What causes conjunctivitis?
Infection is the most common cause of conjunctivitis. This leaflet only concerns infective conjunctivitis. However, conjunctivitis may also be due to:
- Allergy: many people with hay fever (pollen allergy) have a red and inflamed conjunctiva - the thin, clear covering over the white part of the eyes and inside of the eyelids. See separate leaflet called Allergic Conjunctivitis for more details.
- Irritation: irritant conjunctivitis sometimes occurs. For example, your conjunctiva may become inflamed after getting some shampoo in your eyes. Chlorine in swimming pools is a common cause of mild irritant conjunctivitis. Crowd control substances like tear gas cause a severe form of irritant conjunctivitis.
What types of infective conjunctivitis are there?
Most cases of infective conjunctivitis are caused by the same germs that cause coughs and colds, and conjunctivitis commonly develops when you have a cold or cough.
In the vast majority of cases, infective conjunctivitis is not serious.
- Most bacterial conjunctivitis is mild. It usually clears within a week or so without antibiotics.
- Most viral conjunctivitis is caused by a virus called adenovirus. This tends to cause a very red and more prolonged conjunctivitis. It is very contagious and typically lasts several weeks. Whilst it is not a serious condition, the fact that it lasts so long (and the eye can be very red) means that you should seek medical advice to rule out other causes.
- Molluscum contagiosum virus can cause a mild conjunctivitis which, like adenovirus, can persist for several weeks. This is more common in children, and typically the little mole-like bumps of molluscum are visible on the eyelids or fingers.
More serious types of infective conjunctivitis
Rarely, infective conjunctivitis can more serious, with some rarer germs being capable of spreading into, and damaging, the cornea and the main part of the eye:
- Some bacteria can cause more serious infective conjunctivitis.
- Conjunctivitis in newborn babies can be caused by germs called chlamydia or gonorrhoea. These are sexually transmitted infections (STIs) in adults; babies can acquire them from the birth canal during childbirth. They cause a severe, very sticky conjunctivitis which can spread rapidly to the cornea and permanently damage the eye. They need urgent treatment. Note: this is different to the very common sticky eye of newborn babies, caused by a blocked tear duct. A blocked tear duct with sticky eye does not cause redness and inflammation of the conjunctiva. See separate leaflet called Tear Duct Blockage in Babies for more details.
- Some adults can develop conjunctivitis due to chlamydia or gonorrhoea. These STIs cause a severe conjunctivitis, with extremely profuse discharge. They need urgent treatment as they can spread quickly to the cornea and can affect vision.
- Some viruses can cause more serious viral conjunctivitis. These tend to cause marked pain, particularly if the cornea is also affected. They include:
- Cold sore virus (herpes simplex virus), which in addition to causing a conjunctivitis, can cause keratitis and painful ulcers on the surface of the eye.
- Shingles (varicella-zoster virus), which can cause inflammation of the inside of the eye and can affect vision.
- Trachoma is conjunctivitis caused by a different form of chlamydia to the one that causes STIs in the UK. Repeated infection particularly affects the underside of the eyelids and gradually leads to scarring and loss of vision. It is widespread in many of developing countries but is rarely seen in the UK.
- Conjunctivitis can sometimes be one part of a more serious infection of the cornea or of deeper structures of the eye. This is suggested by symptoms of eye pain, reduced vision, or swelling around the eye.
- There are several serious eye conditions which are not caused by infection, but which make the eye red. These conditions include acute glaucoma and uveitis. These conditions generally markedly affect vision, and most also cause severe pain.
Further reading and references
Guidance on infection control in schools and other childcare settings; Public Health England (September 2017)
Sheikh A, Hurwitz B, van Schayck CP, et al; Antibiotics versus placebo for acute bacterial conjunctivitis. Cochrane Database Syst Rev. 2012 Sep 12(9):CD001211. doi: 10.1002/14651858.CD001211.pub3.
Conjunctivitis - infective; NICE CKS, August 2015 (UK access only)
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