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Eye: ocular discharge – overview

ISSN 2398-2950

Synonym(s): Serous discharge or Wet eye; Purulent discharge


  • Ocular discharge is a common presenting clinical sign for dogs and cats with ocular disease. Although there are only a few principal types of ocular discharge, the underlying causes for these are numerous. It is therefore essential that a thorough and systematic approach is adopted so that the correct diagnosis is reached.
  • Ocular discharge can be divided into two main types, serous discharge or the wet eye, and purulent discharge. The latter is often further described with terms such as mucoid or mucopurulent discharge, although the clinical distinction is often less precise.

Serous discharge (the Wet Eye)

  • Investigation of any ocular disease begins with a general and ocular history. Relevant points about the nature of the discharge include duration, whether unilateral or bilateral and appearance (color and consistency).
  • A wet eye may be attributed to:
    • Excessive lacrimation.
    • Inadequate tear distribution.
    • Reduced tear drainage (epiphora).
    • Conditions unrelated to the tear film.

Excessive lacrimation

  • Lacrimation is the secretion and discharge of tears. Basal tear production is augmented with reflex tear production in response to irritation of the ocular surface. The ocular surface receives sensory innervation via the ophthalmic branch of the trigeminal nerve (cranial nerve V), except for the lateral canthal area which is supplied by the maxillary branch. Trigeminal nerve stimulation causes an increase in the total tear film volume that will overload capacity of the nasolacrimal drainage apparatus and is thus manifested as a wet eye.
  • Conjunctivitis Conjunctivitis, eyelid and eyelash disorders and ulcerative keratitis Ulcerative keratitis are the most common extra-ocular causes for increased lacrimation due to irritation or pain.
  • The superficial cornea is more richly innervated than the deeper corneal stroma. The clinical relevance of this is that deep stromal ulcers and descemetoceles may appear relatively nonpainful and with little lacrimation; this is particularly true in brachycephalic breeds which often have reduced corneal sensitivity and low or marginal tear production. Superficial corneal lesions such as indolent ulcers Indolent ulcer are almost invariably associated with marked pain and lacrimation.

Conditions causing increased lacrimation 

Eyelash disorders

  • Distichia are abnormal, extra eyelashes usually exiting the meibomian gland orifices. They are rarely responsible for marked ocular pain or ulcerative keratitis.
  • They may however cause excessive tear production by a constant low grade stimulation of the conjunctival and corneal nerve endings. In contrast, ectopic cilia invariably incite acute and severe ocular discomfort manifested by excessive lacrimation and blepharospasm.
  • Ectopic cilia Ectopic cilia emerge from the palpebral conjunctiva and so immediately come into direct contact with the corneal surface, causing pain and focal ulceration. The history may be of an intermittently painful, wet eye as relief is provided when the hair falls out during the natural cycle. Careful examination with magnification, often under general anesthesia, may be required to confirm the diagnosis.

Eyelid disorders

  • Trichiasis (normal hairs in abnormal contact with the ocular surface) Trichiasis causes variable levels of discomfort rather than pain and may only cause a mild to moderate increase in tear production.
  • Trichiasis is also often seen in association with cicatricial eyelid lesions, traumatic and iatrogenic, and in feline eyelid agenesis Eyelid: abnormality. Some breeds are also predisposed to keratoconjunctivitis sicca Eye: keratoconjunctivitis sicca, so that excessive lacrimation is absent despite the presence of multiple irritating causes such as nasal fold trichiasis, lower medial entropion, distichiasis and atopic conjunctivitis.


  • Ocular surface trauma and foreign bodies Eye: ocular foreign body are typically accompanied by acute and excessive lacrimation. The serous discharge usually becomes copious and purulent within 24 hours and careful examination of the conjunctival sac is indicated.
  • Entropion is invariably associated with serous ocular discharge, which may become mucoid or mucopurulent. The site of the peri-ocular wetness is an indication for the position of the entropic region. There are many causes for conjunctivitis but allergic conjunctivitis associated with systemic atopy is a common cause of increased lacrimation.
  • In the cat, chlamydial conjunctivitis typically causes marked serous ocular discharge in conjunction with severe chemosis.

Intra-ocular causes of lacrimation

  • Excessive lacrimation is not only observed in relation to extra-ocular conditions. Painful intra-ocular conditions frequently have serous discharge at the medial canthus, although it is the pain, rather than the discharge, that is usually the most prominent feature.
  • Intra-ocular and orbital conditions, for which inflammation is the major abnormality, include uveitis Anterior uvea: traumatic uveitis, glaucoma Glaucoma, endophthalmitis Endophthalmitis/Panophthalmitis, posterior scleritis, abscess and cellulitis.

Reduced tear drainage

  • Tear overflow is called epiphora Epiphora. A patient with normal tear production but an abnormal nasolacrimal system will present with a wet eye. Once STT Schirmer tear test values establish normal tear production, the function and patency of the nasolacrimal drainage apparatus must be assessed.
  • The nasolacrimal system Nasolacrimal duct disease in the cat consists of upper and lower punctae that empty into the upper and lower canaliculi which then merge as the single nasolacrimal duct exiting via the distal nasal ostium on the ventrolateral aspect of the nares. The application of fluorescein dye Fluorescein test to the conjunctival sac enables the patency and function of the system to be assessed by observation of the flow of green stained tears at the nostrils within approximately five minutes. Observation of stained tears at the nostril confirms a patent, functioning duct. The absence of stained tears may be due to a genuine obstruction, or may represent a false negative result. The latter may be caused by insufficient dye applied, extra openings into the nasopharynx and the patient licking away the dye exiting the nares. A negative result warrants further investigation by direct examination with good illumination and magnification, cannulation, irrigation and possibly radiography. Sedation and general anesthesia will be required for cannulation and flushing of the duct in almost all cats.

Congenital and acquired conditions that affect nasolacrimal drainage

  • Developmental defects:
    • Lacrimal punctal aplasia/imperforate punctum.
    • Micropunctum. 
    • Aplasia of other parts of nasolacrimal system.
  • Acquired:
    • Trauma.
    • Foreign bodies.
    • Dacryocystitis Dacryocystitis.
    • Neoplasia (intra/extraductal).
    • Cysts.

Imperforate punctum

  •  The condition may be unilateral or bilateral, and may affect the upper or lower punctum, or both. Affected patients present with rust-coloured epiphora at the medial canthus from approximately two months old and have a negative fluorescein patency test. The condition is corrected surgically with the three-snip technique. Patients with micropunctum also present with epiphora and delayed rather than absent fluorescein drainage. The micropunctum is a round rather than elliptical opening, and can also be simply treated surgically.
  • Aplasia of other parts of the lacrimal drainage system is rare and referral to an ophthalmologist for a more detailed investigation and treatment is advised if possible.


  • Trauma of the nasolacrimal system requires good illumination, magnification and microinstruments to achieve adequate surgical repair.
  • Dacryocystitis Dacryocystitis presents with a purulent rather than serous ocular discharge in most patients and is mentioned further below.

Anatomical predisposition

  • Epiphora with obvious tear staining is often of cosmetic concern to the owner. 
  • Bilateral rust-coloured staining of the medial canthal skin and hair is recognized from approximately two to three months of age, and is especially noticeable in light-coated animals. Ocular irritation and conjunctival hyperemia are usually absent.
  • A thorough examination is required to eliminate increased lacrimation and/or abnormal tear drainage, but the syndrome is generally a result of a combination of the following anatomical features:
    • Shallow orbit with prominent globe.
    • Close apposition of eyelids to the globe, creating a  shallow lacrimal lake and functional occlusion of the lower punctum.
    • Medial lower entropion.
    • Misplaced lower punctum.
    • Hair at medial canthus, on eyelids and lacrimal caruncle (trichiasis).
    • Distichiasis.
    • Tight medial canthal ligaments.
  • Medical treatment is not always indicated for these patients as it is often only a cosmetic problem. Regular cleansing and hair trimming reduce the chance of localized dermatitis. Surgical correction of some of the anatomical features may be considered in severe cases. The severity of tear staining is alleviated by oral metronidazole Metronidazole or tetracycline Tetracycline; the mechanism for this is not understood but may be related to the interaction of the antibiotics with tear porphyrins.

Wet eye unrelated to the tear

  • Leakage of fresh, unclotted aqueous humour will cause a wet eye. Once the aqueous begins to clot (within several minutes), the discharge becomes more mucoid and tacky and is often mistaken for pus. Any eye with a penetrating foreign body or injury, or suspected dehiscence of a surgical wound must be examined carefully if clotted aqueous is present.
  • If wiped away without care, the deficit plugged by the aqueous clot will leak, resulting in further leakage, hypotony and uveitis. The presence of small full thickness lesions can be assessed by the Seidel test; fluorescein dye applied to the ocular surface is observed as bright apple-green rivulets draining from the site of leakage.

Purulent discharge

  • Purulent ocular discharge manifests as a thick, yellow to green discharge.
  • Common causes include:
    • Keratoconjunctivitis sicca (dry eye).
    • Foreign body (conjunctival sac, cornea, nasolacrimal system).
    • Dacryocystitis.
    • Melting ulcerative keratitis.
    • Bacterial conjunctivitis.
    • Eyelid margin disease (blepharitis Blepharitis).

Keratoconjunctivitis sicca

  • A Schirmer I tear test (STT) Schirmer tear test is an essential part of the diagnostic investigation in the examination of any eye with a mucoid or mucopurulent discharge to eliminate a diagnosis of keratoconjunctivitis sicca (KCS). If the initial STT reading is equivocal, eg between 10-15 mm/minute, the test should be repeated at future examinations and the clinical signs considered.
  • Secondary bacterial infection is common in dry eyes and is likely to be a contributing factor to the purulent discharge.
  • Bacteriology and cytology should also be performed in an eye with purulent discharge. Swabs should be submitted for aerobic culture from the affected eye(s). Be aware that topical anaesthetic preparations contain preservatives, although it is unlikely that this alters bacterial cultures in a clinically relevant way. Bacteriology samples should ideally be collected before antibiotic treatment is started; however organisms that persist in the face of antimicrobial use are also relevant.  Small tipped swabs, such as those available for contagious equine metritis (CEM), are particularly useful for ocular sampling. Differentiation of normal ocular flora from pathogenic flora may be difficult, although normal flora tend to be represented by more than one isolate and usually appear in lighter growth.
  • Cytology specimens can be obtained by a swab, spatula or cytobrush. Each method of collection has advantages and disadvantages. Cytological specimens can be examined following the application of a Romanowsky-type stain such as Diff-Quick, or a Gram stain. This allows rapid in-house assessment and immediate initiation of appropriate treatment, but samples should also be sent to an experienced cytologist for review.

Ocular foreign bodies

  • Foreign bodies such as grass seeds within the conjunctival sac or the nasolacrimal system are very common causes of purulent ocular discharge. When the focus of infection originates within the nasolacrimal duct (dacryocystitis), the thread of purulent discharge can be seen exiting from the lower punctum with careful direct examination, and will increase in volume if pressure is applied to the medial canthal area.
  • Retrograde flushing of the nasolacrimal system via the distal ostium at the nares, rather than from the proximal end, should be considered if a foreign body is suspected. This prevents a proximal foreign body from being flushed distally into the less accessible, osseous portion of the duct.


  • In cases of severe dacryocystitis, nasolacrimal (normograde or retrograde) flushing is best performed under sedation or general anesthesia as the procedure may be painful and iatrogenic duct rupture should be avoided. Repeat flushing is required to prevent duct stenosis and placement of soft tubing to maintain duct patency is necessary in some cases. Systemic as well as topical broad-spectrum antibiotic therapy is generally indicated for 4-6 weeks.


  • In the dog bacterial conjunctivitis is usually secondary to other underlying dysfunction, such as poor eyelid anatomy or dry eye syndromes. In the cat, primary conjunctivitis is most commonly caused by infectious agents, most notably the respiratory tract viruses ( feline herpesvirus and calicivirus), chlamydia and Mycoplasma spp. These agents cause ocular surface inflammation that is associated with profuse serous discharge that becomes mucopurulent during the course of the disease. The clinical significance of other bacterial isolates in the cat is not clearly defined.

Melting ulcers

  • Purulent ocular discharge is often seen in eyes with melting ulcers (keratomalacia or liquefactive stromal necrosis). An imbalance of collagenolytic enzymes originating from an exogenous bacterial infection such as Pseudomonas spp, from the cornea itself or from inflammatory cells, results in rapid dissolution of the corneal stroma. This condition is an ocular emergency and aggressive management is required.

Other types of ocular discharge

Crocodile tears
  • Increased tearing in response to stimulation of taste receptors (gustolacrimal reflex) is a rare condition described in people. This condition is not recognized in animals with the exception of single case report in a cat that exhibited tear overflow every time it ate for no apparent reason (Hacker 1990).
Medial canthal pocket syndrome
  • Medial canthal pocket syndrome describes the chronic conjunctivitis and mucoid discharge occurring in breeds with deep orbits, narrow skulls and inadequate tear drainage. A grayish, nondescript, jelly-like discharge accumulates in the medial canthus and is best managed by regular flushing from the ventral fornix with eyewash to minimize recurrent conjunctivitis. Specific medical treatment is only indicated if there is significant conjunctival inflammation and secondary bacterial infection.
Black ocular discharge
  • An intra-ocular uveal melanoma that has become extensive enough to erode through the scleral tunic results in rupture of the globe, manifesting as a sudden onset ink black ocular discharge.
Discolored tears
  • Brown to black discolored tears at the medial canthus and dried on periocular hairs are frequently observed in brachycephalic cats, notably Persians. The discoloration may be due to tear porphyrins although its exact origin and nature is unknown (see above). Reddish-brown discharge may be misinterpreted for hemorrhagic discharge by some owners. Regular cleansing is indicated. Unless profuse, this is considered a normal finding in such breeds and no specific treatment is usually indicated.
Red discharge
  • True hemorrhagic discharge is rarely seen in small animals but may be observed with ocular neoplasms such as a conjunctival hemangiosarcoma or ulcerated squamous cell carcinoma of the eyelid margin.

Techniques used in the investigation of ocular discharge

  • The aqueous component of the precorneal tear film is quantitatively assessed by the Schirmer I tear test (STT) Schirmer tear test.
  • As many of the causes of increased lacrimation involve the cornea, corneal integrity must be assessed by the application of fluorescein dye Fluorescein test. Fluorescein is best administered with a sterile, single-use impregnated paper strip. The strip should be moistened with saline or artificial tears and touched to the dorsal bulbar conjunctiva rather than the cornea. The latter may lead to erroneous interpretation as a false positive result, particularly if the examination includes blue light (Woods light) and/or magnification (slit lamp biomicroscope).
  • Application of a topical anesthetic agent may be necessary to allow examination of a very painful eye.
  • Proxymetacaine 0.5 % in sterile, single-use vials is ideal. Proxymetacaine will anesthetize the ocular surface almost immediately and only one to three drops need to be instilled over 2-3 minutes, depending on the severity of the pain.
  • All topical anesthetics are epitheliotoxic and often create a stippling effect on the cornea, similar to orange peel, if applied before fluorescein; it is important not to interpret such changes as ulcerative keratitis.

Further Reading


Refereed papers

Other sources of information

  • Petersen-Jones E S & Crispin S (eds) (2002) BSAVA Manual of Small Animal Ophthalmology. 2nd edn.