NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Pink Eye

PINK EYE IS A COLLOQUIAL TERM WHICH DESCRIBES INFLAMMATORY DISEASES OF THE EYE, USUALLY DUE TO INFECTIOUS KERATOCONJUNCTIVITIS, LISTERIA OR TRAUMA.

OVINE INFECTIOUS KERATOCONJUNCTIVITIS (OIKC)

Infectious keratoconjunctivitis is seen in sheep of all ages, but tends to be more severe in ewes than in lambs. Outbreaks affecting up to 25% of the flock are commonplace. The disease can become particularly important when pregnant ewes are involved.

The first sign of OIKC is tear staining extending from the corner of one or both eyes, associated with conjunctivitis. As the disease progresses the cornea becomes cloudy and blood vessels are seen prominently at the periphery of the eye. The ocular discharge becomes thicker and pus-like as the disease progresses. When both eyes are severely affected sheep become temporarily blind, resulting in handling difficulties and losses due to misadventure. In most cases, healing occurs over a period of several weeks, eventually leaving only a faint corneal scar. However, in extreme cases the anterior chamber of the eye may ulcerate, rupture and become secondarily infected, resulting in permanent blindness. Acquired immunity following infection is poor and many cases relapse.

The primary causative organisms are believed to be Chlamydia psittaci and Mycoplasma conjunctiviae, either alone or in combination, but several other bacteria may also be involved in a secondary role. Close contact of sheep when trough feeding enables rapid spread of infection and probably accounts for the high incidence of the disease during winter months. Outbreaks in recently weaned lambs may be associated with high stocking rates, dust, long grass and flies which have been contaminated by tear secretions of infected animals. Handling of the face and head of lambs when drenching may also provide a means of spread. Clinically recovered animals may harbour the causative organisms for several months and may be a source of re-infection of other animals in subsequent years when their immunity has waned.

THE DISEASE IS PAINFUL AND PROMPT TREATMENT OF AFFECTED EWES IS USUALLY RECOMMENDED TO MITIGATE AGAINST THE RISK OF PERMANENT BLINDNESS.

Treatment is tedious and usually involves the topical application of aureomycin as a puffer or ointment. There is some evidence to support the use of intramuscular injections of long acting oxytetracycline, which may be a practical method for the treatment of OIKC in some flocks.

LISTERIA IRITIS (silage eye)

Infection of the anterior chamber of the eye is commonly seen in sheep (and cattle) of all ages associated with the feeding of big bale silage. The disease is associated with Listeria monocytogenes infection and up to 25% of the flock can be affected in severe outbreaks.

The initial presenting signs are excessive tear production, frequent blinking and avoidance of bright light involving one or both eyes. On closer examination the iris is swollen and thrown into a series of radial folds. Within two to three days, more severe inflammatory changes develop, with blueish white corneal opacity starting at the border and spreading inwards. Focal aggregations of fibrin in the anterior chamber of the eye are seen as accumulations of white material beneath the cornea. In more advanced cases widespread corneal opacity and vascularisation is often seen. If untreated, complete healing of this painful condition may take one to three weeks.

The precise cause of ovine iritis has not been experimentally proven. Outbreaks are invariably linked to the feeding of big bale silage. The consistent isolation of L.monocytogenes from conjunctival swabs taken from affected animals suggests that the organism has an important role, although in some surveys an equal incidence of L. monocytogenes has been shown in conjunctival swabs taken from both normal and affected eyes.

Subconjunctival injection of a combination of oxytetracycline and dexamethasone usually halts the progression of clinical signs when administered in the acute stages of the disease and the eyes usually return to normal within two to three days.

Your vet can advise you about the treatment and management of pink eye in your flock.

Neil Sargison BA VetMB DSHP FRCVS