Expert: Dr. Louis N. Gotthelf - 9/27/2009

Question
My yellow lab had a severe allergic reaction to what we think were red ant bites a couple of months ago. We had to give her prednisone for the reaction. I have noticed that since then the skin in her arm pits and on her belly is turning black. Could this be due to the allergies or a secondary infection? What questions should I ask the vet when I take her in?

Answer
In those areas, I would think of chafing causing the skin to thicken and pigment in her armpits.  Dogs that have skin problems often pigment because there is a very close similarity between 2 hormones that are secreted from the pituitary gland.  In response to the allergy, the brain produces the Adrenal Corticotopic Hormone (ACTH), which is very similar to a pigment producing hormone called melanocyte stimulating hormone (MSH), which makes the pigment in the skin grow. Some dogs with low thyroid hormones will do that.