What is aspergillosis?
Aspergillosis is an infection caused by the Aspergillus fungus. This fungus grows on dead leaves, stored grain, compost piles, and other decaying vegetation. The Aspergillus species includes more than 180 types of molds that occur widely in indoor and outdoor environments. Aspergillus is commonly found as a lacy mold that grows on foods.
Although most Aspergillus molds are harmless, a few can cause serious illnesses in people and in dogs with weakened immune systems, underlying lung disease, or asthma. The most common species of Aspergillus that cause problems in dogs are Aspergillus fumigatus and Aspergillus terreus, although several other species can also cause illness.
How do dogs get aspergillosis?
Aspergillus is an opportunistic pathogen and usually affects animals whose immune systems are suppressed or compromised. Aspergillus fungi shed microscopic spores that float in the air and are easily inhaled. Fortunately, dogs have several protective mechanisms built into their bodies that help prevent infection.
In the nose, the nasal sinuses trap debris, as does the mucus layer that lines the nasal passages. A dog can also sneeze to expel inhaled debris and foreign bodies. In the remainder of the respiratory tract, the mucus lining of the passages traps debris that enters the airways, and tiny hair-like cilia constantly move to “sweep” the mucus and trapped debris up and out of the airways. A healthy immune system provides active protection of the entire body, preventing invasion and fighting infections within the body.
If Aspergillus spores, called conidia, are inhaled and manage to evade or bypass these protective respiratory mechanisms, the spores can begin to grow, which makes them more difficult for the body to remove. Some strains of the fungus are more likely to cause disease than others, and a dog may simply be exposed to so many spores that its defenses are overwhelmed.
Aspergillosis begins when susceptible dogs inhale mold spores. In some dogs, the spores trigger an allergic reaction. In others, the spores cause an infection. Most infections are localized in the nose (nasal aspergillosis), but some dogs develop mild to serious lung infections as well. The deadliest form of aspergillosis, called systemic or disseminated aspergillosis, occurs when the infection spreads beyond the lungs to other organs. Even when discovered and treated early, systemic aspergillosis is often fatal.
What are the clinical signs of aspergillosis?
The different forms of aspergillosis affect dogs in different ways.
Nasal aspergillosis is the most commonly diagnosed form of aspergillosis in dogs. Most cases of nasal aspergillosis are invasive, meaning that the fungus destroys the delicate bones of the sinuses. The less common and less invasive forms of nasal aspergillosis create an accumulation of mucous and fungus commonly referred to as a “fungal ball” or aspergilloma.
A pet owner will observe a chronic nasal discharge that often has a strong odor, lasts for weeks to months, does not respond to antibiotics or other common therapies, and typically involves only one nostril. Nosebleeds may occur intermittently, and the edges of the nostrils are often rough, inflamed, and ulcerated (the tissue breaks open and bleeds due to the discharge, which is very irritating). The dog may also sneeze and paw at its nose or face. Aspergillosis should be considered as a possible cause for any dog with a chronic or long-term nasal infection or condition.
"…recent studies show that retrievers and rottweilers
have the highest incidence of aspergillosis infection."
Affected dogs often have a long nose (e.g., collie, greyhound, dachshund), although recent studies show that retrievers and rottweilers have the highest incidence of aspergillosis infection. It is possible, however, that the affected dogs lived in an area with high levels of pathogenic (disease-causing) Aspergillus. It is important to note that any dog can develop nasal aspergillosis.
In systemic or disseminated aspergillosis, the fungus enters the body through the respiratory tract and travels to other organs via the bloodstream, creating a more serious generalized or systemic fungal infection. According to some studies, German shepherds may be predisposed to systemic aspergillosis.
The clinical signs of systemic aspergillosis depend on the location where the fungus settles in the body. Common sites of infection in dogs include the bones and the intervertebral discs of the spine. If these areas are infected, clinical signs include lameness, weakness, and incoordination.
Many dogs develop draining tracts (holes in the skin with pus or bloody discharge) in the areas of infection. Fever, weight loss, appetite loss, and uveitis (deep inflammation of the eye) are also commonly associated with systemic aspergillosis. Most dogs with systemic aspergillosis do not have nasal symptoms. Unfortunately, most dogs are terminally ill by the time they exhibit clinical signs.
How is aspergillosis diagnosed?
To diagnose nasal aspergillosis, at least two of the following four criteria must be met for most dogs:
- Radiographs (X-rays), CT (computed tomography) scans, or MRI (magnetic resonance imaging) will indicate a fungal infection. MRI and CT are considered more diagnostic than X-rays.
- Fungal plaques or aspergillomas may be visible with rhinoscopy (a technique in which a narrow, needle-like camera is inserted into the nose).
- Aspergillus organisms are seen in a tissue biopsy or cultured from a tissue biopsy. The fungus is not usually seen in nasal discharge but may be recovered by flushing the nasal cavity.
- A blood test is positive for antibodies against an Aspergillus species. These tests must be interpreted with caution, however, as the different species of Aspergillus do not cross-react on tests, so separate tests are needed for each species.
Fungal species are sometimes identified using DNA testing on cultured fungal colonies or biopsy samples, but these tests can also be difficult to interpret.
General anesthesia is necessary for diagnostic imaging (X-rays or CT scans), as well as for rhinoscopy. In many cases, the advanced equipment required to perform these tests is only available at specialty referral practices or veterinary teaching hospitals. In some cases, the yellow Aspergillus plaques can be seen within the nasal passages without specialized diagnostic equipment.
The diagnosis of systemic aspergillosis can be more challenging. Patterns of bone lysis (bone destruction) on X-rays or a CT scan may suggest fungal infection. Unfortunately, there is currently no blood test accurate enough to diagnose systemic aspergillosis outright, but blood tests can be used to support the diagnosis of the disease.
There is a urine test, called a galactomannan antigen test, that can test for the fungus, although false positive and negative results have been reported, making definitive diagnosis difficult. In some cases, the draining fluid or a tissue sample may contain Aspergillus organisms that are visible microscopically. If the organism is not visible, it may be cultured from fluid or tissue samples.
How is aspergillosis treated?
Fungal infections generally require extended treatment and many of the drugs traditionally used to treat aspergillosis have toxic side effects. Advances in techniques and medications have improved the success of nasal aspergillosis treatment, but systemic aspergillosis is still challenging to treat.
Treatment of nasal aspergillosis
After anesthetizing the dog, an endotracheal tube (breathing tube) is placed in the trachea (windpipe), allowing the patient to breathe a mixture of anesthetic gases and oxygen. The area at the back of the throat is packed off with gauze sponges and an inflatable balloon catheter known as a Foley catheter. A 1% solution of clotrimazole or 1% bifonazole with enilconazole (topical antifungal agent) is infused into the nose and frontal sinuses and the nostrils are sealed. The solution incubates for an hour, during which time the patient is periodically turned to ensure the solution contacts all surfaces of the sinuses. At the end of the incubation period, the solution is suctioned or drained out through the nostrils.
This treatment is highly effective, with a reported 86% success rate, although approximately one-third of patients require several treatments. In most cases, the nasal discharge will resolve within two to four weeks. If there is still evidence of continuing infection one month after the treatment, another treatment should be performed.
If there is evidence that the infection has eroded through the bones of the nasal sinuses and has entered the brain, treatment with oral antifungal medication, such as itraconazole (Itrafungol®, Sporanox®), fluconazole (Diflucan®), or terbinafine (Lamisil®) is required. Several months of therapy are needed and a 60% to 70% success rate has been reported.
Treatment of systemic aspergillosis
Treatment of systemic or disseminated aspergillosis in dogs is very difficult. Antifungal treatments are available, although they can be expensive. Long-term treatment (for months to years) is often required. Drugs such as itraconazole have proven successful in some cases. Systemic aspergillosis in humans has been treated with terbinafine and posaconazole (Noxafil®). The effectiveness of other antifungal drugs used in human medicine is less clear.
What is the prognosis?
The prognosis is generally good for cases with localized nasal aspergillosis. The prognosis for systemic aspergillosis is guarded (uncertain) to poor, especially if there is evidence of sinus destruction.
Can I get aspergillosis from my dog?
Birds are the most common source of infection in humans. While you can contract aspergillosis from your dog, it is less likely if you practice good personal hygiene, especially handwashing, and avoid handling any discharges. If your dog has been diagnosed with aspergillosis, ask your physician for advice.
