As stated before, fleas feed on the blood of their host. In extreme situations, populations can be so large, and so many bites that the dog will suffer from a loss of red blood cells. This more often occurs in small puppies because of their size, but larger dogs, often sickly, can be effected as well. Anemia, or low blood count, can be very dangerous, and even result in death.
The number one symptom of anemia in dogs is the color of their gums. If they appear whitish or pale, seek veterinary care immediately. Other symptoms, such as paleness in the eyes and ears, discolored skin or mucous membranes, low energy, black stool. eating dirt and vomiting can be signs that something is wrong. Again, your vet can give you a correct diagnosis and treatment if anemia is playing a role in your dogs health.