[AngelZoo]

There are actually about 6,000 different types of fleas, and each perfering a different host.
Canin fleas would rather not live on a cat or a human, and vice versa, and there are even human fleas

Just one possibility.

Have you tried Frontline, or Advantage for your dogs fleas?

http://www.zin.ru/Animalia/Siphonaptera/index.htm