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Fungal agents cause a variety of diseases that range from mild skin infections to life-threatening systemic illness. Fungi normally live in the soil or environment, where they often perform important roles in the ecology, such as helping in the breakdown of organic debris (rotting food, stool or plant material). Fungi are not designed to live peacefully inside an animal's body, however. When a pet becomes infected, fungal organisms often cause discomfort, ill health and possibly even death. 


The definitive diagnosis of fungal disease is possible if we can view the organism under a microscope. In some cases, however, it can be difficult to capture and observe the fungus causing the illness. In those cases, histopathology or culture may give a definitive diagnosis. In addition, serologic tests may give a strong presumptive diagnosis. Other diagnostic tools, such as hematology (studying the cells of the blood), serum biochemical profiling, and radiographs (X-rays) may give the veterinarian clues as to cause and severity of disease, but these methods cannot definitively determine that a fungal organism is the cause of the pet's illness. 


In general terms, "Cytology" means the study of the formation, structure and function of cells. In diagnostic medicine, however, the term usually refers to examining fresh cells under a microscope in order to see if infectious agents are present. Veterinarians use several techniques to collect samples that they suspect might harbor infectious organisms: 

Impression Smear: A clean microscope slide is pressed against the skin, wound, or affected area. The debris collected is allowed to dry, then is stained to enhance the contrast between normal cells and infectious organisms. Examination under a microscope follows. 

Fine Needle Aspirate: A small needle is attached to a syringe and is inserted into a wound, a mass, or into a body cavity. The veterinarian pulls back on the syringe's plunger, creating a vacuum that withdraws fluid or cells. The needle is then removed from the animal, and the collected cells are deposited on a clean microscope slide, allowed to dry, then stained and examined. 

Lavage: This is most often performed when attempting to collect cells from the lungs, trachea or nasal cavity. In a lavage, a catheter is inserted into the area of interest and a small amount of sterile fluid is dripped into the body in order to moisten the tissues, and to cause foreign organisms to become suspended in the fluid. A syringe is attached to the outer part of the catheter, and the plunger is withdrawn, applying pressure and sucking the fluid back into the catheter. A few drops of fluid is placed on a clean microscope slide, allowed to dry, then stained and examined.

Under the microscope, most cells look uniformly pale and are difficult to identify. By applying certain chemicals, called stains, the cells become colored and are easier to see. Each cell or a part of a cell may react differently to these chemicals, so it is possible to not only identify foreign cells, but it also becomes possible to see the internal structure of normal body tissues. 

In order to identify fungal organisms on a slide, the veterinarian often chooses stains that either kill and clear all cells except the well-protected fungal elements, such as potassium hydroxide (KOH), or a stain that binds to the proteins that are unique to fungal cell walls. Fungal organisms stained in this way are very easy to identify. Stained cytology specimens often give a rapid and definitive diagnosis of fungal disease. 


Histopathology is, literally, the study of abnormal tissues. In regards to diagnosing fungal diseases, it is very similar to cytology, except the sample is usually ollected by a surgical biopsy and is preserved in formalin before being examined. A surgical biopsy allows for a larger sample when compared to cytology collection techniques. Like cytology, any disease causing fungal organism found inside tissues gives a definitive diagnosis of the cause of the pet's disease. 

Culture: When it is difficult to observe organisms under the microscope, a fungal culture may be performed. This is an excellent way to diagnose infections, because culturing will allow just a very few hard-to-find organisms to grow into a large, easily examined colony. Cultures are usually done in a laboratory, although dermatophyte (ringworm) cultures are easily performed in most veterinary hospitals. In order to get a meaningful culture, it is essential to collect a sample that is likely to contain the infectious organism. In the case of ringworm, this is easily done by plucking hair and crusts inside a suspicious lesion, or by using a sterile toothbrush to comb the coat in order to collect spores. In the case of deeper fungal infections, a sterile swab may be dipped into a draining wound or introduced into a more solid mass in order to collect infectious fungal cells. 

Once the sample is collected, it is transferred to an appropriate culture medium. This is usually a tube or covered Petri dish that has been filled with a gelatin-like substance (the media). This media is designed to contain all the proper nutrients to optimize fungal growth. There are many types of culture media available, and it is important to select the correct one for the suspected infection. While most fungal organisms grow easily on routine fungal media or blood agar, some have special nutritional requirements, and therefore must be grown on special media. For example, Blastomyces and Sporothrix grow best on Sabourad's dextrose agar, a medium that contains the extra sugars required for their nutrition. 

After the sample is placed on culture media, it is stored in an area best suited for fungal growth. This is usually in a warmer designed to mimic the body temperature of an animal. Depending on the organism, small patches (called colonies) will appear on the media within 2 days to four weeks. Once growth occurs, samples of the colonies are collected and examined under the microscope. Identification of a fungus gives a definitive diagnosis of the cause of the pet's disease. 

Although fungal culture confirms the cause of disease, it can have drawbacks. First, it can take a long time to get results - an average of two weeks for most organisms, and longer for some. If treatment is delayed, especially if the pet is infected with a dangerous systemic fungal disease such as Blastomycosis, the delay can allow life-threatening symptoms to develop. In this case, treatment should be begun before the culture confirms the diagnosis. A second drawback is that some cultured fungal organisms, such as Coccidioides, are extremely infectious to people. Due to the hazards of catching zoonotic diseases, dangerous organisms are not routinely cultured. 


Serology is the study of serum, which is the watery part of the blood that is collected after whole blood coagulates. The blood clot contains the cells and solid proteins (such as fibrin), while the remaining fluid (the serum) contains water and active water-soluble proteins. The immunologically active soluble proteins can be identified and quantified in order to tell if the body has been exposed to a specific disease. Immunologically active refers to the capacity of these proteins to fight infection or foreign substances. 

While the technology involved in serologic studies can be quite complex, the basis for all such studies lies in identifying either antigens (a foreign substance such as a fungal protein that causes an immune system reaction) or an antibody (the protein the body forms to fight foreign substances) in the serum of a sick pet. Most tests rely on what's known as an antigen-antibody reaction, which is the chemical binding together of these two agents whenever they come in contact. A common serology test kit may contain a paper-like substance that has been filled with a known amount of fungal antigen and a coloring agent that is activated only when an antibody-antigen reaction occurs. When a serum sample rich in appropriate antibodies is passed over the antigen fixed in the test paper, the antigen and antibody bind together, and the color is revealed. This would be a "positive" test, indicating exposure to the disease. 

A positive test is strong evidence that the organism is causing disease, but (especially in fungal diseases) it does not always confirm that the fungus has caused the pet's illness. Antibodies are produced whenever the body fights off an unusual organism. A positive test confirms the pet has seen and reacted to the fungus, but it may have successfully fought off infection rather than becoming ill. For example, where Blastomycosis is common, many dogs have positive serology tests but never become ill. A similar result is seen in vaccination: antibodies rise after vaccination and are a measure of successful immunity, not illness. Another problem with serologic tests is the possibility of cross-reaction. The body may produce antibodies that have the ability to fight more than one disease, and will thus react to more than one antigen. For example, a pet's body can creates antibodies that are capable of fighting both Aspergillus and Penicillium (a normally harmless organism). One of the serology tests used to diagnose Aspergillus (known as the counterimmunoelectrophoresis test, or CIE) will give a positive result if the pet has been exposed to either antigen. Fortunately, other tests are available to sort out specific exposure. 

Despite the fact that no serologic test is considered 100% diagnostic for fungal disease, the tests can be very helpful. Matched with the clinical signs, a positive serologic test can give a strong presumptive diagnosis, allowing treatment to begin. One of the most reliable serologic tests for fungal disease is the LCAT - the Latex Cryptococcal Antigen Test. A positive test is not only considered nearly diagnostic, but the strength of the reaction to the test often matches the severity of disease. After effective treatment, this test should become negative. 

Fungal disease can take on many forms in our pets. To review the agents of infection in a summary form, please see below. 

Author: Joanne Howl, DVM 
Editor: Walt Ingwersen, DVM, DVSc, Diplomate, ACVIM 
Web Site

Agents of Infection
Disease Species Systems affected Signs noted Transfer to humans?
Aspergillosis Dog, cat, birds Respiratory, sometimes whole body Sneezing, nasal drainage No
Blastomycosis Dog, rarely cats Respiratory, musculoskeletal, nervous, skin Coughing, draining skin wounds, seizures, fever No, but caution warranted
Candidiasis dog, cat Skin, gastrointestinal including oral cavity White cottony growths in mouth, crusty skin lesions, diarrhea No
Coccidioidomycosis Dog, cat Respiratory, skeletal, often other organs Cough, lameness No, but caution warranted
Cryptococcosis Dog, cat, human Upper respiratory, nervous, eye Nasal swelling and drainage, trouble walking, circling, seizures, dilated pupil or swollen eye Unlikely but caution warranted
Dermatophilosis Dog, cat, human Skin, oral cavity Crusty wounds on the body, growths in mouth, draining abscesses Possibly
Dermatophytosis (Ringworm) Dog, cat, human Skin Patchy hair loss, crusty skin Yes
Histoplasmosis Dog, cat, human Respiratory Cough, fever Not directly, but caution warranted
Malassezia Dog, cat Skin, ears Yellowish, greasy patches on skin or in ears, often itchy No
Rhinosporidiosis Dog Nose Sneezing, mass protruding from nostril No
Sporotrichosis Dog, cat, human Skin, may spread throughout body Draining wounds Yes
Author: Joanne Howl, DVM 
Editor: Walt Ingwersen, DVM, DVSc, Diplomate, ACVIM 
Web Site
 Types of Fungal Diseases in Canines

Most agents of parasitic fungi exist as an organism in soil, decaying vegetation and feces, and on decaying animal matter and tissues. The soil and the air is the primary source of most infections, which can be acquired by ingestion, inhalation, or even through the skin. Those fungi capable of producing infection in hosts, such as histoplasmosis, coccidioidomycosis, and blastomycosis are regarded as primary systemic mycoses. Opportunistic fungi, such as aspergillosis, and cryptococcosis, usually require a host that is debilitated, or immune-suppressed, to establish infection, but stronger strains have been doing just the opposite. Clinical diagnosis of fungal infections can only be reached by both the culture and microscopic identification of the organism. Serology may also be used as a diagnostic tool for some fungal disease. 

Aspergillosis is a fungal infection seen in mostly young dogs--in particular, those breeds with a long nose. The fungus affects the nasal chambers but lesions can occur in several organs, including the eye. More and more cases are being reported due to housing problems with toxigenic moulds. It first begins in the posterior region of the nasal cavity with a nasal discharge that may last for months before becoming purulent and bloody. Nasal pain, sneezing and lethargy accompany these symptoms, and as the disease progresses the fungus may destroy and replace the spongy nasal passages with masses of fungi growth. If not diagnosed and treated, the fungi continue to grow into the cranium and the soft tissue around the eye. Since diagnosis of the disease from culture is not always easy, microscopic evaluations of the fungus from areas of the nasal cavities and membranes is also needed for positive identification. Radiographs may show tissue destruction since frontal sinus osteomyelitis will be apparent. Treatment of aspergillosis with systemic drugs and sometimes surgical scraping of the nasal passages is involved, expensive, and of long duration. Iodine flushes have been used with some success followed by systemically administered drags such as ketoconazole, thiabendazole, or itraconazole. 

Cryptococcosis is another systemic fungal disease that affects the respiratory tract, eyes, skin, and central nervous system of dogs. The fungus exists in the environment and in tissues in a yeast form. The fungus is found in soil and fowl dropping, especially pigeons. Transmission is by the inhalation of the spores or the contamination of open wounds. Dogs, especially young dogs under one year, tend to have the infection quite severely, with the infection first showing up as lesions on the lungs, facial regions, legs, and the cerebral cavities. It rapidly spreads to their central nervous system and eyes. Organs most usually involved include the kidneys, lymph nodes, liver, spleen, thyroid, adrenals, pancreas, heart valves, tonsils, GI tract, and muscles. If the brain is involved the dog will show signs of circling, staggering, behavior changes, accompanied by sneezing, coughing, nasal and eye discharge, and in some cases, blindness. There is little success in treatment when the infection has spread to this point. 

Cutaneous lesions which have not disseminated can be removed surgically, and the dog treated with amphotericin, given IV three times a week until evidence of success is shown. The lesions show up as slimy, mucoid, and ulcerating, accompanied by tissue swelling. Diagnosis of the disease can be confirmed by microscopic examination of the fluid. Ketoconazole, itraconazole or fluconazole have also been successful as treatment. 

Canine histoplasmosis is a non-contagious systemic fungus that grows in the soil in tropical and subtropical regions, especially endemic in the Mississippi and Ohio River valleys and the Appalachian Mountain range. Bird, chicken and bat manure provide a rich environment for the fungus, but other origins have recently been reported.

Some forms of the disease are mild and self-limiting, or involve only the lungs and lymph nodes of the chest. The organisms enter the body through the respiratory tract and become disseminated throughout the body, with the signs of the disease varying, depending on the various organs involved. Many dogs have a long course of weight loss due to severe diarrhea, characterized by fresh blood, straining, and mucous, high fever, and anemia. The dog may have a chronic cough and show respiratory difficulty due to obstructions and enlargements of the bronchial lymph nodes. Dissemination may involve ocular disease, weeping ulcerated nodules on the skin, ulcerated intestinal walls, enlargement of the liver and spleen, accompanied by depression, anorexia, lameness and ocular disease. 

Presence of the fungus is determined by rectal scrapings in dogs with diarrhea or needle aspirations of the lymph nodes and lungs. The more acute and widespread the infection is, the less promising is any drug therapy, but with early diagnosis, antifungal drug therapy with ketoconazole may improve the outlook. Since Histoplasmosis is primarily a chronic diarrhea disease, the illness can progress so rapidly that not even the antifungal drug can help if treatment and diagnosis is delayed... For severe cases, concurrent treatment with amphotericin has been used with success. 

Hypoallergenic diets must contain ingredients not previously encountered by the patient and all other potential sources of offending substances should be excluded, including rawhide chews, table scraps, vitamin and mineral supplements. Response to the hypoallergenic diet rarely occurs within the first week. If the dog's skin condition improves by the twenty-first day, diet is probably at fault, and the dog will be put back on his original diet. If the animal's itchiness increases, there is no doubt about what the problem is. In this case you will need to find a diet that is both nutritionally sound and free of the offending substances. 

North American Blastomycosis is a fungal disease that is most usually found in the Mississippi River area and around the Great Lakes, along with areas in the South and Southwest where soil is moist, acidic, and rich in decaying vegetation. Rain, dew, or fog may play a critical role in aerosolizing the fungus. The primary entry is the respiratory tract by way of inhalation of spores from sites of fungal growth. For some reason the dogs most affected are usually young intact males, two to four years of age, with hunting breeds most likely to pick up the fungus than other dogs. 

The disease is characterized by lesions in various tissues with organ involvement. Lesions and nodules consist of numerous, variable sized, irregular pus-filled granulomas that multiply and ulcerate through the skin as cutaneous pustules covered with yellow scabs. The bronchial lymph nodes enlarge and the dog will show signs of a fever and respiratory problems including coughing, and a nasal discharge. Dry, harsh lung sounds from lung lesions are commonly found. The infection may also involve the urinary tract, with blood in the urine, and difficult urination. If the disease spreads to the eyes and growths develop in the eyeballs, glaucoma, retinal detachment, and blindness can result. Weight loss is evident, accompanied by anorexia, and lameness from nodular lesions on bones. 

Blastomycosis should be considered when a dog has both respiratory problems and draining nodules on the skin, along with extreme weight loss. Diagnosis can be made from radiographic findings in the lungs or from aspirated samples from lesions. This fungus does not respond well to treatment unless caught early, and the lesions removed surgically. The prognosis is best for dogs without lung disease or even with only a moderate lung disease. With the use of Ketoconazole and Amphotericin B, some symptoms can be reduced without a chance of relapse, but the treatment will require extensive veterinarian management. 

Valley Fever
Valley Fever is a fungal disease found mostly in the dry areas of the South-western U.S., and northern Mexico. Dogs are most commonly affected, although many warm-blooded animals can contact it. It is not spread from one animal to another; it is not highly contagious; and can not spread from animal to man. Most dogs develop an immunity to it and never show outward signs of illness unless stressed with a lowered immune system. 

Infection occurs by the dog inhaling spores from the soil. If infection happens, the inhaled spores burst in the lungs and start growing, usually in the lymph nodes. If the immune system does not kill the fungal organism here, they spread to many parts of the body, involving bone and eye tissue. Hunting dogs are particularly at risk of being exposed to the fungal spores, but even indoor dogs have a chance of exposure following a dust storm when billions of spores become airborne. A few years ago after a severe earthquake in the Los Angeles area disturbed dust, sand, and old buildings, several dogs were reported as having contacted the disease. However, the months of greatest exposure to the spores are May through August, when the fungi stop growing and produce spores to survive the dry, hot summer. 

Most diagnosis of Valley Fever is based on a history of travel in prevalent areas, clinical signs, and laboratory testing. Clinical signs include a chronic, dry, persistent cough, breathing problems, depression, fever, decreased appetite, lethargy, weight loss, and in some cases, blindness. The lymph nodes may be swollen and enlarged along with extreme pain and lameness as the disease involves the bones or spinal column, and enlarged joints. Since it takes several months for these signs to become apparent, diagnosis is often delayed until the disease has progressed to the point where the dog is very ill. A blood test can confirm Valley Fever and treatment started, with the drug of choice being Ketoconazole, given twice a day. This medication is usually tolerated if given with food. Continuous use is necessary for at least six months before major results can be seen. Most dogs do respond to treatment and go on to live a healthy life, if the owner can handle the expensive medications and treatments.

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