Q: My son has a gerbil about 2 1/2 years old that has developed a knot at his neck. It is off to one side. It has gotten quite large. He does sleep a lot more than he used to and has lost a lot of weight. He still eats and drinks and chews and, when we take him out of the cage, he is still very active. What could this knot be? Could it be a tumor? Is there anything we can do for him? He doesn't act as though he is in pain unless the knot is touched.

A: Since your gerbil is showing other signs of disease, sleeping more and losing weight, it is likely this “knot” is associated with these signs. If so, the knot may be an infection or cancer. If you visit a veterinarian, in most cases, he or she can tell what this is by taking a few cells from the knot and looking at those cells under a microscope. If this is an infection, it could possibly be treated with an antibiotic, so it is worth bringing your gerbil to a veterinarian. If this knot is not an infection or cancer, your veterinarian may be able to tell you that, too.