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Thanks lisa74656, and apologies Woofgang for misspelling your name earlier. I was so intrigued by this stuff that I looked up the active ingredient. I was sure that it was zapping the fleas before they even had a chance to bite. Lisa, my puss react to the bites and you can feel them n his skin, and he has a tendency to scratch, thus risking infection. He does not mind my applying the anti-flea drops, however.
I did some research on the chemical responsible and found this explanation:
Frontline contains Fipronil. It remains effective even after bathing or swimming. Frontline Top Spot kills most fleas before they bite, which is great for pets suffering from flea allergies.
Fipronil, the powerful active ingredient, collects naturally in the hair follicles and oil-producing glands of the skin, where it remains protected from removal by shampooing or swimming. Then it continually reapplies itself to the hair, providing long lasting control for a month or more.
So, there we have it. Case solved; ooh, and a contented happy puss, by the way!