Core Concept Biodiversity Published: April 19, 2024

Fleas: Amazing Jumpers That Can Carry Pathogens


Have you ever seen some small dark brown bugs jumping off your pets? Those creatures might have been fleas. Fleas are insects specialized for feeding on blood, with long, strong legs for jumping and claws for holding onto your pet tightly. Fleas use many animals, including humans, as “hotels” and “restaurants”. Even if pets try to scratch and shake to kick out these unwelcome guests, it does not work! They hang on with their claws and keep eating for free. In addition to being nasty, fleas can carry diseases that they can transmit to animals when the fleas feed, or if the animal eats them accidentally. So be careful if these tiny insects decide to visit your pet’s skin! Do you want to learn more? Keep reading, and we will introduce to you some curious facts about the lives of fleas.

A Brief History of Fleas

Fleas are tiny incredible insects that live among us. Although the history of fleas is poorly studied, there are a few known fossils of these organisms from the mid-Mesozoic Era, more than 66 million years in the past. The evolution of fleas is interesting since fossils from the Jurassic period of the Mesozoic Era show that the ancestors of fleas were nasty-looking creatures and, in some cases, these ancestors looked very different from today’s fleas [1]. The fact that they may have been on Earth since the time of the dinosaurs makes fleas fascinating insects.

What Do Fleas Look Like?

Maybe you have never seen a flea up close—you might only know them as tiny, dark brown spots jumping off your pet. Fleas are small (1.5–3.2 mm long), dark brown, oval-shaped insects with thin, flat bodies. They cannot fly because they do not have wings, but they are among the best jumpers in the animal world [2]. Fleas have six long legs, and the hind pair is extra-long, giving fleas amazing jumping ability. Some species can jump up to 200 times their body length. In practical terms, if 1.5 m tall children had the same jumping ability as a flea, they could jump about 300 m into the air—the height of the Eiffel Tower!

Under a magnifying glass, you can see more details of the flea’s body (Figure 1). The body is divided into the head, thorax, and abdomen. The head is helmet-shaped, with mouthparts that are specialized for piercing the host’s skin, allowing the flea to suck blood. Fleas have two eyes that can detect light and a pair of antennae. Some species have rows of spines on their bodies, called the ctenidium. A flea’s body is covered with hard, smooth plates, like armor, and they have bristles on those plates that all point in the same direction to move through the host’s fur more easily. On both sides of the flea, there are pores called spiracles that the flea breathes through. Fleas also have a sensory structure called the pygidium that detects air currents.

Figure 1 - Main body structures of the cat flea, as seen through a laboratory magnifying glass.
  • Figure 1 - Main body structures of the cat flea, as seen through a laboratory magnifying glass.

Some scientists believe that fleas may communicate with each other by producing high-frequency sounds through their spiracles. These sounds might be picked up as vibrations by the pygidium bristles of another flea. Generally, the backs of female fleas are convex (curved outwards), while males’ backs are nearly straight. Male fleas have complex genitals, with the phallus (male sexual organ) normally coiled up within the abdomen [3]. When male and female fleas reproduce, the male’s sex cells must reach the female’s sex cells to produce eggs. Females have a structure called the spermatheca, which is responsible for storing the sex cells that are deposited by the male flea.

Not All Fleas are the Same

At first glance, all fleas look the same in body shape and coloration, but if we look more closely, we can see that some have different shapes and structures. Organisms that share common characteristics and can make babies together are considered the same species. Each species is given a scientific name. Currently, more than 2,500 flea species are known worldwide, most of which infest animals, including dogs, cats, rabbits, rats, foxes, pigs, goats, raccoons, and some birds. A few flea species even infest humans [4].

Where are Fleas Found?

Most flea species live in tropical regions, where the warmth and humidity help them to develop. By hitching a ride on animals, fleas have been transported to various environments and have adapted to live in many conditions. Did you know that fleas can even live in very, very cold places like Antarctica? Although fleas prefer warm weather, there are some super-tough fleas that can infest seabirds, and they can be found in Antarctic and sub-Antarctic regions. In summary, while fleas are more common in areas with warmer climates, they are established in almost every environment on Earth. So, whether you are on a tropical sunny beach, high in a snowy mountain, deep in a humid rainforest, or a desert, there is a chance you might encounter these pesky little insects!

Life Cycle of Fleas

Fleas go through a lifecycle that consists of four stages (Figure 2):

Figure 2 - Life cycle of fleas, from eggs to adulthood.
  • Figure 2 - Life cycle of fleas, from eggs to adulthood.
  • The entire life cycle generally takes 21–28 days.


Female fleas lay up to 50 eggs per day on their host animal. Those eggs do not stick to the host and usually fall down. This means that flea eggs can be found anywhere inside our houses (for example, in carpets, sofas, or bedding) or in outdoor areas (in grass, sand, dirt, and nests). Flea eggs are tiny and hard to see, so it is almost impossible to find one. When conditions of temperature and humidity are right, eggs hatch within a few days.


Once the eggs hatch, they develop into larvae. Larvae look like tiny white worms, and they really do not like sunlight, so they crawl into dark, humid hiding places where they can have their favorite meal: organic matter from the environment. When the eggs do not fall off the host animal, the larvae feed on their parents’ poop (yuck!) or skin cells from the host.


After about 1–2 weeks of feeding, larvae start the metamorphosis into the pupal stage. Each larva spins a cocoon made of a silk-like material to protect itself. The cocoon is very sticky and helps camouflage the pupa. Pupae can remain in cocoons for several weeks to months since they do not have to feed and can wait until environmental conditions are perfect to emerge as adults.


The adult flea emerges from the cocoon once the weather is warm and potential hosts are nearby. At this stage, fleas are fully developed and ready to start looking for hosts to feed on for blood. Both male and female fleas take several blood meals daily. Once they enter their new home, they start the life cycle again by feeding, reproducing, and laying eggs.

Generally, adult fleas can live for 2 or 3 months, but they can only survive a few days or weeks without attaching to a host. The entire life cycle of cat fleas, for example, usually is completed in 21–28 days. The cycle can be influenced by humidity, temperature, amount of light, and host availability. The knowledge about the lifecycle of fleas is essential for developing effective flea control and prevention.

Are Fleas Dangerous?

Fleas can transmit pathogens, like harmful bacteria, to humans and their pets, and they can also cause itching and redness of the skin, called allergic dermatitis. Cat fleas are responsible for transmitting the bacterium Bartonella henselae to humans, which can cause an illness called cat scratch disease. In this case, the bacteria are released in flea poop when they take a blood meal from an infected cat, which usually is not sick. When the flea-infested cat scratches and grooms itself to get rid of fleas, contaminated flea poop get under its claws and into its mouth. The cat can then transmit the bacteria to humans through bites and scratches [4]. As you can see, fleas can be a health risk, and the close relationships between humans and animals create situations where these diseases are more likely to occur. In addition to bacteria, cats and dogs can be infected by parasites called tapeworms if they eat fleas that are infected with tapeworm larvae [5]. Infected pets can have diarrhea and they usually rub their butts on the floor to show they are not feeling well.

Protecting Ourselves and Our Pets From Fleas

Controlling the presence of fleas is essential to keep humans and their pets healthy and protected from flea-borne diseases. It is not enough to control fleas only on pets—fleas, eggs, and larvae must also be eliminated in the environment. So, it is important to clean indoor and outdoor spaces to remove the fleas at all stages of their life and organic matter that feeds flea larvae. Veterinarians can prescribe anti-flea sprays, soaps or shampoos, oral medications, and collars to prevent flea infestation [6].

Take Home Message

Fleas have characteristics and a life cycle that allows them to survive in the environment and on animals. The adult fleas you may see on pets represent 5% of the fleas in the area where that pet lives. The remaining 95% of fleas are in the environment as eggs, larvae, and pupae. When these fleas become adults, they look for hosts to feed on. Therefore, to prevent flea infestations, it is important to treat pets and clean the environment where our pets live. When fleas are taking their blood meals, they transmit pathogens to animals and humans. But do not worry! Now you can recognize fleas and know how to prevent them. Be sure to inform your parents if there is any signs of fleas in your home or on your pets. If you notice that your pet is sick after a flea infestation, you should visit the veterinarian to help your pet get healthy and prevent any problems.


Ctenidium: Sharp and pointy spines arranged as a comb, allowing the flea to hold onto their host while they feed.

Spiracles: Tiny holes that many insects use to breathe, including fleas.

Pygidium: A sensorial structure located in the rear end of fleas that help them feel the environment.

Spermatheca: Kind of a storehouse where female fleas and other insects produce and keep their eggs.

Species: Groups of living things that look similar to each other, like dogs or birds.

Organic Matter: Any material or residues from a living or non-living organisms.

Metamorphosis: The period when an animal undergoes a transformation of their body; for example, when a caterpillar changes into a butterfly or when a pupae changes into an adult.

Pathogens: Very tiny germs that can make humans or animals sick.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


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[2] Sutton, G. P., and Burrows, M. 2011. Biomechanics of jumping in the flea. J. Exp. Biol. 214:836–47. doi: 10.1242/jeb.052399

[3] Saari, S., Näreaho, A., and Nikander, S. 2019. Canine Parasites and Parasitic Diseases. Cambridge, MA: Academis Press, 159–85. doi: 10.1016/B978-0-12-814112-0.00008-8

[4] Bitam, I., Dittmar, K., Parola, P., Whiting, M. F., and Raoult, D. 2010. Fleas and flea-borne diseases. Int. J. Infect. Dis. 8:3667–76. doi: 10.1016/j.ijid.2009.11.011

[5] Rousseau, J., Castro, A., Novo, T., and Maia, C. 2022. Dipylidium caninum in the twenty-first century: epidemiological studies and reported cases in companion animals and humans. Parasit. Vectors 15:131. doi: 10.1186/s13071-022-05243-5

[6] Hnilica, K. A. 2011. “Hypersensitivity disorders”, in Small Animal Dermatology: A Color Atlas and Therapeutic Guide, 3rd ed., ed. K. A. Hnilica (Oxford: Elsevier Saunders), 210–212.