What Is the Difference Between Dog Fleas & Head Lice?

Dog fleas and human head lice, both wingless, flattened, six-legged insect parasites of about the same size, feed on blood or skin debris and cause intense itchiness to the skin of their hosts. That's what they have in common -- but how they differ is more important. Although head lice are highly contagious among humans, human lice can't survive on dogs. Fleas, on the other hand, represent a much greater potential health threat. Even though dog fleas prefer canine blood, in a pinch, they will happily sink their mouthparts into human flesh, creating he potential for transmitting some serious diseases.

Head lice infestations are common in the United States, especially among children, who often make close contact with each other's scalps when playing. Dogs, cats and other animals also get lice but the different strains of these parasites are host-specific, meaning they can't jump the species barrier. Mature lice are about 1/8 inch long, with oval-shaped bodies, two antennae and a short, retractable proboscis, which they sink into the skin of their hosts. When unfed, the bodies of head lice are grayish-white but turn deep red as they fill with blood. Lice larvae, called nymphs, are smaller, paler versions of adults. Female lice produce a sticky substance to "glue" their white eggs, called nits, to the base of hairs.

Dogs don't contract lice as often as fleas but when they do, infestations make them just as miserable. In North America, this condition, called canine pediculosis, can be caused by two species of lice. One, a chewing louse, feeds directly on tissue and the other, a sucking louse, feeds on blood, like a mosquito. Even though these lice can't live on humans, they look and behave similarly to head lice, also attaching nits to the base of hairs. Infested dogs often scratch themselves so aggressively that they lose hair in patches, usually around the ears, neck, groin, shoulders and backside. Severe infestations can lead to anemia, especially in puppies and small dogs.

The cat flea isn't the only species that parasitizes dogs and cats in North America but it is the most common. Once fleas get into a dog's fur, backward-facing bristles on their legs and bodies help to make them difficult to dislodge. Their powerful hind legs allow them to jump up to 150 times their own body length. Female fleas can lay 40 to 50 eggs daily, which drop off the dog. After they hatch, fleas hop aboard and immediately start feeding on the blood of their hosts. If dogs aren't around, humans will do just fine. Flea-borne diseases include plague, typhus and tularemia. Fleas are also intermediate hosts to tapeworm, which can be transmitted to humans, most commonly in children who have swallowed an infected flea on dirty hands.

Pulex irritans is often referred to as the "human" flea but that's misleading, say the authors of a paper published in the August 2010 "International Journal of Infectious Diseases." Many of the more than 2,500 known flea species will feed on human blood and none is exclusive to humans. But what makes P. irritans a special threat as a disease vector is that this flea actually prefers human and pig blood to that of other hosts it infests, which include dogs, cats, rats, wildlife and livestock. P. irritans is found all over the world but in the U.S., is most common in the Midwest, South and Pacific Coast regions. According the the Division of Agriculture at the University of Arkansas, this flea has been implicated in plague outbreaks.