The term conjunctivitis is used generically to describe any inflammation of the conjunctiva. From an anatomical point of view the areas of conjunctiva involved may be the palpebral conjunctiva, the orbital (or bulbar) conjunctiva and the part lining the third eyelid internally or externally.
CLINICAL SIGNS
Conjunctivitis is always accompanied by hyperaemia, which is manifested by redness of varying intensity depending on the severity of the pathology (Fig. 1). It is important to differentiate the superficial conjunctival vessels from those of the deep scleral and episcleral venous plexus in order to determine the site of the ocular disorder.
The presence of conjunctival folds and thickening indicates chemosis or oedema of the conjunctiva. Another characteristic sign is an ocular discharge. This may be serous, mucous, mucopurulent, purulent or haemorrhagic in nature. It can be localised to the medial palpebral commissure or, in organized forms (mucus-pus, pus), may be firmly attached to the conjunctival or palpebral surface. In cases of follicular conjunctivitis, small organized follicles may be observed, particularly on the bulbar surface of the conjunctiva of the third eyelid, giving rise to the characteristic “cobblestone” appearance. Photophobia and blepharospasm are uncommon in canine conjunctivitis and are only present in particularly severe or deep forms and in cases in which the cornea is also involved (keratoconjunctivitis). An animal with conjunctivitis may rub the peri-ocular area and harm itself because of the itching and burning present.
DIAGNOSIS
A generic diagnosis of conjunctivitis can be made simply from the clinical examination, in which Schirmer’s test may be of additional help. However, diagnostic tests such as conjunctival cultures, scrapes and biopsy are useful for determining the aetiology. When carrying out bacterial cultures it should be remembered that bacteria, some of which are potentially pathogenic, are present physiologically on the conjunctival surface in completely asymptomatic subjects. It is, therefore, thought that in many cases the bacteria grown in cultures may not be responsible for the conjunctivitis. The Gram-positive bacteria commonly isolated include Staphylococci, Bacilli, Streptococci and Corynebacteria; Gram-negative bacteria are less common and it is rare to find anaerobes. Some fungi, such as Cladosporium and Curvularia, can be isolated from the conjunctival sac.
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| These may be particularly important in the case of failure of initial antibiotic therapy. The technique involves placing sterile swabs, previously moistened with sterile saline solution, in contact with the lower or upper conjunctival fornix taking care not to touch the surface of the palpebra. The swabs are then immerged in a selective growth medium. |
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| Conjunctival scrapes stained with dyes such as Giemsa or Diff-Quick enable examination of cytological features (facilitating the differential diagnosis between neoplastic and non-neoplastic cells) and show any intracytoplasmic inclusions. Some stains, including the most commonly used Gram stain, allow cocci to be differentiated from bacilli and Gram-positive micro-organisms from Gram-negative ones. |
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| Using thin surgical tweezers and tenotomy scissors, together with repeated administration of a topical anaesthetic, a small specimen of conjunctival tissue can be taken. The wound heals by second intention. Local application of an antibiotic for a few days is usually advised. The sample, fixed in formalin, must then be sent to a histopathology laboratory to obtain the diagnosis. |
CLASSIFICATION
The various forms of conjunctivitis can be classified on the basis of their onset as acute, subacute or chronic, on the basis of the type of ocular discharge present, or according to the possible aetiological cause.
AETIOLOGY
Conjunctivitis in the dog can be primary or secondary to numerous disorders of the adnexae and eyeball. The primary forms of conjunctivitis are classified generically into infectious, non-infectious and neoplastic. The infectious forms include bacterial, viral, fungal, parasitic and rickettsial conjunctivitis. The non-infectious primary forms of conjunctivitis can be due to allergy, hypersensitivity or immune-mediated reactions, be related to quantitative or qualitative defects of the tear film, be the consequence of radiotherapy or be associated with contact with toxic or chemical substances or the presence of foreign bodies. Besides being secondary to any disorder of the adnexae of the eyeball and orbit, conjunctivitis can also be the consequence of a systemic disease.
Primary bacterial conjunctivitis is fairly rare in the dog, since the bacterial infection is usually secondary to other disorders of the eye or adnexae. Culture studies are important for diagnostic purposes in particularly resistant or doubtful cases. From a cytological point of view, a conjunctival scrape from an animal with acute conjunctivitis usually contains neutrophils, numerous bacteria, degenerated epithelial cells and a few mononuclear cells; in chronic conjunctivitis there are neutrophils, an increased number of mononuclear cells, and keratinising or degenerated epithelial cells.
Viral causes of conjunctivitis include the distemper Paramyxovirus, Adenovirus and, possibly, Herpesvirus. In the case of Paramyxovirus, the ocular form of the infection, which can also involve the chorion, retina and optic nerve, is manifested by a mucopurulent discharge, often associated with systemic respiratory symptoms with dejection, poor appetite and fever. In some subjects, this viral conjunctivitis can give rise to keratoconjunctivitis sicca. From a diagnostic point of view, the techniques of choice besides a complete clinical assessment are an immunofluorescence test or polymerase chain reaction analysis on a sample of conjunctival epithelial cells and the observation of intracytoplasmic inclusion bodies. The Adenovirus responsible for conjunctivitis may be either type 1 (CAV 1) or type 2 (CAV 2). Both viruses cause a bilateral form of conjunctivitis characterized by hyperaemia and a serous or sero-mucous discharge.
Fungal forms of conjunctivitis are rare and often depend on environmental and climatic factors; they occur predominantly during local or systemic immunosuppressive therapy. Some species of Nocardia and yeasts were detected in the conjunctival sac of 17% of subjects with chronic conjunctivitis. The significance of this finding is not, however, clear since it is also possible to isolate fungi and bacteria from healthy subjects. Nodular formations may be observed in the case of infection by Blastomyces dermatidis. The diagnosis is made from cytological and histological studies.
The parasitic forms of conjunctivitis are secondary to the presence of a nematode of the genus Thelazia and are manifested y conjunctival hyperaemia, a mucous or mucopurulent ocular discharge and presence of larvae in the superior or inferior conjunctival fornix (Fig. 2).
Allergic conjunctivitis is secondary to contact with allergens such as dusts, bacterial toxins and pollens and can by present alone or in association with atopic dermatitis. The conjunctiva is hyperaemic and oedematous, there is a serous ocular discharge and the affected animal usually has severe pruritus, particularly in the peri-ocular region.
Follicular conjunctivitis is a result of chronic antigenic stimulation and characteristically occurs in young animals. The signs include follicles on the internal surface of the third eyelid but, in particularly severe cases, other parts of the conjunctiva may also be involved. Follicular conjunctivitis is usually associated with hyperaemia and a mucous discharge. The diagnosis is based on the clinical appearance and detection of lymphoid tissue in conjunctival scrapes.
Conjunctivitis secondary to neoplasms is uncommon in dogs. There are descriptions of cases of melanoma (Fig. 3), squamous cell carcinoma, angioendothelioma, mast cell malignancy, transmissible venereal tumour, haemangioma, haemangiosarcoma, lymphosarcoma, histiocytoma, angiokeratoma and papilloma. Some of these neoplasms have a malignant behaviour, with local recurrences or distant metastases being extremely common.
In all cases of primary conjunctivitis not responding to common, broad-spectrum antibiotic treatment it is very important to examine the eyelids and nasolachrymal system, determine the quality and quantity of tear production, measure ocular pressure, instil fluorescein and rose Bengal in order to demonstrate any lesions of the ocular surface, and carry out a full ophthalmological evaluation as well as a careful clinical examination to exclude any systemic disorders.
TREATMENT
The treatment depends on the aetiology. Cases of primary and secondary bacterial conjunctivitis should initially be treated with broad-spectrum antibiotics (tobramycin, chloramphenicol, erythromycin and ciprofloxacin) with or without corticosteroids (dexamethasone and betamethasone). Corticosteroids are, however, contraindicated in animals with corneal ulcers. Cultures or histological studies may be useful in chronic conjunctivitis and other forms that respond poorly to treatment. It is also very important to clean the ocular and peri-ocular areas carefully and, in some cases, topical use of artificial tears may be helpful.
For cases of fungal conjunctivitis it has been suggested that intravenous antifungal agents, such as miconazole, could be used, or topical treatment with itraconazole, fluconazole or natamycin.
The treatment of choice for parasitic conjunctivitis is removal of the larvae, topical administration of solutions of moxidectin 1-2.5% and imidacloprid 10% or tetramisole, and systemic use of ivermectin.
Follicular conjunctivitis in young animals does not require treatment, in part because the lymphoid follicles represent a physiological defence raised by the immune system. In cases of severe concomitant infections or a tendency to rub the eyes and cause self-harm, compounds based on antibiotic-corticosteroid combinations can be used, even though the condition usually recurs as soon as the treatment is suspended. If the condition persists into adulthood, removal of the follicles – by surgical scraping or cauterisation with chemicals (copper sulphate crystals) – may be appropriate.
In cases of secondary conjunctivitis it is essential to resolve the underlying cause of the disorder.
Suggested readings
- Lavach JD, Thrall MA, Benjamin MM, Severin GA. Cytology of normal and inflamed conjunctivas in dogs and cats.J Am Vet Med Assoc. 1977 Apr 1;170(7):722-7.
- JacksonJA, Corstvet RE.Study of nictitating membranes and genitalia of dogs with reference to lymphofollicular hyperplasia and its cause.Am J Vet Res. 1980 Nov;41(11):1814-22.
- Gerding PA Jr, McLaughlin SA, Troop MW. Pathogenic bacteria and fungi associated with external ocular diseases in dogs: 131 cases (1981-1986).J Am Vet Med Assoc. 1988 Jul 15;193(2):242-4.
- Martin CL,Kaswan R. Distemper-associated keratoconjunctivitis sicca. J Am Anim Hosp Ass 1985;21:355-359
- Slatter D. Canine conjunctiva and nictitating membrane. In: Slatter D, ed. Fundamentals of Veterinary Ophthalmology. 4rd ed. Philadelphia: Saunders WB, 2007: 662-689.