Viral, bacterial and allergic conjunctivitis compared

The three types share a red eye, so the question is what else is going on. Allergic conjunctivitis is the itchy one, usually with watery eyes and often tied to hay fever or other allergens, and it is not contagious. Viral conjunctivitis is the most common infectious type, and infectious conjunctivitis in general (viral or bacterial) can cause a gritty, burning feeling and a discharge that may make the lids stick together on waking.

Even so, NICE Clinical Knowledge Summaries say that it is difficult to differentiate viral and bacterial conjunctivitis clinically. That is not a failing of the clinician but a feature of the condition, and it is why most guidance emphasises self-care, hygiene and review if there are warning signs rather than trying to put a precise label on every red eye.

The NHS says conjunctivitis is an eye condition caused by infection or allergies that usually gets better in a couple of weeks without treatment. It is also known as red or pink eye and usually affects both eyes, making them red, burning or gritty, producing pus that sticks to the lashes, itchy and watery.

NICE CKS describes infective conjunctivitis as inflammation of the conjunctiva due to viral, bacterial or parasitic infection. The most common bacterial causes are Streptococcus pneumoniae, Staphylococcus aureus and Haemophilus influenzae. Allergic conjunctivitis is a group of conditions linked to an IgE hypersensitivity reaction, including seasonal allergic conjunctivitis (tree and grass pollen), perennial allergic conjunctivitis (house dust mites, mould spores or animal dander), and rarer, more severe forms such as vernal and atopic keratoconjunctivitis and giant papillary conjunctivitis, which can be linked to contact lens wear.

The table brings together what the NHS and NICE CKS describe. Individual cases do not always follow the pattern.

NICE CKS says that characteristic features of infective conjunctivitis include redness of the conjunctiva, discomfort described as grittiness, a foreign body feeling or burning, and watering and discharge that may blur vision briefly. Purulent or mucopurulent discharge may cause the lids to stick together on waking. Swabs should not be taken routinely but may be appropriate if the person does not need to be referred to ophthalmology and has not responded to initial treatment.

With allergic conjunctivitis the most consistent feature is itching, and CKS also lists redness, watery or mucoid discharge and swelling of the conjunctiva and eyelid. The NHS gives a visual guide: conjunctivitis that produces sticky pus is contagious, conjunctivitis with red, gritty eyes is also usually contagious, and conjunctivitis caused by allergies such as hay fever makes the eyes red and watery but is not contagious.

For infective conjunctivitis, the NHS suggests washing the eyelids with boiled and cooled water and a clean cotton wool pad for each eye, holding a cold flannel on the eyes to cool them and not wearing contact lenses until the eyes are better. A pharmacist can suggest eyedrops or antihistamines, although a child under 2 needs a prescription from a GP. NICE CKS adds lubricating drops or artificial tears and a judicious approach to topical antibiotics such as chloramphenicol or fusidic acid, where a delayed treatment strategy may be appropriate.

For allergic conjunctivitis, NICE CKS lists avoiding allergens and eye rubbing, cold compresses and ocular surface lubricants, with topical antihistamines or mast cell stabilisers considered, and topical diclofenac as an adjunct if needed.

To stop infectious conjunctivitis spreading, the NHS suggests washing hands regularly, washing pillow cases and face cloths in hot water and detergent, covering the mouth and nose when sneezing and not sharing towels and pillows or rubbing the eyes. It says you do not need to stay away from work or school unless you or your child are feeling very unwell. Some sexually transmitted infections can cause conjunctivitis, and the NHS says this type takes longer to get better.

The NHS says to ask for an urgent GP appointment or call NHS 111 if you have pain in your eyes, sensitivity to light, changes in your vision such as wavy lines or flashing, very red eyes in one or both eyes, or a baby less than 30 days old with red, sticky eyes. These can be signs of a more serious eye problem. NICE CKS says that serious causes of red eye, such as acute glaucoma, keratitis, iritis and trauma, must be excluded, and that red flag features include pain, photophobia, a change in visual acuity and a pupillary abnormality.

The NHS says to see a GP if your baby has red, sticky eyes, if you wear contact lenses and have conjunctivitis symptoms as well as spots on your eyelids (you might be allergic to the lenses), or if symptoms have not cleared up within 7 days. NICE CKS notes that contact lens wearers and immunocompromised people have the greatest risk of complications such as keratitis, and that severe, recurrent or persistent conjunctivitis may need discussion with an ophthalmologist.