Eye diseases in foals

Knowledge of the different morphology and function of the neonate eye is essential for examining foals and understanding the different diseases. In general, eye diseases of foals can be categorised in congenital, inherited and acquired with congenital and inherited disease probably diagnosed more often in foals than in adult horses.

Specifically, it should be considered for examining foals, that the menace response is not a reflex and develops later in live, approximately within the first 2 weeks after birth. But depending for example on postnatal problems this period can be longer. The dazzle reflex is present from birth, also the pupillary light reflex, although this might be sluggish and incomplete for the first days of life. In neonates the globe appears to have a mild ventromedial orientation, which resolves after two to four weeks. Furthermore, eyelids might appear droopy and bilateral conjunctival congestion may appear for the first three weeks of life. Conjunctival haemorrhage, located dorsally, extending to the corneoscleral junction might be seen uni- or bilaterally and usually resolve spontaneously within the first two weeks. Bilaterally a perilimbal vascular ring, associated with diffuse corneal oedema has been reported in 1-day old foals, thought to follow the insult of amniotic or allantoic fluid over the cornea during the last days of gestation. The iris might be slightly grey in colour and the pupil is rounder than in adult horses. Lens suture lines are sometimes more prominent and hyaloid artery remnants might be seen. The optic disc appears more rounded than later in life. Optic nerve head congestion and multifocal haemorrhage within the tapetal area have been reported to occur in neonates, probably following increased intracranial pressure during birth or due to birth trauma. The cornea of foals has a decreased sensitivity, which explains why pain and discomfort are less evident compared to adults.

Congenital diseases can affect all parts of the globe, with some diseases being more common than others. A common congenital disease is entropion, but also other problems like ankyloblepharon, megalocornea, anterior segment dysgenesis, cataract, or different types of colobomas have been reported to occur in neonates. Congenital (sometimes inherited) diseases can affect one ocular structure or multiple structures, like MCOA (multiple congenital ocular abnormalities). For some of the diseases, like MCOA or CSNB (congenital stationary night blindness) there are clear predispositions, like coat colour or breed.

Acquired diseases like traumatic injuries, ulcerative or non-ulcerative keratitis are similar to the ones seen in adult horses. But some disease like uveitis associated with septicaemia are more common in foals.