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Selected Eye Diseases of Sheep and Goats
CHAPTER 86 Selected Eye Diseases of Sheep and Goats Moore CP, Miller RB: Infectious and parasitic diseases of cattle. In Howard JL, editor: Current veterinary therapy: food animal practice, ed 3, Philadelphia, 1993, Saunders. Rebhun WC: Diseases of dairy cattle, Baltimore, 1995, Williams & Wilkins. Rebhun WC: Ocular manifestations of systemic diseases in cattle, Vet Clin North Am Large Anim Pract 6:623, 1984.
CHAPTER
427
Snowder GD, VanVleck LD, Cundiff LV et al: Genetic and environmental factors associated with incidence of infectious bovine keratoconjunctivitis in preweaned beef calves, J Anim Sci 83:507, 2005. Whittaker CJ, Gelatt KN, Wilkie DA: Food animal ophthalmology. In Gelatt KN, editor: Veterinary ophthalmology, ed 2, Philadelphia, 1991, Lea & Febiger.
86
Selected Eye Diseases of Sheep and Goats HARRIET J. DAVIDSON and J. PHILLIP PICKETT
S
heep and goats are uncommonly examined for primary ocular disease and few systemic diseases have ocular lesions as part of the clinical presentation. Because of the behavior of small ruminants and management techniques, the eyes are rarely closely examined. For these reasons data on ocular diseases and treatment in small ruminants are scarce. The anatomy of sheep and goat eyes is similar. The eyelids are pliable and thin, although sheep have slightly thinner skin than goats, and the lids fit tightly to the globe. Both lids have cilia; superior lid cilia are longer than on the inferior lid. The lacrimal puncta are located approximately 5 mm from the medial canthus superiorly and inferiorly. The third eyelid is in the medial canthus and moves passively in the superior lateral direction. The third eyelid contains lymphatic nodules within and beneath the conjunctiva. The cornea is generally oval in the horizontal plane, with a slightly broader medial aspect. The limbus is frequently pigmented. The iris ranges in color from yellow to brown with variations in color and a slightly folded appearance. The pupil is generally oval when contracted, being broader medially and more round in shape when dilated. The corpora nigra is prominent and placed forward on the pupil margin. The fundus has a tapetum that may vary in color from yellow to orange to green to blue. The optic nerve is oval with a pinkish white color. Generally, four main blood vessels radiate in each quadrant.
ORBIT/GLOBE DISORDERS Inherited microphthalmos is seen in conjunction with multiple ocular anomalies in British, Dutch, and German Trexel sheep. The mode of inheritance appears to be autosomal recessive. Ingestion of the insect chemical sterilant apholate by pregnant ewes can cause congenital microphthalmos and other ocular anomalies, as well as malformation of periorbital bones. Chronic ingestion of plants containing high levels of selenium (>3 ppm) by pregnant ewes can also cause microphthalmos, ocular cysts, and other ocular malformations. Overzealous selenium supplementation to pregnant ewes by herdsmen can have the same effect. Consumption of the plant Veratrum californicum (“skunk cabbage”), most common in the high mountain pastures of western North America, by ewes and does can cause cyclopia (presence of one central eye) in lambs and kids. Other facial skeletal malformations frequently referred to as “monkey face” and absence of the pituitary gland and other brain malformations can also be seen. The plant must be ingested on day 14 of gestation to result in this facial/ocular malformation. Consumption of the plant later in gestation normally results in embryonic death. Cyclopia has been seen in the eastern United States and Canada in areas where V. californicum does not grow. The etiology of the facial/ocular malformation in these areas is unknown.
428
CHAPTER 86 Selected Eye Diseases of Sheep and Goats
Glaucoma and megaloglobus are rare in sheep and goats and are usually secondary to ocular puncture wounds or severe uveitis. Enucleation is recommended when the enlarged globe has exposure keratitis or is painful. Orbital cellulitis occurs rarely in sheep and goats. Causes include puncture wounds, migration of foreign plant material from the mouth to the orbit, and, rarely, caseous lymphadenitis (Corynebacterium pseudotuberculosis).
EYELID/THIRD EYELID Congenital Anomalies Dermoids are uncommon, but they may occasionally be identified. These have not been shown to be genetic in small ruminants. A dermoid is a section of skin either with or without hair in an incorrect location. The most frequently identified location is the superior or lateral limbal region. This skin may contain hairs that rub on the conjunctival or corneal surface, causing chronic irritation. Clinical signs are swelling of the conjunctiva, corneal opacities, ulcers, and an ocular discharge. If these signs occur, the dermoid should be surgically removed. Entropion is the most common congenital eyelid disorder in sheep. Afflicted lambs usually have inrolling of the lower lid, which causes irritant keratoconjunctivitis with pain, ocular discharge, and potential blindness from corneal perforation or scarring. The disorder has a hereditary predisposition but may be augmented by factors such as windy or dusty environmental conditions. Immediate therapy is aimed at reducing the frictional irritation and protecting the cornea. Having the shepherd frequently manually roll the lids out in conjunction with the application of lubricant antibiotic ointment may be too labor intensive for a large flock. Temporary eversion of the lid margin with skin clips (Michel clips), skin staples, or vertical mattress sutures or injection of penicillin (1-2 ml) into the lid to evert the lid margin may break the cycle of irritation. Most animals will do well with a single temporary eversion technique, but some animals may require a Hotz-Celsus procedure to remove eyelid skin and permanently repair the defect. This should only be done in older animals (4-6 months or older) when temporary techniques have failed. This is a relatively simple method of making an incision 3 to 5 mm from and parallel with the eyelid margin. A second curvilinear incision is made beginning and ending at the same place as the first incision, outlining a crescent-shaped area of skin. This crescent-shaped area of skin is sharply dissected using scissors. The deep subcutaneous fat is also removed. The wound is closed with simple interrupted sutures in the skin. Sutures should be removed in approximately 10 days. Entropion afflicted animals should not be bred, and sires and dams producing multiple afflicted offspring should be culled.
Blepharitis Numerous infectious and parasitic diseases of the skin may cause blepharitis in sheep. The viral infections that cause cutaneous ecthyma and sheep pox, ulcerative dermatosis, and blue tongue may cause exudative skin lesions
about the eyelids. Dermatophilus congolensis, Actinobacillus lignieresii, and Trichophyton verrucosum infections can cause exudative, scablike eyelid lesions. Clostridium novyi infection (“big head”) can cause eyelid and facial edema. Sarcoptic and demodectic mange, as well as keds, lice, and ticks, can cause blepharitis along with facial dermatitis. Ocular therapy for these disorders is supportive (the eyelids should be kept cleansed and the cornea protected with ophthalmic ointment), and precautions should be taken to prevent the spread of these infections to other animals and/or humans. As with cattle, photosensitization and keratoconjunctivitis sicca (KCS) can occur in sheep and goats secondary to the ingestion of toxic substances. St. John’s wort, buckwheat, and spineless horsebrush are common plants that contain these toxins. Locoweed ingestion causes KCS and palpebral nerve paralysis, which can in turn cause severe exposure keratoconjunctivitis. Although biopsy of the third eyelid is one method used in detection of scrapie, there are no ophthalmic lesions associated with the disease. Samples may be collected by restraining the sheep, placing a topical ophthalmic anesthetic on the surface of the eye and eyelids, and grasping the third eyelid and rolling it outward, exposing the bulbar conjunctiva. Scissors are used to snip the biopsy sample. No treatment is necessary post sample collection.
CONJUNCTIVAL AND CORNEAL DISORDERS In sheep, infection with Chlamydophila pecorum (Chlamydia psittaci is the former taxonomic name) and Moraxella ovis (Branhamella ovis is the former taxonomic name) is most common. Mycoplasma spp. and other aerobic bacteria, notably Neisseria ovis, may also cause conjunctivitis. In goats, mycoplasmal infections are most common, although aerobic bacteria have also been isolated. Although much of the syndrome in young goats is caused by Mycoplasma agalactiae, it may also be caused by Mycoplasma conjunctivae. For sheep and goats, in which chlamydophilal and mycoplasmal infections are most likely, topical ophthalmic tetracycline, oxytetracycline/polymyxin B, or erythromycin ointments are the treatments of choice. These preparations should be applied 3 to 4 times daily. When topical therapy is not practical, an injection of long-acting oxytetracycline (10-20 mg/kg, IM) or the addition of oxytetracycline to the feed (80 mg/head/day) may be beneficial. In sheep and goats, concurrent polyarthritis may be present. In goats, mammary gland and uterine infection may also occur simultaneously with keratoconjunctivitis. Disease rarely advances beyond a mild corneal opacity, with the occasional accompanying ulcer and conjunctivitis. Relapse may occur at any stage of recovery, but late lesions are not as severe as initial lesions. Thelazia californiensis infection is widespread in North America, but clinical disease is uncommon. Conjunctival or nasolacrimal duct involvement with the nasal botfly Oestrus ovis may cause irritant conjunctivitis and epiphora. Systemic organophosphate anthelmintics and ivermectin are effective in killing the parasite. Treatment is best performed in the fall when the larvae are small. Manual extraction of the larvae from the conjunctival
CHAPTER 86 Selected Eye Diseases of Sheep and Goats cul-de-sacs and flushing the nasolacrimal ducts may also be useful in severe cases. The filarid nematode Elaeophora schneideri afflicts sheep, deer, and elk in the high mountain regions of western North America. The disease is sometimes called “sore head” or clear-eyed blindness. Adult worms live in the carotid and internal maxillary arteries, but microfilaria can lodge in the capillary beds of the face and orbit and produce an immunologic reaction with facial swelling, alopecia, encrustations, conjunctival swelling, and corneal ulcerations. Diagnosis using skin biopsy is difficult and best confirmed at necropsy and evaluation of the carotid or internal maxillary arteries. Therapy may include oral piperazine (50 mg/kg-220 mg/kg once daily) or diethylcarbamazine (100 mg/kg). The efficacy of ivermectin therapy is unknown.
ANTERIOR UVEAL DISORDERS Clinical signs of inflammation of the uvea, or uveitis, include aqueous flare (hazy opacity to the anterior chamber), hypopyon (white cells in the anterior chamber), swollen irregular iris, misshapen pupil, cataracts, chorioretinitis, and optic neuritis. Uveitis may be caused by septicemia. Mycoplasma agalactiae infection is most common, but neonatal septicemias caused by coliforms, Staphylococcus spp., and Streptococcus spp. have been reported. Listeriosis in 4- to 6-month-old feedlot lambs being fed silage may cause anterior uveitis similar to that seen in calves. E. schneideri infection in sheep may cause uveitis in conjunction with the previously described lesions of the lids and conjunctiva. Toxoplasmosis is an uncommon cause of anterior uveitis and chorioretinitis in sheep, goats, and cattle. Treatment is primarily directed at the systemic disease. Medical therapy may include application of topical ophthalmic steroids such as dexamethasone or prednisolone two to six times daily depending on the severity of the ophthalmic lesions and the use of the animal. Atropine ophthalmic ointment may be applied one to three times daily to control ciliary spasms, which are painful and help to stabilize the blood aqueous barrier. These medications should be decreased in frequency or discontinued as the clinical symptoms improve.
LENS A cataract is any opacity of the lens. Inherited cataracts have been described in New Zealand Romney sheep. Cataracts have been reported as sequelae to uveitis secondary to E. schneideri infestation or M. agalactiae infection. Cataracts in general are rarely identified in small ruminants. No medical therapy is available to restore the lens
429
to its normal clarity. Cataract surgery is a possibility but is generally cost prohibitive for production animals. Pet animals that have lost vision from cataracts may benefit from referral to a veterinary ophthalmologist for possible surgery.
FUNDUS Bluetongue virus exposure during the first half of gestation (either as a viral infection or as a vaccination with modified live virus vaccine) may cause in utero necrosis of the developing retina with subsequent postpartum signs of multifocal retinal dysplasia, vision impairment, and cerebellar hypoplasia and hydrocephalus. Neonatal septicemias, listeriosis, and toxoplasmosis may cause chorioretinitis. E. schneideri infestation may cause chorioretinitis and optic neuritis during acute disease and chorioretinal scars, optic nerve atrophy, and blindness as sequelae to the acute disease. Scrapie virus has been reported to cause a retinopathy that accompanies the neurologic disease. Multifocal, oval, raised “blisterlike” lesions less than 1 disk diameter in size scattered throughout the tapetal fundus have been described. Chronic bracken fern (Pteridium aquilinum) ingestion causes “bright blindness” in sheep. After many months of grazing, progressive, generalized retinal degeneration results in blindness. In western Australia ingestion of “blind grass” (Stypandra imbricata) causes a similar retinal atrophy in sheep. Chronic ingestion of locoweed causes retinal atrophy in sheep, the same as in cattle. Hexachlorophene (used in countries outside the United States to treat liver flukes) can cause acute optic nerve–head edema within 24 hours of ingestion of a toxic dose, with subsequent generalized retinal atrophy, optic nerve atrophy, and blindness.
Recommended Readings
Moore CP, Wallace LM: Selected eye diseases of sheep and goats. In Howard JL, editor: Current veterinary therapy: food animal practice, ed 3, Philadelphia, 1993, Saunders. O’Rourke KI, Duncan JV, Logan JR et al: Active surveillance for scrapie by third eyelid biopsy and genetic susceptibility testing of flocks of sheep in Wyoming, Clin Diagn Lab Immunol 9:996, 2002. Prince JH, Diesem CD, Eglitis I: Anatomy and histology of the eye and orbit in domestic animals, Springfield, Ill, 1960, Charles C. Thomas. Smith MC, Sherman DM: Goat medicine, Philadelphia, 1994, Lea & Febiger. Webb AI, Bayenes RE, Craigmill AL et al: Drugs approved for small ruminants, J Am Vet Med Assoc 224:520, 2004.
CHAPTER
427
Snowder GD, VanVleck LD, Cundiff LV et al: Genetic and environmental factors associated with incidence of infectious bovine keratoconjunctivitis in preweaned beef calves, J Anim Sci 83:507, 2005. Whittaker CJ, Gelatt KN, Wilkie DA: Food animal ophthalmology. In Gelatt KN, editor: Veterinary ophthalmology, ed 2, Philadelphia, 1991, Lea & Febiger.
86
Selected Eye Diseases of Sheep and Goats HARRIET J. DAVIDSON and J. PHILLIP PICKETT
S
heep and goats are uncommonly examined for primary ocular disease and few systemic diseases have ocular lesions as part of the clinical presentation. Because of the behavior of small ruminants and management techniques, the eyes are rarely closely examined. For these reasons data on ocular diseases and treatment in small ruminants are scarce. The anatomy of sheep and goat eyes is similar. The eyelids are pliable and thin, although sheep have slightly thinner skin than goats, and the lids fit tightly to the globe. Both lids have cilia; superior lid cilia are longer than on the inferior lid. The lacrimal puncta are located approximately 5 mm from the medial canthus superiorly and inferiorly. The third eyelid is in the medial canthus and moves passively in the superior lateral direction. The third eyelid contains lymphatic nodules within and beneath the conjunctiva. The cornea is generally oval in the horizontal plane, with a slightly broader medial aspect. The limbus is frequently pigmented. The iris ranges in color from yellow to brown with variations in color and a slightly folded appearance. The pupil is generally oval when contracted, being broader medially and more round in shape when dilated. The corpora nigra is prominent and placed forward on the pupil margin. The fundus has a tapetum that may vary in color from yellow to orange to green to blue. The optic nerve is oval with a pinkish white color. Generally, four main blood vessels radiate in each quadrant.
ORBIT/GLOBE DISORDERS Inherited microphthalmos is seen in conjunction with multiple ocular anomalies in British, Dutch, and German Trexel sheep. The mode of inheritance appears to be autosomal recessive. Ingestion of the insect chemical sterilant apholate by pregnant ewes can cause congenital microphthalmos and other ocular anomalies, as well as malformation of periorbital bones. Chronic ingestion of plants containing high levels of selenium (>3 ppm) by pregnant ewes can also cause microphthalmos, ocular cysts, and other ocular malformations. Overzealous selenium supplementation to pregnant ewes by herdsmen can have the same effect. Consumption of the plant Veratrum californicum (“skunk cabbage”), most common in the high mountain pastures of western North America, by ewes and does can cause cyclopia (presence of one central eye) in lambs and kids. Other facial skeletal malformations frequently referred to as “monkey face” and absence of the pituitary gland and other brain malformations can also be seen. The plant must be ingested on day 14 of gestation to result in this facial/ocular malformation. Consumption of the plant later in gestation normally results in embryonic death. Cyclopia has been seen in the eastern United States and Canada in areas where V. californicum does not grow. The etiology of the facial/ocular malformation in these areas is unknown.
428
CHAPTER 86 Selected Eye Diseases of Sheep and Goats
Glaucoma and megaloglobus are rare in sheep and goats and are usually secondary to ocular puncture wounds or severe uveitis. Enucleation is recommended when the enlarged globe has exposure keratitis or is painful. Orbital cellulitis occurs rarely in sheep and goats. Causes include puncture wounds, migration of foreign plant material from the mouth to the orbit, and, rarely, caseous lymphadenitis (Corynebacterium pseudotuberculosis).
EYELID/THIRD EYELID Congenital Anomalies Dermoids are uncommon, but they may occasionally be identified. These have not been shown to be genetic in small ruminants. A dermoid is a section of skin either with or without hair in an incorrect location. The most frequently identified location is the superior or lateral limbal region. This skin may contain hairs that rub on the conjunctival or corneal surface, causing chronic irritation. Clinical signs are swelling of the conjunctiva, corneal opacities, ulcers, and an ocular discharge. If these signs occur, the dermoid should be surgically removed. Entropion is the most common congenital eyelid disorder in sheep. Afflicted lambs usually have inrolling of the lower lid, which causes irritant keratoconjunctivitis with pain, ocular discharge, and potential blindness from corneal perforation or scarring. The disorder has a hereditary predisposition but may be augmented by factors such as windy or dusty environmental conditions. Immediate therapy is aimed at reducing the frictional irritation and protecting the cornea. Having the shepherd frequently manually roll the lids out in conjunction with the application of lubricant antibiotic ointment may be too labor intensive for a large flock. Temporary eversion of the lid margin with skin clips (Michel clips), skin staples, or vertical mattress sutures or injection of penicillin (1-2 ml) into the lid to evert the lid margin may break the cycle of irritation. Most animals will do well with a single temporary eversion technique, but some animals may require a Hotz-Celsus procedure to remove eyelid skin and permanently repair the defect. This should only be done in older animals (4-6 months or older) when temporary techniques have failed. This is a relatively simple method of making an incision 3 to 5 mm from and parallel with the eyelid margin. A second curvilinear incision is made beginning and ending at the same place as the first incision, outlining a crescent-shaped area of skin. This crescent-shaped area of skin is sharply dissected using scissors. The deep subcutaneous fat is also removed. The wound is closed with simple interrupted sutures in the skin. Sutures should be removed in approximately 10 days. Entropion afflicted animals should not be bred, and sires and dams producing multiple afflicted offspring should be culled.
Blepharitis Numerous infectious and parasitic diseases of the skin may cause blepharitis in sheep. The viral infections that cause cutaneous ecthyma and sheep pox, ulcerative dermatosis, and blue tongue may cause exudative skin lesions
about the eyelids. Dermatophilus congolensis, Actinobacillus lignieresii, and Trichophyton verrucosum infections can cause exudative, scablike eyelid lesions. Clostridium novyi infection (“big head”) can cause eyelid and facial edema. Sarcoptic and demodectic mange, as well as keds, lice, and ticks, can cause blepharitis along with facial dermatitis. Ocular therapy for these disorders is supportive (the eyelids should be kept cleansed and the cornea protected with ophthalmic ointment), and precautions should be taken to prevent the spread of these infections to other animals and/or humans. As with cattle, photosensitization and keratoconjunctivitis sicca (KCS) can occur in sheep and goats secondary to the ingestion of toxic substances. St. John’s wort, buckwheat, and spineless horsebrush are common plants that contain these toxins. Locoweed ingestion causes KCS and palpebral nerve paralysis, which can in turn cause severe exposure keratoconjunctivitis. Although biopsy of the third eyelid is one method used in detection of scrapie, there are no ophthalmic lesions associated with the disease. Samples may be collected by restraining the sheep, placing a topical ophthalmic anesthetic on the surface of the eye and eyelids, and grasping the third eyelid and rolling it outward, exposing the bulbar conjunctiva. Scissors are used to snip the biopsy sample. No treatment is necessary post sample collection.
CONJUNCTIVAL AND CORNEAL DISORDERS In sheep, infection with Chlamydophila pecorum (Chlamydia psittaci is the former taxonomic name) and Moraxella ovis (Branhamella ovis is the former taxonomic name) is most common. Mycoplasma spp. and other aerobic bacteria, notably Neisseria ovis, may also cause conjunctivitis. In goats, mycoplasmal infections are most common, although aerobic bacteria have also been isolated. Although much of the syndrome in young goats is caused by Mycoplasma agalactiae, it may also be caused by Mycoplasma conjunctivae. For sheep and goats, in which chlamydophilal and mycoplasmal infections are most likely, topical ophthalmic tetracycline, oxytetracycline/polymyxin B, or erythromycin ointments are the treatments of choice. These preparations should be applied 3 to 4 times daily. When topical therapy is not practical, an injection of long-acting oxytetracycline (10-20 mg/kg, IM) or the addition of oxytetracycline to the feed (80 mg/head/day) may be beneficial. In sheep and goats, concurrent polyarthritis may be present. In goats, mammary gland and uterine infection may also occur simultaneously with keratoconjunctivitis. Disease rarely advances beyond a mild corneal opacity, with the occasional accompanying ulcer and conjunctivitis. Relapse may occur at any stage of recovery, but late lesions are not as severe as initial lesions. Thelazia californiensis infection is widespread in North America, but clinical disease is uncommon. Conjunctival or nasolacrimal duct involvement with the nasal botfly Oestrus ovis may cause irritant conjunctivitis and epiphora. Systemic organophosphate anthelmintics and ivermectin are effective in killing the parasite. Treatment is best performed in the fall when the larvae are small. Manual extraction of the larvae from the conjunctival
CHAPTER 86 Selected Eye Diseases of Sheep and Goats cul-de-sacs and flushing the nasolacrimal ducts may also be useful in severe cases. The filarid nematode Elaeophora schneideri afflicts sheep, deer, and elk in the high mountain regions of western North America. The disease is sometimes called “sore head” or clear-eyed blindness. Adult worms live in the carotid and internal maxillary arteries, but microfilaria can lodge in the capillary beds of the face and orbit and produce an immunologic reaction with facial swelling, alopecia, encrustations, conjunctival swelling, and corneal ulcerations. Diagnosis using skin biopsy is difficult and best confirmed at necropsy and evaluation of the carotid or internal maxillary arteries. Therapy may include oral piperazine (50 mg/kg-220 mg/kg once daily) or diethylcarbamazine (100 mg/kg). The efficacy of ivermectin therapy is unknown.
ANTERIOR UVEAL DISORDERS Clinical signs of inflammation of the uvea, or uveitis, include aqueous flare (hazy opacity to the anterior chamber), hypopyon (white cells in the anterior chamber), swollen irregular iris, misshapen pupil, cataracts, chorioretinitis, and optic neuritis. Uveitis may be caused by septicemia. Mycoplasma agalactiae infection is most common, but neonatal septicemias caused by coliforms, Staphylococcus spp., and Streptococcus spp. have been reported. Listeriosis in 4- to 6-month-old feedlot lambs being fed silage may cause anterior uveitis similar to that seen in calves. E. schneideri infection in sheep may cause uveitis in conjunction with the previously described lesions of the lids and conjunctiva. Toxoplasmosis is an uncommon cause of anterior uveitis and chorioretinitis in sheep, goats, and cattle. Treatment is primarily directed at the systemic disease. Medical therapy may include application of topical ophthalmic steroids such as dexamethasone or prednisolone two to six times daily depending on the severity of the ophthalmic lesions and the use of the animal. Atropine ophthalmic ointment may be applied one to three times daily to control ciliary spasms, which are painful and help to stabilize the blood aqueous barrier. These medications should be decreased in frequency or discontinued as the clinical symptoms improve.
LENS A cataract is any opacity of the lens. Inherited cataracts have been described in New Zealand Romney sheep. Cataracts have been reported as sequelae to uveitis secondary to E. schneideri infestation or M. agalactiae infection. Cataracts in general are rarely identified in small ruminants. No medical therapy is available to restore the lens
429
to its normal clarity. Cataract surgery is a possibility but is generally cost prohibitive for production animals. Pet animals that have lost vision from cataracts may benefit from referral to a veterinary ophthalmologist for possible surgery.
FUNDUS Bluetongue virus exposure during the first half of gestation (either as a viral infection or as a vaccination with modified live virus vaccine) may cause in utero necrosis of the developing retina with subsequent postpartum signs of multifocal retinal dysplasia, vision impairment, and cerebellar hypoplasia and hydrocephalus. Neonatal septicemias, listeriosis, and toxoplasmosis may cause chorioretinitis. E. schneideri infestation may cause chorioretinitis and optic neuritis during acute disease and chorioretinal scars, optic nerve atrophy, and blindness as sequelae to the acute disease. Scrapie virus has been reported to cause a retinopathy that accompanies the neurologic disease. Multifocal, oval, raised “blisterlike” lesions less than 1 disk diameter in size scattered throughout the tapetal fundus have been described. Chronic bracken fern (Pteridium aquilinum) ingestion causes “bright blindness” in sheep. After many months of grazing, progressive, generalized retinal degeneration results in blindness. In western Australia ingestion of “blind grass” (Stypandra imbricata) causes a similar retinal atrophy in sheep. Chronic ingestion of locoweed causes retinal atrophy in sheep, the same as in cattle. Hexachlorophene (used in countries outside the United States to treat liver flukes) can cause acute optic nerve–head edema within 24 hours of ingestion of a toxic dose, with subsequent generalized retinal atrophy, optic nerve atrophy, and blindness.
Recommended Readings
Moore CP, Wallace LM: Selected eye diseases of sheep and goats. In Howard JL, editor: Current veterinary therapy: food animal practice, ed 3, Philadelphia, 1993, Saunders. O’Rourke KI, Duncan JV, Logan JR et al: Active surveillance for scrapie by third eyelid biopsy and genetic susceptibility testing of flocks of sheep in Wyoming, Clin Diagn Lab Immunol 9:996, 2002. Prince JH, Diesem CD, Eglitis I: Anatomy and histology of the eye and orbit in domestic animals, Springfield, Ill, 1960, Charles C. Thomas. Smith MC, Sherman DM: Goat medicine, Philadelphia, 1994, Lea & Febiger. Webb AI, Bayenes RE, Craigmill AL et al: Drugs approved for small ruminants, J Am Vet Med Assoc 224:520, 2004.