18 For convenience, this chapter is divided by major species (i.e., canine, feline, equine, and food and fiber animals). We selected the more common systemic diseases or those in which the ophthalmic manifestations assist in establishing the clinical diagnosis. Additional information about these diseases is available in the species‐related chapters (canine, Chapters 4–13; feline, Chapter 14; equine, Chapter 15; and food and fiber animals, Chapter 16). There are a large number of popular breeds that have the merle gene and coat. Excessive white or partial albinism occurs in the canine breeds with the merling gene. In animals homozygous for the incomplete dominant merling gene (usually the result of breeding two merled parents), the hair coat is primarily white, iridal heterochromia occurs with multiple eye anomalies, and sometimes deafness occurs. The ocular anomalies have been studied extensively in the Australian Shepherd breed, and include microphthalmia, iridal heterochromia, cataract formation, large and variable equatorial staphylomas, retinal dysplasia, and retinal detachments (Figure 18.1; see also Figures 4.1, 10.2, and 10.5). The eye defects are often asymmetrical, and blindness is uncommon. However, cataract progression and/or retinal detachments can impair vision in later life. Dog fanciers should be advised that breeding of merled dogs will result in at least 25% of affected offspring; hence, it is recommended to breed merled dogs to a non‐merle animal. Figure 18.1 In the homozygous merle Australian Shepherd, microphthalmia and multiple ocular anomalies occur. Note the bilateral heterochromia irides and asymmetrical microphthalmia in this puppy. Dwarfism‐associated ocular defects occur in the Samoyed and Labrador Retriever breeds. In the Labrador Retriever, the eye defects are inherited as an incomplete dominant trait while the skeletal defects are transmitted as an autosomal recessive trait (Figure 18.2). In affected animals the ocular anomalies range from focal or geographic retinal dysplasia usually dorsal of the optic disc (thought to be heterozygous) to multiple ocular anomalies (microphthalmia, cataract formation, hyaloid artery remnants, retinal dysplasia, and complete retinal detachments, thought to be homozygous). Figure 18.2 Oculoskeletal dysplasia is inherited in the Labrador Retriever. In this mildly affected puppy, note the numerous areas of retinal dysplasia in the tapetal fundus. The eye defects in dwarfism in the Samoyed breed are thought to be inherited in an autosomal recessive mode and are similar, although not as severe, to those in affected Labrador Retrievers. Breeding of affected animals as well as adult animals known to have had affected offspring is not recommended. Congenital hydrocephalus can cause enlargement of the developing calvarium and displacement of the orbits (Figure 18.3). The result is displacement of both globes ventrolaterally, resulting in a change in the animal’s facial features. Papilledema is occasionally present. Figure 18.3 Hydrocephalus in the dog can cause enlargement of the calvarium and in this Chihuahua has displaced the eyes laterally. The canine distemper virus (RNA paramyxovirus) affects the ophthalmic structures in several ways, resulting in mucopurulent conjunctivitis, acute keratoconjunctivitis sicca, iridocyclitis, chorioretinitis, optic neuritis, and central (occipital cortex) blindness (Figure 18.4). Conjunctivitis, keratoconjunctivitis sicca, and iridocyclitis occur with the other systemic (gastrointestinal and respiratory disease) signs of distemper while the retinal, optic nerve, and central nervous system (CNS) abnormalities occur several days after the initial viremia when the virus invades the neural tissues. Figure 18.4 (A) Canine distemper frequently causes mucopurulent conjunctivitis. In some of these eyes, reduced tear production will also develop. (B) In this dog, canine distemper has caused an acute chorioretinitis. Note the round to oval translucent inflamed areas (arrows) within the tapetal fundus. (C) A chorioretinal scar in a dog that has recovered from infection with canine distemper virus. Sometimes, these scars are referred to as “gold medallion” lesions. However, they are not pathognomonic for distemper. (D) Optic neuritis associated with canine distemper. Note the hyperemic optic disc and its fuzzy, indistinct margins. The retinal vessels appear to be raised up on to the protruding disc. Chorioretinitis is characterized by multifocal round to oval lesions scattered throughout the tapetal and nontapetal fundi. In one study, 41% of the dogs with the neurologic form of disease had ocular fundus lesions, while another report on dogs with chronic leukoencephalopathy syndromes noted 83% of the dogs had chorioretinal lesions. Hence, dogs with neurological diseases require a complete fundus examination. Optic neuritis can cause temporary to complete blindness when both nerves are involved. Mydriasis and blindness are the presenting signs. The inflamed papillae appear raised or elevated, and peripapillary hemorrhages and retinal vascular congestion can occur. Often, the inflammation extends a few disc diameters into the peripapillary retina (neuroretinitis). Optic nerve degeneration follows weeks later, and appears as ophthalmoscopically as depressed, smaller than normal, darkened optic discs with retinal vessel attenuation. (For other distemper ophthalmic manifestations see Figures 3.20, 3.21, 13.6A,B,D, and 13.19.) Infectious canine hepatitis (ICH) caused by canine adenovirus‐1 (CAV‐1) has been associated with ophthalmic disease since its initial report in 1947 (Figure 18.5). Anterior uveitis and profound corneal edema occur with the natural disease (20% of cases) or following vaccination with CAV‐1. Vaccination with canine adenovirus‐2 (CAV‐2) conveys protection against ICH while eliminating the adverse ocular reactions that occur with CAV‐1. Figure 18.5 The ocular signs of infectious canine hepatitis include iridocyclitis with profound corneal edema. Ophthalmic disease is thought to be due to a delayed Arthus reaction, and occurs 7–21 days post‐CAV‐1 contact. The local immune response is directed at virus in the corneal endothelium and anterior uvea. The anterior uveitis is bilateral in 12–28% of patients, and usually resolves without sequelae. However, persistent corneal edema, secondary glaucoma, and phthisis bulbi occasionally result. Treatment is symptomatic with topical corticosteroids and mydriatics (see Figure 10.10). The canine papilloma virus can result in focal papillomas affecting the mucus membranes of the mouth, eyelids, conjunctiva and nictitating membrane, and cornea (Figure 18.6). Younger dogs are most frequently affected. Most oral papillomas regress spontaneously, but the ophthalmic tumors may persist. The tumors appear as solitary raised cauliflower‐like masses that are sometimes slightly pigmented. Recommended therapy is wide excision of the papilloma and cryotherapy of its base. Figure 18.6 Focal papilloma affecting the medial canthus of a St. Bernard puppy. Rickettsial infections such as ehrlichiosis and infectious cyclic thrombocytopenia (Ehrlichia canis, Ehrlichia platys, and Ehrlichia equii) and Rocky Mountain spotted fever (Rickettsia rickettsii) produce ocular signs (Figure 18.7; see also Figures 3.15E and 10.8). The ocular lesions produced by E. canis and R. rickettsii infections are very similar, and include conjunctivitis, chemosis, anterior uveitis, and retinal vasculitis. Petechial hemorrhages occur in the conjunctiva, iris, and retina. The systemic antibiotic of choice is oral doxycycline or tetracycline. Ophthalmic therapy is supportive and usually includes corticosteroids and mydriatics. When they occur, ophthalmic complications are usually related to intraocular hemorrhage and inflammation and include secondary glaucoma, cataracts, and retinal detachments. Figure 18.7 (A) Rocky Mountain spotted fever can cause hemorrhages in many of the body’s tissues and the retina is no exception. Note in this dog, recent and resorbing preretinal hemorrhages are present. (B) In this dog affected by Rocky Mountain spotted fever there are small hemorrhages throughout the iris stroma. Brucella canis is an infrequent cause of anterior uveitis and endophthalmitis in the dog (Figure 18.8). Systemic signs include abortion, infertility, and vaginal discharges in bitches, and sterility and testicular and epididymal disease in males. The ophthalmic disease is usually a mild but persistent uveal inflammation. This disease is considered zoonotic, and neutering of an affected animal should be encouraged. Figure 18.8 Canine brucellosis (Brucella canis) in a Miniature Dachshund dog has produced anterior uveitis that responded poorly to broad‐spectrum antibiotics. Note the episcleral injection, corneal vascularization and edema, swollen iris, and miosis. B. canis was cultured from the aqueous humor. Mycotic infections or dermatophytosis can affect the canine eyelids, and are frequently associated with Microsporum canis, Microsporum gypseum, or Trichophyton mentagrophytes (Figure 18.9; see also Figure 5.16). Mycoses usually present as dry crusty alopecia affecting the eyelid and face. Diagnosis is made by Wood’s lamp inspection, culture, and/or microscopic examinations of scrapings. Figure 18.9 (A) Mycotic lid infections or dermatophytosis are common in puppies and appear as dry and crusty alopecia of the eyelids. (B) Mycotic corneal ulcerations are rare in the dog, and often associated with foreign bodies or systemic immunosuppression. In this dog, tissue plaque of necrotic tissue, inflammatory cells, and fungal elements are present in the axial cornea and there is peripheral corneal vascularization, corneal edema, and miosis, which signals anterior uveitis. Fungal corneal ulcers are uncommon in the dog (in contrast to the horse and human), and are often associated with corneal foreign bodies, local or systemic immunosuppression, or chronic antibiotic therapy (see Figure 8.7). Mycotic corneal ulcers have a variety of clinical appearances ranging from nonhealing superficial ulcers to progressive antibiotic‐resistant corneal ulcers with malacia or surface plaques that appear pigmented. Diagnosis is usually made with cytology and fungal culture. Treatment consists of topical antifungals and antibiotics, and in some cases surgical debulking and stabilization is necessary. With severe anterior uveitis, topical mydriatics and systemic antifungal agents should be considered. Blastomycosis is endemic in the river valleys and also occurs in Canada, Europe, Mexico, Latin America, and Africa. Canine blastomycosis (Blastomyces dermatitidis) is a common regional systemic mycotic infection in the USA that often affects the ophthalmic tissues. At least 40% of affected dogs will show ophthalmic changes (Figure 18.10). Ophthalmic lesions include anterior uveitis (miosis, conjunctival hypermia, episcleral injection, aqueous humor flare, low intraocular pressure, and iridal swelling), posterior uveitis, panophthalmitis, and retinal detachments (see Figure 10.11A). Posterior segment granulomas are common. Most cases are bilateral, but can be asymmetrically affected. Spread to the eyes is via the hematogenous route, hence the disease primarily affects the choroid initially. Although the signs of a “red eye” or anterior uveitis may attract the attention of the pet owner and result in presentation for diagnosis and therapy, the most serious inflammation involves the posterior uvea, which can result in vitreal inflammation, exudative retinal detachments, and blindness. Cataract formation, secondary glaucoma, and phthisis bulbi are frequent sequelae. Treatment consists of long‐term systemic antifungals and local anti‐inflammatory therapy and can be expensive. Eyes with exudative retinal detachments do not usually regain vision. Enucleation may become necessary for patient comfort and can make control of the systemic condition easier. Figure 18.10 (A) Early blastomycosis in a dog causing chorioretinitis. Note the numerous pigmented foci in the tapetal fundus, and a single white granuloma (arrow) in the nontapetal fundus. (B) A blastomycosis granuloma in the peripheral fundus of a dog. Note the profound subretinal exudate underlying the retinal vessels. Coccidioidomycosis (Coccidiodes immitis), or Rift Valley fever, is a regional mycotic infection in the dog that occurs primarily in the southwestern USA, Mexico, and Central America. The arthrospores usually enter via the respiratory tract, and spread to infect the bones, skin, eyes, visceral organs, testicles, CNS, and the heart. In one study, 42% of patients with eye lesions had no systemic clinical signs, and most (80%) patients had unilateral infections (see Figure 10.11C). Clinical abnormalities include keratitis (49%), anterior uveitis (43%), and glaucoma (31%). Posterior segment involvement occurs in about 50% of patients, and can be masked by the anterior uveitis and/or opaque media (Figure 18.11). Treatment consists of long‐term systemic antifungals and local anti‐inflammatory medications. Eyes with exudative retinal detachments do not usually become visual. Figure 18.11 (A) Coccidioidomycosis in a dog causing anterior uveitis and secondary glaucoma. Note the inflammatory debris in the anterior chamber, corneal edema, episcleral congestion, enlarged globe (buphthalmos), and deep corneal vascularization in the peripheral cornea. (B) Coccidioidomycosis also affects the posterior segment. In this dog note the diffuse chorioretinitis and focal granulomas (arrows) in the tapetal fundus. Courtesy of Ron L. Sigler. Histoplasmosis (Histoplasma capsulatum) is an infrequent ophthalmic infection in small animals which occurs in endemic areas in the Ohio, Missouri, and Mississippi river valleys. Systemic signs usually affect the respiratory or gastrointestinal systems. The ophthalmic manifestation is typically pyogranulomatous choroiditis that extends into the retina and can result in exudative retinal detachments (Figure 18.12). Diagnosis is by cytology, biopsy, or agar gel immunodiffusion test. Figure 18.12 Histoplasmosis in a dog causing chorioretinitis. Note the pigmented areas within the fundus. Courtesy of Charles Martin. Cryptococcosis (Cryptococcus neoformans) occurs worldwide, and affects a variety of animal species. Both CNS and ophthalmic clinical signs result, although not necessarily together. The ophthalmic signs are usually acute onset of mydriasis and blindness, related to granulatomatous or pyogranulomatous optic neuritis and chorioretinitis (Figure 18.13). The optic neuritis probably results from the direct extension of the organism from the brain and cerebrospinal fluid (CSF). Diagnosis can be made with CSF tap or vitreocentesis demonstrating the thick‐walled spheroidal yeast organisms. If attempted, long‐term antifungal therapy is necessary. Figure 18.13 (A) Cryptococcosis in the dog tends to affect primarily the posterior segment causing chorioretinitis, optic neuritis, and exudative retinal detachment. In this dog, note the inflammatory retinal detachment and the dark foci of chorioretinitis. (B) Cryptococcosis in dog with multiple granulomas within the retina. The highest granuloma concentration is adjacent to the disc. Aspergillosis also affects the dog, and many variants are involved (Aspergillus terreus, Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger, and Aspergillus nidulans). For reasons not yet defined, systemic aspergillosis seems to predominantly affect the German Shepherd breed (Figure 18.14). Systemic clinical signs often include diskospondylitis (vertebral pain, paresis, and paralysis). Ophthalmic signs include chorioretinitis, anterior uveitis, retinal detachments, and endophthalmitis (see Figure 10.11B). The prognosis is very poor. Figure 18.14 Ocular aspergillosis in a German Shepherd female primarily affecting the posterior segment, causing retinal detachment and diffuse inflammation of the vitreous (causing the vitreous to appear yellow). Aspergillus organisms were aspirated from the vitreous. Toxoplasmosis (Toxoplasma gondii), a protozoan, causes disease worldwide in many species of animals, including humans. The disease occurs clinically more frequently in cats than in dogs; however, affected dogs have been reported. Anterior uveitis, retinitis, choroiditis, extraocular myositis, scleritis, and optic neuritis have been associated with toxoplasmosis infections (Figure 18.15). Toxoplasmosis can occur concurrently with canine distemper. Figure 18.15 Toxoplasmosis in infrequent in the dog, and can cause anterior and posterior segment inflammations. In this dog, episcleritis, anterior uveitis, keratic precipitates, and darkened iris are present.
Systemic Diseases with Ophthalmic Manifestations
Ophthalmic Manifestations of Canine Systemic Diseases
Orbital Diseases
Merle Ocular Dysgenesis
Dwarfism and Ocular Defects
Congenital Hydrocephalus
Infectious Diseases
Infectious Canine Hepatitis
Canine Viral Papilloma
Rickettsial Infections
Canine Brucellosis
Mycoses
Blastomycosis
Coccidioidomycosis
Histoplasmosis
Cryptococcosis
Aspergillosis
Toxoplasmosis
Leishmaniasis
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