Nasal Aspergillosis

Nasal aspergillosis is a fungal infection of the nasal cavity and paranasal sinuses, most commonly caused by the saprophytic fungus Aspergillus fumigatus. It is a significant cause of chronic nasal disease in dogs, particularly in dolichocephalic and mesaticephalic breeds. The infection is typically non-invasive and confined to the nasal passages, but can extend into the frontal sinuses. In contrast, sinonasal aspergillosis in cats is often more invasive and can involve the orbit and central nervous system. The disease is characterized by fungal plaque formation on the turbinates, mucosal inflammation, and destruction of nasal structures. Clinical signs include chronic mucopurulent nasal discharge, epistaxis, sneezing, and depigmentation of the nasal planum. Diagnosis is based on imaging, rhinoscopy, and serology. Treatment involves topical antifungal therapy, often combined with systemic antifungals, and surgical debridement in refractory cases.

The primary causative agent is Aspergillus fumigatus, a ubiquitous saprophytic fungus found in soil, decaying vegetation, and indoor environments. Other Aspergillus species, such as A. niger, A. flavus, and A. terreus, are less commonly isolated. The fungus produces conidia (spores) that are inhaled and deposited in the nasal cavity. In immunocompetent dogs, the infection typically remains localized, suggesting a local immune deficiency or anatomical predisposition rather than systemic immunosuppression. Virulence factors include thermotolerance, the ability to adhere to epithelial cells, and the production of proteases and toxins that damage mucosal tissue. The fungus forms mats of hyphae that invade necrotic tissue and bone, leading to turbinate destruction. Transmission is through inhalation of airborne conidia; direct transmission between animals is not reported.

Nasal aspergillosis is primarily a disease of dogs, with a higher prevalence in dolichocephalic and mesaticephalic breeds such as German Shepherds, Golden Retrievers, Labrador Retrievers, and Rottweilers. Brachycephalic breeds are less commonly affected, possibly due to differences in nasal anatomy and airflow. The disease is rare in cats, but when it occurs, it is often more invasive and severe. There is no clear sex predilection, but middle-aged dogs (2-7 years) are most commonly affected. The condition is seen worldwide, with no strong geographic or seasonal variation, although some studies suggest a higher incidence in humid climates. The exact incidence is unknown, but it accounts for a significant proportion of chronic nasal disease cases in dogs, with some referral centers reporting up to 30% of chronic nasal disease cases.

The pathogenesis of nasal aspergillosis involves inhalation of Aspergillus conidia, which are normally cleared by mucociliary action and innate immune responses. In susceptible individuals, the fungus germinates and forms hyphae that invade the nasal mucosa and underlying bone. The infection is typically non-invasive in dogs, with fungal growth confined to the nasal cavity and sinuses, but it causes extensive tissue destruction due to the production of proteolytic enzymes and mechanical pressure. The host inflammatory response is characterized by a mixed infiltrate of neutrophils, macrophages, and lymphocytes, leading to mucosal edema, necrosis, and fibrosis. Chronic inflammation results in turbinate lysis, septal perforation, and formation of fungal plaques. In cats, the infection can be invasive, with hyphae penetrating into the orbit, cribriform plate, and brain, causing severe neurological signs. The exact mechanism of susceptibility is unknown, but local immunodeficiency, altered mucociliary clearance, or anatomical factors may play a role.

Intrinsic risk factors include breed-related anatomical variations in nasal conchae and airflow dynamics, which may impair mucociliary clearance and create microenvironments conducive to fungal growth. Dolichocephalic breeds have larger nasal passages, which may allow greater deposition of conidia. Age is a factor, with young to middle-aged dogs more commonly affected. Immunosuppression, such as from concurrent disease or corticosteroid therapy, may predispose to invasive disease, but most affected dogs are immunocompetent. Extrinsic factors include environmental exposure to high concentrations of Aspergillus spores, such as in dusty or moldy environments. Poor ventilation and high humidity may increase spore counts. There is no evidence of direct transmission between animals or from soil, but the fungus is ubiquitous.

Clinical signs of nasal aspergillosis are typically chronic and progressive. The most common signs include mucopurulent or serosanguineous nasal discharge, which may be unilateral or bilateral, and is often refractory to antibiotic therapy. Epistaxis is common, especially in advanced cases. Sneezing, often violent and paroxysmal, is frequent. Pain on palpation of the nasal bridge or frontal sinuses may be present. Depigmentation of the nasal planum, ulceration, and crusting can occur. In some cases, facial deformity or swelling over the nasal cavity or frontal sinuses is noted. As the disease progresses, signs of frontal sinusitis, such as a mucocele or chronic discharge from the sinus, may develop. Invasive disease, particularly in cats, can lead to exophthalmos, blindness, seizures, and other neurological deficits. Systemic signs such as fever, lethargy, and anorexia are uncommon unless there is dissemination.

Differential diagnoses for chronic nasal disease in dogs include: 1) Chronic bacterial rhinitis, which typically responds to antibiotics and lacks fungal plaques on rhinoscopy. 2) Nasal neoplasia (e.g., adenocarcinoma, lymphoma), which often presents with progressive facial deformity and is diagnosed via biopsy. 3) Lymphoplasmacytic rhinitis, an immune-mediated condition with similar signs but no fungal elements. 4) Oronasal fistula, which is associated with dental disease and can be ruled out by oral examination. 5) Foreign body rhinitis, which may be acute and unilateral, with a history of foreign body exposure. 6) Allergic rhinitis, which is less common in dogs and often seasonal. 7) Cryptococcosis, a fungal infection more common in cats, which can be differentiated by serology and cytology. 8) Nasal mites (Pneumonyssoides caninum), which cause sneezing and discharge but are diagnosed by direct visualization or nasal flushing. 9) Trauma, which may cause nasal discharge and epistaxis but has a history of injury. 10) Coagulopathies, which can cause epistaxis but are usually associated with other bleeding signs.

The diagnostic approach to suspected nasal aspergillosis should be systematic. 1) History and physical examination: Chronic nasal discharge, sneezing, and pain over the nasal cavity raise suspicion. 2) Baseline laboratory tests (CBC, biochemistry, urinalysis) are usually unremarkable but help rule out systemic disease. 3) Imaging: Skull radiographs may show increased opacity in the nasal cavity, loss of turbinate detail, and lysis of the vomer bone or turbinates. However, CT is the preferred imaging modality, as it provides detailed assessment of the nasal cavity, frontal sinuses, and bone destruction. CT findings include a soft tissue mass, turbinate destruction, and frontal sinus involvement. 4) Rhinoscopy: Direct visualization of fungal plaques (white to gray, necrotic material) on the turbinates is highly suggestive. Biopsy and fungal culture should be obtained during rhinoscopy. 5) Serology: Serum antibody testing for Aspergillus (e.g., agar gel immunodiffusion, ELISA) has high sensitivity and specificity in dogs, with a positive result supporting the diagnosis. 6) Fungal culture: Culture of nasal discharge or tissue can confirm the presence of Aspergillus, but false negatives are common. 7) Histopathology: Biopsy samples showing fungal hyphae with tissue invasion confirm the diagnosis. A definitive diagnosis is made when characteristic lesions are seen on rhinoscopy or CT, and fungal elements are identified on cytology, histopathology, or culture.

Complete blood count (CBC) is typically within normal limits, but mild neutrophilia or eosinophilia may be present. Serum biochemistry is usually unremarkable. Urinalysis is normal. Invasive aspergillosis may show elevated liver enzymes or azotemia if disseminated. Serology: Agar gel immunodiffusion (AGID) for Aspergillus antibodies has a sensitivity of approximately 70-90% and specificity of 90-100% in dogs. ELISA for IgG antibodies is also available and may be more sensitive. False negatives can occur in early or localized disease. Polymerase chain reaction (PCR) for Aspergillus DNA in nasal tissue or discharge is highly sensitive and specific but is not widely available. Fungal culture of nasal discharge or tissue is often positive, but negative cultures do not rule out the disease. Cytology of nasal discharge may show fungal hyphae, but this is not sensitive. In cats, serology is less reliable, and diagnosis often relies on imaging and histopathology.

Radiography: Skull radiographs (open-mouth ventrodorsal, lateral, and frontal sinus views) may show increased soft tissue opacity within the nasal cavity, loss of turbinate detail, and lysis of the vomer bone or nasal septum. However, radiographs are less sensitive than CT. Computed Tomography (CT) is the imaging modality of choice. CT findings include: 1) Unilateral or bilateral soft tissue opacity within the nasal cavity, 2) Destruction of the nasal turbinates, 3) Lysis of the vomer bone, 4) Frontal sinus involvement (soft tissue opacity or fluid), 5) Thickening of the nasal mucosa, 6) In chronic cases, reactive new bone formation. CT is also useful for planning surgical debridement or topical therapy. Magnetic Resonance Imaging (MRI) is less commonly used but can provide similar information, with better soft tissue contrast. Rhinoscopy is an essential imaging-adjunct, allowing direct visualization of fungal plaques and biopsy.

Cytology: Nasal discharge or swabs may be examined cytologically. Fungal hyphae are occasionally seen, but this is not sensitive. Fine needle aspiration of a nasal mass may yield fungal elements. Histopathology: Biopsy samples obtained during rhinoscopy or via trephination of the frontal sinus are the gold standard. Histological findings include: 1) Fungal hyphae (septate, branching at acute angles) within necrotic tissue, 2) Mucosal ulceration and necrosis, 3) Mixed inflammatory infiltrate (neutrophils, macrophages, lymphocytes, plasma cells), 4) Fibrosis and granulation tissue, 5) Invasive hyphae in blood vessels in severe cases. Special stains such as Gomori methenamine silver (GMS) or Periodic acid-Schiff (PAS) highlight fungal elements. The presence of hyphae within tissue confirms the diagnosis.

Treatment of nasal aspergillosis aims to eliminate the fungal infection and restore nasal function. The primary treatment is topical antifungal therapy, often combined with systemic antifungals. Topical therapy is typically performed under general anesthesia, with the dog positioned in dorsal recumbency and the hard palate elevated. A catheter is placed into the nasal cavity or frontal sinus, and a 1% clotrimazole solution is infused and allowed to contact the mucosa for 1 hour. This is repeated in the other nostril. The procedure may be repeated after 2-4 weeks if needed. Systemic antifungals, such as itraconazole (5-10 mg/kg PO q24h) or fluconazole (5-10 mg/kg PO q12h), are often used concurrently, especially if there is sinus involvement or invasive disease. In refractory cases, surgical debridement of fungal plaques and necrotic tissue may be necessary, followed by topical antifungal infusion. Supportive care includes pain management, antibiotics for secondary bacterial infections, and nutritional support if anorexia is present. In cats, treatment is more challenging due to the invasive nature; systemic antifungals are the mainstay, and topical therapy may be attempted, but prognosis is guarded.

The prognosis for nasal aspergillosis in dogs is generally good with appropriate treatment. Success rates for topical clotrimazole therapy are reported to be 60-90%, with many dogs requiring one to three treatments. Factors associated with a poorer prognosis include: 1) Extensive frontal sinus involvement, 2) Invasive disease, 3) Presence of fungal plaques that are difficult to access, 4) Concurrent systemic disease. Recurrence is possible, and long-term follow-up is recommended. In cats, the prognosis is guarded to poor, especially if there is orbital or central nervous system involvement. Mortality is low in dogs, but morbidity can be significant if treatment is delayed.

Follow-up is essential to assess response to treatment and detect recurrence. Re-evaluation is typically performed 2-4 weeks after topical therapy. Clinical signs should improve, with reduction in nasal discharge and sneezing. Repeat rhinoscopy or CT may be performed to assess resolution of fungal plaques. If signs persist, additional topical treatments may be indicated. Long-term monitoring includes periodic assessment of clinical signs and, if systemic antifungals are used, monitoring liver enzymes and drug levels. In dogs, a positive serology may persist for months after successful treatment, so it is not a reliable indicator of cure. Owners should be advised to monitor for recurrence of nasal discharge, epistaxis, or sneezing, and to seek veterinary attention if these occur.

Pearls: 1) CT is superior to radiography for diagnosing nasal aspergillosis and assessing the extent of disease. 2) Rhinoscopy with biopsy is the most definitive diagnostic tool. 3) Topical clotrimazole is the treatment of choice; ensure proper positioning and contact time for optimal results. 4) Combine topical and systemic therapy for severe or refractory cases. 5) In cats, suspect invasive aspergillosis if there is exophthalmos or neurological signs. Pitfalls: 1) Do not rely solely on serology; false negatives are common. 2) Avoid using systemic antifungals alone, as they are often ineffective for localized nasal disease. 3) Do not perform nasal flushing or biopsy without imaging, as this can cause hemorrhage or spread of infection. 4) Be aware that fungal culture can be negative despite active infection. 5) Do not overlook frontal sinus involvement, as it can lead to treatment failure.

Topical Clotrimazole: 1% solution, infused into the nasal cavity and frontal sinuses under general anesthesia. Dose: 50-100 mL per nostril, left in contact for 60 minutes. Repeat in 2-4 weeks if necessary. Systemic Itraconazole: 5-10 mg/kg PO q24h, with food. Duration: 2-3 months. Monitor liver enzymes. Systemic Fluconazole: 5-10 mg/kg PO q12h. Duration: 2-3 months. Alternative: Voriconazole (4-5 mg/kg PO q12h) for refractory cases, but with careful monitoring for hepatotoxicity. In cats, itraconazole (5-10 mg/kg PO q12h) or fluconazole (10 mg/kg PO q12h) is used. For secondary bacterial infections, appropriate antibiotics (e.g., amoxicillin-clavulanate 12.5-25 mg/kg PO q12h) may be prescribed. Analgesics such as tramadol (2-5 mg/kg PO q8-12h) or NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h) can be used for pain. Always adjust dosages for renal or hepatic impairment and monitor for drug interactions.

Several studies have evaluated the efficacy of topical clotrimazole for nasal aspergillosis. A landmark study by Mathews et al. (1998) reported a success rate of 87% in dogs treated with topical clotrimazole infusion. Another study by Zonderland et al. (2002) found that combining topical clotrimazole with oral itraconazole improved outcomes. A more recent study by Schuller et al. (2015) compared topical clotrimazole alone versus combined with systemic antifungals and found no significant difference in success rates, but the combined group had fewer recurrences. ACVIM consensus guidelines on the diagnosis and treatment of fungal respiratory diseases (2014) recommend topical clotrimazole as the first-line treatment for nasal aspergillosis in dogs, with systemic antifungals reserved for invasive or refractory cases. In cats, systemic antifungals are recommended, but the prognosis is poor. Overall, the evidence supports the use of topical therapy as the cornerstone of treatment, with a good prognosis for dogs.