Aspergillosis

Aspergillosis is an opportunistic fungal infection caused by saprophytic molds of the genus Aspergillus, primarily affecting the respiratory tract of dogs and, less commonly, cats. In veterinary medicine, the disease manifests in two main clinical forms: sinonasal aspergillosis (SNA), which is typically localized to the nasal cavity and frontal sinuses, and disseminated or systemic aspergillosis, which is rare but severe, often involving the lungs, bones, kidneys, and central nervous system. The disease is characterized by granulomatous and necrotizing inflammation, with fungal hyphae invading tissues and blood vessels, leading to thrombosis and infarction. In dogs, SNA is the most common form, usually affecting young to middle-aged, large-breed, dolichocephalic dogs, while disseminated aspergillosis is more frequently seen in German Shepherd Dogs with underlying immunosuppression. In cats, aspergillosis is less common but can present as sinonasal, orbital, or disseminated disease, often associated with retrovirus infections or other immunocompromising conditions. The clinical course can range from chronic, localized nasal disease to acute, fatal systemic infection, depending on the immune status of the host and the virulence of the fungal strain.

The primary causative agents are molds of the genus Aspergillus, with Aspergillus fumigatus being the most common species isolated in canine sinonasal aspergillosis, accounting for over 90% of cases. Other species, including A. niger, A. flavus, A. terreus, and A. nidulans, are less frequently implicated. In cats, A. fumigatus is also common, but A. terreus and A. niger have been reported, particularly in disseminated infections. These fungi are ubiquitous in the environment, found in soil, decaying vegetation, hay, straw, and dust. They produce conidia (asexual spores) that are aerosolized and inhaled. The virulence factors of Aspergillus species include thermotolerance (growth at 37°C), the ability to adhere to epithelial cells, and the production of proteases, phospholipases, and mycotoxins (e.g., gliotoxin) that damage host tissues and impair immune responses. The organism's small conidia (2-5 µm) can evade mucociliary clearance and reach the lower airways, but in immunocompetent hosts, alveolar macrophages and neutrophils effectively eliminate them. In sinonasal aspergillosis, the fungus colonizes the nasal turbinates and frontal sinuses, forming fungal mats (mycetomas) without deep tissue invasion in most cases, suggesting a localized immune dysregulation rather than systemic immunosuppression. In disseminated aspergillosis, angioinvasion leads to hematogenous spread to multiple organs, particularly the kidneys, bones, and central nervous system.

Aspergillosis occurs worldwide, with no strong geographic predilection, but it is more commonly diagnosed in regions with temperate climates. In dogs, sinonasal aspergillosis is the most common fungal respiratory infection, accounting for approximately 30% of all canine nasal diseases. It predominantly affects young to middle-aged dogs (mean age 3-7 years), with a slight male predisposition. Large-breed, dolichocephalic dogs are overrepresented, including breeds such as German Shepherd Dogs, Golden Retrievers, Labrador Retrievers, Rottweilers, and Collies. The German Shepherd Dog is also at increased risk for disseminated aspergillosis, which is often associated with a genetic immunodeficiency (e.g., IgA deficiency or defective neutrophil function). In cats, aspergillosis is less common, but the incidence is increasing, particularly in brachycephalic breeds (e.g., Persian, Himalayan) and in cats with concurrent viral infections such as feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV). Sinonasal aspergillosis in cats is often associated with chronic rhinosinusitis and may be secondary to anatomical abnormalities or previous nasal trauma. Disseminated aspergillosis in cats is rare but has been reported in immunocompromised individuals. There is no seasonal variation, but exposure to dusty environments, moldy hay, or construction sites may increase the risk of inhalation of conidia.

The pathophysiology of aspergillosis varies between the localized sinonasal form and the disseminated form. In sinonasal aspergillosis, inhaled conidia adhere to the nasal mucosa, where they germinate into hyphae in the presence of impaired local immunity. The fungus forms a dense mat of hyphae on the surface of the nasal turbinates and within the frontal sinuses, causing chronic inflammation, mucosal ulceration, and necrosis. The hyphae do not typically invade deep tissues but produce proteolytic enzymes and mycotoxins that damage the epithelium and underlying connective tissue. The host inflammatory response is characterized by a mixed infiltrate of neutrophils, macrophages, and lymphocytes, leading to granulomatous inflammation. Over time, the fungal mat can cause turbinate destruction, septal perforation, and extension into the frontal sinuses, orbit, or cranial cavity. In disseminated aspergillosis, the fungus invades blood vessels (angioinvasion), leading to thrombosis, infarction, and hematogenous spread to distant organs. The kidneys are commonly affected, resulting in pyelonephritis and renal failure. Bone involvement causes osteomyelitis, particularly in the vertebral bodies and long bones. Central nervous system involvement leads to meningoencephalitis or granulomas. The pathogenesis of disseminated disease is often linked to underlying immunosuppression, such as in German Shepherd Dogs with inherited immunodeficiencies or in cats with retroviral infections. The fungus can also produce gliotoxin, which inhibits phagocytosis and impairs T-cell function, further compromising the host's ability to clear the infection.

Predisposing factors for aspergillosis include both intrinsic and extrinsic factors. In sinonasal aspergillosis, anatomical factors such as a long nasal cavity in dolichocephalic breeds may predispose to impaired mucociliary clearance and fungal colonization. Chronic nasal diseases, such as allergic rhinitis, nasal foreign bodies, or previous nasal trauma, can damage the mucosa and create a favorable environment for fungal growth. Immunosuppression is a major risk factor for disseminated aspergillosis, including congenital immunodeficiencies (e.g., IgA deficiency, complement deficiencies, defective neutrophil function) in German Shepherd Dogs, and acquired immunosuppression due to retroviral infections (FeLV, FIV) in cats, or long-term glucocorticoid or cytotoxic drug therapy. Concurrent diseases such as diabetes mellitus, hyperadrenocorticism, or neoplasia can also predispose to fungal infection. Environmental factors, such as exposure to moldy hay, straw, or dusty environments, increase the likelihood of inhaling large numbers of conidia. Poor ventilation and high humidity in kennels or shelters may also contribute. In cats, brachycephalic conformation and chronic rhinosinusitis are significant risk factors for sinonasal aspergillosis.

Clinical signs of sinonasal aspergillosis in dogs typically develop insidiously over weeks to months and include chronic mucopurulent to serosanguineous nasal discharge, sneezing, epistaxis, depigmentation or ulceration of the nasal planum, and facial pain or swelling. As the disease progresses, signs may include decreased nasal airflow, stridor, and, in severe cases, extension into the frontal sinuses causing frontal sinus pain or swelling. Neurological signs may occur if the cribriform plate is eroded, leading to meningitis or brain abscess. In cats, sinonasal aspergillosis presents similarly, with chronic nasal discharge, sneezing, and sometimes orbital involvement causing exophthalmos or epiphora. Disseminated aspergillosis is characterized by systemic signs such as fever, lethargy, anorexia, weight loss, and lameness (due to osteomyelitis). Renal involvement may cause polyuria, polydipsia, and signs of renal failure. Neurological signs include seizures, ataxia, and paresis. In some cases, cutaneous lesions (nodules, draining tracts) may be present. The clinical course of disseminated disease is often rapidly progressive and fatal if untreated.

Differential diagnoses for sinonasal aspergillosis include: (1) Chronic bacterial rhinitis, which typically responds to antibiotics and may be secondary to dental disease or foreign bodies; (2) Nasal neoplasia (e.g., adenocarcinoma, lymphoma, fibrosarcoma), which is more common in older dogs and often associated with unilateral discharge and facial deformity; (3) Nasal foreign body, which causes acute onset of sneezing and discharge, often with a history of plant material exposure; (4) Lymphoplasmacytic rhinitis, an idiopathic inflammatory condition that can mimic aspergillosis but lacks fungal elements on imaging and histopathology; (5) Nasal polyps, which are rare in dogs but can cause similar signs; (6) Cryptococcosis, a fungal infection that can cause nasal and systemic signs, particularly in cats; (7) Rhinosporidiosis, a rare fungal infection causing polypoid nasal masses; (8) Oronasal fistula, which is usually associated with dental disease and causes unilateral nasal discharge. For disseminated aspergillosis, differentials include other systemic fungal infections (e.g., histoplasmosis, blastomycosis, coccidioidomycosis), bacterial sepsis, and neoplasia with metastasis. Definitive diagnosis relies on imaging, rhinoscopy, histopathology, and fungal culture.

The diagnostic algorithm for suspected aspergillosis begins with a thorough history and physical examination, including assessment of nasal airflow and oral examination. If sinonasal aspergillosis is suspected, advanced imaging (CT or MRI) of the nasal cavity and frontal sinuses is recommended to evaluate for turbinate destruction, soft tissue masses, and extension into the frontal sinuses or orbit. Rhinoscopy is then performed to visualize the fungal plaques and obtain biopsy samples for histopathology and fungal culture. In cases where rhinoscopy is not feasible, a blind nasal swab or biopsy can be taken. Serological tests, such as serum Aspergillus-specific IgG or galactomannan antigen, can support the diagnosis but are not definitive. For disseminated aspergillosis, a complete blood count, serum biochemistry, urinalysis, and urine culture for Aspergillus may be helpful. Imaging of the thorax and abdomen (radiographs, ultrasound, CT) can identify pulmonary, renal, or bone lesions. Definitive diagnosis is made by histopathological identification of fungal hyphae in tissue samples and positive fungal culture. PCR-based assays on tissue or body fluids are increasingly used for rapid species identification.

In sinonasal aspergillosis, routine laboratory findings are often unremarkable, with no consistent abnormalities on CBC or serum biochemistry. Mild leukocytosis or eosinophilia may be present in some cases. In disseminated aspergillosis, common laboratory abnormalities include non-regenerative anemia, leukocytosis with a left shift, and thrombocytopenia. Serum biochemistry may reveal azotemia (due to renal involvement), elevated liver enzymes, and hyperglobulinemia. Urinalysis may show proteinuria, hematuria, and pyuria if the kidneys are affected. Fungal culture of urine can be positive for Aspergillus in cases of renal aspergillosis. Serological tests, such as agar gel immunodiffusion (AGID) for Aspergillus-specific antibodies, have high sensitivity and specificity for sinonasal aspergillosis in dogs. Galactomannan antigen testing (ELISA) on serum or bronchoalveolar lavage fluid is used in humans and may be helpful in disseminated disease, but its utility in veterinary medicine is still being evaluated. PCR assays for Aspergillus DNA in tissue or fluid samples are highly sensitive and specific and can confirm the diagnosis.

Imaging plays a crucial role in the diagnosis and management of aspergillosis. In sinonasal aspergillosis, computed tomography (CT) is the imaging modality of choice, as it provides detailed evaluation of the nasal cavity, turbinates, and frontal sinuses. Characteristic CT findings include loss of normal turbinate architecture, soft tissue opacification, and destruction of the nasal septum or cribriform plate. In chronic cases, there may be evidence of frontal sinus involvement, with fluid accumulation or soft tissue masses. Magnetic resonance imaging (MRI) may be used if there is suspicion of intracranial extension. Radiography of the skull can be useful but is less sensitive than CT. In disseminated aspergillosis, thoracic radiographs may reveal interstitial or nodular pulmonary infiltrates, and abdominal ultrasound may show renomegaly or renal infarcts. Skeletal radiographs or CT can identify osteolytic lesions in the vertebrae or long bones. Advanced imaging, such as CT or MRI, is essential for surgical planning and monitoring response to therapy.

Cytological examination of nasal discharge or fine-needle aspirates from affected tissues may reveal fungal hyphae, which are septate, branching at acute angles (45°), and often stain with periodic acid-Schiff (PAS) or Gomori methenamine silver (GMS). However, cytology has low sensitivity, and a negative result does not rule out aspergillosis. Histopathological examination of biopsy samples is the gold standard for diagnosis. In sinonasal aspergillosis, histopathology typically shows fungal mats on the mucosal surface, with minimal tissue invasion. The underlying mucosa exhibits chronic inflammation, ulceration, and necrosis. In disseminated aspergillosis, histopathology reveals granulomatous inflammation with central necrosis, and fungal hyphae are often seen invading blood vessels (angioinvasion), causing thrombosis and infarction. Special stains, such as GMS and PAS, are essential for visualizing the fungal elements. Fungal culture of biopsy samples or nasal swabs can confirm the species, but culture may be negative in up to 30% of cases, so histopathology is often more reliable.

Treatment of sinonasal aspergillosis in dogs primarily involves topical antifungal therapy, often combined with surgical debridement. The most effective topical treatment is clotrimazole (1% solution) infused into the nasal cavity and frontal sinuses under general anesthesia, at a dose of 1 g per nostril, with a contact time of 60 minutes. This is typically performed once, but may be repeated if necessary. Alternatively, enilconazole (10% solution) can be used topically. Systemic antifungal therapy with oral itraconazole (5-10 mg/kg q12-24h) or fluconazole (5-10 mg/kg q12-24h) is often added, but topical therapy is considered the mainstay. In cases of frontal sinus involvement, trephination and sinus lavage may be required. For disseminated aspergillosis, systemic antifungal therapy is essential, using agents such as voriconazole (4-6 mg/kg q12h) or posaconazole (5-10 mg/kg q24h), which have better CNS penetration. Amphotericin B (0.5-1 mg/kg IV three times per week) may be used in severe cases, but nephrotoxicity is a concern. In cats, treatment is more challenging, and topical therapy is often less effective; systemic therapy with itraconazole or voriconazole is recommended. Surgical debridement of fungal plaques or granulomas may be necessary in some cases. Supportive care includes fluid therapy, nutritional support, and management of concurrent infections.

The prognosis for sinonasal aspergillosis in dogs is generally good, with a success rate of 70-90% following topical clotrimazole therapy, especially if the cribriform plate is intact. However, recurrence can occur in up to 20-30% of cases, requiring repeat treatment. The prognosis for disseminated aspergillosis is poor, with a mortality rate of over 80% despite aggressive therapy, particularly in immunocompromised animals. Negative prognostic indicators include involvement of the central nervous system, renal failure, and underlying immunosuppression. In cats, the prognosis for sinonasal aspergillosis is guarded, with a lower response rate to topical therapy and a higher risk of recurrence. Early diagnosis and aggressive treatment improve the chances of a favorable outcome.

Follow-up for sinonasal aspergillosis involves re-evaluation at 2-4 weeks after treatment to assess clinical improvement. Repeat rhinoscopy or CT may be performed to confirm resolution of fungal plaques. If clinical signs persist or recur, a second topical treatment may be indicated. Long-term monitoring includes regular assessment of nasal airflow, nasal discharge, and sneezing. For disseminated aspergillosis, serial monitoring of renal function, liver enzymes, and imaging (e.g., radiographs, ultrasound) is necessary to evaluate response to therapy and detect adverse effects of antifungal drugs. Serum drug levels may be monitored for voriconazole or itraconazole to ensure therapeutic concentrations and avoid toxicity. The duration of systemic antifungal therapy is typically 6-12 months, with gradual tapering based on clinical and imaging improvement.

Pearls: (1) In dogs with chronic nasal discharge, aspergillosis should be a primary differential, especially in young, large-breed dogs; (2) CT is essential for assessing cribriform plate integrity before topical therapy; (3) Topical clotrimazole is highly effective for sinonasal aspergillosis, but ensure adequate contact time and drainage; (4) In cats, consider underlying immunosuppression (FeLV/FIV) and treat accordingly; (5) For disseminated aspergillosis, early aggressive therapy with voriconazole may improve outcomes. Pitfalls: (1) Relying solely on cytology can lead to false negatives; histopathology is essential; (2) Systemic antifungal therapy alone is often ineffective for sinonasal aspergillosis; topical therapy is required; (3) Failure to assess cribriform plate integrity can lead to intracranial complications during topical infusion; (4) In disseminated aspergillosis, delay in treatment is often fatal; (5) Do not use glucocorticoids in aspergillosis, as they exacerbate immunosuppression.

For sinonasal aspergillosis in dogs: Topical clotrimazole (1% solution) at 1 g per nostril, infused under general anesthesia for 60 minutes, with the dog positioned in sternal recumbency and the head elevated. This may be repeated at 2-4 weeks if needed. Alternatively, enilconazole (10% solution) can be used topically. Systemic therapy: Itraconazole (5-10 mg/kg PO q12-24h) or fluconazole (5-10 mg/kg PO q12-24h) for 4-8 weeks. For disseminated aspergillosis: Voriconazole (4-6 mg/kg PO q12h) or posaconazole (5-10 mg/kg PO q24h) for 6-12 months. Amphotericin B (0.5-1 mg/kg IV three times per week, up to a cumulative dose of 8-12 mg/kg) may be used in severe cases, with close monitoring of renal function. In cats, itraconazole (5-10 mg/kg PO q12-24h) or voriconazole (4-6 mg/kg PO q12h) is recommended. All azole antifungals can cause hepatotoxicity, so liver enzymes should be monitored regularly. Drug interactions: Azoles inhibit cytochrome P450 enzymes, so concurrent use of drugs metabolized by these enzymes (e.g., cyclosporine, cisapride) should be avoided or dose-adjusted.

Evidence-based literature supports the use of topical clotrimazole as the treatment of choice for canine sinonasal aspergillosis, with success rates of 70-90% in multiple studies (e.g., Mathews et al., 1990; Davidson et al., 2006). A study by Zonderland et al. (2002) reported a 87% success rate with a single infusion of clotrimazole. For disseminated aspergillosis, voriconazole has shown promise in case reports, but the prognosis remains poor. A retrospective study by Schultz et al. (2008) found that German Shepherd Dogs with disseminated aspergillosis had a median survival time of only 21 days despite treatment. In cats, a study by Tomsa et al. (2011) reported a 50% success rate with systemic itraconazole for sinonasal aspergillosis. Consensus guidelines from the ACVIM (e.g., 2018) recommend topical therapy for sinonasal aspergillosis and systemic azoles for disseminated disease. Further research is needed to optimize treatment protocols for feline aspergillosis and to evaluate the role of newer antifungals such as posaconazole.