Ringworm/Dermatophytosis In Dogs And Cats

Dermatophytosis is a condition of dogs and cats (or people, or other animals) having skin, nails or hair infected by certain fungi.

The condition is commonly called by the ancient term “ringworm,” because, centuries ago, people thought there was a worm living in the circular lesions the fungus sometimes produces. There is no worm associated with ringworm infection.

This is a Dermatophyte Test Medium kit, brand new.
This is a Dermatophyte Test Medium kit, brand new.

“Dermato” refers to the skin and its adnexa. “Phyto” means plant, and fungi may be broadly categorized as plant material. “Osis” refers to an abnormal or diseased condition.

Dermatophytes cannot live in healthy skin. They thrive in and feed on dead surface skin cells and hair, which is a fairly inactive structure which shares little or no body defense mechanisms.

Skin fungi fall into a number of categories, but there are three main ones that affect dogs and cats.

Trichophyton mentagrophytes is a soil-based organism, but is right at home on your dog, your cat, or your self. It is likely to affect animals whose skin and/or hair is stressed by disease or lesions. Light abrasion would create good conditions for a skin-infecting fungus to invade. Likewise, if the body’s immune system is compromised the organism could more easily effect entry. If a pet with an abrasion lay in a grassless area of the yard where T. mentag was growing, that pet could easily become infected.

This is a DTM kit with a negative test result.
This is a DTM kit with a negative test result.

Microsporum gypseum is another fungus that can infect mammals, but it is more geophilic (soil-loving) than T. mentag. Similar conditions would be conducive to infecting your pet as described above.

Microsporum canis, on the other hand, is obligated to live on animals. This is the fungus you or your child is most likely to get from cuddling with a new pet in the household, at the pet shop or in a neighbor’s home.

Fungi can also be transmitted by fomites, such as combs, brushes, collars, harnesses and leashes. Non-porous fomites may be disinfected with a 1:9 dilution of household chlorine bleach and water. Porous items, such as nylon-webbed collars, should be discarded if they are suspected to be carrying dermatophytes.

Veterinarians test for dermatophytes with, what else, dermatophyte test medium, or DTM. DTM is agar infused with nutrients dermatophytes like, so they grow quickly when infected hair or scales of dead skin are applied to the culture. In addition, DTM uses a color indicator, phenol red, to indicate the growth of dermatophytes, as well as some other fungi. When dermatophytic fungi metabolize, they produce an alkaline (elevated) pH. Phenol red, then, responds by changing the color of the culture medium from its natural yellow to pink, then red as these metabolites appear. While this is supposed to be diagnostic of dermatophytes, it’s actually diagnostic only of a pH change. If, for example, a dog had some high-pH product applied to it right before the culture was taken, and the person preparing the sample didn’t clean it first, a false pH change could occur.

Note the strong positive phenol red color change and presence of fungi.
Note the strong positive phenol red color change and presence of fungi.

Far more accurate for diagnosis is microscopic evaluation of the culture. When a fungus begins to grow on a DTM, a sample can be placed on a microscope slide, then stained, and examined under low to medium power. Having studied the structures of fungi, your pet’s doctor can then determine whether he has grown M. gypseum, M. canis, T. mentagrophytes, some other dermatophyte, or a saprophyte. Saprophytes are non-pathogenic fungi which are simply on the skin, consuming dead skin cells, hair and debris.

Treatment is aimed at removing the fungus from the body. Small, localized lesions may be treated locally, using an ointment. Widespread fungal infection can be treated with special dips, most of which are harmless to the patient but smell perfectly terrible. Systemic treatment is often used when large areas of the body are affected, but have the disadvantage of working from the inside out. As these fungi are zoonotic, the fungus on the outside continues to be able to infect family members, visitors and other pets until the medication reaches the outer layers of the body. This usually takes about 30 days. Therefore, pets administered oral medications for fungal infections of the skin usually have some form of topical treatment also.

Rare is the case that cannot be cured. Treatment may be long and difficult, but it pays off in time.



  1. Topical antifungals are cornerstones of dermatophyte infections. Available over-the-counter, they are effective, efficient and safe; there are very few undesired effects and unlikely to “stop-up” (occlude) pores.

    • All true, LF, but the reader’s pet’s diagnosis was in question as to whether it even had a dermatophyte problem, and the “topical” it was being treated with was coconut oil and camphor oil. The use of a sticky, oily topical on skin that might have had bacterial infection was my biggest concern. Thanks for reading, Dr. Randolph.

  2. Thanks, Roxanne. Always worry for every pet because each pet is unique for his owner. My lab is doing fine. Hope your beagle is in good health.

  3. Hello Doctor Randolph, I am so glad to find your site. My 8 month old beagle has Dermatophytosis and reading through this article and correspondence with Bijal (thank you as well!) is very helpful. Thank goodness for doctors like you! Roxanne

    • Roxanne, I’m glad you found the information helpful and I hope your beagle is making a quick and full recovery. If you search for “Beagle” in the search pane on any MyPetsDoctor.com page you can read about Beagles (Sam and Blossum) I’ve owned as well as Beagles I have treated. Best wishes, Dr. Randolph.

  4. Thanks for the response, Sir. He is perfectly fine. Very playful and active. His coat is getting well. The spots from which hair fell are growing. But the skin dryness is occuring. He has no itching problem. I’m checking his skin regularly and brushing daily.

  5. Sir, again some information please. The Veterinarian continued the treatment for another five days. and now it has been completed. My dog has improved a lot. But something for your kind knowledge, maybe it can be my conclusion. That besides this treatment, I’ve used coconut oil mixed with camphor and applied all over the affected area daily and medicated shampoo bath twice a week with “Neem” (I dont know English name) leaves boiled water. A homeopathy tab. sulphur twice a day for five days. He recovered a lot. Now the new spots are very less or none. But I’m applying the coconut oil regularly. Is it ok? Or should I go with Cephalexin tablet more for seven days?

  6. Sir, my dog’s treatment is going on. He is recovering but today I noticed some spots again. Is it a situation to worry? He is playful, healthy and energetic. How will it take time to recover fully? Can this infection catch him again frequently? How should I take care?

    • I didn’t want to put myself in the position of telling your veterinarian what to do, but I was concerned when you mentioned that your dog was scheduled for just five days of Cephalexin. We commonly treat such cases for no less than 3 weeks and sometimes the worst cases may get two months of antibiotics, or even more. Ask your pet’s doctor about extending the time period of the medicine, as well as asking whether she has access to 10% benzoyl peroxide shampoo for treating pyoderma. Thank you for the update. We are truly eager for your dog to be well and you to be at peace. For instructions on how to use medicated shampoos go to http://www.mypetsdoctor.com/pet-bathing-technique-and-frequency .

  7. Thanks for your response sir. I’m from Delhi, India. I’ve changed Veterinarians and the new doctor clarified it to be dermatitis and dispensed CEPHALEXIN tablets for 5 days with vitamin supplements. She said that it will be ok but also said that there is a tendency to develop this allergy frequently, once it happens to a dog. So we should have to take more care. Next visit is due after a week. Lets hope for good. My dog’s coat is looking good but if these symptoms don’t stop then it will create a problem. He is getting bald patch where these spots occur. Eager to hear from you. Thanks again.

    • It sounds like you’re on the right track. Don’t be disappointed if you need to give the antibiotics for a longer time. It is not unusual for us to administer Cephalexin for 1-2 MONTHS so as to completely knock out infections. If your veterinarian is able to determine what your pet is allergic to she may be able to help you reduce relapses. Thank you for the update, Dr. Randolph.

  8. My 3 year old black male lab developed fungal dermatitis around his neckline first. Now it has spread over hip area also. The veterinarian treated him with shots of Prednisolone Acetate suspension scabisole injection daily with Amoxicilin orally with LIVCET tab. The infection spread very fast within 3-4 days. It seems that it caught him in a garden area where he used to play and sit in humid, wet weather. The veterinarian says it is non-recoverable. I’m so worried. Should I change Veterinarians? Suggest some remedy please.

    • Bijal, I am so sorry to hear that your lab is so ill. Dermatophytosis is rarely, almost never, a life-threatening or untreatable disease unless the immune system has experienced total collapse. Therefore, I suspect your dog’s condition is a systemic fungal infection such as coccidioidomycosis, blastomycosis, histoplasmosis (there are many more). Corticosteroids (such as prednisolone acetate) are not a usual medication for treating fungal infections, so I wonder if there might be a miscommunication on the actual diagnosis. Biopsy of the affected area might be appropriate, as well as bacterial and fungal culture if the offending organism hasn’t been specifically identified. A second opinion certainly isn’t going to hurt, and, if you have access to a board-certified internist and/or dermatologist (such as at a teaching hospital or referral center) they might be better qualifed to determine the actual cause of the problem. Please be in touch and keep us up to date with progress reports. Best wishes, Dr. Randolph.

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