Funguses (also called fungi) are parasitic, spore-producing organisms. They obtain their nourishment by absorbing food from the hosts on which they grow. Many species of fungus exist in the environment, but only a very few cause infections. The primary source of most infections is soil. Fungal infections can be acquired by inhalation, ingestion, or through the skin (for example, through a cut or wound).
Some fungal infections can cause disease in otherwise healthy animals, while others require a host that is incapacitated or immunocompromised (by, for example, such stresses as captivity, poor nutrition, viral infections, cancer, or drugs like steroids) to establish infection. Prolonged use of antimicrobial drugs or immunosuppressive agents appears to increase the likelihood of some fungal infections. The infection itself may be localized or may affect the entire body (systemic or generalized).
Aspergillosis is a fungal infection caused by several Aspergillus species. It is found worldwide and in almost all domestic animals as well as in many wild species. It is primarily a respiratory infection that may become generalized; however, the susceptibility to fungal infections varies among species.
In dogs, aspergillosis is typically localized to the nasal cavity or paranasal sinuses and is usually caused by infection with Aspergillus fumigatus. Nasal aspergillosis is a relatively common disease in dogs. The disease usually remains confined to the nasal cavity or the sinuses, but it causes destruction of the delicate nasal bony structures known as turbinates. Occasionally, a very invasive infection may affect the orbit of the eye and the skull. Most cases of nasal aspergillosis occur in dogs with normal immune systems that are in otherwise good health.
Nasal aspergillosis is mainly seen in breeds of dogs with long, narrow heads and noses (such as Collies and Greyhounds). Signs of infection include lethargy, a bloody nasal discharge, nasal pain, sneezing, ulceration surrounding the nostrils, and nosebleed. Your veterinarian will perform a number of tests before providing a diagnosis. Tests may include x-rays, computed tomography (CT scan), endoscopy of the nose, and laboratory tests. Most cases of nasal aspergillosis can be successfully treated with antifungal drugs, which are typically infused into the nasal cavity. Oral antifungal drugs may also be recommended. Relapses are possible, but are uncommon.
Disseminated aspergillosis, a more generalized form of infection, is more common in German Shepherds than in other breeds. Aspergillus tereus, Aspergillus deflectus, and Aspergillus niger are the most common species of organism responsible. The fungus likely enters through the respiratory tract and goes to the lungs, then travels through the bloodstream where it spreads throughout the body, including the intervertebral discs of the spine or the kidneys. Other organs, muscles, or bones may also be affected.
Disseminated aspergillosis usually takes several months to develop. Signs of this disease may include neurologic signs, such as back pain progressing to partial or complete paralysis or lameness of a limb with pronounced swelling. Other, less specific, findings include poor appetite, weight loss, muscle wasting, fever, weakness, lethargy, limping, vomiting, blood in the urine, urinary accidents, and lymph node enlargement. Severely ill dogs have a poor prognosis. Most dogs die from the disseminated aspergillosis although a few have been treated successfully.
Candidiasis is a localized fungal disease affecting the mucous membranes, the skin, and the gastrointestinal tract. It is distributed worldwide in a variety of animals and is most commonly caused by species of the yeast-like fungus, Candida albicans. Candidiasis is rare in dogs. Factors that may predispose an animal to infection include injury to any of the mucous membranes, the use of catheters, administration of antibiotics, and immunosuppressive drugs or diseases.
Signs of infection are variable and nonspecific (for example, diarrhea, weakness, skin lesions) and may be associated more with the primary or predisposing conditions than with the candidiasis infection itself. An ointment or topical application may be useful in the treatment of oral or skin candidiasis. Your veterinarian may also recommend different drugs given by mouth or through the vein for treatment of infected dogs.
Coccidioidomycosis (Valley Fever)
Coccidioidomycosis is a dustborne, noncontagious infection caused by the fungus Coccidioides immitis. Infections are limited to dry, desert-like regions of the southwestern United States and the valleys of southern California. Similar geographic areas of Mexico and Central and South America are also affected. While many species of animals, including humans, are susceptible, only dogs are affected significantly. Inhalation of fungal spores (often carried on dust particles) is the only established mode of infection. Epidemics may occur when rainy periods are followed by drought, resulting in dust storms. Coccidioidomycosis is primarily a chronic respiratory disease, but canine infections can spread throughout the dog’s body infecting many tissues, especially eyes, joints, and bone. Signs can vary greatly, depending on which organs are involved and the severity of infection. Dogs with disseminated disease may have a persistent cough, poor appetite, general ill health and malnutrition, lameness, enlarged joints, fever, and intermittent diarrhea. The skin may become ulcerated. Many dogs exposed to the fungal spores will fight off the infection without showing any signs at all.
Coccidioidomycosis is diagnosed using x-rays and laboratory tests. In some dogs that develop signs, the disease will resolve without treatment. However, if chronic respiratory signs occur or the infection involves many body organs and tissues, longterm antifungal treatment is needed. In cases where the infection has spread, treatment of at least 6 to 12 months is typical. With treatment, the likelihood of recovery from the primary (respiratory) form is quite good. Recovery in the disseminated case varies with the location and severity, and must be considered guarded. There is no known prevention other than decreasing your pet’s exposure to desert soil and dust in areas where the fungus occurs.
Cryptococcosis is a fungal disease that may affect the respiratory tract (especially the nasal cavity), central nervous system, eyes, and skin. The causal fungi, Cryptococcus neoformans and Cryptococcus gattii, are found worldwide in soil and bird manure, especially in pigeon droppings. Transmission is by inhalation of spores or contamination of wounds. Immunosuppressed humans and animals are at increased risk for developing cryptococcosis.
In dogs, the disease often spreads throughout the body, with central nervous system or eye involvement. Signs are most commonly related to dysfunction in these areas. Central nervous system signs include head tilt, the back-and-forth eye movements called nystagmus, paralysis of the facial nerve leading to inability to blink, poor coordination, circling, and seizures. Eye problems may include bleeding in the retina and inflammation of the eye. The disease is also often found in the nasal passages and the lungs. Other tissues that may be affected include the kidneys, lymph nodes, spleen, liver, thyroid, adrenal glands, pancreas, bone, gastrointestinal tract, muscles, heart, prostate, and tonsils.
Cryptococcosis is diagnosed with laboratory tests, usually by taking samples of the affected area or any discharge. Tests may also be performed on urine, blood, or cerebrospinal fluid. Various antifungal drugs may be prescribed for the treatment of cryptococcosis; but most affected pets require prolonged treatment (up to several months), depending on the severity and extent of the disease. Treatment for cryptococcosis may include surgery to remove lesions in the nasal cavity or on the bridge of the nose. The outlook for recovery is poor when infected dogs have widespread nervous system involvement.
Geotrichosis is a rare fungal infection caused by Geotrichum candidum, a fungus of soil, decaying organic matter, and contaminated food. This fungus is part of the normal flora of the mouth and intestinal tract in humans. The organism can cause generalized disease in dogs. Signs of infection vary based on the organs involved and may include coughing, fever, poor appetite, excessive intake of water, progressive difficulty in breathing, vomiting, and jaundice (yellow appearance of the skin and mucous membranes). The disease may progress rapidly and is usually fatal.
Treatment for disseminated geotrichosis may be attempted with antifungal drugs but most often is not successful.
Histoplasmosis is a noncontagious infection caused by the fungus Histoplasma capsulatum, which is found worldwide. The organism responsible for the disorder is a soil fungus that is widely distributed (particularly by bird and bat populations) in the midwestern and southern United States, especially in river valleys. Infection occurs when spores in the air are inhaled. The lungs and the lymph nodes in the chest are the sites of primary infection, although the gastrointestinal tract may be affected in dogs. The organisms enter the bloodstream from the primary site of infection and become dispersed throughout the body; they may localize in bone marrow or the eyes.
The signs vary and are nonspecific, reflecting the various organ involvements. Chronic digestive signs, especially diarrhea, are usually most obvious in dogs. Many dogs have a long course of weight loss (which can progress to emaciation); coughing; fever; anemia; enlarged liver and spleen, or lymph nodes; and digestive ulcers. Difficulty breathing, skin sores, joint disease, and eye conditions can also be seen. Sudden histoplasmosis can cause death in 2 to 5 weeks. Diagnosis requires identification of the fungus in body fluids or tissues. Blood, urine, or cerebrospinal fluid tests may also be used.
Treatment of disseminated histoplasmosis is difficult. It requires the use of antifungal drugs and supportive treatment such as adequate nutrition, additional liquids (hydration), and control of secondary bacterial infections. Antifungal treatment must be continued for prolonged periods of time and may be expensive.
Mycetomas are infections of the skin and underlying tissues that have the appearance of nodules or tumors. When such lesions are caused by funguses, they are known as eumycotic mycetomas. The fungus proliferates in the lesions and organizes into aggregates known as granules or grains. Granules may be of various colors and sizes, depending on the species of fungus involved.
Most eumycotic mycetomas are confined to the tissue beneath the skin, but some may be extensions of fungal infections in the abdomen. Inflammation of the membrane lining the abdominal walls or abdominal masses is typically seen with cases of white-grain mycetomas. Black-grain mycetomas are usually characterized by skin nodules on the legs and feet or on the face. When the feet or limbs are involved, the infection may extend to the underlying bone.
A mycetoma is diagnosed by identifying the fungus within its contents. A tissue biopsy may be necessary. The outlook for abdominal mycetomas is guarded because tissue involvement is usually extensive. Skin mycetomas, while not life-threatening, are often difficult to resolve. Surgical removal, including limb amputation, may be the only effective treatment for some cases of skin mycetomas. Antifungal medication has been reported as effective in only a few cases.
North American blastomycosis, caused by the fungus Blastomyces dermatitidis, is generally limited to geographically restricted areas in the Mississippi, Missouri, Tennessee, and Ohio River basins and along the Great Lakes and the St. Lawrence Seaway. It also occurs in the Pacific Northwest. Beaver dams and other habitats where soil is moist, acidic, and rich in decaying vegetation may serve as the ecologic niche for the organism, which has also been found in pigeon and bat feces.
This fungus exists in 2 different forms: a mycelial form (this form is present in the environment and is contagious) and yeast (this form is found in the tissues and is not contagious). The mycelial form of blastomycosis can easily infect both animals and humans. When respiratory defenses are overwhelmed or immunosuppressed, the infection spreads from the lungs through the bloodstream. Infection can occur through the skin, but spread from a lung infection is much more common.
Young male dogs, especially hunting dogs, are at increased risk presumably because of increased contact with contaminated soil. The prognosis depends on the extent and severity of lung involvement. In dogs, blastomycosis most commonly affects the lungs (up to 85% of cases), eyes, skin, and bones.
Most affected animals have signs such as coughing, fever, lethargy, loss of appetite, and weight loss. In addition to the cough, lung involvement leads to exercise intolerance and difficulty breathing. The animal’s peripheral lymph nodes often are enlarged. These are found under the neck, in the shoulder region, and behind the knee. Bone involvement may cause pain that results in lameness. Infection of the urogenital tract (for example the prostate gland in male dogs) occasionally may occur and cause signs like blood in the urine or difficulty urinating. Eye involvement can lead to pain, light sensitivity, and glaucoma. Involvement of the retina may lead to blindness. Multiple draining nodules may be found in the skin.
Your veterinarian may suspect blastomycosis based on your dog's signs of illness, especially coughing, trouble breathing, or draining skin nodules. Chest x-rays are often performed. The diagnosis can be made by identifying the fungus in affected tissue or with laboratory blood or urine tests.
Treatment of blastomycosis is based on the severity of the condition and other factors that must be evaluated by a veterinarian. Treatment is aimed at relief of specific signs (such as difficulty breathing, coughing, or eye problems) and elimination of the fungus from the body. Treatment may include one or more antifungal drugs, which are given for an extended period (2 months or more) until active disease is not apparent. The infection can recur in about 20% of treated dogs. The recurrence can occur months to years after treatment. Most dogs will again respond to treatment. The outlook is best for dogs without severe lung disease. The outcome is more guarded for dogs with moderate to severe lung disease, and is poorest for dogs with central nervous system involvement.
Oomycosis (Pythiosis and Lagenidiosis)
Oomycosis (also called pythiosis) is a disease caused by Pythium insidiosum, which is not a true fungus but a water mold related to algae. It occurs in some tropical and subtropical areas of the world and is seen in warmer to temperate sections of the US. In dogs, pythiosis is most often encountered in Southeast Asia, eastern coastal Australia, South America, and the United States, especially along the Gulf coast. In the US, the disease most often is seen in fall and winter months.
These organisms cause disease when animals come into contact with the infecting zoospores in water, usually by ingestion. Pythiosis most often affects the gastrointestinal tract in young adult dogs, especially Labrador Retrievers. The usual site of infection is the stomach and the adjacent intestinal tissues, but any part of the gastrointestinal tract may be infected. Affected dogs often will have a history of upper gastrointestinal tract obstruction and may have an abdominal mass the veterinarian can feel during examination. Common signs include vomiting, weight loss, and loss of appetite. The weight loss can be severe, but affected dogs usually do not appear generally ill until the infection is well advanced.
Less frequently, dogs will have a pythiosis infection concentrated in the skin and adjacent tissues. A wound has usually allowed the entry of the infective zoospores into the skin tissues. Many of these animals have a history of swimming or being in “swampy” areas. Non-healing, tumor-like nodules may be present. They often grow rapidly. Some lesions may have a focused area of dead cells and a spongy appearance. Such lesions are usually on the legs, neck, near the place where the tail connects to the body, or on the perineum (the space between the anus and scrotum (male) or vulva (female)). They can, however, be seen anywhere on the body.
Lagenidiosis is a disease involving the skin, the area beneath the skin, and multiple parts of the body, most often the legs, mammary glands, perineum, or trunk. It has been recently reported in dogs from the southern United States. Lagenidiosis also is caused by a water mold (Lagenidium species). There are at least two species of Lagenidium that affect dogs, one of which progresses quickly. The slower developing form of lagenidiosis is very similar to the skin form of pythiosis. The faster developing form can involve multiple, organs across the body. Internal lymph nodes, the lungs, and large blood vessels (such as the aorta or vena cava) may be affected. This can lead to large bulges in the wall of the blood vessels (called aneurysms) that can rupture and cause sudden death.
Dogs exposed to warm, standing fresh water are more likely to be in contact with the infectious zoospores and may have increased risk for both pythiosis and lagenidiosis. Reports of animals being infected with no known history of being near water suggests that animals may be infected by contact with resistant spores that form in wet soil and on grass.
The diagnosis of lagenidiosis and pythiosis is made with laboratory tests including cultures and blood tests. The treatment of choice for pythiosis and lagenidiosis has been aggressive surgery to remove infected tissue, but the disease is often too extensive at the time of diagnosis to allow complete removal. Whenever possible, complete amputation of affected limbs may be curative. Your veterinarian may recommend surgery followed by treatment with antifungal drugs. Approximately 20% of dogs will respond to longterm antifungal treatment. For most infections the outlook is guarded to poor, even though there have been advances in treatment. In some dogs, a therapeutic vaccine (one given to treat a disease rather than prevent it) has helped cure the condition.
Phaeohyphomycosis is a general term for an infection by any of a number of funguses of the family Dematiaceae. This type of fungal infection is uncommon in dogs. The funguses known to cause phaeohyphomycosis have been recovered from decaying vegetative matter, water, and soil all over the world.
Infection may result from fungal implantation into tissue at the site of an injury. Signs of infection are uncommon and almost invariably occur in animals that are sick or that have weakened immune systems. The most common signs include ulcerated skin nodules, and nasal or paranasal masses. Slowly enlarging masses or nodules beneath the skin are found on the head, linings of the nasal passages, limbs, and chest. In most cases, the infection is confined to the skin and tissues beneath the skin. Diagnosis is made by identifying the fungus in tissue or fluid samples.
Phaeohyphomycosis is generally not very responsive to treatment. Surgical removal of the lesion, followed by antifungal drugs, can be a cure. Treatment with antifungal drugs may be considered in cases when surgery is not possible. In dogs being treated with drugs that suppress the immune system, the outlook may be better if the drugs can be discontinued.
Rhinosporidiosis is a longterm, nonfatal, infection, primarily of the lining of the nasal passages and, occasionally, of the skin. It is caused by the fungus Rhinosporidium seeberi. Uncommon in North America, it is seen most often in India, Africa, and South America.
Infection is characterized by polyp-like growths that may be soft, pink, crumbly, lobular with roughened surfaces, and large enough to obstruct or close off the nasal passages. The skin lesions may be single or multiple, attached at a base, or have a stem-like connection.
Diagnosis is made by identifying the fungus in affected tissue. Surgical removal of the lesions is considered to be the standard treatment, but recurrence is common.
Sporotrichosis is a sporadic, longterm disease caused by Sporothrix schenckii. The organism is found around the world in soil, vegetation, and timber. In the United States S. schenckii is most commonly found in coastal regions and river valleys. Infection usually results when the organism enters the body through skin wounds via contact with plants or soil or penetrating foreign objects, such as a sharp branch. Transmission of the disease from animals to humans can occur.
The infection may remain localized to the site of entry (involving only the skin) or may spread to nearby lymph nodes. Although generalized illness is not seen initially, longterm illness may result in fever, listlessness, and depression. Rarely, untreated or mistreated infections will spread through the bloodstream or tissue from the initial site of inoculation to the bone, lungs, liver, spleen, testes, gastrointestinal tract, or central nervous system. This widespread form of sporotrichosis can be life threatening.
Cultures are typically necessary to diagnose the disease in dogs. Longterm treatment with antifungal drugs (continued 3 to 4 weeks beyond apparent cure) is usually recommended. Because sporotrichosis can be passed from your pet to you, strict hygiene must be observed when handling animals with suspected or diagnosed sporotrichosis. If your pet is diagnosed with sporotrichosis, be sure to ask your veterinarian about the precautions you and every member of your family should take while your pet is ill.