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How to tell if dog has ringworm

Sunday, January 5, 2003


By Dr. John Koch

Question: I was watching the evening news on television a few days ago and someone from the Humane Society was talking about crusty circular round lesions on a dog being ringworm. That is exactly what my puppy has on it. I went to a vet to get some medication for it and they wouldn't give me anything. They said they needed to see the dog first because it probably wasn't ringworm at all. They said it was probably another type of infection, which I didn't understand. What is going on anyway? Were they just trying to give me the run-around?

Answer: The vast majority of crusty round circular lesions seen on dogs are not ringworm as is often thought. These lesions are actually a bacterial infection, which is totally different from ringworm. Ringworm is a fungal infection. The appropriate treatment of the two problems is also different and that is why the veterinarian wanted to see the lesions before prescribing medication.

It is true that some crusty round circular lesions are ringworm so making a diagnosis based strictly on appearance is often difficult. Sometimes a black light is used to help with the diagnosis. The most common type of ringworm spore will fluoresce while under the light. Bacterial infections will not. Most lesions suspected of being ringworm do not fluoresce. If they do not, then culturing for ringworm spores is the best diagnostic technique. The problem with culturing is that it may take up to two weeks before the culture can be accurately read. Two weeks is a little long to wait to start treatment.

Over my many years of practice the great majority of these cultures have been negative for ringworm indicating that the problem is bacterial. Because of this, in our practice we usually treat lesions negative to the black light test as bacterial. If after two weeks, the culture is positive for ringworm then we change treatment. Other veterinarians may assume a different posture for such therapy.

Skin diseases are often difficult to diagnose because the way they appear can be so deceiving. If you insist upon making your own diagnosis and then prescribing your own treatment, you do have some statistical data on your side. Approximately 80 percent will get better even if you treat with tincture of cow pie. The problem is that you also have a 20 percent chance of making a bad problem a lot worse.

Dr. Koch is a Cape Girardeau area veterinarian.

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