Along with viruses and bacteria, fungal organisms can cause disease in dogs. Probably the most common one pet owners are familiar with and have heard about is dermatophytosis, or ringworm. In addition, yeast infections can be a common problem in the ears of dogs. These types of yeast and fungi that affect mainly the outer skin surfaces are termed superficial mycoses.
The most prevalent fungal disease that afflicts dogs is ringworm. Three different organisms — Microsporum canis, Trichophyton mentagrophytes, and Microsporum gypseum — can actually cause ringworm. The first two are contracted from infected animals; the third, from contaminated soil. In dogs, ringworm causes patchy hair logs with or without an accompanying lesion on the skin beneath. Since humans can be susceptible to the same type of ringworm, reddened, circular lesions occurring on the owner as well might support a diagnosis. Diagnosis of ringworm is confirmed by a fungal culture. Treatment can consist of iodine or chlorhexidine shampoos and topical and/or oral antifungal medications.
In contrast to ringworm, fungal and yeast infections involving the deeper tissues of the body are termed subcutaneous or deep mycoses, depending on the depth of tissue involvement. These diseases, including sporotrichosis, aspergillosis, blastomycosis, histoplasmosis, coccidioidomycosis, and cryptococcosis, can be quite severe, and even life-threatening at times. Spores from these organisms are inhaled, are ingested, or penetrate the skin via wounds. Depending on the organism involved, clinical signs can include weight loss, coughing, breathing difficulties, draining skin lesions, tender skin masses, lameness, diarrhea, and nervous system impairment (i.e., blindness).
History, physical exam findings, and laboratory tests, including radiographic X rays, and biopsies of affected regions can lead the veterinary practitioner to a tentative diagnosis of a fungal infection within the body. Microscopic examination of body fluids or drainages for fungal spores or yeast can also be helpful. In most cases, a definitive diagnosis is made by testing a blood serum sample for antibodies against the fungal organisms in question, or, less commonly, by culturing for growth.
A number of antifungal medications are available and can be used to treat such infections in dogs. Depending on which agents are used (many are used in combination with one another), duration of treatment required is often 1 to 3 months to afford a complete cure.
Radiographs and special immunologic tests can be used to monitor the response to treatment. In many cases, surgical excision of those regions infected with the fungus can afford faster recovery. The prognosis for dogs is good when fungal infections are detected and treated in their early stages but guarded to poor if dissemination throughout the body has occurred.
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