Dogs with yeast infections are usually brought to a veterinarian with a history of intense skin itchiness, with accompanying scratching and chewing at affected areas. The most common cause of canine yeast skin infections is a fungus called Malassezia pachydermatis, which can be tricky to diagnose definitively. A yeast infection should be considered in any dog presenting with extreme itchiness. Initially, the veterinarian will perform a thorough physical examination, checking the dog from nose to tail tip and everywhere in between, paying special attention to areas of skin that are red, scratched, oozing, flaky, greasy, bumpy or smelly. If the dog does have a yeast infection, its ears will almost always be affected. The veterinarian will note any areas of patchy hair loss. He probably will ask the owner a string of questions about the dog’s overall health, diet, vaccination status, travel history and exposure to other domestic and wild animals. The initial database often includes taking blood and urine samples for a complete blood count (CBC), serum biochemistry profile and urinalysis. The results of these tests can help identify internal diseases that may predispose the dog to developing yeast infections, although they won’t identify a yeast infection itself.
The single most important diagnostic procedure for dogs with skin infections is to take samples from affected areas and evaluate them under a microscope, looking for evidence of yeast, other fungi, bacteria, viruses, mites or other organisms that may be responsible for or contributing to the dog’s condition. This process is called cytology. Direct impressions can be taken by pressing a glass slide onto skin sores or by pressing acetate tape to the lesions and then applying the tape to the surface of a clean microscope slide. The slides are examined microscopically, with or without special stains and using either high power (40x) or oil immersion (100x), to look for the budding forms of yeast. Samples of waxy debris from infected ears can be taken with a cotton swab, rolled on a glass slide and examined in the same way to identify yeast, bacteria, mites or other possible causes of the dog’s discomfort. Deeper skin scrapings, and bacterial or fungal culture on specific growth media, can be used to identify other bacterial or fungal organisms and certain external parasites, such as Demodex mites.
Skin biopsies can be taken and examined by a laboratory process called histopathology. Unfortunately, while histopathologic findings may suggest the presence of yeast, they are not always diagnostic. Various types of allergy testing, including food allergy trials, are available. More advanced blood tests can be conducted to assess the integrity of the dog’s endocrine system.
If a dog’s ears are infected, it is extremely important to figure out whether the eardrums (tympanic membranes) are intact, before any liquids, gels, cleansers or other medications are applied. If one or both eardrums are ruptured, putting those products into the ear canals can damage the sensitive structures of the middle and inner ears.