Fungi of the Aspergillus genus are well known to most people as the lacy, fluffy mold that grows on foods kept a bit too long. There are over 180 species of Aspergillus fungi and they generally do not cause disease unless the host has a compromised immune system or has a big exposure to the fungus. The most common species to cause problems in veterinary patients are Aspergillus fumigatus and Aspergillus terreus.
Under the microscope Aspergillus looks like a beautiful flower but note the spores (called conidia) it sheds. These float in the air to colonize organic matter or infect a host when they land.
There are two common types of Aspergillus infection in dogs:
- Nasal (usually caused by Aspergillus fumigatus)
- Systemic (usually caused by Aspergillus terreus)
Dog with nasal discharge From Aspergillosis. Photo by Dr. Kimberly Morrissey
Sinonasal aspergillosis is the most common manifestation of the Aspergillus infection. Most infections are invasive and actually destroy the delicate bones of the sinuses while less invasive (and very rare) infections form a big mucous wad of fungus called a fungal ball or aspergilloma. What the owner usually sees is a snotty nasal discharge that often stinks, lasts for months, does not respond to antibiotics, and classically only comes from one nostril. Nose bleeds occur intermittently and the edges of the nostrils are often ulcerated. Classically, the affected dog is of a breed with a long nose (collie, greyhound, dachshund, etc.) although in one study, retrievers and Rottweilers showed the highest numbers (possibly suggesting they do more sniffing in fungal contaminated areas). Any age dog could be come infected.
To diagnose sinonasal aspergillosis, two out of the following criteria must be met:
- Radiographs or other imaging (CT or MRI) should be compatible with a fungal infection.
- Fungal plaques should be visible with rhinoscopy (a technique where a narrow needlelike camera is inserted in the nose).
- Aspergillus organisms are seen in or cultured from either a tissue biopsy or the nasal discharge.
- A blood test (either antibody level, test for fungal cell wall galactomannan or PCR test) or a urine test for galactomannan antigen is positive against an Aspergillus species.
General anesthesia is necessary for imaging as well as for rhinoscopy. Most veterinary hospitals can manage nasal radiographs but rhinoscopy may require referral (though sometimes the yellow Aspergillus plaques can be seen with less specialized equipment). Non-invasive testing (i.e. blood testing) simply has not panned out to be adequately accurate for Aspergillus, but several tests are available and may contribute to making a diagnosis.
The treatment of aspergillosis has been challenging for many years; fungal infections as a rule are slow to clear and for a long time the only available drugs had toxic side effects. Today, sinonasal aspergillosis has a good prognosis thanks to a unique treatment plan. The patient is anesthetized and the back of the throat and is closed off with gauze and inflatable balloon catheters called Foley catheters. The nostrils are also closed off (the patient breathes through an endotracheal tube that is placed through the mouth) and a one percent solution of clotrimazole, a topical antifungal lotion, is infused into the nose and frontal sinuses. The solution incubates for an hour and the patient is periodically turned to be sure all the nooks and crannies of the sinuses are treated. At the end of the incubation, the clotrimazole is drained through the nostrils and suctioned out.
This treatment is highly effective with an 86 percent success rate though about one-third of patients require several treatments. Most of the time the nasal discharge has resolved within a couple of weeks, but if there is still evidence of continuing infection one month after the treatment, the patient should be rechecked and another of bout of treatment will most likely be needed.
If there is evidence that the infection has eroded through the sinus bones and has penetrated the brain, the treatment described above cannot be used and oral medication is needed. Common medications used are itraconazole and fluconazole. Several months of therapy are needed and a 60 to 70 percent success rate has been reported.
Aspergillus Stain from CDC
Under the microscope Aspergillus looks like a beautiful flower but note the spores (called conidia) it sheds. These float in the air to colonize organic matter or infect a host when they land. Photo by CDC.
As shown in the photograph, Aspergillus fungi shed microscopic spores that float in the air and which we inhale all the time. Fortunately, we have an assortment of safety mechanisms built into our bodies that prevent infection (sinuses to trap inhaled debris, ability to sneeze, mucus lining of the entire tract to trap debris, cilia to remove mucus with trapped debris, and an immune system to fight infection). If Aspergillus spores called conidia escape these mechanisms and begin to grow, they become more difficult for the body to remove. Some strains of fungi are more virulent than others or one may simply be exposed to a large number of spores. In disseminated aspergillosis, the fungus penetrates the respiratory tract and may travel to other organs via the bloodstream, creating a more serious fungal invasion. German Shepherd Dogs seem to be predisposed to this type of dissemination.
Symptoms experienced by the patient depend on where the fungus settles. Bone infection is common and the intervertebral discs of the spine also tend to be favorite spots for Aspergillus. This means that clinical features commonly include lameness, weakness, and incoordination. Frequently there are draining tracts (holes 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 features. Sadly, most dogs are already terminally ill by the time they are first examined.
Diagnosis is tricky. Bone destruction patterns on radiographs may be suggestive of fungal infection. If you’re lucky, the organism can be identified in some of the draining fluid or in a tissue sample. If the organism is not visible, it may be cultured from fluid or tissue sample. Ideally, there would be a blood test for Aspergillus antibodies that could make the diagnosis non-invasively and, while such antibody tests exist, as mentioned, it is hard to interpret them. There are accuracy issues plus different species of Aspergillus do not cross-react so one must test separately for each Aspergillus species.
Treatment is particularly frustrating for disseminated aspergillosis but some new drugs have been promising. Amphotericin B is an older antifungal drug, largely replaced by newer drugs because of its high potential to cause kidney damage. The problem here is that the newer drugs, with the possible exception of long courses of itraconazole (several years), are not particularly effective in aspergillosis. To minimize the kidney effects, amphotericin B can be encapsulated in liposomes (microscopic fatty envelopes) or other lipid formulations. These have been used in humans with some success but are extremely expensive. Terbinafine and voriconazole are newer antifungals that are still being investigated for this use.
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