Cat has glowing scabs, negative ringworm test. Is it false negative?

Question

My cat has lesions on his ears and head that the vet and I both thought were ringworm. The culture came back negative and she said she didn’t see anything w her wood’s lamp. I have a black light and his scabs on his ears glow a pale yellow-green color. The vet says she didn’t see anything and what I use isn’t a woods lamp and the culture is negative, it’s probably an allergic reaction. Is it possible the scab is just glowin and it’s not ringworm? I’m concerned the culture was a false negative. Pet's info: Cat | Bengal | Male | unneutered | 4 months and 23 days old | 6 lbs

Dr. B. DVM, DACVIM (Oncology) · Accepted Answer

It is very common for scabs/debris around the site to glow a bit under the lamp. A positive result would be fluorescent hairs from a metabolite the fungus secretes on to the hairs. Unfortunately only half of the strains of the most common cause of ringworm will glow however. A DTM medium fungal culture is a good way to diagnose ringworm but false negatives are always possible with any test. DTM medium is nice because it contains a color indicator as well. If the index of suspicion is high for ringworm another culture could be performed making sure to pluck hairs from many different sites. It is also possible that this is not ringworm but some other skin related issue of which there are many (secondary infections (bacteria, yeast), allergy (food, hypersensitivity), etc. I hope his helps and thank you for using Petco Pet Education Center, formerly Petcoach!

Cat has glowing scabs, negative ringworm test. Is it false negative?

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