Pseudallescheria boydii Species Complex Fungal Rhinitis and Sinusitis in a Dog

Introduction

The most common cause of fungal rhinitis/sinusitis in dogs is infection with Aspergillus spp. Uncommonly, other fungal organisms are responsible for clinical signs. Pseudallescheria boydii species complex is rarely reported as a cause of nasal disease in dogs, but can be easily confused with Aspergillus spp. due to similar clinical presentation and morphologic features on histopathology. P. boydii is an emerging pathogen in both immunosuppressed and nonimmunocompromised humans and has a high mortality rate. Recent genetic studies have shown that Pseudallescheria does not represent a single species, but is actually a complex composed of at least eight species. The Pseudallescheria species show different behavior and antifungal susceptibility. Infection with P. boydii should be considered as a differential diagnosis in any dog with fungal rhinitis/sinusitis, and a fungal culture should be performed even if aspergillosis is suspected. If P. boydii is identified, antifungal susceptibility testing is indicated. The purpose of this article is to report (to the best of the authors' knowledge) the first documented case of fungal rhinitis/sinusitis in a dog due to P. boydii in the United States.

Case Report

A 9 yr old, 30.5 kg, male castrated Australian shepherd mixed-breed dog was referred to Sage Centers for Veterinary Specialty and Emergency Care with a 3 mo history of intermittent, right-sided epistaxis that had recently become more frequent. Right nasal biopsy had been performed by the referring veterinarian 2 mo prior to referral. Histopathology of the nasal mucosa revealed severe lymphoplasmacytic eosinophilic rhinitis with turbinate remodeling. No etiologic agents were identified. Aerobic bacterial culture yielded Staphylococcus spp. and Pasteurella multocida. Treatment with enrofloxacin^a (2.2 mg/kg per os [PO] q 12 hr for 2 wk) and prednisone (0.65 mg/kg PO q 24 hr for 5 days then 0.65 mg/kg PO q 48 hr for 6 wk) was initiated, which resulted in temporary improvement of clinical signs.

Five days prior to referral, an episode of severe right-sided epistaxis occurred. Amoxicillin trihydrate/clavulanate potassium^b (9.0 mg/kg PO q 12 hr) was prescribed, prednisone was discontinued, and the dog was referred.

Physical examination at the time of referral to Sage Centers for Veterinary Specialty and Emergency Care was unremarkable. There was no evidence of nasal pain or discharge, and no depigmentation or ulceration of the nares was present. Results of a complete blood count and serum biochemistry panel were within reference ranges. Lateral and dorsoventral thoracic radiographs revealed no abnormal findings.

Rhinoscopy and nasal computed tomography (CT) were recommended. Rhinoscopy was declined by the dog's owner because multiple nasal biopsies had already been obtained. Nasal and frontal sinus CT was performed using 5 mm contiguous transverse slices after administering intravenous contrast medium^c. Additional 1 mm slices were made through the cribriform plate. CT revealed focal turbinate loss in the middle to caudal dorsolateral aspect of the right nasal cavity. A nonenhancing soft-tissue mass was present in the right frontal sinus (Figure 1). Mild focal thinning of the right frontal bone and right dorsal aspect of the cribriform plate was also present. CT findings were consistent with right-sided destructive rhinitis and associated sinusitis, which were thought to be most likely associated with infection with Aspergillus.

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The dog returned 9 days later for rhinoscopy to allow for debridement of fungal plaques, as well as frontal sinus trephination for combined clotrimazole irrigation and depot therapy. Rhinoscopy revealed marked turbinate destruction in the caudal right nasal cavity. Several small white plaque lesions were identified and removed.

The dog was placed in sternal recumbency, and the pharynx was packed with moistened laparotomy sponges. Right frontal sinus trephination was performed. In-house cytology of impression smears made of material collected from the right frontal sinus showed mats of fungal hyphae. Given the suspicion for infection with Aspergillus, the dog was treated with a combination of clotrimazole irrigation and depot therapy, as previously described.¹ Briefly, 50 mL of a 1% clotrimazole solution^d was infused over 5 min through 10 Fr red rubber catheters^e placed and secured into the frontal sinuses after bilateral trephination. In addition, 10 mg of 1% clotrimazole cream^f was then deposited into each of the frontal sinuses. The rubber catheters were removed and the skin incisions were sutured. The tissue collected from the right frontal sinus was submitted for histopathology, aerobic and anaerobic bacterial cultures, and fungal culture. The dog was discharged the following day and was prescribed tramadol^g (1.6 mg/kg PO q 8–12 hr for 7 days) and was continued on the amoxicillin trihydrate/clavulanate potassium^b (as prescribed previously) pending culture results.

Histopathology results were returned 2 days later showing evidence of severe lymphoplasmacytic neutrophilic inflammation and submucosal gland hyperplasia. Large numbers of tangled fungal hyphae were present (Figure 2). The fungal hyphae were thought to be most consistent with Aspergillus spp. Fungal cultures were complete 7 days after clotrimazole therapy, and Scedosporium sp. was identified. The bacterial cultures were negative. The fungal culture was submitted to an outside laboratory^h for speciation and susceptibility testing against amphotericin B, fluconazole, itraconazole, ketoconazole, voriconazole, and clotrimazole. The owner reported at this time that the dog's clinical signs had completely resolved.

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The outside laboratory reclassified the fungal organism as P. boydii species complex. DNA sequencing of the organism was requested. The organism was speciated as P. boydii by internal transcribed spacer and D1/D2 sequencing. Antifungal susceptibility testing indicated resistance to amphotericin B and itraconazole. The organism was sensitive to fluconazole, ketoconazole, and voriconazole. The minimum inhibitory concentration for clotrimazole was 2 μg/mL (range, 0.03–16 μg/mL). The outside laboratory reported that interpretative standards have not been established for this organism with regard to clotrimazole.

At a follow-up examination performed 3 mo after clotrimazole treatment, the dog remained free of clinical signs. Upon questioning the dog's owner regarding his environment, it was reported that the dog had occasional exposure to turkeys.

Discussion

P. boydii (anamorph S. apiospermum) is a ubiquitous, filamentous, saprophytic fungus with a worldwide distribution.² P. boydii has been associated with soil, polluted water (including sewage and ponds frequented by waterfowl), and poultry and cattle manure.³ P. boydii has undergone numerous name changes over the years and has been referred to in both the human and veterinary literature using the teleomorph name (Pseudallescheria) and the anamorph name (Scedosporium), as well as being referred to as Allescheria boydii. There is currently debate regarding appropriate nomenclature.

P. boydii was previously thought to represent a single species; however, recent work has demonstrated that P. boydii is actually a complex of at least six known and two cryptic species. These six species (P. boydii, P. angusta, P. ellipsoidea, P. fusoidea, P. minutispora, and S. aurantiacum) were genetically differentiated after DNA sequencing. These six species and the two cryptic species are currently referred to as P. boydii species complex. Previously, only P. boydii had been considered a significant pathogenic species. The eight species comprising the P. boydii species complex likely all show pathogenic behavior individually, although the specific degree of virulence of each of the recently discovered species has not been established.

In recent years, P. boydii has been increasingly associated with severe infections in humans and is associated with a high mortality rate.² P. boydii species complex infection is often initially mistaken for Aspergillus spp. and Fusarium spp. because the organisms cannot be readily distinguished either clinically or histologically. As a result, patients are often treated inappropriately.

The P. boydii species complex has only rarely been associated with clinical disease in dogs. Three previous reports describe P. boydii complex as a cause of fungal rhinitis.^5–7 Two of the dogs lived in Spain, and one dog lived in New Zealand. One of the dogs was reported to have clinical improvement after treatment with ketoconazole.⁵ A second dog was treated initially with itraconazole, with no clinical improvement after 5 wk of treatment. This second dog had complete resolution of its symptoms after topical clotrimazole therapy was subsequently performed.⁶ The third dog required no antifungal treatment after a fungal mass was removed via rhinoscopy.⁷ Susceptibility testing was performed on isolates from two of these three dogs. In the first case, the organism was susceptible to ketoconazole, was intermediate in resistance to clotrimazole, and was resistant to amphotericin B, 5-flucytosine, fluconazole, and itraconazole.⁵ In the second case, the organism was susceptible to clotrimazole, itraconazole, miconazole, ketoconazole, and natamycin but was resistant to terbinafine.⁷

DNA sequencing was not performed in the previously reported cases; therefore, it is unclear what species in the P. boydii species complex was responsible for causing the infections in any of these three dogs. The lack of DNA sequencing may account for the variable antifungal susceptibility testing for the dog described in the current report compared with the previously published cases.

In addition to the three dogs reported with fungal rhinitis associated with P. boydii, there are also four previous case reports of P. boydii causing disseminated disease in dogs and one report of fungal keratitis. ^8–12 Dogs reported to have disseminated disease had variable systemic involvement (i.e., bone, pulmonary, intestinal, pancreatic, cranial mediastinal, and hepatic), and two of the dogs had intra-abdominal wall mycetoma. All dogs with disseminated disease died or were euthanized shortly after diagnosis.

Fungal sinusitis secondary to infection with the P. boydii species complex in humans is rare. Only 26 cases have been reported in the literature.¹³ Only one patient was reported to have nasal cavity involvement. P. bodyii can either result in clinical disease via mucosal invasion or can cause allergic fungal sinusitis, which is an immune reaction to noninvading fungal colonies. The majority of human patients with invasive disease are immunocompromised.

Definitive identification of the organism via culture is imperative to allow for selection of appropriate treatment. Amphotericin B is the recommended treatment of invasive fungal sinusitis in humans with aspergillosis, but it is not the treatment of choice for P. boydii. Surgery, often in combination with antifungal therapy, is generally recommended. The mortality rate among immunocompromised patients with P. boydii species complex mucosal invasion is high, even with treatment.¹³

An increased incidence of disease has also been identified in human patients undergoing immunosuppressive or antineoplastic therapy, in patients infected with the human immunodeficiency virus, and in patients who have undergone organ or bone marrow transplantation.¹⁴ It is noteworthy that infection with P. Bodyii has been frequently documented in nonimmunocompromised patients as well.^2,15 Three clinical syndromes are described: localized disease after traumatic inoculation; symptomatic and asymptomatic colonization of body cavities; and systemic disease.² Disseminated infection after near drowning, pulmonary infection, central nervous system infection, bone and joint infection, endocarditis, sinusitis, keratitis, and other infections have been widely reported.

Optimal therapy for infection with the P. boydii species complex in human patients has not been established. In cases of localized infection, surgical resection of the affected tissue is the recommended treatment, sometimes in combination with antifungal drugs.² Numerous studies evaluating the efficacy of antifungal drugs have shown marked species variability in susceptibility; therefore, no clear consensus has been made.^16–18 One recent study evaluated the efficacy of 11 antifungal drugs against each of the species comprising the P. boydii species complex.¹⁹ Voriconazole was the most active drug, and posaconazole showed activity against most (but not all) species. Itraconazole showed poor activity against most species tested. Amphotericin B was not active against any of the species.

Clotrimazole therapy appears to be a potential treatment option for dogs diagnosed with P. boydii rhinitis/sinusitis, as indicated by this dog's response to treatment and the low mean inhibitory concentration reported for clotrimazole. Given the limited information regarding treatment of this fungal disease in veterinary patients, it cannot be assumed that all patients presenting with rhinitis/sinusitis will respond to clotrimazole. Limited antifungal testing performed on veterinary isolates has shown variable susceptibility to clotrimazole. Unfortunately, fungal susceptibility to clotrimazole has not been investigated in studies on human clinical isolates. Whether intranasal enilconazole infusion would be a treatment option as it is for nasal aspergillosis is unknown. Although itraconazole may result in resolution of nasal aspergillosis based on a nonpeer-reviewed publication, itraconazole would not be recommended for treatment of P. boydii species complex infections given the general resistance of the organism to this drug.²⁰ Voriconazole may be a viable oral treatment option; however, this triazole would be cost prohibitive for most veterinary patients. If indicated by antifungal susceptibility testing, ketoconazole or fluconazole may be less expensive options. Appropriate duration of treatment of P. boydii species complex rhinitis/sinusitis with oral antifungal drugs in dogs has not been established.

Conclusion

The P. boydii species complex should be considered as a differential in any dog diagnosed with fungal rhinitis and/or sinusitis. Given that it is an emerging pathogen in humans, it may become a more clinically relevant infection in dogs as well. A fungal culture should always be performed in dogs with fungal rhinitis/sinusitis, even if infection with Aspergillus spp. is suspected based on the results of rhinoscopy, CT, or histopathology, because the P. boydii species complex cannot be readily distinguished from Aspergillus spp. If the P. boydii species complex is diagnosed, susceptibility testing is indicated due to species differences in response to antifungal drugs. To allow for further characterization and to determine the importance of these species in veterinary patients, DNA sequencing should also be considered.