Retinal Changes
A number of lesions may be recognized in the course of a thorough ophthalmoscopic examination of the retina. Cellular infiltration may appear focal or diffuse or it may follow blood vessels.
Infiltrations are white or gray. Edema has a similar appearance, but may be better demarcated. Hemorrhages have different shapes depending on the layer of the retina where they are located. Thus, focal or round hemorrhages are deep within the retina, linear lesions (flame hemorrhages) are in the nerve fiber layer, and crescent-shaped (keel) hemorrhages are preretinal (beneath the vitreous).Papilledema
Papilledema is a non-inflammatory swelling of the optic disc. It is an important sign of increased intracranial pressure. Causes in ruminants include hypovitaminosis A, acquired and congenital hydrocephalus, space-occupying brain lesions, meningitis, encephalitis, coenurosis, and hexachlorophene toxicosis (Whittaker et al. 1999). Papilledema is usually bilateral. The arterioles are more thread-like (and redder) than the congested retinal venules. Hemorrhages may be present on the disc or retina.
Chorioretinitis
Retinitis has been described in association with elaeopho- rosis in sheep. Trypanosomosis of sheep has caused severe uveitis, chorioretinitis, and optic neuritis. Natural infection with bluetongue or use of an attenuated vaccine in ewes in the first half of pregnancy has caused necrotizing retinopathy in lambs, according to Wyman (1983). Toxoplasmosis, as discussed under anterior uveitis, can cause a granulomatous chorioretinitis in ruminants. These conditions apparently have not been reported in goats.
Chorioretinopathy
This term suggests that active inflammation is not (or is no longer) present. One example would be scars from an earlier septicemia. Retinopathy is manifested by tapetal hyperreflectivity and pale depigmented areas in the non-tapetal fundus.
Retinal atrophy has been reported after congenital infection with Akabane virus. A spontaneous retinopathy possibly caused by rod-cone dysplasia has been reported in a young Toggenburg goat (Buyukmihci 1980). Four other closely related Toggenburg kids in Canada were blind from birth and showed a similar tapetal hyperreflectivity and marked retinal vessel attenuation; an inherited disorder was suspected (Wolfer and Grahn 1991). Other causes of retinal degeneration in ruminants, even if unreported in goats, are mentioned below for completeness.Scrapie
Scrapie, at least in sheep, can cause a central blindness. However, multifocal retinal elevations have also been described. These are blister-like, hyperreflective, with a dark edge, and scattered in the tapetal fundus. They vary from one-fourth to three-fourths the size of the optic disc (Barnett and Palmer 1971). Accumulation of lipofuscin in the retina is thought to be responsible for increased fluorescence intensity in the eyes of sheep with scrapie (Adhikary et al. 2010).
Border Disease
Because of parallels with lesions identified in cattle affected in utero with bovine virus diarrhea (BVD) virus, border disease might be expected occasionally to be accompanied by retinal changes. Abnormalities reported in cattle include grayness of the optic disc, vascular attenuation, hyperre- flective areas of the tapetal fundus, unusual admixtures of tapetal colors, and multifocal depigmentation of the non- tapetal fundus. Cataracts may occur and normal pupillary light reflexes may be absent. Such findings have not yet been reported in sheep or goats.
Bright Blindness
Sheep consuming bracken fern (Pteridium spp.) for many months may develop progressive bilateral blindness (Barnett and Watson 1970). The tapetum develops increased reflectivity, and the neuroepithelium degenerates. The pupils become circular and react poorly to light. The arteries and veins of the retina appear narrower than normal. The shepherd notices the blindness when a sheep becomes separated from the flock and runs around with elevated head and a high-stepping gait.
This clinical presentation has not been reported in goats, but goats do excrete ptaquilo- side, the associated toxin, in their milk (Virgilio et al. 2015).Blind Grass
Stypandra imbricata, or blind grass, is a plant found in western Australia. Goats and sheep that survive acute exposure become permanently blind, with degeneration of photoreceptor cells, optic nerves, and optic tracts. Clinical examination several weeks after blindness develops reveals foci of pigment epithelium hypertrophy, especially prominent in the non-tapetal fundus near the optic disc. Pupillary light reflexes are absent (Main et al. 1981). A related Australian plant, Stypandra glauca (nodding blue lily), has been associated with blindness in goats consuming the plant during its flowering stage. Acute cerebral edema with subsequent compression of the optic nerve within the optic canal may be important to pathogenesis (Whittington et al. 1988).
Other Plant Toxicities
Helichrysum argyrosphaerum toxicity has caused papilledema and retinal changes in sheep and cattle in Namibia, with spongiform lesions occurring in brain and optic fasciculi as well (Kellerman et al. 2005). Goats appear to be resistant (Basson et al. 1975), but in one blind goat with cerebral and brain stem vacuolation, a slight increase in tapetal reflectivity and pigmented foci in the non-tapetal fundus were noted (Van der Lugt et al. 1996).
Some species of Astragalus (locoweed) cause a retinal degeneration, at least in cattle and sheep. Many cells in the body are vacuolated, including neurons in the brain and inner ganglionic layer of the retina and secretory cells in the lacrimal gland. The eye appears dull and vision is impaired (Van Kampen and James 1971). Swainsona (darling pea) in Australia causes similar signs. Both plants contain substances that inhibit lysosomal mannosidase, thereby producing a storage disease. Similar vacuolation of retinal cells has been seen in goat kids with hereditary beta mannosidosis (Render et al.
1989). Astragalus species have also caused blindness, ataxia, and weight loss (blind staggers), possibly caused by high concentrations of selenium (Hosseinion et al. 1972), although it has been speculated that other toxins from the plants were involved and some cases of blind staggers were actually sulfur- related polioencephalomalacia (O'Toole et al. 1996; Whittaker et al. 1999). This condition is discussed in Chapter 5.Dryopteris filix-mas (male fern) ingestion in cattle causes acute retrobulbar neuropathy that may progress to optic atrophy. Hemorrhages on or around the optic disc and papilledema are noted acutely. Chronic lesions include optic nerve atrophy and reduced retinal vasculature; these animals are blind.
Retinal Detachment
Detachment occurs when transudates or exudates accumulate between the retinal pigment epithelium and the receptor layers. A total detachment results in blindness and is recognized by the presence of a billowing structure containing blood vessels in the vitreous. Case reports in goats are lacking.