Canine Systemic Fungal Infections

INFECTIOUS DISEASE AND THE EYE

0195-5616/00 $15.00 + .00

CANINE SYSTEMIC FUNGAL INFECTIONS Sheryl Greve Krohne, DVM, MS

The diagnosis of ocular involvement from systemic mycotic infections in the dog is extremely important for preserving vision and often for saving the life of the dog. The four most common infections are blastomycosis, cryptococcosis, coccidioidomycosis, and histoplasmosis. These disorders combine to cause more than 95% of ocular disease from systemic mycotic infections. 30 Ocular involvement is frequently the presenting clinical sign that owners recognize first in these diseases. The typical presentations of these ocular diseases should immediately signal the clinician that a disseminated fungal disease may be responsible for the ocular changes. A specific diagnosis should be pursued as soon as possible because these diseases are life threatening and difficult and expensive to treat, and because the prognosis improves dramatically with early detection. BLASTOMYCOSIS Transmission

Most infections occur from the dog inhaling the fungal spores of Blastomyces dermatitidis into the lungs. The spores are produced as the fungus grows in a saprophytic mycelial form in the soil. The exact ecologic niche for Blastomyces has not been identified; however, soil where Blastomyces survives is sandy, acidic, and usually near a waterway. 2• 19• 27 In a Wisconsin study, 95% of dogs with the disease lived within 400 meters of water. 3 This association was confirmed in a Louisiana study in which 89% of dogs lived close to water. The odds of a dog with blastomycosis living near water was ten times that of a control dog without blastomycosis. 2 Moisture of all types (fog, dew, mist, rainfall, body of water) is thought to have a major role in the dissemination of infective spores, 14 thus it is

From the Department of Veterinary Clinical Sciences and the Veterinary Teaching Hospital, School of Veterinary Medicine, Purdue University, West Lafayette, Indiana

VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 30 • NUMBER 5 • SEPTEMBER 2000

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not surprising that blastomycosis is endemic in the Mississippi, Missouri, and Ohio River valleys, the mid-Atlantic and southern states, and southeast Canada along large waterway systems. 19 The organism is rarely isolated froin the environment. Because dogs are more likely to develop blastomycosis than are humans, they serve as a sentinel for this disease. Outbreaks seem to occur in groups of dogs and humans from "point source" exposure. 2• 16• 27 Aerosol transmission of the yeast phase from infected animals to humans is not possible; however, penetrating wounds can spread the disease. Possible modes of transmission to humans include dog bites from infected animals, contamination from necropsy procedures, and contaminated needles. 19

Clinical Signs Systemic

Historically, male dogs have been more frequently affected; however, in a large series of cases from Louisiana, males and females were affected at the same frequency as the control population. 2 The average age of affected dogs is young, but disease has been reported in dogs aged 6 months to 17 years; therefore, no age can be ruled out except very young puppies. Retrievers and Doberman Pinschers were overrepresented when compared with the control population in one study. 2 Nonspecific clinical signs of this disease include anorexia, weight loss, fever, and depression. 19• 29 The onset of signs ranges from several days to several months. Frequently diagnosed organ system problems include respiratory disease, ocular disease, lameness, lymphadenopathy, and proliferative or draining skin lesions. Systems less frequently affected include the CNS, mammary glands, testes, and the prostate. 2 Dry harsh lung sounds are found in 85% of dogs. Dogs with more severe involvement may be exercise intolerant or have dyspnea. 19 Diffuse lymphadenopathy is seen in 40% to 60% of dogs with blastomycosis. 19• 29 Ocular

Ocular involvement occurs in 20% to 52% of systemic blastomycosis cases.>· 6• 7

It is bilateral in approximately 50% of cases. 25 The reported lesions in one

study in decreasing order of frequency were chorioretinitis, anterior uveitis, retinal detachment, and secondary glaucoma (Figs. 1 to 3). 2 Endophthalmitis (Fig. 4) has been reported in 26% and optic neuritis in 10% of affected eyes. 19 Another study evaluating 108 eyes diagnosed 22% with posterior segment disease, 6% with anterior uveitis, and 72% with endophthalmitis. 6 Other reported lesions include retinal and vitreal hemorrhages (Fig. 5) and orbital cellulitis. 25 Ocular disease as the only presenting abnormality has been reported, although that presentation is rare. 6• 7 Ocular disease was the only clinical abnormality (initially) in a case of orbital wall proliferative osteomyelitis diagnosed at the author's clinic. Blastomyces organisms were found in the bony inflammatory reaction (Fig. 6). The affected eye presented with exophthalmos and optic neuritis, whereas the lungs were radiographically normal. Ocular lesions develop initially in the choroid secondary to septicemia. A subretinal pyogranulomatous exudate causes a white-to-gray granuloma to form under the retina (Fig. 7). 6• 8 Individual lesions are common early in the ocular involvement and may slowly grow to a large size (see Fig. 1A). Two or more lesions may coalesce to form a large lesion. Early lesions frequently form in the

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Figure 1. A, Subretinal focal granuloma caused by Blastomyces dermatitidis chorioretinitis in a mixed-breed dog. Anterior segment examination was normal, and vision was present in this eye. B, Severe anterior uveitis from blastomycosis causing miosis, conjunctival and iris congestion, corneal neovascularization, and edema. This is the opposite eye from the one shown in A.

Figure 2. Fundus photographs of two different types of retinal detachments. A, Complete retinal detachment secondary to blastomycosis chorioretinitis in a Rottweiler. The retina is lifted approximately halfway into the vitreous, and the retinal blood vessels are visible. The fold in the retina results from the retinal attachment to the optic nerve. 8, Complete retinal detachment in a Samoyed. The retina can be seen during the anterior segment examination immediately behind the lens. The folds where the retina is attached to the optic nerve and the retinal vessels can also be seen in th is example. The center white area is a flash artifact.

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Figure 3. Glaucoma secondary to blastomycosis endophthalmitis in a Brittany Spaniel. Corneal edema, neovascularization, and cellular infiltrates are present. Epiphora is also present and is caused by pain from the uveitis and glaucoma.

Figure 4. Another case of endophthalmitis from blastomycosis in a Retriever mixed-breed shows conjunctival and iris swelling and congestion. The third eyelid is slightly elevated secondary to enophthalmos from pain. Cloudiness in the posterior segment can be seen through the pupil and is attributable to vitritis and chorioretinitis.

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Figure 5. The pyogranulomatous chorioretinitis shown in this view of the fundus of a Labrador Retriever is accompanied by multifocal retinal and subretinal hemorrhages. (Courtesy of Jean Stiles, DVM, West Lafayette, IN.)

Figure 6. Photomicrograph of frontal bone proliferative osteomyelitis with granulomatous inflammation containing Blastomyces organisms (hematoxylin-eosin, original magnification X 1).

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Figure 7. Two small subretinal granulomas can be seen in this fundus photograph. They are located in the peripheral nontapetum of this eye and were not visible until the pupil was dilated. This Coonhound was diagnosed with blastomycosis and had respiratory signs and a normal ocular examination except for these fundic lesions.

nontapetum (see Fig. 7). These lesions can be seen ophthalmoscopically and are suggestive of blastomycosis. If the le sion is peripheral and sm all, adequate mydriasis will be needed to see it. A single chorioretinitis lesion signals early ocular involvement and may be asymptomatic; therefore, all dogs with systemic blastomycosis should undergo complete ocular examinations to evaluate fundus involvement, the ocular and systemic prognosis, and the response to therapy. Anterior u veitis usually occurs secondarily as the posterior disease becomes more severe and eventually progresses to endophthalmitis (see Figs. lB and 4).6' 7 ' 8 Progressive ocular disease will eventually result in glaucoma secondary to the uveitis (s ee Fig. 3). B e cause the posterior segm ent usually cannot be visualized at this stage, it is difficult to d etermine the severity of involvement. 6' 25 In the subset of Blastomyces-infected dogs with ocular disease, dogs with chorioretinitis only have the best systemic and ocular prognosis (see Figs. l A, 5, and 7). Dogs with endophthalmitis and optic neuritis have the worst prognosis.6, 7 However, one study reported optic neuritis in two dogs (three eyes) without systemic neurologic involvement, and the prognosis for cure and vision w as favorable.6 Diagnostics

The inhaled spores transform into the yeast form in animal tissues, typically starting in alveolar macrophages.2' 18 The yeast organism ranges from 5 to 20 J.Lm in diameter and is characterized by a thick, refractile, double cell wall. It does not have a capsule (Fig. 8). Diagnosis is based on finding this organism in

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Figure 8. Photomicrograph of the lung aspirate cytology of a dog showing the inflammatory reaction with budding Blastomyces organisms (Wright's stain, original magnification x 100). (Courtesy of Armando Irizarry, DVM, West Lafayette, IN.)

cytologic preparations from fine-needle aspirates or touch preparations. In one study, cutaneous impression smears (34%) and lymph node aspirates (31%) were diagnostic for 65% of 115 cases. 2• 18 These aspirates are typically easy to obtain and are the first tissues that should be examined carefully for affected sites. Lung aspirates (see Fig. 8), vitreous aspirates, subretinal taps, transtracheal wash, bone aspirates, and bronchoalveolar lavage have also been diagnostic in smaller numbers of cases; however, these techniques are much more invasive and may threaten organ integrity (loss of vision) or death from respiratory complications. Vitreous aspirates have been shown to be the most productive, with 100% positive diagnostic results (Fig. 9), although only five samples were submitted in one study. 2• 18 Ocular ultrasonography is useful for determining whether an aspirate is indicated and for directing needle placement when the ocular media are opaque. A complete detachment that i s not disinserted ("morning glory" type, Fig. lOA) or a smaller detachment can be diagnosed. A vitreous or subretinal aspirate is more likely to be diagnostic when a large detachment is present. Damage to the posterior segment from the procedure is less relevant because extensive damage has already occurred from the infection. An aqueous humor tap may also produce a diagnostic cytology or culture specimen if the anterior segment is inflamed; however, this procedure is frequently nondiagnostic because the organism is rarely found in the anterior chamber. 25 Organisms are typically found within a pyogranulomatous or suppurative inflammatory reaction in the cytology preparation (Fig. lOB and C). Aspirates may also produce a diagnosis based on culture results. Such results may require 1 to 4 weeks to obtain. It may be necessary to perform a biopsy if cytology results are nondiagnostic. Histopathologic abnorm alities are the same as the findings seen with cytology.29 Radiographic assessment is helpful in most cases because lung involvement

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Figure 9. Photomicrograph of a vitreous aspirate cytology from a dog showing several Blastomyces organisms in a proteinaceous substance (Wright's stain, original magnification x 100). (Courtesy of Dennis Denicola, DVM, West Lafayette, IN.)

Figure 10. A, Ultrasonogram showing conical (morning-glory) retinal detachment and continuation of abnormality along the optic nerve, which is swollen. The swelling is the same echogenicity as the subretinal changes. Illustration continued on opposite page

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c Figure 10 (Continued). B and C, Photomicrographs of pyogranulomatous inflammatory reaction surrounding Blastomyces organisms in the subretinal aspirate from the eye shown in A. B is focused on the inflammatory cells. C is focused on the organism demonstrating the double cell wall and the relative size (Wright's stain, original magnification x 100). (Courtesy of Dennis Denicola, DVM, West Lafayette, IN.)

is common. Thoracic films may show diffuse involvement even without respiratory-related clinical signs. Radiographic changes typically show a nodular interstitial or bronchointerstitial pattern, often referred to as the "snowstorm pattern," that is characteristic of blastomycosis. 2• 18• 19 There may also be solitary or multiple cystic lesions or solid nodules or masses; however, these findings are rare. 19 Serology is based on an antibody response to the fungal organism and is not the best choice for diagnosing blastomycosis. It is also not reliable for

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assessing response to therapy. A positive test may be used to diagnose the disease if other tests fail; however, a false-negative test may occur early or late in the disease, even when the disease is active.Z9 Therapy

Itraconazole (5 mg/kg orally once daily) has been the drug of choice for dogs with blastomycosis. Dogs respond as rapidly to itraconazole as they do to amphotericin B, and it has less toxicityP The initial dose should be 5 mg/kg orally twice daily for 5 days to maximize blood concentrations, followed by a reduced dose of once daily for the duration of treatment. 19 Itraconazole (and ketoconazole) requires an acid environment for maximal oral absorption; therefore, it should be given with food. Oral antacid treatments inhibit the oral bioavailability of these drugs. 29 Treatment should be continued for at least 60 days and 1 month after the resolution of clinical signs. The recurrence rate is 20% using this protocol and usually occurs in the first year. The reported cure rate is 54% with itraconazole. The once-daily dose frequency had less toxicity and the same cure rate as a twice-daily frequency. 19 Dogs with posterior segment disease have responded to itraconazole treatment; therefore, it has some ocular penetration, although the amount is not known.' Dogs presenting with or who develop optic neuritis do not respond well to itraconazole therapy. In one study, 100% of these animals were euthanized or died.' This poor prognosis may be related to disseminated CNS infection, with optic neuritis as a secondary manifestation. In animals with severe fulminating infection, the protocol using injectable amphotericin B and oral ketoconazole should be used. 19 In a study evaluating this treatment for ocular blastomycosis, optic neuritis responded well, and vision was retained. The two dogs evaluated and treated did not have generalized CNS disease. 6 Fluconazole has not yet been studied in dogs with ocular blastomycosis. It has been used by clinicians to treat dogs with CNS, ocular, and prostatic involvement because it penetrates blood-brain, blood-{)cular, and bloodprostatic barriers better than itraconazole. Anecdotal reports suggest that it is effective for CNS disease,' and several systemic cases with ocular involvement treated at the author's center have responded and have been cured. The reader is referred to the cryptococcosis section for the complete fluconazole dosage and treatment protocol. Amphotericin B lipid complex (Abelcet) is a mixture of two phospholipids and amphotericin B. The lipid complex drug has been used successfully to treat blastomycosis in dogs without resulting in the renal complications that occur with amphotericin B. 15 Traditional amphotericin B therapy as part of a multipledrug protocol is still warranted in some cases of severe, life-threatening pulmonary involvement because of its rapid onset of action. 2 Ocular Therapy

In addition to treatment with a specific systemic antifungal drug, specific ocular therapy is necessary. Goals of this therapy are to preserve vision, decrease and eliminate pain, and retain a cosmetic globe. Systemic prednisone (1 mg/kg orally once to twice daily) should be administered when posterior segment ocular inflammation is moderate or severe, or when focal retinal lesions do not respond to systemic antifungal therapy used alone. Systemic glucocorticoids can also dramatically improve respiratory symptoms. This treatment should be

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continued until the lesions improve or resolve, which may take several months. It may be slowly tapered as lesions improve. The level of intraocular inflamma-

tion should be monitored as antifungal therapy is initiated because fungal cell death may increase the inflammatory response in the eye when compared with the presentation at initial evaluation. Oral prednisone may not be required initially but should be added to the therapeutic regimen if ocular inflammation develops. Glucocorticoid therapy can improve the prognosis for vision, especially if the chorioretinitis is severe or extensive. Bilateral optic neuritis was improved and vision maintained when prednisone (4 mg I kg orally, once daily for 7 days, then tapered) was added to systemic ketoconazole therapy. 6 Two-week intervals (or less) are necessary to monitor chorioretinitis. If the chorioretinitis worsens while systemic signs are improving, oral corticosteroids should be discontinued for an interval and the response of the ocular disease to systemic antifungal treatment evaluated. Oral nonsteroidal anti-inflammatory agents have not been evaluated for reducing or controlling the intraocular inflammation associated with fungal infections. Although they may be helpful, they probably would not be as effective as glucocorticoids against the cellmediated immune reaction that causes the pyogranulomatous retinal detachment. Topical treatments are important for several complications, including anterior segment inflammation and glaucoma. Recommended anti-inflammatory agents include topical 0.1% dexamethasone suspension or 1.0% prednisolone acetate suspension four to six times a day for moderate-to-severe uveitis and nonulcerative keratitis.' Topical 1% atropine should also be used to effect. Secondary glaucoma may be treated topically with a beta-blocker (0.5% timolol) or a carbonic anhydrase inhibitor (2.0% dorzolamide). Systemic carbonic anhydrase inhibitors such as methazolamide or acetazolamide can also be used if they are not contraindicated by the dog's systemic condition. Glaucoma therapy may be ineffective at reducing the intraocular pressure to normal. Enucleation of severely infected eyes may be necessary if they develop glaucoma, are unresponsive to glaucoma therapy, and are chronically painful. Lack of response is common owing to the severity of the intraocular damage, even if the intraocular organisms are destroyed with systemic therapy. Enucleation cannot be performed until the dog is clinically stable and lung disease is minimal. Some cases of glaucoma will have resolved on their own by that time, and the eye will become phthisical. Debate continues concerning the need for enucleation to prevent the eye from becoming a nidus of infection for disease recurrence. In a recent study, the dogs that developed phthisis bulbi did not have a recurrence of infection. 6 It was recommended not to enucleate blind eyes unless they were inflamed or painful. Other clinicians have successfully placed intrascleral prostheses in previously infected eyes after the initial infection has responded, the animal is stable, and chronic antifungal therapy is used. CRYPTOCOCCOSIS Transmission

Infection usually occurs when dogs inhale Cryptococcus neoformans spores found in the environment. The spores may initiate disease in the nasal cavity or lungs. The organism is large enough that it may settle out in the nasal cavity or nasopharynx. It may then be asymptomatic, locally symptomatic, or spread to other tissues in the body. One study documented that cryptococcal organisms

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could be cultured from nasal washings of 14% of clinically normal dogs. 21 Smaller forms may be inhaled into the lungs and cause primary pulmonary disease. Dissemination by direct extension or blood-borne infection most commonly affects the skin, eyes, or CNS. 29 Cryptococcosis affects dogs throughout the United States, although it is more common in the south. One should not overlook this disease in the differential diagnosis for intraocular infections in the northern half of North America. C. neoformans is a saprophytic round fungus that resembles a yeast in laboratory conditions. In tissues, it has a thick capsule. The organism may be viable up to 2 years in the environment. The two subspecies identified in infections are C. neoformans var. neoformans, which may be associated in the environment with bird droppings, and C. neorformans var. gattii, which is found in debris under eucalyptus trees. 23 The pigeon has been implicated as an important vector of C. neoformans. The organism is often found in roosts, bam lofts, hay mows, and around roofs of buildings where pigeons sit. The thick capsule of Cryptococcus inhibits phagocytosis, plasma cell function, and leukocyte migration by the immune system of the host.>9 Individual immune system reactions result in differences in disease severity between dogs. Immune suppression is an important factor in the development of this disease in humans; however, it has not been shown to be a predisposing factor in dogs. Several cases have been reported following chronic glucocorticoid therapy; however, this therapy was not proven to predispose the dogs to infection. The availability of high numbers of infective organisms in the environment and exposure are the highest risk factors for dogs. 16• 29 Clinical Signs Systemic

The clinical signs of dogs with cryptococcosis depend on the organ system that is involved. The most common organ involvement in the dog is CNS (50% to 80%)/ upper respiratory (50%)/3 ocular (20% to 40%)/9 and cutaneous (10% to 20%). 29 Upper respiratory tract infection can cause nasal cavity and frontal sinus involvement. Signs may include stridor, nasal discharge, which can be serous to purulent, sneezing, epistaxis, and firm swellings over the dorsum of the nose. 29 Granulomatous lesions can be protruding from the nares or seen in the skin covering the nose. Nasal cavity infection is less common in the dog than in the cat. Disseminated disease causes depression and anorexia, but fever is not a common sign. 29 CNS cryptococcosis usually has multifocal signs that are secondary to meningitis alone or that accompany encephalomyelitis.B These signs can include ataxia, head tilt, nystagmus, facial paralysis, circling, paresis, paraplegia, or tetraplegia (usually upper motor neuron), cervical hyperesthesia, and seizures. 13• 16• 29 CNS involvement uncommonly occurs in the spinal cord alone. 29 Ulcerative and nodular draining skin lesions may be found on the head (Fig. 11 ), footpads, nail beds, and mouth. In one study, cryptococcosis was diagnosed most frequently in purebred dogs of medium to large size. Dogs younger than 4 years were most commonly affected, although the age ranged from 1 to 7 years. 16 No sex predilection for this disease has been documented. 23 Cocker Spaniels are overrepresented in North America.•· 23 Ocular

Ocular abnormalities include multifocal granulomatous or pyogranulomatous lesions of the choroid that can produce a subretinal exudate that subse-

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Figure 11. Clinical view of a draining tract from a lymph node on the face of a dog. Cryptococcosis was diagnosed from cytologic examination of the discharge.

quently detaches the retina. These lesions may appear similar to blastomycosis lesions; however, they are usually smaller and multifocal at presentation (Fig. 12). Retinal hemorrhage may be present. 13 Multifocal retinal scarring may be seen in addition to the active chorioretinitis (Fig. 13). Optic neuritis is also a

Figure 12. Fundus photograph from a 2·year-old German Shepherd Dog with active chorioretinal lesions associated with systemic cryptococcosis. (Courtesy of Jean Stiles, DVM, West Lafayette, IN.)

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Figure 13. Fundus photographs from a Springer Spaniel with confirmed CNS cryptococco· sis. A, The dark appearance of the tapetal lesions is illustrated. 8, In the same eye, the nontapetal lesions appear white or light gray. The active lesions in the lower half of the photographs have a subretinal cellular infiltrate causing this color change, and they appear fuzzy. The lesions in the upper half of the photographs are depigmented and appear more chronic.

common clinical sign and may result from chorioretinitis progressing to the optic nerve, 25 or the disease may enter the eye from the optic nerve via the nasal cavity, cribriform plate, and meninges.29 Optic neuritis is the most common cause of blindness in this disease, and dogs may present with dilated unresponsive pupils. A retrobulbar abscess with lysis of the bone of the orbit has been reported in one dog." Diagnostics

As is true in blastomycosis, cytologic identification is the most common way to diagnose cryptococcosis. Fine-needle aspirates or swabs for cytology and culture should be obtained from affected tissues (Fig. 14). This method is simple and noninvasive, especially if the skin is involved. Cultures are generally productive because the organisms grow readily; however, the results may not be available for 2 days to 6 weeks. Positive culture results from the nasal cavity should be interpreted with caution becau se organisms may be present without cau sing disease.21 Subretinal, v itreal, or aqueous centesis are recommended if inflammation is present in those areas of the eye and the diagnosis cannot be confirmed from aspirates elsewhere. Ultrasound may be helpful for evaluating the extent of ocular involvement and for localizing the area to ta p if the ocular media is cloudy (see Fig. 10). Organisms are found using cytology in 75% of cases of cryptococcosis, allowing for a timely diagnosis. 29 If CNS signs are present, a cerebrospinal fluid tap is indicated, and fluid should be submitted f or culture and cytology. Cytospin preparations are recommended for these cytology examinations because the sample will be concentrated, and the cells will not be damaged. Organisms are seen in approximately 90% of dogs with CNS cryptococcosis,5 and cultures may be positive when organisms are not seen.

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Figure 14. Photomicrograph of Cryptococcus organisms found in a conjunctival scraping from an infected eye. The thick capsule is evident in this preparation (Wright's stain, original magnification x 100). (Courtesy of Armando Irizarry, DVM, West Lafayette, IN.)

Histopathology of biopsy specimens should be performed if cytology is negative. Hematology results may show a mild nonregenerative anemia with a neutrophilia with or without a left shift; however, the complete blood count and serum chemistry results may be normal. Thoracic and skull radiographs are indicated if there are clinical signs referable to these areas. Serology is important for documenting infection and for following the response to therapy. Cryptococcus capsular antigen tests should be used instead of antibody titers because most infected animals do not have a humoral response. Serum, cerebrospinal fluid, and urine can be tested. Serum provides the best results in dogs without CNS signs and cerebrospinal fluid the best results in dogs with CNS signs. 29 Therapy

Fluconazole (5 mg/kg orally one to two times daily) is effective and probably the first choice of treatment for dogs with ocular and CNS involvement.'· 13• 23• 29 Fluconazole has better bioavailability than itraconazole and is widely distributed with good cerebrospinal fluid penetration (cerebrospinal fluid to serum ratio of 60% to 80%)Y· 24 At the author's clinic, dogs with CNS and ocular cryptococcosis have had a good response to fluconazole, and other anecdotal reports have documented curesY Fluconazole is eliminated by the kidney; therefore, dogs with renal disease should have decreased dosages and be frequently monitored. Itraconazole (5 mg/kg orally one to two times daily) is effective but seems to be less helpful than fluconazole in dogs with CNS disease. 5 The higher dose is recommended as the initial dose but should be reduced if toxicity (usually hepatic) develops. One advantage of itraconazole is that it may be effective as a once-a-day dosage after the loading dose (documented in the treatment of

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blastomycosis). 19 Once-daily dosing reduces the cost of treatment significantly even though the per milligram cost of itraconazole is actually higher than the cost of fluconazole. At current prices, treating a 20-kg dog for the first month would cost approximately $225 for itraconazole (twice daily loading for 5 days, and once daily for 25 days) versus $350 for fluconazole (twice daily for 30 days) if the tablets are purchased in 100 count. Treatment using either of these agents should be initiated for a minimum of 60 to 90 days and continued until the antigen titer is negative and for 2 months after the resolution of clinical and radiographic signs. Serial evaluations of the antigen serum titer are helpful in determining the response to therapy during the treatment period but are not useful for determining the prognosis. 20 Rechecks should be performed monthly during treatment and should include a chemistry panel to evaluate liver and kidney function in addition to the latex agglutination antigen titer. An initial titer decline of two to fourfold per month usually corresponds to adequate clinical improvement-29 Amphotericin B can be used if oral treatment with the azole antifungal agents listed previously is not possible. Amphotericin B, itraconazole, and fluconazole were shown to be equally effective in treating CNS cryptococcosis in humans. 11• 29 Subcutaneously administered amphotericin B has been used in combination with the azole antifungals or flucytosine to treat canine cryptococcosis successfully. 22 Amphotericin B (0.5 to 0.8 mg/kg) is diluted in 0.45% saline containing 2.5% dextrose. A volume of 500 mL for dogs weighing less than 20 kg and 1000 mL for larger dogs is administered subcutaneously two to three times per week. There is reduced toxicity with this protocol. Ocular involvement sometimes necessitates treatment in addition to the antifungal drug chosen. This therapy is described in the section on ocular therapy for blastomycosis. Although rare, glaucoma can be a complication in cryptococcosis. COCCIDIOIDOMYCOSIS Transmission

Coccidioidomycosis is contracted in a similar way to blastomycosis. It is not a contagious disease. 21· 29 Spores (arthroconidia, arthrospores) from Coccidioides immitis in the environment are inhaled into the lungs and induce an initial neutrophil reaction. 16 Inhalation of less than ten arthrospores can produce disease.29 Later in the disease, infiltrates of monocytes, lymphocytes, and plasma cells are found around the spore in the tissue, where a spherule is formed. The response of the immune system controls the severity of the clinical disease. 29 In most cases, the infection is cleared by the host's cell-mediated immune response. In a few cases, the infection may become systemic because the spores are not removed and spread from the peribronchial tissue to the subpleural space. They are then disseminated throughout the body by the lymphatic or circulatory systems. Infection can also occur directly via wound contamination by the spores. 16 C. immitis is found in sandy alkaline soils in the low-elevation deserts of the Americas. Endemic areas include Arizona, New Mexico, Utah, Nevada, southwest Texas, California, Mexico, Central America, and South America. Other names for the disease coccidioidomycosis are valley fever and San Joaquin Valley fever. The spores are variable in size, ranging from 2 to 4 IJ.m wide and 3 to 10 fJ.ID long. They are formed by the mycelial phase of the geophilic

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dimorphic fungus and come to the surface after a rainfall. They are then spread by the wind. Epidemics have been reported in years when rainy periods are followed by drought conditions that result in dust storms. 21• 29 An increased incidence of coccidioidomycosis has also been documented following years of higher rainfall in the desert. As is true for blastomycosis, moisture (rainfall) is important in the transmission of this disease. 16 Clinical Signs Systemic

Many of the animals who are exposed to C immitis are subclinically infected. They may or may not show respiratory signs, and an effective immune response results in a spontaneous recovery.29 Animals that develop systemic clinical disease can have dissemination to the thoracic lymph nodes, bones and joints (65% to 90% ), liver, spleen, kidneys, heart and pericardium, testicles, brain, and spinal cord. 1• 9• 29 Young dogs are more commonly affected and are typically housed outdoors.29 Breeds that may be overrepresented include the Boxer, Pointer, Australian Shepherd, Beagle, Scottish Terrier, Doberman Pinscher, and Cocker SpanieP1• 29 In the tissues, spherules can be 20 to 200 IJ.m in diameter and contain endospores. These large spherules are pathognomonic for coccidioidomycosis (Fig. 15).29 The endospores can be released by cleavage, forming new spherules in the infected tissue. Following endospore formation, an intense inflammatory response may occur that results in respiratory signs. These signs will develop 1 to 3 weeks after exposure; disseminated disease will occur up to 4 months after exposure.16 Disseminated disease is less common than in other systemic mycoses. 29

Figure 15. Photomicrograph of Coccidioides immitis organism surrounded by inflammatory cells from a vitreal aspirate from a dog. The large size is pathognomonic (Wright's stain, original magnification x 100). (Courtesy of Armando Irizarry, DVM, West Lafayette, IN.)

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Common clinical signs of respiratory disease include a chronic dry harsh cough or a wet moist productive coughP Dyspnea and crackles, wheezes, or muffled lung sounds may also be present from pleural effusion. 16• 29 Frequently, mild clinical signs are present for 1 to 6 months before a diagnosis is made. Bone and joint infection is common and presents as lameness. Draining tracts or skin ulcers usually are the result of a bone or joint infection and not a primary dermatology lesion. 16• 29 CNS disease usually presents as depression, seizures, ataxia, or behavioral changes. Generalized peripheral lymphadenopathy is rare, although regional nodes may be involved near cutaneous or osseous lesions. 21 Ocular

The incidence of ocular disease in disseminated coccidioidomycosis has been infrequently measured. Ocular disease may be the only clinical sign, representing 42% of cases in one study. Eighty percent of these cases were unilateral, which is unusual in systemic disease. 1• 25 When ocular involvement is part of the systemic disease, it frequently occurs in the posterior and anterior segments of the eye (Fig. 16). Ocular disease is not as common as in blastomycosis; however, the lesions appear similar. Granulomatous chorioretinitis is the most common single ocular abnormality (Figs. 17 and 18).25 If the chorioretinitis progresses, retinal detachment will occur secondary to subretinal exudate (see Fig. 16). At this stage, the dog's owner might notice visual deficits in the eye. All dogs with systemic coccidioidomycosis should undergo fundic examinations to evaluate the extent of disease in the posterior segment. If not treated, the disease will progress forward in the uveal tract until it causes iritis or granulomatous anterior uveitis. 29 Mild-to-moderate anterior segment involvement may also be present by this time, and the dog will present with secondary conjunctivitis. Most dogs with anterior involvement will already

Figure 16. Endophthalmitis secondary to systemic coccidioidomycosis in a dog. Conjunctival and iridal congestion and swelling are present. Posterior synechia is present, and a detached retina can be seen behind the lens. This dog had respiratory clinical signs. (Courtesy of Ron Sigler, DVM, Phoenix, AZ.)

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Figure 17. Fundus photograph showing granulomatous chorioretinitis secondary to systemic coccidioidomycosis in a dog. A large subretinal lesion surrounding the optic nerve is visible, as are several other smaller lesions that are white and appear fluffy because of the tremendous inflammatory cell reaction around the organisms. (Courtesy of Ron Sigler, DVM, Phoenix, AZ.)

Figure 18. Fundus photograph showing a medium-sized granuloma in the nontapetum from systemic coccidioidomycosis in a dog. The dissection of the exudate under the retina is evident at the edges of the lesion . (Courtesy of Ron Sigler, DVM, Phoenix, AZ.)

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have posterior segment disease; however, a few will present with anterior uveitis alone. If the disease is allowed to continue its progression, glaucoma will occur secondary to angle blockage or closure from anterior uveitis. Glaucoma is reported in 31% to 50% of affected eyes,'' 29 which is indicative of a late presentation for this systemic disease. Because the systemic signs can be vague and intermittent, dogs usually have infection and systemic disease for months before they are presented with ocular disease signs, which may be the first clinical signs severe enough to warrant attention. Diagnostics

Definitive diagnosis of coccidioidomycosis occurs with the identification of organisms. Spherules may be found in lymph node aspirates or in impression smears of exudate from draining tracts. False-negative findings are common with transtrachael wash or bronchoalveolar lavage fluid; however, if alveolar disease is present, results are more likely to be positive.U Organisms are uncommonly seen in bone aspirates. 29 The organism is easily identified by staining the cytology preparation with Wright's, Papanicolaou, or periodic acid-Schiff stains. Large numbers of neutrophils may be present and surround the spherules (see Fig. 15), sometimes obscuring them. 12 In addition, low numbers of spherules make diagnosis from cytology more difficult than in other systemic fungal infections. 29 Histopathologic evaluation of biopsy samples is often necessary because the organism may not be found on cytology. Multiple biopsy samples should be taken at the same time cytology specimens are obtained if the animal is sedated or anesthetized and coccidioidomycosis is suspected. 29 Microabscesses are the best source of histopathology specimens, and routine hematoxylin and eosin are adequate to stain the organisms. Special stains including periodic acid-Schiff or silver will highlight the organisms. A pyogranulomatous reaction occurs around the organism. 12 Cultures grow readily, and samples can be obtained at the same time cytology specimens are prepared. These samples should be sent to commercial laboratories for culture and identification because the mycelial growth in culture produces arthroconidia that are highly infectious.t> Radiographs of the chest are useful because they often show the typical changes associated with this disease, which include a diffuse interstitial or peribronchiolar pattern. The hilar and central lung regions are usually affected. 29 Hilar lymphadenopathy is diagnosed radiographically in 80% of affected dogs. Radiographic evidence of pleural effusion or pleural thickening is seen in 65% of dogs. 29 Radiographs of limbs with grossly identifiable lesions during physical examination will also show changes consistent with osteomyelitis. Serologic testing is performed using antibody tests but should not be the first or only diagnostic method chosen. Serology can be performed to make a presumptive diagnosis when clinical signs are suggestive but organisms cannot be found on cytology or biopsy. 29 Several techniques are available, including tube precipitin and an agar gel immunodiffusion (ACID) test for precipitin (IgM). 16, 29 These tests may be positive 2 to 4 weeks after infection and then become negative. Complement fixation tests or the ACID test can be performed to estimate complement fixation antibodies that are IgG. The complement fixation titer increases with the severity of disease. Titers less than 1:16 may indicate early, chronic, localized, or past infection; titers greater than 1:32 suggest active or disseminated disease; and higher titers are seen in more severe disease. 29 False-positive results from the complement fixation test can occur. The assays

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can cross-react with antibodies against Histoplasma capsulatum and Blastomyces dermatitidis. 16 Some of these tests have not been extensively evaluated in dogs and cannot be confirmed based on other diagnostic techniques; therefore, the results can be confusing or inaccurate. Antigen tests may soon be available.

Therapy

Ketoconazole (5 to 15 mg/kg orally twice daily) remains the drug of choice to treat coccidioidomycosis. 11,16• 29 1traconazole (5 to 10 mg / kg orally once daily) has also been used.29 Comparative studies of efficacy are not available. Although many cases of lung involvement resolve spontaneously, most cases that present to the veterinarian require systemic treatment because they have systemic dissemination. The decision to treat should be based on clinical signs or a positive diagnosis and not solely on serology results. 12 Therapy should be continued for at least 2 months after the resolution of clinical signs. 12• 16• 29 When the disease is disseminated, treatment frequently takes a minimum of 1 year. Relapses are common, especially if the drug is not administered daily for extended periods of time.'2 Ocular lesions should be observed for response to therapy (Fig. 19). Serologic tests repeated after 4 t o 6 weeks of therapy are helpful to determine whether therapy is effective. Anecdotal evidence su ggests that fluconazole or itraconazole may be more effective in some cases; in others, they fail to treat the disease, and ketoconazole must be used. For CNS coccidioidomycosis, fluconazole may prove to be more effective because it has better CNS penetrationY The prognosis is very good for the resolution of clinical signs (90%) when dogs are treated with ketoconazole; however, maintenance therapy is necessary in many of these animals.'2 Overall recovery rates can approach 100% for dogs w ith

Figure 19. Fundus photograph showing resolved and resolving tapetal chorioretinitis lesions from systemic coccidioidomycosis in a dog. Hyperpigmentation is present, and there are hyper-reflective rings around some of the resolved lesions that are now scars. (Courtesy of Ron Sigler, DVM, Phoenix, AZ.)

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respiratory disease only. The prognosis is guarded for disseminated disease, and the recovery rate approaches 0% with multiple bone involvement. 12' 29 HISTOPLASMOSIS Transmission

Similar to the infections resulting from the dimorphic fungi discussed previously, infection from Histoplasma capsulatum usually occurs by inhalation of the microconidia. Histoplasmosis is not a contagious disease. 29 The fungus is endemic in large areas of the world, with most clinical cases in the United States occurring in the Ohio, Missouri, and Mississippi River valleys. 25' 32 Because clinical disease can be related to the concentration of the inoculum, local outbreaks have been reported following exposure to heavily contaminated areas such as chicken coops, bat habitats, or starling roosts.Z9 After they are inhaled, the microconidia are small enough to reach the lower respiratory tract. They convert to the yeast phase in the lung, are phagocytized, and replicate in mononuclear cells. They are usually found intracellularly.29 The infection may stay in the pulmonary system or become systemic from hematogenous or lymphatic spread. 32 Most cases are subclinical; however, large doses of spores or infection an immunocompromised animal may result in the manifestation of clinical signs. 16 The incubation period is 12 to 16 days, and the cell-mediated immune response determines the severity of the clinical disease. Granulomatous inflammation surrounding the organism results from a cellular immune response against the infection. Primary infection may also occur in the gastrointestinal system from ingestion; however, this route has not been proved.29, 32

Clinical Signs Systemic

Dogs affected are usually less than 4 years old; however, histoplasmosis can affect dogs of any age. 29 Inappetence, weight loss, and fever of unknown origin are typical presentations of disseminated histoplasmosis. Respiratory signs may be present, including coughing, abnormal lung sounds, or dyspnea in severe cases. 16' 29 Usually, dogs with clinical disease present with gastrointestinal symptoms. 29, 32 These symptoms include large-bowel diarrhea with tenesmus, mucus, and fresh blood in the feces. Extensive infiltration of the organism throughout the bowel can cause voluminous, watery stool with a protein-losing enteropathy. Other accompanying signs can include hepatomegaly, enlarged abdominal lymph nodes, splenomegaly, icterus, and ascites. Occasionally, the disease may disseminate to bone causing lameness, to the peripheral lymph nodes causing lymphadenopathy, and to the CNS and skin. Subcutaneous nodules rarely drain or ulcerate as is commonly seen in cryptococcosis or blastomycosis.16 Ocular

Ocular involvement from this disease, when it naturally occurs, seems to be rare. 32 Experimental histoplasmosis caused ocular lesions in 66% of dogs.

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These lesions were seen in the anterior choroid, iris, ciliary body, and sclera.28 The choroid seems to be the target tissue of the eye in naturally occurring cases, and multifocal subretinal pyogranulomatous lesions develop that may coalesce and produce a retinal detachment.'5 These lesions are different in appearance from the lesions seen in blastomycosis. They are smaller and multifocal early in the disease. Rarely, optic neuritis may occur (an extension of CNS disease) and may result in blindness. When anterior uveitis occurs, it is nongranulomatous.25 Diagnostics

Nonspecific clinical, laboratory, and radiographic abnormalities are combined to suggest the diagnosis of histoplasmosis. Histoplasmosis should be suspected in dogs that live in endemic areas and that have typical respiratory or gastrointestinal signs unresponsive to antibiotics. 25 Serology is not reliable to diagnose histoplasmosis, thus identification of the organism is necessary for confirmation.29• 30• 32 Organisms can be recovered from transtracheal wash or bronchoalveolar lavage specimens if respiratory signs are present, and from colonic biopsies if gastrointestinal signs are present. 29• 32 Other productive areas can include rectal scrapings and aspirates of liver, lung, spleen, mediastinal masses (Fig. 20), and bone marrow. 32 Routine cytology stain~/including Wright's or Giemsa are adequate and will demonstrate small (2-4 fLih) round intracellular organisms with basophilic centers and a halo. A histopathologic diagnosis requires special stains such as periodic acid-Schiff, silver, or Gridley's fungal because organisms are hard to detect with routine hematoxylin and eosin. 32 Useful tests include thoracic radiographs, which may show a diffuse or

Figure 20. Photomicrograph of a fine-needle aspirate from a mediastinal mass from a dog with disseminated histoplasmosis. The ocular disease was not severe enough in this dog to suggest the eye as a site for cytology samples. The small organisms are round, intracellular, and have basophilic cente rs and a halo (Wright's stain, original magnification x 100). (Courtesy of Armando Irizarry, DVM, West Lafayette, IN.)

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linear interstitial pattern. Hilar lymphadenopathy may be evident. Calcified pulmonary infiltrates may indicate previous disease in dogs and are intermittently seen at the author's center.

Therapy

Itraconazole (10 mg/kg orally once to twice d~ily) is currently the drug of choice for histoplasmosis in dogs. Even though this drug has poor penetration into the eye, it has cured ocular histoplasmosis.U· 32 Treatment should be continued for 60 to 90 days or until clinical evidence of disease has been resolved for at least 1 month. 16• 32 Amphotericin B can be used for dogs with life-threatening or severe gastrointestinal tract disease. 16 The historical drug ofchoice has been ketoconazole, and studies have proved its efficacy.29 Nevertheless, it has more systemic complications than the azole derivatives such as itraconazole. Fluconazole has been shown to be effective in some animals. Pulmonary disease may be self-limiting; however, most clinicians believe it should be treated with systemic antifungal agents to stop dissemination to other organ systems. Histoplasmosis has a fair-to-good prognosis for a cure. The reader is referred elsewhere for a detailed description of treatments in addition to systemic antifungal drugs for gastrointestinal histoplasmosis, including diet and antibiotics. 29 Ocular treatment should include the chosen systemic antifungal agent and specific ocular anti-inflammatory treatments. These treatments are discussed in detail in the section on blastomycosis ocular therapy. Although histoplasmosis does not present frequently with ocular lesions, when such involvement occurs, the prognosis is poor. 25 Long-term therapy is necessary, and relapses are common.

OTHER FUNGAL INFECTIONS

Other fungal organisms cause the remaining 5% of ocular infections seen in dogs. 30 Some ocular infections do not result from systemic spread to the eye but occur from local invasion from a sinus, the nasal cavity, or infected head or neck wounds. These infections include aspergillosis, candidiasis, geotrichosis, paecilomycosis, penicilliosis, pseudallescheriasis, pythiosis, and sporotrichosis. 30 The best documented systemic syndrome from this group of diseases involves disseminated opportunistic Aspergillus sp. These infections in young to middle-aged German Shepherds can cause ocular lesions of chronic anterior and posterior uveitis (Fig. 21A). 10• 25 This disease has been described in Europe, the United States, and Australia. The respiratory system is thought to be the entry route for the fungus. Vertebral pain, paresis, and paralysis from diskospondylitis are the typical presenting clinical signs (Fig. 21B). Ocular disease may be present for months before the orthopedic and neurologic disease is diagnosed, offering a method for early detection and diagnosis. 10 Diagnosis is made based on culture and cytology of urine or vitreous and aqueous humor aspirates (Fig. 21C). Treatment is long-term imidazole therapy. 10 Systemic glucocorticoids should not be used in the treatment of systemic aspergillosis in dogs. In vitro studies show that these agents may increase the growth rate of Aspergillus sp. and impair the immune response needed to clear this fungus from the body.'"· 31 Severely ill dogs have a poor prognosis. 10

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Figure 21. A, Anterior uveitis in a German Shepherd Dog showing diffuse iris hemorrhage and corneal neovascularization. Aspergillus terreus was cultured from both vertebral and aqueous humor fine-needle aspirates. Illustration continued on following page

SUMMARY

The principal route of infection for the disseminated fungal diseases discussed in this article is inhalation. In some cases, direct wound contamination and ingestion may also have an important role in the pathogenesis of the disease, especially in histoplasmosis. Another common theme of these diseases is the response of the immune system. If the inoculum is small and the animal is not immunocompromised, the infection may be limited to the respiratory tract and may resolve with few or no clinical signs. Dogs are usually presented to the veterinarian when the fungus has disseminated throughout the body via the circulatory or lymphatic systems, thus causing clinical signs secondary to specific organ infection. Draining skin tracts and lymphadenopathy occur in several of the diseases. The ocular location that is frequently affected is the choroid, where the organisms cause cell-mediated chorioretinitis. Early detection of these changes is important for saving vision and for diagnosing the systemic nature of the disease. Treatment is often effective, especially early in the disease, although it is expensive and long-term, with many animals needing over a year of treatment. Sometimes the treatment must continue lifelong. Ocular disease may not respond to treatment even when respiratory and other organ system clinical signs are rapidly improving. This isolation of the eye is similar to that of the CNS and requires regular monitoring of ocular disease, especially in the fundus, to ensure that systemic drugs are penetrating into the eye. Once the disease progresses to the anterior segment, the ocular prognosis worsens. Better penetration of the blood-retinal and blood-aqueous barriers may be achieved with fluconazole when compared with the other antifungal drugs. Secondary inflammatory ocular disease must also be monitored and treated appropriately to prevent scarring, which may cause vision loss or glaucoma.

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Figure 21 (Continued). B, Photomicrograph of the cytologic preparation from the vertebral fine-needle aspirate from the clinical case shown in A. Aspergillus organisms can be seen in the center of the photograph, and the inflammatory reaction is mixed (Wright's stain, original magnification x 100). C, Photomicrograph of the cytologic preparation from the aqueous humor aspirate from the clinical case shown in A and B. A pyogranulomatous inflammatory reaction can be seen, and Aspergillus organisms were cultured from the fluid (Wright's stain, original magnification x 100). (Courtesy of Armando Irizarry, DVM, West Lafayette, IN.)

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26. Rosser EJ Jr, Dunstan RW: Sporotrichosis. In Greene CE (ed): Infectious Diseases of the Dog and Cat, ed 2. Philadelphia, WB Saunders, 1998, pp 399-402 27. Rudmann DG, Coolman BR, Perez CM, et al: Evaluation of risk factors for blastomycosis in dogs: 857 cases (1980-1990). JAm Vet Med Assoc 201:1754-1759, 1992 28. Salfelder K, Swartz J, Akbarian M: Experimental ocular histoplasmosis in dogs. Am J Ophthalmol59:290-299, 1965 29. Taboada J: Systemic mycoses. In Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine, ed 5. Philadelphia, WB Saunders, 2000, pp 453-476 30. Ward DA: Oculomycosis. In Bonagura JD (ed): Kirk's Current Veterinary Therapy XII Small Animal Practice. Philadelphia, WB Saunders, 1995, pp 1257-1261 31. Willis AM, Martin CL, Stiles J: Sino-orbital aspergillosis in a dog. J Am Vet Med Assoc 214:1644-1647, 1999 32. Wolf AM: Histoplasmosis. In Greene CE (ed): Infectious Diseases of the Dog and Cat, ed 2. Philadelphia, WB Saunders, 1998, pp 378-383

Address reprint requests to Sheryl Greve Krohne, DVM, MS VCS Lynn Hall Purdue University West Lafayette, IN 47907