Q. What's the difference between flea control that I can buy at the pet store or grocery store versus buying it from my veterinarian?

A. Until 10 years ago, the best products available for flea control were pyrethrins and permethrins. Because they didn't last very long, we had to treat both the pet and the house with sprays and foggers. Permethrins are also not safe to use on cats, and they wash off quite easily. Not to mention that after being in use for several decades, many flea populations have become resistant to permethrins.

In 1996 everything changed when Bayer introduced Advantage, a completely different kind of insecticide. Both Advantage and a similar product, Frontline, work in a different way than permethrins. They are a more effective and much less toxic. Advantage and Frontline are insecticides, not prescription drugs. The companies that sell them have made the decision to make them available only through veterinarians.

The companies that make permethrins have realized the selling power of the topical "spot" treatment. They are now producing flea treatments that look similar to Advantage and Frontline, however they still contain the same permethrins as before. (Some have added an insect growth regulator as well.) Unfortunately many people do not realize that by paying less for the over-the-counter flea product at the pet or discount store, they are getting a less effective and less safe product.

Problems can range from not working at all to hair loss to central nervous system toxicity (including death). In fact, the EPA successfully sought the removal of certain Hartz Flea & Tick Drops after many complaints of these problems in cats and kittens. If you do choose to use a product purchased over the counter, read and follow the label instructions on these "spot" treatments very carefully. The irony is that the safe and effective products are only available through veterinarians, while the potentially more toxic insecticides can be bought anywhere.

If your pet shows signs of toxicity including salivation, trembling, or seizures following application of any topical product, contact your veterinarian immediately. Adverse reactions should be reported to the National Pesticide Information Center at (800) 858-7378. For more information about pet pesticide products, see the EPA's Web site http://www.epa.gov/pesticides/health/pets.htm

Q. Our 6-year-old Labrador continues to have heat cycles. A recent one has been continuing for three weeks. Is this normal? Should we have her spayed?

A. Unlike humans, dogs do not go into menopause — they will continue to have heat cycles throughout their life. Normal heat cycles can last from six days to six weeks. But usually the dog's cycles tend to be about the same length throughout their life. A longer than normal (for the individual dog) heat can be a sign of an ovarian cyst or tumor. I would recommend that you have your dog spayed as soon as possible. Your veterinarian will no doubt want to run some blood tests beforehand to screen for any major health problems and check for other causes of the vaginal bleeding.