Aspergillus species are saprophytic fungi commonly found in compost piles, stables, and barns. They were first described in the environment in 1729 and have been associated with diseases in companion animals since 1829. Despite the ubiquitous nature of these organisms, disease occurs in only a small percentage of exposed animals.
In general, this fungal infection in dogs is either sinonasal or disseminated. The two presentations are caused by different Aspergillus species and are clinically distinct, as sinonasal aspergillosis does not lead to disseminated infection and solitary infections outside of the nasal passageways are rare.1
In this article, we review these two classic manifestations of canine aspergillosis and discuss the diagnosis and treatment of affected patients.
Aspergillus fumigatus is the most prevalent isolate in patients with sinonasal disease, although infection can occur with other species including Aspergillus flavus, Aspergillus niger, and Aspergillus nidulans.1 Although disease severity varies, most dogs with sinonasal aspergillosis have marked destruction of turbinate bones and mucosa. In severe cases, destruction of the frontal bones with invasion into the periorbital soft tissues and penetration through the cribriform plate into the central nervous system may occur.
Unlike the disseminated form, canine sinonasal aspergillosis occurs in apparently immunocompetent dogs. However, there is speculation that mucosal immune dysfunction predisposes certain dogs to infection.1-3 In addition, fungal virulence factors may potentiate infection through interference with mucociliary clearance and macrophage phagocytosis.
Sinonasal aspergillosis affects primarily mesaticephalic and dolichocephalic breeds; brachycephalic dogs are rarely affected.2 Most dogs with sinonasal aspergillosis are young to middle aged, with a mean age of 4.4 years (range = 1.5 to 8 years). This is substantially younger than dogs with nasal neoplasia, with a mean age of 9.5 years (range = 4 to 12 years).4
Consider sinonasal aspergillosis in any dog with nasal discharge and other compatible clinical findings. Differential diagnoses include nasal neoplasia, idiopathic lymphoplasmacytic rhinitis, a nasal foreign body, tooth root abscessation, and oronasal fistulation.1,2
The physical examination should include facial palpation to check for symmetry and detect pain. Also evaluate airflow through each nostril by using a chilled microscope slide or a cotton wisp.5 Loss of airflow suggests a mass lesion and is not consistent with primary aspergillosis. Perform careful retropulsion of each globe since asymmetric retropulsion may suggest a mass lesion. Examine the hard and soft palates, tonsils, and teeth for evidence of neoplastic erosion or invasion or dental disease.
It is essential to definitively diagnose nasal aspergillosis before starting treatment. As most of the available diagnostic tests have limitations, a combination of tests is often necessary to confirm a diagnosis. These tests include serology, imaging studies, rhinoscopy, cytologic and histologic examinations of affected tissues, and fungal culture. Before inducing anesthesia, perform a complete blood count, serum chemistry profile, and urinalysis to rule out concurrent systemic disease.
Serology. Tests that can detect serum antibodies against Aspergillus species include agar gel immunodiffusion (AGID), complement fixation, and ELISA techniques. The antibodies detected with AGID vary depending on the laboratory; however, most commercial laboratories detect antibodies to A. fumigatus, A. niger, and A. flavus.6 This test is widely available through veterinary diagnostic laboratories and is probably the most commonly performed fungal serologic test at this time. However, a recent study indicated a test sensitivity of only 67% in dogs with sinonasal aspergillosis, suggesting that one-third of affected dogs would have a negative result.7 This reduced sensitivity may be due to infection with a less common Aspergillus species. The test specificity was high at 98%, indicating that false positive results are unlikely. It is important to remember, however, that a positive result with any serologic test does not eliminate the possibility of nonfungal rhinitis and is not enough evidence for a definitive diagnosis of sinonasal aspergillosis.7
In human patients, a sandwich enzyme immunoassay (Platelia Aspergillus EIA—Bio-Rad Laboratories, Marnes-la-Coquette, France) has been used to confirm infection with Aspergillus species by the detection of a cell wall component called galactomannan. This component can be identified in serum and other body fluids such as cerebrospinal fluid or bronchoalveolar lavage fluid.8 Limited information is available on the reliability of antigen detection tests in veterinary medicine, but in a recent study, the sensitivity of this test in dogs with sinonasal disease was only 23.5%.6 The poor sensitivity may reflect the noninvasive nature of this disorder, with limited release of antigens into the circulation.
A recent study evaluated the use of real-time broad-spectrum polymerase chain reaction test fungal DNA detection in whole blood or tissue samples. Detection of fungal DNA in blood was of little diagnostic use because of low positive and low negative predictive values, while identification of Penicillium and Aspergillus species DNA in nasal tissues lacked specificity for sinonasal aspergillosis.9
Imaging studies. All imaging studies should be completed before rhinoscopy and the collection of biopsy samples as hemorrhage can limit lesion detection. Radiographs of the nasal cavity and frontal sinus can be diagnostically useful, but the patient must be anesthetized during the radiographic examination to permit proper positioning. Ideally, lateral, ventrodorsal (both open- and closed-mouth) and rostrocaudal views should be obtained. Common radiographic changes associated with aspergillosis are areas of increased radiolucency, which suggest turbinate destruction.10 Opacification of the nasal cavities and frontal sinuses may also be noted.
Although the frontal sinuses may be accessed by using a rostral nasal approach, it can be technically challenging. Alternatively, trephination may be performed to permit insertion of a rigid endoscope.5 Trephination prolongs anesthesia time and increases postoperative discomfort but may provide essential diagnostic information in veterinary patients without rhinoscopic evidence of fungal disease. In one study of 46 dogs with sinonasal aspergillosis, 17% had fungal plaques in the sinuses but not in the nasal cavity.12
In classic cases, the results of CT or MRI studies along with the identification of fungal plaques with rhinoscopy or sinuscopy are enough to establish a diagnosis of aspergillosis, and treatment (described later) can be performed during the same anesthetic event.
Fungal culture. Fungal culture has been used in combination with other tests in the diagnosis of sinonasal aspergillosis in veterinary patients. However, false positive and false negative results confound its diagnostic use. Aspergillus species are ubiquitous in the environment and can be cultured from normal dogs and those with neoplastic or inflammatory nasal diseases.1 If a culture is warranted to confirm Aspergillus species vs. other fungal agents, positive results are more likely if the material is obtained by direct sampling of fungal plaques during rhinoscopy.14
Topical antifungals. Topical antifungal medications are regarded as the treatment of choice if the cribriform plate is intact. When used topically, both enilconazole and clotrimazole appear to be more effective in the treatment of sinonasal aspergillosis than oral antifungal agents are. The topical azoles have poor solubility and minimal intestinal absorption and are fungicidal (rather than fungistatic) at higher concentrations.
The first described technique for topical therapy in dogs required surgically placing tubes into the frontal sinus followed by enilconazole instillation twice a day for one or two weeks. This method was 90% effective in dogs without extranasal infection.15
An alternative topical method involves one infusion via both nares of either clotrimazole or enilconazole, performed while the patient is under general anesthesia. A detailed description is beyond the scope of this article, so veterinarians unfamiliar with this procedure should consult additional resources or refer patients to a specialist. In a study to determine the efficacy of nonsurgically placed intranasal catheters, topical clotrimazole was shown to resolve clinical disease in 65% of dogs after one treatment and in 87% of dogs after two treatments.16 A similar study using topical enilconazole in dogs reported 57% resolution of clinical disease after one treatment and 94% after one to three treatments.17 This study also evaluated the role of extensive rhinoscopic débridement before enilconazole infusion and concluded that it improved patient outcome.
As topical administration of antifungal agents is only appropriate if the cribriform plate is intact, a CT scan is warranted before each treatment.
Systemic therapy. Although oral antifungal medications are expensive and side effects are common, systemic therapy is recommended if there is fungal invasion of extranasal structures. Several azole drugs have been used to treat dogs with sinonasal aspergillosis, but the success rates are moderate at best. One treatment protocol using ketoconazole and surgical débridement was curative in only three of seven dogs, while six of 10 dogs with fungal rhinitis due to either Aspergillus or Penicillium species responded to fluconazole.18,19 One report describes the successful treatment of a dog with sinonasal aspergillosis using itraconazole alone, while others have reported positive responses in dogs given itraconazole after surgical and topical therapy.20,21 Anorexia, vomiting, and hepatotoxicosis have been reported with long-term use of these three antifungal agents in dogs, so monitor serum alanine transaminase activity regularly.1
The prognosis of dogs with sinonasal aspergillosis depends on the extent of extranasal involvement and the response to initial therapy. Most dogs treated with topical antifungals do well, although a series of treatments may be necessary. In some patients with extensive turbinate damage, nasal discharge may persist, and bacterial rhinitis occurs in up to 25% of dogs after successful resolution of the primary fungal disease.1 The response to treatment is indicated by a reduction of clinical signs and is supported by repeat CT examination. The use of serology to assess response to treatment is limited, as antibody titers remain high for up to five years after successful therapy.1
In a recent report of three dogs successfully treated for frontal sinus aspergillosis, sinonasal tumors were diagnosed 13 to 30 months after standard treatment with topical clotrimazole.22 Although it was hypothesized that the infection, inflammatory response, or drug may have been carcinogenic, no causal associations were evident and no conclusions can be made from this small veterinary case series.
In general, dogs without extranasal involvement carry a good prognosis, and recurrence of aspergillosis after successful treatment is rare.
Disseminated aspergillosis is relatively rare in dogs compared with the sinonasal form. Infection is thought to occur through the respiratory tract with subsequent hematogenous spread to other sites, including intervertebral disks, kidneys, and irises as well as other organs, muscles, and long bones.1
Although several Aspergillus species have been isolated from dogs with disseminated disease, Aspergillus terreus and Aspergillus deflectus predominate.23 Many affected dogs have underlying immunocompromise, such as diabetes mellitus or bacterial infections, or are receiving immunosuppressive medications, such as glucocorticoids or chemotherapeutics. Genetic factors may also play a role, as German shepherds are substantially predisposed to this disease.1,23
Imaging studies. Radiographs and CT may reveal lesions associated with diskospondylitis (collapsed disk spaces, proliferative bony changes adjacent to the intervertebral disk spaces, sclerosis) or lysis and destruction of long bones (Figures 6-8).
Confirmatory testing. Cytologic identification of Aspergillus species can be made by sampling affected tissues (urine, blood, synovial fluid, lymph node, bone or intervertebral disk material).1,23 In addition, Aspergillus species may be cultured from the urine of affected dogs.
Supportive therapy is often necessary initially, including fluid therapy and pain relief. Once a diagnosis has been established, initiate systemic antifungal therapy either empirically or guided by the results of fungal culture. Traditional antifungal treatments include amphotericin B and itraconazole (Table 1).1 Amphotericin B is a macrolide polyene drug with a broad spectrum of antifungal activity. It appears to be primarily fungistatic but may be fungicidal against some organisms depending on the drug concentration.25 Marked nephrotoxicosis has been reported in veterinary patients after administration of this agent, although the more expensive lipid-based formulations appear to be less toxic and have improved tissue penetration. Some controversy exists in the human literature about the concurrent administration of amphotericin B and azole drugs, since amphotericin B binds to ergosterol within the fungal cell membrane causing cellular contents to leak, while azole drugs inhibit the synthesis of ergosterol. Theoretically, therefore, the efficacy of amphotericin may be blunted in patients receiving azole therapy.
Prolonged clinical remission has been reported in dogs given oral itraconazole, and it is probably the most commonly administered azole drug in dogs with disseminated aspergillosis.23,26 Although fluconazole would be a less expensive option, we are not aware of any reports of successful management of canine disseminated aspergillosis using this agent. In addition, several studies in the human field suggest poor efficacy of fluconazole for this condition.27,28
Many new azoles have been developed in recent years, including voriconazole (Vfend—Pfizer). Voriconazole appears to be more effective against invasive Aspergillus species in people and is associated with fewer adverse events than amphotericin B.29 Other new azoles, including posaconazole and ravuconazole, have been shown to be efficacious against aspergillosis in vitro but their use in companion animals has not been explored.30
Additional antifungal options include terbinafine and caspofungin. Terbinafine is a squalene epoxidase inhibitor and may work synergistically with azole drugs.25 It is generally well-tolerated by dogs, and positive results have been reported in dogs with Malassezia species dermatitis.31 Caspofungin is the first member of a new class of antifungal drugs called the echinocandins. It prevents fungal cell wall synthesis by inhibiting a glucan synthase and has been used successfully in people with invasive aspergillosis who fail to respond to conventional antifungal medications.25 However, we are unaware of any reports of a positive response after the administration of either agent to dogs with disseminated aspergillosis.
Survival times for dogs with disseminated aspergillosis are variable, but owners should generally be given a guarded prognosis. Dogs with mild disease can have prolonged survival after antifungal drug therapy, although many months of treatment are necessary and relapse may occur if medications are prematurely discontinued. Sadly, more severely compromised dogs often succumb to complications of their infection or are euthanized because of quality of life concerns.
Whitney Nelson, DVM
Audrey K. Cook, BVM&S, MRCVS, DACVIM, DECVIMCA
Department of Small Animal Clinical Sciences
College of Veterinary Medicine & Biomedical Sciences
Texas A&M University
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