What is nasal aspergillosis?
Nasal aspergillosis is a fungal infection of the nasal passages. It is a well-recognised cause of chronic, mucopurulent discharge in the dog. Clinical reports have suggested that the disease may contribute to between 12 and 34% of all nasal disorders.
Nasal aspergillosis remains an incredibly complex and frustrating disorder to treat. The nasal cavity is a very tortuous space, providing lots of sites where the fungus can ‘hide’ from treatment. This means the apparent ‘simplicity’ of many current treatments can sometimes result in poor clinical results if remnants of fungal growth manage to escape contact with the drug. When the disease appears to be recalcitrant to treatment, good outcomes are still possible but much more dedicated treatment strategies are required.
What causes nasal aspergillosis?
The fungus that colonises the nose is caused by infection with either Aspergillus or Penicillium spp. The majority of clinical cases are associated with infection by A. fumigatus, an organism commonly found in compost piles, stables and barns. The organism has a world-wide distribution, and is the cause of similar clinical disease syndromes throughout the world. It is the same organism that causes mould to develop on stale bread.
Aspergillosis in the dog is usually limited to the nasal and paranasal sinuses. Colonisation of the fungus in the nasal and paranasal sinuses results in progressive destruction of the nasal tissues through the combined influence of a local vasculitis, and direct destructive effects of the fungal products.
Other forms of the disease affecting the lungs, or other organs systems have been reported but are very uncommon. The German Shepherd appears to be at particular risk of developing these systemic forms of the disease, and often there is an underlying immunosuppression.
What dogs are most commonly affected by aspergillosis?
Nasal aspergillosis is typically confined to dog breeds with longer noses. Male dogs appear to be at a higher risk of developing disease than females. Disease has been reported in dogs ranging from one year of age to twelve years of age. However, more than two-thirds of dogs are less than 8 years of age at the time of diagnosis. A seasonal incidence of infection has not been reported, and environmental studies have shown no significant fluctuation in spore counts during the year. Anecdotal evidence suggests that rural dogs, or dogs walked regularly in the country-side, are at greater risk of developing infection than their urban companions. There is no evidence to explain why affected dogs have become infected. Although immunosuppression is an important risk factor for humans that develop aspergillosis infection, most dogs are otherwise healthy with no evidence of underlying disease or risk factors.
What are the signs of aspergillosis?
Clinical infection of dogs with any of the fungal agents results in a nasal disease characterised by a profuse mucopurulent nasal discharge, intermittent sneezing and, usually, facial pain or discomfort. Often the animal is depressed and withdrawn, as the disease can cause considerable pain and discomfort. Epistaxis is an occasional finding. Ulceration and depigmentation of the nostrils is considered a pathognomonic feature of nasal aspergillosis, although this is not a consistent finding in all cases (25 – 77%).
How is aspergillosis diagnosed?
Diagnosis of the disease is usually based on the outcome of clinical examination, radiology, rhinoscopy and serological testing. Diagnosis can sometimes be challenging as discrete colonies of fungal growth are not always seen. Typically, a confident diagnosis can be achieved by observing a consistent combination of signs on examination, radiology and rhinoscopy.
CT examination of the nose provides the best opportunity to evaluate the extent of destruction to the nasal tissues, and for inspection of the paranasal sinuses.
Typical radiological features of the disease include:
- turbinate destruction
- areas of increased radiolucency
- areas of mixed opacity due to the thick nasal discharge
- punctate lucencies in the supporting bones.
Careful examination of the bones surrounding the sinuses is essential, looking particularly for erosion or thinning of the cranial vault, and osteomyelitis or periosteal reaction of the frontal bone.
Characteristic features of rhinoscopy include:
- significant areas of turbinate loss, particularly in the areas of the ethmoid turbinates
- discrete, white fungal plaques on the surface of the swollen turbinate mucosa.
Biopsies are usually obtained at the time of rhinoscopy to aid in the diagnosis of the disease.
Serological testing can be a useful aid, but false-positive and false-negative results can occur in about 15% of patients, so complete reliance on this diagnostic aid is not encouraged.
How is aspergillosis treated?
Over the years, a variety of agents with reported anti-fungal effects have been used in the treatment of the disease. Treatments via the systemic and local route have been associated with mixed success. In the last ten years, treatment with newer generations of the imidazoles has resulted in greater therapeutic efficacy.
1. Topical Treatments
The first line treatment for most dogs with nasal aspergillosis is instillation of topical anti-fungal cream into the frontal sinus. This strategy is similar to how superficial fungal infections such as athletes foot and candidiasis are treated in the human – indeed, the same medicated cream is being used for the nasal infection in the dog. For athletes foot etc, daily application of cream is required for 2-3 weeks to ensure the infection is eliminated.
Because we can’t apply cream to the inside of the nose every day, a large amount of cream is placed into the frontal sinus cavity instead, from where it slowly leaks into the nose. This technique aims to provides continuous contact of the drug onto the fungal growth for several days after administration. To ensure complete elimination of the infection, this treatment needs to be repeated every 2-3 weeks for 3-4 treatments. At the third treatment, rhinoscopy is usually repeated to ensure all active fungal growth has been eliminated.
2. Systemic Treatments
Systemic treatment with anti-fungal drugs may still be required in some cases. If the dog is showing signs of blepharospasm, significant osteomyelitis of the frontal or nasal bones, or obvious extension of the fungal disease beyond the paranasal sinuses carries a poorer prognosis for treatment success. These animals will benefit from concurrent systemic treatment with itraconazole (5 mg/kg, BID) for up to 6 weeks, or longer.
Numerous studies have shown that systemic treatment alone does not give consistent clinical cures. As well as their low efficacy, systemic anti-fungal drugs are also associated with a high incidence of systemic side-effects. Hepatotoxicity, nausea, and cutaneous eruptions have all been reported, and necessitate early withdrawal of the drug.
Current recommendations in the treatment of nasal aspergillosis limit the use of systemic medication to cases where the disease is considered to have extended beyond the nasal cavity, on the basis of clinical, radiological, or computer tomographic findings. Prolonged treatment is often required – consisting of several months of medication.
What is the prognosis for dogs with aspergillosis?
Only about 65% of dogs respond to a single treatment, and up to four treatments may be required to obtain an 87% overall success rate. Severely affected dogs may require more invasive treatment strategies to be more certain of success.
Even after successful treatment, some animals may have a residual serous nasal discharge, and intermittent sneezingdespite successful resolution of fungal disease. This appears to be related to severity of turbinate destruction and alteration of the nasal microenvironment. There is a correlation between the occurrence of persistent signs with disease duration prior to treatment.