I am not a veterinarian and I do not play one on TV but in the future if I can help you save some money with a natural remedy please PM me. I use veterinarians for emergencies like dying otherwise I would be twice as poor!
Fleas are species specific, yes that's right there are cat fleas and dog fleas. Although cat fleas can get on dogs they prefer cats and visa versa. Unless you are killing all the flea eggs on the dogs, cats and in your house and in your yard you will get fleas every year. A cheap fix for your yard is one part bleach to 2 parts water, in a sprayer. Do this twice 3 weeks apart in the spring. In your house if you do not have any toddlers use Ovitrol. $21 a bottle that shit works! Use topical flea killers every month on the critters. They are cheaper online then at your vets. I have not had fleas in 15 years, on my dogs and I do not use any topicals. Roaming cats are what bring the fleas to your house. Once you have killed them on your property you are less likely to have fleas. Just some unasked for advice I can shove if you don't want it. =)