Introduction

Keratitis is the inflammation of the cornea, the clear, protective layer covering the front of the eye, including the pupil and iris. The cornea protects the eye from injury and infection and maintains the eye’s shape, allowing light to enter the eye to provide clear vision. Keratitis can be infectious or non-infectious. Infectious causes of keratitis include bacterial, viral, fungal, and parasitic infections. Non-infectious causes include injury, issues affecting the function of the eyelid, allergic and autoimmune responses, vitamin A deficiency, and more. It is important to recognise the different causes of keratitis to best address how to treat the condition. This article will outline both infectious and non-infectious causes of keratitis. 

Infectious causes of keratitis

Infectious keratitis occurs when infection by bacteria, viruses, fungi, or parasites causes inflammation in the cornea. The inflammation is a result of the recruitment of inflammatory immune cells, such as macrophages, to the cornea.¹ These inflammatory cells release specialised proteins that damage the cornea, leading to a corneal ulcer, or an open sore on the outside of the cornea.^2,3

Infectious keratitis is divided into categories: microbial keratitis, referring to keratitis caused by bacteria, fungi or parasites; and viral keratitis.⁴ These infections may arise from being exposed to contaminated water or soil or from injury to the cornea. Infectious keratitis causes blindness if left untreated, and some infections can be transmitted from person to person. This transmission can occur from coughing, sneezing, or touching contaminated objects.⁴

Bacterial keratitis

Bacterial keratitis is caused by bacterial infection of the cornea and is the most common type of keratitis. Common bacteria that cause keratitis are:

• Pseudomonas aeruginosa
• Staphylococcus aureus
• Streptococcus pneumoniae^1,4  

Other pathogens that can cause bacterial keratitis are Moraxella,⁵ Nocardia,⁶ and atypical Mycobacteria.⁷ Marginal keratitis is inflammation of the peripheral cornea. It is caused by antigens from Staphylococcus rather than the bacteria themselves.⁸ 

Bacterial keratitis is often associated with wearing contact lenses for extended periods of time.^1,4 Treatment for bacterial keratitis consists of antibiotics such as fluoroquinolones.⁴

Viral keratitis

Viral keratitis is caused by a viral infection of the cornea. Viruses known to cause viral keratitis include:

• Herpes simplex virus type 1 (HSV-1)
• Varicella-zoster virus (VZV)
• Cytomegalovirus (CMV)
• Epstein-Barr virus (EBV)
• Adenovirus

Herpesviruses are the most common cause of viral keratitis.⁹ Less common viral causes of keratitis are Rhabdovirus, Coxsackievirus, and human herpesvirus 7 (HHV-7).⁹  

Herpesviruses that cause viral keratitis can remain latent, or inactive, in sensory neurons or lymphoid tissues, depending on the species.^9,10 These viruses can later reactivate in infected cells, triggering the body’s immune response that leads to inflammation in the cornea and the development of keratitis.⁹ Viral keratitis is a primary cause of a cloudy cornea in a condition known as corneal opacity, which can lead to vision loss.⁹

Getting diagnosed as early as possible for viral keratitis is best for the management and treatment of the condition. Treatments include antiviral eye drops or oral medications.⁹

Fungal keratitis

Fungal keratitis results from fungal infection in the cornea and commonly occurs in tropical climates. You can develop fungal keratitis when you injure your cornea with plant material. Immunocompromised people are susceptible to fungal keratitis if they are exposed to fungi. It is also possible to get fungal keratitis through improper use of contact lenses.^1,11

Common organisms that cause fungal keratitis include:

• Fusarium
• Aspergillus
• Candida (yeast)^1,11

Other fungi associated with fungal keratitis include Cladosporium, Alternaria, Curvularia, and Microsporidia.¹ Additionally, fungal-like organisms called oomycetes are known to cause keratitis. The oomycete species Pythium insidiosum causes keratitis that resembles fungal keratitis.¹²

Antifungal eye drops or antifungal oral medications are typical treatment options for fungal keratitis.¹¹

Parasitic keratitis

Parasitic keratitis is caused by parasitic organisms infecting the cornea, either directly or indirectly through neighboring organs.¹³ The most common causes of parasitic keratitis are amoebas from the Acanthamoeba genus and fungi-like unicellular parasites called Microsporidia.¹³ 

River blindness, also known as Onchocerciasis, is caused by the parasitic worm Onchocerca volvulus. River blindness can lead to sclerosing keratitis and corneal opacity.^1,13 Common transmission routes of parasitic keratitis are poor contact lens hygiene and exposure to contaminated water.^13,14

Keratitis can also result from leishmaniasis, a disease caused by protozoan Leishmania parasites spread through sand fly bites.¹⁵

Parasitic keratitis can mimic other forms of infectious keratitis or non-infectious keratitis. Parasites that cause keratitis can evade detection.^13,14 Treatments include antiparasitic eye drops or oral medications, though some parasites may be resistant to medications.¹³ In severe cases, a corneal transplant may be needed.¹⁴

Non-infectious causes of keratitis

Injury or trauma

Injuries that can lead to the development of keratitis include:

• Corneal abrasion or trauma – a scratch or injury on the cornea surface
• A foreign object in your eye
• Surgical injury
• Contact lens-related injury – contact overwear or misuse^1,16

Exposure keratopathy

Exposure keratopathy or exposure keratitis is a condition where the cornea becomes dry and damaged due to extended exposure to air. It can result from problems closing the eyelids, such as lagophthalmos, where it is impossible to close the eye. This can lead to pain, irritation in the eyes, blurry vision, and light sensitivity. Managing exposure keratitis includes covering the cornea and keeping it lubricated.¹⁷

Allergic and autoimmune-mediated keratitis

Allergic keratitis arises from inflammation of the cornea due to an allergic reaction. The term keratoconjunctivitis describes inflammation of both the cornea and the conjunctiva, the layer that protects the white part of your eye. 

Seasonal allergies to allergens in the air can cause vernal keratoconjunctivitis (VKC). Atopic keratoconjunctivitis (AKC) and phlyctenular keratoconjunctivitis (PK) are also allergic forms of keratoconjunctivitis.¹⁸

Peripheral ulcerative keratitis (PUK) is a condition where the outer edge of the cornea becomes inflamed and develops ulcers. PUK is often connected to autoimmune diseases such as rheumatoid arthritis and lupus.¹⁹

Neurotrophic keratitis

Neurotrophic keratitis is a degenerative corneal disease caused by loss or impairment of the sensory nerve connections, or innervation, in the cornea.^20,21 Loss of sensory innervation is detrimental to the health of the cornea, with loss of feeling in the cornea, development of ulcers, and increased vulnerability to damage and infection as possible consequences. Neurotrophic keratitis occurs due to damage to the trigeminal nerve, the largest cranial nerve.^{20, 21}

Chemicals and toxins

Exposure to chemicals or toxins can also cause keratitis and keratoconjunctivitis.^{1, 22} The following are some possible chemicals that can lead to keratitis:

• Exposure to irritants in the environment ^1,22
• Household cleaning agents ^{1, 22}
• Toxic medications²³

Vitamin A deficiency

Xerophthalmia is a collection of eye symptoms caused by vitamin A deficiency. Vitamin A is important for your eyes and vision. The eyes require vitamin A to keep the cornea moist. A lack of vitamin A leads to dry corneas and subsequent eye damage or blindness.^{1,4, 24}

Vitamin A is needed for the production of eye pigments that allow for healthy retinal function. Without sufficient vitamin A, the eyes have trouble making these pigments. This can result in night blindness or trouble seeing in the dark or low-light conditions.⁴

Treatment for xerophthalmia is taking vitamin A supplements.^1,4

UV light exposure

Ultraviolet (UV) light can damage both the cornea and conjunctiva. Photokeratitis is keratitis caused by ultraviolet (UV) light exposure. Photokeratitis can be acute or chronic, depending on how long and intense the exposure to UV light is.²⁵ 

Notable types of photokeratitis are:

• Snow blindness – Occurs when UV light is reflected by snow or ice^25,26
• Welder’s arc – Occurs when a welder’s eyes are unprotected from UV light coming from a welding torch^25,26

Sources of UV light include:

• Sunlight reflected from surfaces such as snow, ice, and sand
• UV lamps
• Tanning beds
• Light from lasers

Pain and irritation from photokeratitis can be alleviated with eye drops, eye rest, and reduced UV light exposure. Protective eyewear such as sunglasses, snow goggles, and welder’s helmets can block UV light exposure to the eyes.²⁵ 

Symptoms

Common symptoms of keratitis are:

• Eye pain
• Redness of the eyes
• Sensitivity to light
• Blurry vision
• Watery eyes¹⁶

Diagnosis

The following methods are used to diagnose keratitis:

• Clinical eye examinations^{1, 16}
• Corneal staining with fluorescein dye^{1, 16}
• Laboratory testing of corneal scrapes^1,16
• Polymerase chain reaction (PCR) tests¹
• In vivo confocal microscopy¹

Treatment and prevention

Treatment for infectious keratitis involves taking medical eye drops or oral medications (antibacterial, antiviral, antifungal, or antiparasitic) that combat the specific microorganism causing the infection.^1,16  For non-infectious keratitis, it is best to consult your healthcare provider as soon as possible to address the underlying cause of the condition. In serious cases, surgery or a cornea transplant may be needed.¹⁶

Proactive steps you can take to reduce the risk of keratitis include:

• Practising proper contact lens hygiene
• Washing your hands before touching your eyes
• Wearing protective eyewear to prevent injury, UV light, or infection¹⁶

Summary

Keratitis is the inflammation of the cornea, the clear, protective layer that covers the front of the eye. Causes of keratitis are either infectious or non-infectious. Infectious causes of keratitis can be bacterial, fungal, viral, or parasitic. Non-infectious forms of keratitis arise from injury to the cornea, issues with closing your eyelids, allergic or autoimmune responses, loss of sensation in the cornea, exposure to chemicals or toxins or UV light, and vitamin A deficiency. Some forms of infectious keratitis spread from person to person, whereas non-infectious keratitis is not contagious. While keratitis can commonly occur from contact lens overwear or misuse and exposure to contaminated water or soil, it is still important to get a timely diagnosis for the condition to receive proper treatment. Persistent or untreated keratitis can lead to vision problems or blindness. Proper contact lens hygiene and protection against eye injury can help prevent keratitis.