Both people and dogs can be bothered by many different species of fleas. The most common flea species to bother dogs and humans though is the cat flea (Ctenocephalides felis), according to “Dog Owner’s Home Veterinary Handbook” and the Illinois Department of Health. Although the cat flea, human flea (Pulex irritans) and the dog flea (Ctenocephalides canis) look very much alike, they do possess crucial differences.

Although the adults of all three flea species will feed on whatever they can get, dog fleas prefer the blood of domestic and wild canines and felines. These fleas can be found on wolves, foxes, bobcats and pumas as well as dogs and cats. They also enjoy the blood of raccoons and possums. Human fleas prefer the blood of people, rats, pigs and wild boar.

If fleas cannot get to the animals whose blood they prefer, they will feed on the blood of whatever other creatures are available. If dog fleas cannot get to their preferred species, they also will drink the blood of humans, mice, rats and hedgehogs, according to the Centers for Disease Control and Prevention (CDC). If human fleas cannot get to people or pigs, they will drink the blood of cats, badgers and rats.

Human fleas are a worldwide pest. They are far more common in North America than are dog fleas, and are especially bothersome to people who work with or live near pigs, according to the CDC. Dog fleas are more of a problem in Europe. But the cat flea is even more common than the human flea in both North America and Europe.

Dog fleas not only cause irritating bites, but flea bites can infect both humans and dogs with tapeworms. Dog fleas are, in turn, parasitized by tapeworms. Dogs usually get tapeworms from fleas by eating an adult flea during the course of grooming. Humans may accidentally have fleas hop in their mouths or crush a flea and then wipe their mouths or eat before washing their hands. Although human fleas can sometimes be infected with tapeworm, dog fleas are almost always infected with them. However, human fleas can transmit typhus.

Adult human fleas tend to be larger than dog or cat fleas. Human fleas grow up to 4 millimeters in length while dog fleas average only 2.5 millimeters, according to “Dog Owner’s Home Veterinary Handbook.” The eggs of the two species are the same size, around 0.5 millimeters long, but the size of larvae can vary, with the human flea growing larger than the dog flea.