Shampoos, powders, sprays and collars all aim to control fleas on pets, but the most popular treatments today are the "spot" medications. Squeeze a few drops on the skin along a dog's or cat's back, and the insecticide will control fleas for a month. The products are available only from veterinarians, in doses adjusted for an animal's weight.

The formulations spread by mixing with a pet's skin oils, which migrate as a result of body movement and gravity. Some products flow into the sebaceous glands of hair follicles, where they are stored and secreted over time; others remain on the skin's surface. Advantage (imidacloprid) from Bayer and Frontline (fipronil) from Merial--the two market leaders--will fan out across the body in less than 12 hours and kill more than 90 percent of fleas by then. Tests by Bayer show that after 28 days, concentrations across a dog's body decrease to as little as one part per million, but less than one tenth of that amount is needed to kill fleas, according to Bob Arther, Bayer's manager of parasitology. And because they reside in or on skin, spot compounds do not readily wash off like treatments that stick to an animal's hair.

This article was originally published with the title Killer Drops.

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