What are fleas?

Fleas are small (1/10"-1/5"), biting insects usually associated with cats and dogs but fleas bite people too. Female fleas require blood to lay healthy eggs. Larval fleas are worm-like (lower right photo) and live in the "nest", or pet bedding. Flea larvae do not bite but rather feed on organic debris and adult flea feces which contain digested blood.

Adult fleas have no wings and are flattened side to side. This shape helps them move between the hairs of their host animal. Fleas jump when disturbed. Fleas bites result in red, itchy lesions and can even transmit diseases, parasitic worms (tapeworms) and cause a skin ailment called flea bite dermatitis.

Flea life cycle

The basic flea life cycle is pretty simple. Eggs are laid in the host animal's nest (pet bedding) or fur. Eggs hatch in about a week. Worm-like flea larvae (photo lower right) feed on organic debris and feces from adult fleas. After two to three weeks fully grown larvae pupate in a silken coccoon. After one or two weeks as pupae adult fleas are ready to emerge but this is where things get interesting.

common misspellings: flee, flees, dog flee, cat flee

Flea pupae (the stage between larvae and adults) need a stimulus in order to emerge as adult fleas. Most of the time they need to detect the vibration of a host animal. In the absense of the vibration stimulus the pupae can become inactive and wait for a host animal for months. This explains why an aprtmant that has been empty for months can suddenly spring to life with fleas when a new tenant moves in.

Diseases spread by fleas

Fleas have also had an important and surprising impact on human history. At least three Plague pandemics have been caused by a flea-transmitted pathogen called Yersinia pestis. The pathogen is transmitted from rodents to humans through flea bites. The rat flea, (Xenopsylla cheopis) is the main vector of this pathogen.