Canine Keratomycosis in 11 Dogs: A Case Series (2000–2011)

Introduction

Keratomycosis is commonly reported in horses but rarely observed in small animals. Common fungal isolates from equine patients with keratomycosis include Aspergillus, Fusarium, Candida, Penicillium, Cyclindrocarpon, Scyalidium, and Torulopsis, as well as yeast.^1,2 With regard to canine fungal keratitis, Aspergillus are the most common isolates, with Acremonium, Alternaria, Candida, Cephalosporium, Cladosporium, Curvularia, Fusarium, Hormographiella, Malassezia, Penicillium, Pseudallescharia, Rhodotorula, and Scedosporium occasionally reported.^3–10

Fungal organisms rarely bind to intact corneal epithelium but readily attach to stroma exposed due to a defect or ulcer.¹¹ Therefore, stromal exposure from pre-existing corneal disease has been a factor in previously documented cases of fungal keratitis.¹² Prolonged use of broad-spectrum topical antibiotics and/or topical corticosteroids can also predispose a patient to mycotic keratitis.¹³ Immunosuppression associated with metabolic diseases may also increase the likelihood of acquiring fungal keratitis.¹³ For example, canine patients with diabetes are at risk of developing epithelial defects due to decreased corneal sensitivity and abnormal wound repair.¹⁴ The purposes of this study were to assess the clinical characteristics and outcome for 11 cases of confirmed keratomycosis in the dog and to compare these findings to previous reports in the literature.

Materials and Methods

Dogs with fungal keratitis were identified by searching the medical records and submissions to the diagnostic laboratory at Louisiana State University Veterinary Teaching Hospital. Criteria for inclusion in the study were a diagnosis of corneal ulceration and positive results for fungi on corneal cytology, culture, and/or histopathology from 2000 to 2011. Information extracted from the medical records included date of diagnosis; signalment; Schirmer tear test results; conditions predisposing to corneal ulceration; results of corneal cytology, fungal culture, bacterial culture, and/or histopathology; antifungal treatment and duration; and visual outcome.

Results

For comparison of clinical characteristics and breeds of affected dogs, medical records for 10 dogs with a diagnosis of fungal keratitis between January 2000 and December 2011 from the Louisiana State University Veterinary Teaching Hospital, and 1 outside case from Louisiana that was consulted by Louisiana State University Veterinary Teaching Hospital Ophthalmology Service, were available for this study. All 11 dogs were small breeds, including two Boston terriers, two miniature pinschers, one miniature poodle, one miniature schnauzer, one mixed-breed Pekingese, one shih tzu, one mixed-breed terrier, one toy fox terrier, and one toy poodle. All dogs were adults, ranging in age from 2 yr to 14 yr. Seven of the 11 dogs (64%) were castrated males and 4 of the 11 dogs (36%) were spayed females (Table 1). All patients were unilaterally affected.

TABLE 1 Signalment and Predisposing Factors for 11 Dogs with Keratomycosis

BNP, bacitracin, neomycin, and polymyxin B; CM, castrated male; KCS, keratoconjunctivitis sicca; OD, ocularis dexter; OS, ocularis sinister; NPD, neomycin, polymyxin B, and dexamethasone; SF, spayed female.

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Pre-existing corneal diseases were identified in 6 of the 11 eyes and included corneal degeneration (n = 4), keratoconjunctivitis sicca (n = 1), and lipid keratopathy (n = 1). Ten of the 11 dogs were being administered topical therapy with antibiotics^a,b and/or corticosteroids^c,d for at least 19 days prior to the time of diagnosis (Table 1). In addition to topical corticosteroids, one patient was also treated with cyclosporine 0.2%^e for 3 yr. Tear production as recorded by the Schirmer tear test^f at the time of diagnosis was available for 10 of the 11 patients. All 10 dogs had tear production ≥ 15 mm/min (15–25 mm/min ocularis dexter, n = 6; 16–27 mm/min ocularis sinister, n = 10) as shown in Table 2.

TABLE 2 Clinical Characteristics and Diagnostics for 11 Dogs with Keratomycosis

EN, enucleated; NR, not recorded; OD, ocularis dexter; OS, ocularis sinister; STT, Schirmer tear test.

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The majority (7 of 11) of patients presented with similar corneal lesions. Those were characterized by a superficial to deep corneal ulcer with dense cellular infiltrate, irregular margins, and ciliary flush (Table 2, Figure 1). Pigmented corneal plaques were observed in 5 of the 11 eyes, and nonpigmented plaques were reported in 2 of 11 eyes. Of the 5 eyes with pigmented plaques, 3 yielded positive cultures of dematiaceous fungi including Phialemonium, Curvularia, and Cladosporium spp. (Table 2).

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Yeast or fungal hyphae were detected in 6 of 11 corneal cytology samples (Figure 2). Fungal isolation using Sabouraud’s agar^g was positive for 7 of 11 eyes. Organisms isolated included Curvularia (n = 2) and one each of Aspergillus, Cladosporium, Chrysosporium, Phialemonium, and Penicillium. Bacteria were cultured concurrently with fungi in 3 of 11 eyes and included Enterobacter spp., Streptococcus spp., and an unidentified gram-negative rod. A Staphylococcus sp. was isolated from one eye that was positive for fungal hyphae on corneal cytology (Table 2).

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Fungal keratitis resolved with medical management alone in 6 of 11 patients (Table 3). Miconazole 1–2%^h,i q 4–6 hr was the most frequent topical treatment prescribed, being used for 9 of 11 eyes. Case 3 resolved with the addition of topical itraconazole^j with dimethyl sulfoxide (DMSO), and case 10 resolved with the addition of topical voriconazole^k, topical itraconazole with DMSO, and oral fluconazole^l. Duration of successful antifungal treatment from diagnosis to clinical resolution ranged from 14 days (cases 7 and 8) to 51 days (case 10) as shown in Table 3.

TABLE 3 Treatment and Outcome for 11 Dogs with Keratomycosis

BNP, bacitracin, neomycin, and polymyxin B; EDTA, ethylenediaminetetraacetic acid; NP, not performed; NPG, neomycin, polymyxin B, and gramicidin; PO, per os.

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Ancillary medical treatment included broad-spectrum antibiotics to combat concurrent bacterial infections as most antifungal agents lack antibacterial properties. The topical antibiotics used in this study included ciprofloxacin (n = 7); cefazolin^m (n = 4); moxifloxacinⁿ (n = 1); a combination of bacitracin, neomycin, and polymyxin B (n = 1); and neomycin, polymyxin B, and gramicidin^o (n = 2). Systemic antibiotics (amoxicillin trihydrate/clavulanate potassium^p and enrofloxacin^q) along with intracameral antibiotics (amikacin^r and ceftazidime^s) were used in one patient (case 9) with severe endophthalmitis and dehiscence of a corneal incision following cataract surgery (Table 3). Nonsteroidal anti-inflammatory drugs were also used to control secondary uveitis, including topical flurbiprofen^t (n = 3) and diclofenac^u (n = 8). Two dogs received systemic nonsteroidal anti-inflammatory drugs (carprofen^v and meloxicam^w) due to progression of anterior uveitis despite topical therapy (Table 3). Autologous serum (n = 8) or ethylenediaminetetraacetic acid^x (n = 2) were applied topically to the cornea in cases with evidence of collagenolysis due to their anticollagenase properties.¹⁵ Atropine 1%^y (n = 5) was also prescribed to control ciliary spasm. Artificial tears^z were added to the medication protocol in seven patients for lubrication and ocular comfort (Table 3).

Surgical procedures intended to aid in corneal wound healing were recommended for complicated nonhealing ulcers. Surgical management was performed only in 3 of 11 cases due to lack of owner consent (Table 3). Two of the three cases that underwent a keratectomy resolved without complication. Enucleation was performed at the owner’s request in one case that developed endophthalmitis after conjunctival graft placement (Table 3).

Discussion

There is no pathognomonic clinical sign associated with fungal keratitis because many clinical signs are nonspecific and secondary to ocular pain, breakdown of the blood-aqueous barrier, and subsequent anterior uveitis. Some patients may have a corneal stromal cellular infiltrate with feathery margins, rough texture, raised edges, and satellite lesions on ophthalmic examination, as seen in Figure 1.¹⁵ Pigmented discoloration of the infiltrate is associated with dematiaceous filamentous fungi such as Alternaria spp. and Curvularia spp. that contain melanin in their cell walls.^3,16

The clinical presentation of fungal keratitis is highly variable. Organisms may be located at any level of the corneal stroma; however, fungal hyphae often migrate deep in the cornea to the level of Descemet’s membrane.¹⁷ Hence, there can be few organisms present within the superficial corneal stroma. That factor complicates diagnostic testing for fungal keratitis. Cytologic examination of corneal scrapings is often the first step to a prompt diagnosis. Neutrophilic inflammation is commonly observed in fungal keratitis.¹⁸ Because the material scraped from a corneal lesion is minimal, the diagnostic success rate of this method is approximately 50–80%.¹³ Fungal culture is another important diagnostic tool. Most fungi are isolated within 48–96 hr; however, some may require 4–6 wk of incubation due to slow growth.^5,13 Because a delay in diagnosis can lead to a delay in proper treatment, multiple and alternative diagnostic tools are recommended.

Recently, fungal polymerase chain reaction has been used in veterinary medicine as an adjunctive diagnostic tool.^3,5,19 That technique provides early species diagnosis and eliminates the time required for fungal culture. A 2009 study by Rampazzo et al. was the first published veterinary report where polymerase chain reaction amplification and sequencing of fungal material was used for the diagnosis of canine keratomycosis.⁵

Histopathologic examination can be performed after biopsy samples are obtained by superficial keratectomy. Despite positive culture and/or cytology reports, the presence of fungal elements could not be confirmed histopathologically (even with Gomori’s methenamine silver stain) in two eyes that were enucleated due to endophthalmitis. That may be explained by the use of antifungal medications prior to enucleation, thereby reducing or eliminating the number of fungal organisms present.

Treatment of fungal keratitis can be prolonged and expensive because 6–8 wk of medical therapy are often required. Antifungal medications, such as miconazole and itraconazole, are imidazoles that function by inhibiting ergosterol synthesis, which alters the permeability of the fungal organism’s plasma membrane and inhibiting growth. Miconazole has good corneal penetration and broad-spectrum activity and is active against filamentous fungi, yeast, and gram-positive bacteria.^3,12 Itraconazole has limited corneal penetration, which can be enhanced when combined with DMSO.²⁰ Due to the retrospective nature of this case series and the relatively recent use of voriconazole in veterinary antifungal therapy, voriconazole was administered in only one case. Voriconazole is a second generation triazole antifungal agent with broad-spectrum activity, good corneal penetration, and low toxicity, making it a well-tolerated and effective option in the treatment of canine keratomycosis.²¹ The majority of patients (6 of 11) responded well to antifungal therapy alone. Adjunctive surgical options exist, including lamellar keratectomy followed by conjunctival pedicle graft, and corneal transplantation.

To date, there are no confirmed canine breed or sex predispositions for fungal keratitis. Seven of the 11 dogs in the current study were castrated males. Marlar et al. (1994) reported a similar finding, with males outnumbering females 3:1 in 13 cases of canine keratomycosis.¹⁰ A similar trend has been reported with male dogs and systemic mycoses.²² A predisposition for males to have fungal keratitis could reflect either behavioral or environmental difference between the sexes resulting in more ocular trauma. The sample sizes in the current study, however, are too small to suggest a sex predisposition.

In the current study, four brachycephalic dogs and seven nonbrachycephalic dogs, including two miniature pinchers, were diagnosed with keratomycosis. A 2006 study by Tolar et al. found an association between brachycephalic breeds and bacterial keratitis.²³ Although miniature pinschers do not share all the characteristics of brachycephalic breeds, miniature pinschers were reported to be at risk for developing bacterial keratitis as well.²³ That may be due to anatomic considerations, such as the relative prominence of their globes. Nonetheless, both of the miniature pinschers in the current study had an underlying metabolic disease that likely contributed to their corneal disease.

Six of 11 dogs in this study were previously diagnosed with either corneal diseases or tear disorders, such as corneal degeneration, keratoconjunctivitis sicca, and lipid keratopathy. Six of the 11 cases had previous cataract surgery, with 5 of those patients on long-term therapy with topical corticosteroids that likely contributed to corneal degeneration and subsequent corneal ulceration. Four of the 11 dogs were diabetic and were therefore at risk for developing corneal epithelial defects, decreased corneal sensitivity, and corneal degeneration.²⁴ In addition, one case was previously diagnosed with hyperadrenocorticism, which could have indirectly contributed to the patient’s nonhealing corneal ulcer because high levels of endogenous glucocorticoids may delay corneal healing.²⁴ This study supports the idea that alteration of the cornea’s protective mechanisms can predispose a patient to developing fungal keratitis.

Use of either topical antibiotics or corticosteroids can alter normal corneal immunocompetency.^3,10,13 Cortisone may alter the host’s defense mechanisms by limiting the phagocytic response of macrophages.²⁵ Antibacterial medications can eliminate the cornea’s normal flora thereby allowing pathogens to proliferate without competition for space or nutrients.^3,26 Three patients were administered topical steroids for at least 1 yr (from 1yr to 3 yr), four were treated with topical steroids for 22–140 days, and two were prescribed topical antibiotics for at least 19 days (from 19 days to 5 mo) prior to the diagnosis of fungal keratitis. Prolonged use of such medications may increase the frequency of fungal keratitis in canine patients.

The fungal isolates from all patients were representative of three major groups of fungi: hyaline hyphomycetes (nonpigmented), dematiaceous (pigmented) filamentous fungi, and yeast. Curvularia spp., which were isolated from two eyes, are dematiaceous fungi known to cause cutaneous lesions in small animals. A pigmented corneal plaque was noted in one of the two eyes, and both lesions resolved with medical management alone.

To the authors’ knowledge, this is the first report of either Phialemonium spp. or Chrysosporium spp. isolated from a dog with keratomycosis. Phialemonium spp. are dematiaceous and widely distributed in the environment. Although historically those species have rarely been reported to cause disease, they are emerging as opportunistic fungal pathogens of both humans and animals.^27–29 For example, Chrysosporium spp., hyaline hyphomycetes, are occasionally isolated from mammal hair and skin due to the keratinophilic properties of some species.³⁰

Conclusion

In summary, the treatment of fungal keratitis can be both prolonged and expensive. Patients with either pre-existing corneal disease or prolonged use of topical corticosteroids and antibiotics may be at an increased risk for the development of fungal keratitis. The prognosis for return of a visual and comfortable eye with keratomycosis is guarded, especially when significant corneal disease is present. Eight of the 11 dogs in this study retained vision. Three eyes were enucleated, and one developed glaucoma that was likely related to chronic uveitis. Although canine keratomycosis is infrequently encountered, it remains a diagnostic and therapeutic challenge to veterinarians. The definitive diagnosis of fungal keratitis in all species is challenging and largely dependent on both clinical and microbiological results. The desirability of rapid diagnostic methods is obvious: early, accurate diagnosis could result in a more rapid recovery and a better prognosis.