Aspergillosis in Dogs

By Malcolm Weir, DVM, MSc, MPH; Krista Williams, BSc, DVM; Ernest Ward, DVM

What is aspergillosis?

Aspergillosis is an infection caused by the Aspergillus fungus. This fungus grows on dead leaves, stored grain, compost piles, or other decaying vegetation. The Aspergillus species includes more than 180 types of mold that occur widely in indoor and outdoor environments. Aspergillus is commonly found as a lacy mold that grows on foods.

Although most of the molds are harmless, a few can cause serious illnesses in people and pets with a weakened immune system, underlying lung disease, or asthma. The most common species of Aspergillus to cause problems in dogs are Aspergillus fumigatus and Aspergillus terreus, although several other species of Aspergillus can cause illness in dogs.

How did my dog get aspergillosis?

Aspergillus is considered 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.

"Aspergillus fungi shed microscopic spores that float in the air and are easily inhaled."

In the nose, these protective mechanisms include nasal sinuses to trap inhaled debris, the mucus layer that lines the nasal passages and traps debris, and the ability to sneeze and 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 that are constantly in motion '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 able to evade or bypass these protective respiratory mechanisms, they can begin to grow and become more difficult for the body to remove. Some strains of the fungus are more likely to cause disease than others, or a dog may simply be exposed to a large number of spores that overwhelm its defenses.

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 will develop mild to serious lung infections as well. The deadliest form of aspergillosis, 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?

The two 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). Sneezing and pawing at the nose or face may also occur.

Many times, the affected dog is of a breed with a long nose (e.g., Collie, Greyhound, Dachshund) although recent studies showed 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.

"...recent studies showed that Retrievers and Rottweilers have the highest incidence of aspergillosis infection."

Aspergillosis should be considered as a possible cause for any dog with a chronic or long-term nasal infection or condition.

With 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 upon the location where the fungus settles in the body. Common sites of infection 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 oozing out) 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 will 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) scan, or MRI (magnetic resonance imaging) will show changes that suggest a fungal infection. MRI or CT are considered more diagnostic than radiographs.

• Fungal plaques or aspergillomas may be visible with rhinoscopy (a technique where a narrow needle-like camera is inserted into the nose).

Aspergillus organisms are seen in 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 don’t cross-react on a test, meaning you must test separately for each species.

DNA testing on cultured fungal colonies or biopsy samples is sometimes used to identify fungal species, but they too can 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 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 radiographs or a CT scan may be suggestive of fungal infection. Unfortunately, there is currently no blood test that is considered 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 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 visible Aspergillus organisms. If the organism is not visible, it may be cultured from fluid or tissue samples.

How is aspergillosis treated?

Fungal infections, in general, require extended treatment and many of the drugs traditionally used to treat aspergillosis have toxic side effects. Advances in techniques and medications have enabled better success with the treatment of nasal aspergillosis, 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 the 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 have resolved 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 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 is very difficult for systemic or disseminated aspergillosis in dogs. Antifungals are available, although they can be expensive and 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 for my dog?

The prognosis is generally good for cases with localized nasal aspergillosis. The prognosis for systemic aspergillosis is guarded 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 the potential to contract aspergillosis from your dog exists, it is less likely if you practice good personal hygiene, especially hand washing, and avoid handling any discharges. If your dog has been diagnosed with aspergillosis, ask your physician for advice.

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