Aspergillosis is a rare fungal infection that sets up shop in dogs’ nasal passageways. This fungus is usually harmless. However, it is widespread, favoring rotten vegetables, moldy hay, wood chips and sewage as breeding grounds.
If dogs inhale aspergillus spores, usually their immune system stops an infection from developing. Clinical aspergillosis normally affects animals with compromised immune systems that are less able to keep infections in check.
A generalized form of aspergillosis exists, where the fungus spreads to the organs, but this is extremely rare.
Dogs with long noses — such as German shepherds, greyhounds, retrievers and dachshunds — are the most commonly affected, possibly because of the greater surface area of mucous membrane within their noses.
The most common symptom is a discharge from one nostril that does not respond to therapy with antibiotics. This discharge is frequently purulent, or “snotty”-looking, and can be present for weeks or months. Later in the infection, the external nose (the leathery part) erodes.
As time goes on, the nose is intermittently stained with blood, the bridge of the nose appears swollen and the dog shows signs of facial pain, such as reluctance to being stroked there.
If left untreated, the fungus can invade the frontal sinuses, and pass across the protective plate dividing the nose from the brain. This can causes neurological symptoms, the exact nature of which depends on the part of the brain affected.
The aspergillus species is made up of around 150 molds that thrive in damp conditions such as leaf mold and decaying vegetation.
The microscopic spores float in the air, and are easily inhaled. However, this is nothing to panic about — in the majority of cases, a healthy dog’s immune system efficiently prevents infection from taking hold.
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Many cases of aspergillosis are mistakenly diagnosed initially as bacterial infections. This is because bacteria take advantage of the damaged tissue in the nose and set up a secondary infection that is spotted first.
But when the dog doesn’t respond to antibiotic treatment, the possibility of aspergillosis must be considered.
There is a blood test that checks for exposure to aspergillosis. Its use is limited, though, because a positive test only tells you that the dog has been in contact with aspergillosis; it cannot differentiate between those dogs who mounted an immune response and fought off infection from those who succumbed.
The blood test does, however, have a place for ruling aspergillosis out completely, since a negative test means the dog has not be exposed, and so he is unlikely to have clinical disease.
Other investigations include radiographs to check for a foreign body up the nose, an oronasal fistula (a hole between the mouth and nasal cavity), tooth root abscesses and cancer of the nose. Radiographs can help tell the difference, although there is a lot of overlap in appearance on X-rays, and a CT scan is much more sensitive.
Most useful of all is rhinoscopy, where a small camera is inserted into the dog’s affected nostril. This allows direct visualization of any fungal plaques, enabling sampling for culture and cytology.
The success rate of treating aspergillosis with oral antifungal medications is disappointing. The fungus is effectively walled off in the nose, and it is difficult to raise the blood levels of the drug high enough to where it is needed.
More successful are a variety of techniques where antifungals are irrigated into the nasal cavity, so the drugs directly contact the fungal plaques and kill them. This involves a general anesthetic, and some methods involve implanting tubes into the dog’s skull, through which to flush the antifungal.
These techniques have an excellent success rate, with some cases only needing a single treatment, while others require repeated flushing.
At the moment, there is no vaccine against aspergillosis. Although the fungal species are widespread, cases are reassuringly rare because each individual dog’s immune system is usually capable of fighting off the infection.
- Clinical Medicine in the Dog and Cat. Michael Schaer. Publ: Manson Publishing.
This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Oct. 2, 2015.