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Q: Why does my dog still have fleas a week after the first dose of medication?

Original Question: "My Toy Poodle who is 7 years old and weighs 8 pounds has fleas. Our vet suggested we use Advantage 2 and a week later he still had fleas. She then said to try Revolution Purple but he is still scratching and we are still finding 1 or 2 fleas on him 4 days after using the Revolution. Everything in the house has been washed and vacuumed numerous times. Are fleas developing a resistance to medication? Is there any thing else we can try? Thanks" - Anne


Vet Certified Answer | Answered: October 11, 2017

Hi Anne,

Thanks for your question.

I really like this question. Both advantage 2 and Revolution will absolutely resolve the flea infestation but you have to understand how it works and why. You also need to understand the life cycle of the flea to know why failures can happen.

The first thing to understand is that the fleas you see on your dog are only 10% of the problem. 90% of the fleas are in the more juvenile forms of larvae and eggs which are present around your home. The fleas you're seeing are only the adults. There are waves of hatching eggs and growing larvae coming up right behind them that will jump on your dog once they reach adulthood.

These medications largely kill the adults. If you apply one dose, then you'll kill the fleas on your dog but this will not resolve the issue because of the presence of the maturing fleas throughout your home. You can wash everything, spray every surface and vacuum, but you will not get rid of all of the larvae and eggs in your home. And even if only a few survive, they will ensure that your dog becomes re-infected.

The medication is completely reliable and will not only resolve the issue on your dog but also in your home. The only way that they work is by applying them multiple times over a period. Most veterinarians recommend an application once a month for 3 months. The first dose will kill the adults currently on your dog. The second dose will kill the adult fleas that grew from the larvae and eggs in your home. The third dose is to be certain that success is achieved. I have never seen this fail with these medications. Giving one dose is absolutely guaranteed to fail because the fleas in juvenile form in the environment will simply jump back on your dog when they mature.

In a case where the 3 applications fail, there are causes but they are rare. The most common would be a source of fleas from another animal that your dog is coming in contact with that keeps spreading fleas to your dog. You could be petting a neighbourhood dog on a consistent basis and fleas are attaching to you and you're bringing them home but this would be extremely uncommon.

I hope this helps and good luck.

Dr. Clayton Greenway


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