I have a 9 week old Chihuahua puppy who has patches of where he is losing hair. I took him to the vet 5 days ago. He checked him under a black light and scraped his skin and examined for mange. Nothing positive showed up but the patches are more prevalent. They are about the size of a pencil eraser and round and not getting larger. The patches are not on his face but on the sides of his body and on the top part of his legs. Behind his ears there is some dark coloration. My baby is not itching and seems very happy and comfortable. He is on a healthy diet and is growing. Do you have any suggestions as to what this may be?
Patchy alopecia is a hard one to figure out sometimes. Your vet did the right things looking for ringworm and for demodectic mange, which are the two most common presentations. Did he do a skin scraping for the mange? Other wise, hormones or food can also play a part in something of this nature. He is really too young for hormones to cause this. What kind of a diet is he on? Is he eating just puppy food? Did you just get him from the breeders? Even if the patches didn't flourese for ringworm you might want to still ask your vet to treat him for it anyway. It doesn't always show up under the Wood's Lamp. Both conditions, ringworm and demodex can show up as round patches but ringworm is almost always a really round circle-hence the name. Demodex can be more irregular in its patches. Neither one might be red and neither one ALWAYS show up on the face or ears. Here is a more definitive description:
What that means is that the Woods Lamp is not the end all for determining whether or not it's ringworm. Now here is a description of the lesions: Lesions in dogs are classically alopecic, scaly patches with broken hairs. Dogs may also develop regional or generalized folliculitis and furunculosis with papules and pustules. A focal nodular form of dermatophytosis in dogs is the kerion reaction. Generalized ringworm in adult dogs is uncommon and is usually accompanied by immunodeficiency, especially endogenous or iatrogenic hyperadrenocorticism. Differential diagnoses in dogs for classic ringworm lesions include demodicosis, bacterial folliculitis, and seborrheic dermatitis. In dogs it is usually found as scaly patches with broken hairs. I would get a re-check and ask for a culture. Let me know what they find.