Conjunctivitis

Anatomy and Physiology of the Conjunctiva

The conjunctiva is the term given to the thin, mobile, mucous membrane which lines the inner surface of the upper and lower lids, the palpebral inner and bulbal surfaces of the third eyelid and the anterior portion of the globe (until it meets the limbus). Diagram 1 depicts the different anatomical areas of the conjunctiva.

The conjunctiva consists primarily of stroma, which is split into a deeper fibrous layer (mainly connective tissue, blood vessels and nerves) and a more superficial adenoid layer, which contains conjunctiva-associated lymphoid tissue.

Above this sits the conjunctival epithelium, which contains goblets cells. The conjunctival epithelium is coated by the pre-ocular tear film, which provides the conjunctiva with protection and nutrition.

Diagram 1 - Anatomy of the conjunctiva

An overview of small animal conjunctivitis

Inflammation of the ocular mucous membranes, or conjunctivitis, is commonly encountered in small animal first opinion practice, and therefore you would assume that diagnosis and treatment would be easy. However, finding the root cause can sometimes be problematic.

The clinical signs of conjunctivitis include:

1. conjunctival hyperaemia
2. ocular discharge
3. conjunctival oedema (otherwise known as chemosis)
4. haemorrhage (both conjunctival and subconjunctival)
5. conjunctival thickening / swelling / ulceration
6. follicle formation
7. pruritus

Unfortunately, these signs are very non-specific and can occur with other disease processes. Equally, given the reactive nature of the conjunctiva, it often becomes inflamed with almost all other ocular disease processes. Conjunctivitis can result in a red eye, however, not every red eye has conjunctivitis. It is essential that any eye with inflammation of the conjunctiva undergoes a full range of diagnostic testing to determine the cause. 

The most useful way to classify conjunctivitis would be by aetiology. There are multiple potential causes of conjunctival inflammation and these can be split into primary causes (including infection, allergic and environmental) and secondary (including uveitis, adnexal disease, trauma, foreign bodies and neoplasia).

In dogs, conjunctivitis is most commonly secondary in nature, as a result of adnexal abnormalities or as a result of keratoconjunctivitis sicca. In contrast for cats, primary infectious disease is more commonly encountered.

In some cases, it is not possible to classify disease by cause, therefore classification can based upon the duration of disease (e.g. acute, chronic, recurrent) or the appearance of any ocular discharge (e.g. mucoid, purulent, haemorrhagic).

Additional Support

The diagrams below details some of the primary and secondary causes of conjunctivitis in dogs and cats, which may be encountered. For further information on other causes, please refer to the references or further reading.

Where you are unsure with regards to any ocular case, conversation with, and/or referral to a veterinary ophthalmologist should be considered.