Q: A skin condition has affected...

Q: A skin condition has affected three of our dogs during the last 18 years. Each successive dog has had its underbelly, hind legs, under-the-tail area and scrotal region change in color from pink to a blackish color. I asked two veterinarians about this, but neither was able to explain it. Do you know what could be causing such a color change? J.N.

A: You gave me few details of the medical history of the color changes, so I can only make an educated guess. The condition could be an allergic contact dermatitis. Many household items have been found to cause such an allergy in dogs--dyes, plastics, wool or nylon carpeting, rubber, flea collars, cleaning agents, petroleum products, plants such as poison oak or ivy, and some skin medications. The areas usually affected are those with sparse hair, areas in direct contact with the agent that causes the allergy. First, the area becomes red and inflamed, but you may have missed seeing that stage. When the condition becomes chronic, the skin thickens and blackens, and hair loss may occur.

Treatment involves avoidance of the agent causing the reaction, and that will require some investigation. You’ll have to determine what agent each successive dog has been exposed to--inside the house or outdoors. There are other causes of skin darkening in dogs, but these do not fit the description provided. Chronic flea-allergy dermatitis, for instance, leads to skin darkening and hair loss, but the area first affected is on the back and the top of the tail. Hypothyroidism--with the neck and back affected first--also leads to skin darkening, but it is unlikely that you would have three dogs afflicted by that condition unless they were closely related to each other. Dr. Clarke welcomes pet-care questions for use in this column, but regrets she cannot answer mail personally. Send your questions to Pet Doctor, Home magazine, Los Angeles Times, Times Mirror Square, Los Angeles 90053.