99 Episodes But an Itch Ain't One

Kelly’s Field Notes

Common Name: Fleas

Order: Siphonaptera

Family: There are 16 families of flea worldwide (NC State, 2015)

Genera: Ctenocephalides, Pulex, Xenopsylla

Species Near You: There are more than 2,200 described flea species worldwide (Cornell University College of Veterinary Medicine, 2026).

•  Cat flea (Ctenocephalides felis) – the most common flea found on both cats and dogs, found throughout the world.

•  Human flea (Pulex irritans) – can bite humans, pigs, and other mammals, found throughout the world.

• Dog flea (Ctenocephalides canis) – less common in the U.S. than the cat flea

• Tropical rat flea (Xenopsylla cheopis) – historically important as a vector of plague, primarily found in tropical and subtropical regions. Also known as the Oriental rat flea but entomology is trying to move away from potentially xenophobic terms.

Description:

Fleas are tiny, wingless insects usually measuring about 1.5–3 mm (1/16 to 1/8 inch) long. Their bodies are dark reddish-brown, laterally flattened (side-to-side), with backward-pointing bristles and spines that help them move through fur and feathers. These spines also help the fleas resist being groomed off by their hosts (OSU Extension, 2026).

Flea hind legs are highly specialized for jumping. A flea can leap many times its own body length (often up to 100–150 times its body size) allowing it to quickly move between hosts and escape danger. Unlike lice, fleas do not live permanently attached to the host; adults jump on to feed and often spend much of the life cycle in bedding, carpets, nests, or soil. Fleas are ectoparasites, meaning they live on the outside of animals and feed on blood. Adults require blood meals to reproduce. Cat fleas are the most important cats and dogs worldwide and are also common household pests for humans who share space with cats and dogs (Rust, 2005; Dryden & Rust, 1994).

Life Cycle:

Flea eggs are small, smooth, and white. They are usually laid on the host but quickly fall off into carpets, pet bedding, cracks in flooring, upholstery, or outdoor resting areas. Their larvae are tiny, worm-like, and avoid light. They live deep in carpet fibers, under furniture, and in animal nests. They feed on organic debris and especially on “flea dirt,” which is dried blood-rich feces produced by adult fleas.

After about 20 days, larvae spin sticky cocoons that collect dust, hair, and debris, making them difficult to detect (CDC, 2024). This stage is one reason infestations are hard to eliminate. Pupae can remain dormant for days to months depending on environmental conditions.

Adults only emerge when they detect heat, vibration, and carbon dioxide from a nearby host. Newly emerged adults must obtain a blood meal quickly to survive and reproduce (CDC, 2024). Most of the flea population in a home is actually not the adults on the pet; it is the eggs, larvae, and pupae hidden in the environment (Rust, 2017).

Infestation:

Unlike ants or bees, fleas are solitary insects and do not form colonies with castes. However, infestations can become enormous because of their rapid reproductive cycle. Female fleas begin laying eggs within about 1–2 days after their first blood meal. An individual female cat flea averages about 13.5 eggs per day, with some producing more than 20 eggs daily (Osbrink & Rust, 1984). Females may lay up to 600 eggs in a lifetime (OSU Extension, 2026).

Disease and Medical Importance:

Fleas can transmit pathogens and parasites. Fleas commonly transmit the dog and cat tapeworm (Dipylidium caninum) which can affect cats and dogs, but also humans (Jiang et al., 2017). Pets become infected when they swallow fleas during grooming. Many cats and dogs also develop severe allergic reactions to flea saliva. Even a few bites can cause intense itching, hair loss, scabs, and skin infections. Cat fleas can also help spread bacteria associated with cat scratch disease and flea-borne spotted fever.

Perhaps the most famous disease issue with fleas, the tropical rat flea (Xenopsylla cheopis) is famous for transmitting Yersinia pestis, the bacterium responsible for several plagues, including the Black Death. This single bacterium can cause different types of plague based on where it is in the body. For example, the Bubonic plague is bacteria that travels through the lymphatic vessels while the Septicemic plague is when the bacterium enters the blood (Barbieri et al., 2020).

Super Powers:

Jumping machine – some species can jump up to 200 times their body length. In practical terms, if 1.5 m tall (4.9 ft) children had the same jumping ability as a flea, they could jump about 300 m (984.3 ft) into the air (the height of the Eiffel Tower) (Sanches, 2024)!

Flattened armor – their side-compressed bodies make them hard to crush and easy to move through fur.

Backwards bristles – spines and combs act like Velcro, helping them stay attached to hosts.

Stealth babies – eggs fall off the host and larvae hide deep in carpets, making infestations easy to miss until they become severe.

Disease vectors – fleas can transmit bacteria, tapeworms, and historically plague.

 References:

Barbieri, R et al. “Yersinia pestis: the Natural History of Plague.” Clinical microbiology reviews vol. 34,1 e00044-19. 9 Dec. 2020, doi:10.1128/CMR.00044-19

“Cat Flea, Human Flea.” OSU Extension, 2026, “Cat Flea, Human Flea.” OSU Extension, extension.okstate.edu/programs/digital-diagnostics/insects-and-arthropods/cat-flea-ctenocephalides-felis-human-flea-pulex-irritans. Accessed 1 May 2026. ‌. Accessed 1 May 2026.

CDC. “Flea Lifecycles.” Fleas, 20 May 2024, www.cdc.gov/fleas/about/flea-lifecycles.html.

‌ Dryden, Michael W., and Michael K. Rust. "The cat flea: biology, ecology and control." Veterinary parasitology 52.1-2 (1994): 1-19.

“Fleas.” Cornell Feline Health Center, Cornell University College of Veterinary Medicine, 2026, https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/fleas

Jiang, Peng et al. “A Human Case of Zoonotic Dog Tapeworm, Dipylidium caninum (Eucestoda: Dilepidiidae), in China.” The Korean journal of parasitology vol. 55,1 (2017): 61-64. doi:10.3347/kjp.2017.55.1.61

Osbrink, Weste LA, and Michael K. Rust. "Fecundity and longevity of the adult cat flea, Ctenocephalides felis felis (Siphonaptera: Pulicidae)." Journal of medical entomology 21.6 (1984): 727-731.

Rust, Michael K. “Advances in the control of Ctenocephalides felis (cat flea) on cats and dogs.” Trends in parasitology vol. 21,5 (2005): 232-6. doi:10.1016/j.pt.2005.03.010

Rust, Michael K. “The Biology and Ecology of Cat Fleas and Advancements in Their Pest Management: A Review.” Insects vol. 8,4 118. 27 Oct. 2017, doi:10.3390/insects8040118

Sanches, Gustavo Seron, et al. “Fleas: Amazing Jumpers That Can Carry Pathogens.” Frontiers for Young Minds, vol. 12, 19 Apr. 2024, https://doi.org/10.3389/frym.2024.1269791.

‌“Siphonaptera.” NC State Agriculture and Life Science, 2015, genent.cals.ncsu.edu/insect-identification/order-siphonaptera/. Accessed 1 May 2026.