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Conjunctivitis: The difference between bacterial and allergic conjunctivitis

Eyes | April 28, 2015 | Author: The Super Pharmacist


Conjunctivitis: The difference between bacterial and allergic conjunctivitis

Conjunctivitis is an extremely common condition. The main clinical features of conjunctivitis are a red eye or eyes producing a discharge. Conjunctivitis can be caused by a microorganism such as a bacterium or virus or it may be a consequence of allergies. In some cases, conjunctivitis is caused by neither allergies nor infection. In most cases, conjunctivitis is self-limiting or easily treatable; however, it is critical to determine the cause of conjunctivitis so that the correct treatment can be given.

Do I have conjunctivitis?

Most cases of a red eye that produces discharge will be diagnosed as conjunctivitis. Nevertheless, conjunctivitis is not the only cause of red eye. A red eye can be caused by something as innocuous as rubbing one's eye too roughly or it can be caused by something as serious as acute glaucoma. If you are concerned about a red eye that is producing discharge and causing pain, it is important to have the physician make sure that the red eye is not caused by something that may threaten vision such as acute angle closure glaucoma, iritis, or infectious keratitis. While a doctor must perform a thorough evaluation to rule out these causes, potentially worrisome symptoms in someone with red eye include:

  • Acute or severe eye pain
  • Foreign body sensation (i.e. feeling something is in the eye when there is not)
  • Photophobia (i.e. pain when looking at light)
  • Vision problems
  • Difficulty keeping the eye open
  • Headache with nausea

What kind of conjunctivitis do I have?

Assuming one has a case of conjunctivitis, it is important to determine what could be causing it. The three main types of conjunctivitis are:

  • Infectious (bacterial conjunctivitis, viral conjunctivitis)
  • Inflammatory (allergic conjunctivitis)
  • Noninfectious, nonallergic conjunctivitis

Bacterial conjunctivitis is more common in children than it is in adults.1,2 Viral conjunctivitis is probably more common than bacterial conjunctivitis, but bacterial conjunctivitis usually causes worse symptoms. Bacterial conjunctivitis usually causes substantial amounts of thick eye discharge that may be white, yellow, or green.3 Viral conjunctivitis, on the other hand, usually causes a thinner discharge that may be clear, slightly white, or pink. It is usually more difficult to distinguish between allergic conjunctivitis and viral conjunctivitis because the discharge can be virtually the same. The main distinguishing feature between these two conditions is that allergic conjunctivitis usually causes itchy eyes, which may be felt as grittiness or burning.

Allergic Conjunctivitis


Allergic conjunctivitis is an inflammatory process, which means the immune system reacts (or overreacts) to allergens. These allergens contact the eye through the air or through touch and, once in contact with the eye, they provoke an immune response. Common allergens include tree pollen, grass pollen, ragweed pollen, and pet dander. Allergic conjunctivitis causes redness in one or (usually) both eyes, watery discharge or tearing, and itchiness. Unfortunately, people tend to rub itchy eyes, which makes the reaction worse. The eyelid may be swollen and the redness may extend beyond the eye itself.

Illness Course

Allergic conjunctivitis is extremely common and may affect as many as 40% of all people. The immune system recognises the allergen at every exposure. Therefore, people who are allergic to tree pollen, for example, can usually expect to have allergic conjunctivitis seasonally, when plant pollen levels are high. A person who is allergic to cat dander may notice an acute conjunctivitis that occurs after petting a cat and then touching his or her own eye. Identifying the source of the allergy can be helpful since the person afflicted with that allergy can learn to avoid or reduce exposure to the allergen.


Allergen avoidance is a key part of managing allergic conjunctivitis; however, completely avoiding some airborne allergens is virtually impossible. Staying indoors and using air conditioning can help reduce airborne allergen exposure. For people allergic to dust mites, frequent cleaning, linen and towel changes, and trading carpets for hardwood or tile floors can help reduce the burden of allergens. Once allergic conjunctivitis has occurred, the patient should not rub their eyes even if they itch because this intensifies the allergic reaction and inflammation. Cool compresses may be helpful for relieving symptoms and swelling. Artificial tears used throughout the day can dilute allergens on the surface of the eye and reduce the reaction. Oral antihistamines may be helpful in treating mild cases of allergic conjunctivitis. Various eye drops can be used to reduce symptoms of allergic conjunctivitis, these are usually used in patients with severe symptoms. These eye drops include antihistamines, mast cell stabilizers, nonsteroidal anti-inflammatories, and glucocorticoids.

Bacterial conjunctivitis


Bacterial conjunctivitis is distinguished from other forms of conjunctivitis by thick secretions and purulent (pus-containing) discharge from the eye. The secretions come so quickly that they may return moments after wiping the eye. People may complain that their eyes are “glued shut” when they wake in the morning because of crusted discharge. Unlike allergic conjunctivitis, bacterial conjunctivitis does not usually cause itchiness.

Illness Course

Bacterial conjunctivitis is caused by Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. Bacterial conjunctivitis is extremely contagious and is transmitted through direct contact to someone's eye with infectious material. In other words, someone with bacterial conjunctivitis may spread the infection by contaminating surfaces that others then touch or, less likely, infecting others through direct contact. There is no universally effective method to prevent the spread of infectious conjunctivitis. Infected individuals should not share materials that contact the face such as towels, cosmetics, linens, or silverware. Many places, especially day care centers and schools, have instituted mandatory “stay home” policies for children with infectious conjunctivitis. Others require antibacterial eye drops for least 24 hours before returning. While these efforts may be helpful in reducing the spread of the infection, transmission may still occur. Moreover, antibacterial eye drops will do nothing to stop viral conjunctivitis, which is also highly contagious.


Fortunately, most cases of bacterial conjunctivitis are self-limited and resolve within a few days to weeks even without the use of antibiotics. Nevertheless, antibiotic eye drops or eye ointment may reduce symptoms and shorten the duration of the infection. Antibiotic treatment possibly reduces the likelihood of infecting others, but this has not been well studied and may not be the case. When indicated, ophthalmic antibiotics such as chloramphenicol, erythromycin or trimethoprim polymyxin B may be prescribed. Ophthalmic antibiotics should only be prescribed after a thorough eye examination to rule out serious causes of red eye and determine if the cause of conjunctivitis is actually bacterial. Importantly, antibiotics have no effect on viral conjunctivitis and allergic conjunctivitis.

Transmission of Bacterial and Viral Conjunctivitis

Both bacterial and viral conjunctivitis are potentially contagious as long as discharge persists. The incubation period for bacterial conjunctivitis is 1 to 7 days, which means a person may be transmitting the disease during this time before conjunctivitis occurs. Once bacterial conjunctivitis starts, the person may pass the infection on to others for up to a week. On the other hand, the incubation period for viral conjunctivitis is 5 to 12 days. Likewise, patients can transmit viral conjunctivitis for up to two weeks after conjunctivitis starts.

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