Fleas Infest Pets and Homes

-- Fleas can be both irritating and dangerous --

Summary: Fleas bite warm-blooded animals like cats and dogs but they will also bite people. Female fleas must feed on blood in order to lay healthy eggs. Eggs hatch into worm-like larvae that live in the bedding of the host animal. Flea bites cause itchy red lesions and may transmit important diseases to both us and our pets.

Common misspelling: flee, flees, dog flee, cat flee

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Jack DeAngelis, ext. entomologist (ret.) revised: 11/2018

Flea identification

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 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, tapeworms, and cause a common skin ailment called flea bite dermatitis.

adult cat flea (original photo by Ken Gray)

Adult cat flea (1/10"-1/5"). Adult fleas move between the host animal and the "nest", or pet bedding.

cat flea larva (original photo by Ken Gray)

Cat flea larvae. Larvae live in the "nest" not on the host animal. Flea larvae do not bite.

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 feed on organic debris and feces from adult fleas and after two to three weeks fully grown larvae pupate in a silken cocoon. After one or two weeks as pupae adult fleas are ready to emerge but this is where things may get interesting (at least to an entomologist!).

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 vibrations made by a host animal. In the absence of the vibration stimulus the pupae can become inactive and wait for a host animal for months. This explains why an apartment that has been empty for months can suddenly spring to life with fleas when a new tenant moves in and their activity stimulates fleas to emerge.

A pandemic disease 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.

Flea control treatments and products

Homeowners can now safely and effectively treat flea infestations with simple-to-use topical medications combined with insect growth regulators that prevent flea larvae from becoming adults. While relatively expensive the medications are so effective that no one, nor any pet, should have to tolerate fleas any longer (see Safely Eliminating Fleas in Homes and On Pets

If you are considering a flea collar for Fido or Miss Puss you may want to take a look at our article Do Flea Collars Work? (Spoiler: most work better against ticks than they do against fleas!)


Professional-level pest control supplies are generally not available in home and garden stores but can be found at DoMyOwn, our affiliate.

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