Abstract

Deep stromal, off center corneal opacities and/or ulcerations are observed in various species of marine mammals both in the wild and in captivity. Histologically, abnormalities of the deep stroma and Descemet's membrane are demonstrable. These data suggest an intraocular rather than an extraocular etiology, and it appears significant that they correspond in location to the highly vascular iris operculum in cetaceans and the peri pupillary vascular arcade in pinnipeds. These vascular structures are markedly prominent during miosis and may even swell to the point wherein the iris tissue touches the cornea. Extreme miosis, then, could cause corneal endothelial damage and a resultant corneal opacity. Light intensity and aerial accommodation are two reasons for marked miosis to occur, and the possible role of these factors in the incidence of corneal opacities in marine mammals warrants consideration.

Notes

Possible etiologies for Corneal Opacities:

Trauma
Pool water quality
Metabolic disorders
Infection
Poor nutrition
Pupil constriction

Prolonged and intensive pupillary constriction can allow continued contact of the pupil with the lens. Unlike land mammals, marine mammals have a shallow slit between the anterior chamber and cornea of the eye. This is because the marine mammal lens is more rounded and the cornea is flattened as there is no need to overcome refraction between the air and the cornea. Thus the lens, iris and cornea of the marine mammal are close together.

The iris of marine mammals has a marked sphincter muscle supplied by large blood vessels. This prevents bright light from entering the eye and increases visual acuity on land by way of the pinhole effect. However, at the same time when the iris is constricted, the lens is squeezed. In addition, swelling of the large vessels during miosis may decrease the size of the anterior chamber even more.

Normally, the pupil does not constrict because in water the eye is protected from light and there is no need to accommodate. Captive marine mammals, however, spend some time out of water where the eyes are exposed to direct daylight and aerial accommodation must occur. In addition, light intensity increases when reflected off brightly painted pool bottoms, causing the iris tissue to swell.

The above factors can damage the corneal endothelium. Eventually permanent damage can occur and deep corneal disease develops. These opacities may disappear with time if they aren't too serious. Bilateral corneal opacities ringing the pupillary aperture are common in the field and in the aquarium.

Descemet's membrane is thin in marine mammals. This membrane gets split when the endothelium is damaged and no longer secretes properly. Cellular infiltrate can then be seen along Descemet's membrane. The result is deep stromal disease with splitting of Descemet’s membrane and development of irregular endothelium.

Recommendations to decrease incidence of corneal opacity

1. Darken pool bottom and sides, i.e. dark blue

2. Rock or sand bottom to decrease amount of reflected light.

3. Shaded areas for:

   a.  pinnipeds in and out of the water

   b.  cetacean training areas and holding pools.

4. Careful scrutiny to detect incidence of corneal disease and conditions under which it occurs.