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Case Study: Fungal Infection

The following article is courtesy of our partner, Banfield Pet Hospital. Used with permission.

Finding the exact cause of a dog’s biting and scratching can be challenging. In the area of dermatology, many different skin diseases often look very much alike. Approaching dermatology cases requires patience, because all too often, pets can have more than one type of disease or infection, complicating the diagnosis. Approaching a dermatology case requires peeling back the layers of symptoms, primary skin disorders and secondary infections that take advantage of compromised skin.

Case Study: Fungal Infection


I can’t tell you how many times we diagnose demodex on a skin scrape, but the pet’s skin remains itchy and patchy for several months after starting treatment. Often these pets get bacterial infections as well, and they need additional antibiotics for complete resolution of symptoms. In other cases we will diagnose dermatophytosis, but the pet will also have seasonal allergies and break out every spring and summer. Without all the pieces to the puzzle, we cannot effectively treat these pets to satisfaction.

Recently we had a case present to us for a second opinion. A 6-month-old Pit Bull was examined at a private vet for alopecia (hair lost from some or all areas of the body) and itchy skin. They performed a skin scrape which was negative for mites and an impression smear which was positive for bacteria. The pet was given the appropriate antibiotic for the appropriate length of time, but he remained itchy. Then they performed allergy testing which came back negative.

The owner stated that the vet was out of ideas and she just wanted the pet to stop itching. Since the pet came to us on the weekend, we were unable to get his medical records. However, I explained to the owner that it would be a good idea to start with the basics, repeat some tests, and see what is on the pet’s skin now, as sources of infection can change over a short period of time.

We started with the black light, and my, oh my, how the pet glowed! The trunk and neck had green-apple flakes throughout the non-haired areas. In addition, the toenails and nail beds were a bright apple-green color.

Apple-green glow from a black light can indicate a possible fungal infection often referred to as ringworm. Unfortunately, not all skin lesions that glow green are caused by fungus and not all fungal infections glow green on the black light test. However, we knew to pay close attention to the rest of the skin tests.

The skin scrape confirmed that there were no Demodex mites, but we still kept in the back of our minds the possibility of a Sarcoptic mite infection that is very difficult to detect on skin diagnostics. The impression smear revealed no yeast or bacteria. However, we scrutinized it very closely, and there, like a needle in a haystack, we saw a lone dermatophytosis spore (often referred to as “canoes” based on their appearance, which differentiates them from non-disease-causing fungal species).

We continued with the fungal culture, which would take 10 days to complete and would confirm a dermatophytosis infection. We also performed the thyroid screen. Some pets with low thyroid levels are very susceptible to secondary skin infections from bacteria, yeast, fungus, and even mites. Without checking that level, the pet could relapse very quickly with another skin infection soon after resolving the present one — very frustrating for owners!

Thankfully, the pet had a normal thyroid level. We gave the owner the good news about the fungal infection and the pet is responding very well to medicated shampoo and anti-fungal systemic medication. Recovery will take several months as the fungus is rooted deep in the nail beds, but we know we are on the right track!

The owner was relieved, for without this diagnosis, the next step would have been a skin biopsy and histopathology — a step she was not financially ready to take and would most likely have come back from the pathologist as inconclusive.

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