04/05/2022
Pinkeye in sheep and goats is an infection of the eye caused by bacteria (Mycoplasma conjunctivae, Chlamydia organisms, and other Mycoplasma sp.), targeting the conjunctiva and cornea. In cattle, pinkeye is mainly caused by Moraxella bovis, but may be associated with other bacteria including Mycoplasma and Neisseria. Note that the bacteria primarily responsible for pinkeye in cattle (Moraxella bovis), has no role in pinkeye in sheep and goats.
Pinkeye generally requires a combination of causative bacteria in the eye and some form of irritation or trauma to the corneal epithelium from things such as dust, dryness or ultraviolet light.
The condition is contagious and can spread rapidly in susceptible groups of animals. Alternatively, it may present in individual animals in groups where most animals are immune.
The condition is painful and weight gains can be negatively affected. Animals left with corneal scars or blindness may have reduced sale value and suitability for purpose. Most cases, whether treated or not, recover within 4 to 6 weeks (cattle) or 2 weeks (sheep and goats). Recovered animals may have small white areas of residual scarring on the cornea. Occasionally the scarring is extensive, seriously reducing vision. Rarely the cornea ruptures and the eye collapses, causing permanent blindness.
British and European cattle breeds are affected more severely than tropically adapted cattle breeds. Goats are mildly affected compared to sheep.
Predisposing factors include:
Crowding allowing close contact and transmission of infection.
Hot, dry, dusty conditions and presence of flies.
Access to feedstuffs containing material that readily enters the eye such as grass seeds, chaff, fibrous stems or stalks, and fodder fines from disintegrating pellets at sea.
Stressors such as excessive handling, undernutrition, crowding, and climatic changes that reduce resistance.
In any export consignment, multiple animals are usually affected and pink eye may be cause for rejection of badly affected individuals. Most infections occur early in the export process. They are often acquired at the assembly point and are expressed in greatest numbers in the first week or two at sea. If infections are not severe, most will have healed by the end of a three week voyage. Animals still affected at destination, may have reduced vision and be difficult to move.
CLINICAL SIGNS AND DIAGNOSIS
One or both eyes can be affected. Signs include bloodshot eyes (hyperaemia), eye discharge staining the face, and closed eyelids.
The cornea may be red initially and then go through cloudy hues of red, blue, grey, and white during sequential stages of healing. Animals with unilateral lesions may be missed at pen inspection since they are likely to stand so their good eye is facing the observer.
Differential diagnoses include foreign body in the eye, salt water in the eye from high pressure hosing, and infectious bovine rhinotracheitis (IBR) in cattle. IBR can cause a severe conjunctivitis and oedema of the cornea at the corneoscleral junction, but corneal ulceration is absent. Trauma to the eye from ear tags of pen mates during trough feeding has been suspected to cause occasional eye problems at sea. Some cases of malignant catarrhal fever and ocular squamous cell carcinoma may be mistaken for pinkeye in cattle.
It is important to examine multiple affected animals to check for the presence of grass seeds or other foreign material in the eye. Outbreaks of grass seeds or chaff in the eye can behave like pinkeye, even though pinkeye bacteria may not be present. Exposing foreign material requires everting the eyelids, including the third eye lid.
TREATMENT
Most cases of pinkeye will heal without treatment and this will typically occur over the same time period as if treatment was administered. This must be considered when there are large numbers of animals affected, where catching and restraining animals is difficult, and where handling may stress animals and may predispose to other problems or increase the spread of pinkeye infection. There is a small risk of severely affected animals suffering long term effects or even permanent loss of eyesight.
However, correct treatment applied early in the disease when there is just epiphora and tear staining below the eye, may shorten the duration of disease and prevent the development of complications including permanent blindness and corneal scarring.
If there are concerns that grass seeds, chaff or other foreign bodies may be involved, then each affected animal should be restrained and undergo close examination of the eye, especially of the conjunctival sacs. Grass seeds and other foreign bodies are easier to remove if topical anaesthetic and forceps are used.
The treatment of choice for pinkeye is a parenterally administered course of antibiotics (oxytetracycline or trimethoprim sulpha) which should be repeated as necessary if signs persist. Injectable antibiotics are probably more effective than topical treatments because they are better able to achieve sustained therapeutic levels of antibiotic in the eye. Note that Mycoplasma are often involved in pinkeye in sheep and goats. Mycoplasma do not have a cell wall and therefore antibiotics that target bacterial cell wall synthesis (beta-lactam antibiotics such as penicillin and cloxacillin), are less likely to be effective.
A single topical application of long-acting, antibiotic eye ointment is commonly used in cattle. Both eyes should be treated even if only one eye is showing signs of disease. Animals may be re-treated at 48 hour intervals. These products often contain synthetic penicillins so may not be effective against Mycoplasma infections in pinkeye in sheep and goats.
Subconjunctival injection of antibiotic and anti-inflammatory drugs may be useful for individual animals with more severe disease and where good restraint is available to ensure safe administration of the treatment.
Severe cases must be handled carefully to prevent bumping the eye and the risk of corneal rupture. They may also benefit from applying an eye patch or suturing the eyelids closed for a period to provide protection while healing occurs.
Topical treatments with sprays and powders are not beneficial and may in fact be detrimental. The use of irritant chemicals in the eyes will cause damage to the cornea and will not help to treat or prevent infection. In large outbreaks, mass medication of drinking water with antibiotics may be attempted, but effectiveness is uncertain. This practice requires the installation of header tanks to achieve correct concentrations, the intake is variable with some animals not drinking enough, and it carries the risk of disrupting rumen function and feed intake.
Separate penning of severely affected animals with easy access to food and water may be warranted.
PREVENTION
Management changes can help to reduce the rate of spread and also to prevent cases. Reducing exposure to grass seeds and airborne feed particles, controlling flies in the local environment, and reducing dust, are all measures that can aid in preventing and controlling pinkeye. Source batches of feed pellets that do not disintegrate easily and produce dust. Minimise stressors such as excessive handling, crowding, and undernutrition.
At sea, avoid feeding chaff and hay in windy conditions, minimise splash into eyes during high pressure hosing of decks, and spread sawdust in pens carefully to prevent it getting into eyes.
A vaccine is available for cattle, but does not cover all strains of Moraxella bovis that are involved in causing pinkeye. It should be administered 3-6 weeks before the onset of a high risk period to provide protection. There is no vaccine available for sheep and goats.
