India has a high number of cases with fungal keratitis, but poor reporting system prevents accurate data collection. Florida in US regularly reports cases of fungal keratitis, with Aspergillus and Fusarium spp. as the most common causes.

The Appaloosa has a higher risk of developing ERU than other breeds; this predisposition has a genetic basis. Appaloosas which develop ERU are more likely than other breeds to have ERU in both eyes, and more likely to become blind in one or both eyes.

Corneal ulcers are a common human eye disease. They are caused by trauma, particularly with vegetable matter, as well as chemical injury, contact lenses and infections. Other eye conditions can cause corneal ulcers, such as entropion, distichiasis, corneal dystrophy, and keratoconjunctivitis sicca (dry eye).

Many micro-organisms cause infective corneal ulcer. Among them are bacteria, fungi, viruses, protozoa, and chlamydia:

- Bacterial keratitis is caused by "Staphylococcus aureus", "Streptococcus viridans", "Escherichia coli", "Enterococci", "Pseudomonas", "Nocardia", "N. Gonorrhoea" and many other bacteria.

- Fungal keratitis causes deep and severe corneal ulcer. It is caused by "Aspergillus" sp., "Fusarium" sp., "Candida" sp., as also "Rhizopus", "Mucor", and other fungi. The typical feature of fungal keratitis is slow onset and gradual progression, where signs are much more than the symptoms. Small satellite lesions around the ulcer are a common feature of fungal keratitis and hypopyon is usually seen.

- Viral keratitis causes corneal ulceration. It is caused most commonly by Herpes simplex, Herpes zoster and Adenoviruses. Also it can be caused by coronaviruses & many other viruses. Herpes virus cause a dendritic ulcer, which can recur and relapse over the lifetime of an individual.

- Protozoa infection like "Acanthamoeba keratitis" is characterized by severe pain and is associated with contact lens users swimming in pools.

- "Chlamydia trachomatis" can also contribute to development of corneal ulcer.

Superficial ulcers involve a loss of part of the epithelium. Deep ulcers extend into or through the stroma and can result in severe scarring and corneal perforation. Descemetoceles occur when the ulcer extends through the stroma. This type of ulcer is especially dangerous and can rapidly result in corneal perforation, if not treated in time.

The location of the ulcer depends somewhat on the cause. Central ulcers are typically caused by trauma, dry eye, or exposure from facial nerve paralysis or exophthalmos. Entropion, severe dry eye and trichiasis (inturning of eyelashes) may cause ulceration of the peripheral cornea. Immune-mediated eye disease can cause ulcers at the border of the cornea and sclera. These include Rheumatoid arthritis, rosacea, systemic sclerosis which lead to a special type of corneal ulcer called Mooren's ulcer. It has a circumferential crater like depression of the cornea, just inside the limbus, usually with an overhanging edge.

A study in Austria found over the course of the testing, a total of 154 cases of "Acanthamoeba" keratitis. The age of the positive tests ranged from 8 to 82 years old and 58% of the people were female. The data showed that 89% of the infected patients were contact lens wearers and 19% required a corneal transplant.

Keratoconjunctivitis is inflammation ("-itis") of the cornea and conjunctiva.

When only the cornea is inflamed, it is called "keratitis"; when only the conjunctiva is inflamed, it is called "conjunctivitis".

There are several potential causes of the inflammation:

- Keratoconjunctivitis sicca is used when the inflammation is due to dryness. ("Sicca" means "dryness" in medical contexts.) It occurs with 20% of rheumatoid arthritis patients.

- The term "Vernal keratoconjunctivitis" (VKC) is used to refer to keratoconjunctivitis occurring in spring, and is usually considered to be due to allergens.

- "Atopic keratoconjunctivitis" is one manifestation of atopy.

- "Epidemic keratoconjunctivitis" is caused by an adenovirus infection.

- "Infectious bovine keratoconjunctivitis" (IBK) is a disease affecting cattle caused by the bacteria "Moraxella bovis".

- "Pink eye in sheep and goat" is another infectious keratoconjunctivitis of veterinary concern, mostly caused by "Chlamydophila pecorum"

- "Superior limbic keratoconjunctivitis" is thought to be caused by mechanical trauma.

- "Keratoconjunctivitis photoelectrica" (arc eye) means inflammation caused by photoelectric UV light. It is a type of ultraviolet keratitis. Such UV exposure can be caused by arc welding without wearing protective eye glass, or by high altitude exposure from sunlight reflected from snow ("snow blindness"). The inflammation will only appear after about 6 to 12 hours. It can be treated by rest, as the inflammation usually heals after 24–48 hours. Proper eye protection should be worn to prevent keratoconjunctivitis photoelectrica.

Some infections may scar the cornea to limit vision. Others may result in perforation of the cornea, (an infection inside the eye), or even loss of the eye. With proper medical attention, infections can usually be successfully treated without long-term visual loss.

The cornea, an avascular tissue, is among the most densely innervated structures of the human body. Corneal nerves are responsible for maintaining the anatomical and functional integrity of the cornea, conveying tactile, temperature and pain sensations, playing a role in the blink reflex, in wound healing and in the production and secretion of tears.

Most corneal nerve fibres are sensory in origin and are derived from the ophthalmic branch of the trigeminal nerve. Congenital or acquired ocular and systemic diseases can determine a lesion at different levels of the trigeminal nerve, which can lead to a reduction (hypoesthesia) or loss (anesthesia) of sensitivity of the cornea.

The most common causes of loss of corneal sensitivity are viral infections (herpes simplex and herpes zoster ophthalmicus), chemical burns, physical injuries, corneal surgery, neurosurgery, chronic use of topical medications, or chronic use of contact lenses.

Possible causes also include systemic diseases such as diabetes, multiple sclerosis or leprosy.

Other, albeit less frequent, potential causes of the disease are: intracranial space-occupying lesions such as neuroma, meningioma and aneurysms, which may compress the trigeminal nerve and reduce corneal sensitivity.

Conversely, congenital conditions that may lead to this disorder are very rare.

FVR is transmitted through direct contact only. It replicates in the nasal and nasopharyngeal tissues and the tonsils. Viremia (the presence of the virus in the blood) is rare. The virus is shed in saliva and eye and nasal secretions, and can also be spread by fomites. FVR has a two- to five-day incubation period. The virus is shed for one to three weeks postinfection. Latently infected cats (carriers) will shed FHV-1 intermittently for life, with the virus persisting within the trigeminal ganglion. Stress and use of corticosteroids precipitate shedding. Most disinfectants, antiseptics and detergents are effective against the virus.

Refractory corneal ulcers are superficial ulcers that heal poorly and tend to recur. They are also known as indolent ulcers or Boxer ulcers. They are believed to be caused by a defect in the basement membrane and a lack of hemidesmosomal attachments. They are recognized by undermined epithelium that surrounds the ulcer and easily peels back. Refractory corneal ulcers are most commonly seen in diabetics and often occur in the other eye later. They are similar to Cogan's cystic dystrophy.

Most household disinfectants will inactivate FHV-1. The virus can survive up to 18 hours in a damp environment, but less in a dry environment and only shortly as an aerosol.

Treatment depends on the cause of the keratitis. Infectious keratitis can progress rapidly, and generally requires urgent antibacterial, antifungal, or antiviral therapy to eliminate the pathogen. Antibacterial solutions include levofloxacin, gatifloxacin, moxifloxacin, ofloxacin. It is unclear if steroid eye drops are useful or not.

In addition, contact lens wearers are typically advised to discontinue contact lens wear and replace contaminated contact lenses and contact lens cases. (Contaminated lenses and cases should not be discarded as cultures from these can be used to identify the pathogen).

Aciclovir is the mainstay of treatment for HSV keratitis and steroids should be avoided at all costs in this condition. Application of steroids to a dendritic ulcer caused by HSV will result in rapid and significant worsening of the ulcer to form an 'amoeboid' or 'geographic' ulcer, so named because of the ulcer's map like shape.

By far the most common cause of IK is syphilitic disease. However, there are two possible causes of the corneal inflammatory response: an infection and/or an immunological response, such as a hypersensitivity type reaction, or (rarely) Cogan syndrome. Infectious causes include syphilis (commonest), followed by other bacterial infections (TB, Leprosy and Lyme disease) and parasitic infections (Acanthamoeba, Onchocerciasis or river blindness, Leishmaniasis, Trypanosoma cruzi or "Chagas disease", Trypanosoma brucei or "African sleeping sickness" and microsporidia)

Interstitial keratitis (IK) is corneal scarring due to chronic inflammation of the corneal stroma. Interstitial means space between cells i.e. corneal stroma which lies between the epithelium and the endothelium. Keratitis means corneal inflammation.

Neurotrophic keratitis (NK) is a degenerative disease of the cornea caused by damage of the trigeminal nerve, which results in impairment of corneal sensitivity, spontaneous corneal epithelium breakdown, poor corneal healing and development of corneal ulceration, melting and perforation.

Neurotrophic keratitis is classified as a rare disease, with an estimated prevalence of less than 5 in 10,000 people in Europe. It has been recorded that on average, 6% of herpetic keratitis cases may evolve to this disease, with a peak of 12.8% of cases of keratitis due to herpes zoster virus.

The diagnosis, and particularly the treatment of neurotrophic keratitis are the most complex and challenging aspects of this disease, as a satisfactory therapeutic approach is not yet available.

According to the American Optometric Association, the following steps can be taken to prevent "Acanthamoeba" keratitis:

- Always wash and dry your hands before handling contact lenses, ordinary water should never come in contact with your lenses.

- Rub and rinse the surface of the contact lens before storing.

- Use only sterile products recommended by your optometrist to clean and disinfect your lenses. Saline solution and rewetting drops are not designed to disinfect lenses.

- Avoid using tap water to wash or store contact lenses.

- Contact lens solution must be discarded upon opening the case, and fresh solution used each time the lens is placed in the case.

- Replace lenses using your doctor’s prescribed schedule.

- Do not sleep in contact lenses unless prescribed by your doctor and never after swimming.

- Never swap lenses with someone else.

- Never put contact lenses in your mouth.

- See your optometrist regularly for contact lens evaluation.

Vernal keratoconjunctivitis (VKC) or spring catarrh is a recurrent, bilateral, and self-limiting inflammation of conjunctiva, having a periodic seasonal incidence.

Punctate epithelial erosions may be seen with different disorders:

- Rosacea

- Dry-eye syndrome

- Blepharitis

- Acute bacterial conjunctivitis

- Trauma

- Exposure keratopathy from poor eyelide closure

- Ultraviolet or chemical burn

- Contact lens-related disorder such as toxicity or tight lens syndrome

- Trichiasis

- Entropion or ectropion

- Floppy eyelid syndrome

- Chemotherapy i.e. cytosine arabinoside

- Thygeson's Superficial Punctate Keratopathy