How Many Fleas do Dogs Normally Pick up on Walks?
photo 2008 T. S. Smith | more info (via: Wylio)
I rescued my 6 lb. Maltese about 3 weeks ago. When I got her she had some fleas (about 10-15) so I immediately applied Frontline Plus and bathed her 5 days later. I walk her outside twice a day and every day she comes in with 2 or 3 fleas crawling around on her. I try to pick them off as soon as we come in so they don't bother her or get on anything in the house. A few times, I have found one dead on the kitchen floor. I know the Frontline eventually paralyzes them but is this something I should worry about? Is it normal for her to pick up fleas so often?
Walnut Creek, CA
Let's cut straight to the answer to your final question: yes, it is common to see a few fleas on dogs who have suffered very heavy flea infestations. And yes, I believe that your dog was very heavily infested when you adopted her.
10 -15 fleas is not a heavy flea infestation. However, you state that you saw 10 - 15 fleas. That means there were many, many more. I'd estimate that only one in 100 fleas ever is seen by an owner. With 10 or 15 visible fleas, it is safe to say that your dog was living with a population of fleas that was actively reproducing.
This means that there are probably hundreds of thousands of flea eggs in the house. Frontline Plus is effective against flea eggs, but nothing can neutralize all of them. That means that plenty of eggs are eventually turning into pupae (which nothing can kill), and then hatching into adults. I suspect that this is the source of the fleas you are finding now. I think it's less likely that your dog is picking the fleas up on her walks.
No product instantly kills adult fleas. Therefore, it's normal to find dying fleas on pets who have received a high quality flea preventative. I imagine that you will continue to find fleas here and there until all of the pupae in the house have hatched. This could take a few months. However, if you stick with the flea preventative for the long haul, you are not likely to suffer another major infestation.