Technically, cat and dog fleas are different species but each can infect the other. The only way you can tell the difference between the two is under a microscope.
Despite it’s name, the most common flea found on dogs and cats in North America is the cat flea, also known as Ctenocephalides felis. This type of flea also relies on humans as a host. It is found on outdoor animals such as skunks, foxes, racoons and opossums.
Dog fleas, known as Ctenocephalides canis, can also affect dogs, cats, humans and other animals. Dog fleas seem to be more common in Europe but can also be found in North America. It is also found on outdoor animals found in the wild such as racoons.
Both cat and dog fleas can carry the tapeworm parasite which can affect both cats and dogs.