How Fleas Work

By: Tracy V. Wilson & Wendy Bowman  | 
Fleas are essentially freeloaders that feed on the blood of other creatures, which typically includes our dogs and cats. Dorling Kindersley/Getty Images

You might be extremely vigilant when it comes to preventing fleas, but your cats and dogs can still can become infested with these small wingless insects at numerous places. The most common include high-contact areas like your own backyard — courtesy of squirrels, deer, rabbits and other wild animals — as well as dog parks, the groomer or doggy daycare facilities. Even your bedding, rugs upholstery and furnishings can be prime breeding grounds.

Essentially freeloaders that live on other creatures, these parasites exist by consuming blood from their hosts, which typically includes dogs and cats. Fleas can live for anywhere from 13 days up to a year, and during that time, they can produce millions of offspring. Although there are many species of fleas, the one that most often affects both dogs and cats in North America is the cat flea (Ctenocephalides felis).


You probably will first spot the effects of fleas when your pet repeatedly scratches, licks and chews, and you occasionally might even see the tiny brown insects moving around on your animal's head, abdomen and base of the tail.

Although newly emerged fleas need to find food within a few days, adults can go for a couple of months without a meal. Flea pupae also can remain in their cocoons for up to a year, waiting to sense the body heat and vibrations that signal the presence of nearby hosts. The ability to live without food is just one of a flea's many adaptations, making it easy for them to move around on their hosts, feed on blood, reproduce and survive when food is scarce. Here, we'll examine how these adaptations make it harder to kill fleas, explore how to keep these pests from invading your home and your pets, and suggest ways to get rid of an infestation if it does happen.

Flea Anatomy

Fleas have a unique anatomy that allows them to suck blood and leap as high as 7 inches! HowStuffWorks

Fleas are minuscule, but anyone who has seen one usually can recognize it with ease. They're tiny, flat, wingless insects that have a knack for jumping away before you can catch them. Their bodies are covered with hard plates called sclerites. So, if you do catch one, squashing it can be a challenge. Their hard, outer shell protects them from everything from an animal's teeth to hitting the floor after a long jump.

To the naked eye, a flea's exoskeleton seems completely smooth, but it's really covered in tiny hairs that point away from the flea's head. Their flattened bodies and these backward-pointing hairs are what enables them to crawl through a host's fur, and if something tries to dislodge them, the hairs act like tiny Velcro anchors. That's why a fine-toothed comb removes fleas better than a brush. The teeth of the comb are too close together for fleas to slip through, so it can pull them from the host's hair, regardless of which way a flea's hairs are pointing.


A flea also has spines around its head and mouth, with the number and shape varying according to the species. The mouth itself is adapted for piercing skin and sucking blood. Several mouthparts unite to form a needlelike drinking tube. Here's a rundown:

  • Labrum and labium make up the "upper" and "lower" lips
  • Labial palps are long, five-segmented sensory organs that come from the labium.
  • Maxillae is a pair of short, wide plates located in front the labial palps.
  • Maxillary palps: A long, four-segmented palpus comes off each maxilla.
  • Fascicle are three long, slender stylets that are supported within the labial palps.
  • Maxillary lacinae: These are the two outer stylets of the fascicle. They're serrated and blade-like.
  • Median epipharynx: This is the central stylet of the fascicle that joins with the maxillae to form a tube-like food canal.

Fleas use their sharp maxillary laciniae to easily puncture the skin of their host. Then blood travels from their host through the tip of the median epipharynx up the flea's food canal. This requires a lot of suction, which comes from pumps in the flea's mouth and gut.

A flea's legs are adapted for jumping. As with all insects, a flea has three pairs of legs that attach to its thorax. The back legs are very long, and the flea can bend them at several joints. The process of jumping mimics the action of a crossbow. The flea bends its leg, and a pad of elastic protein called resilin stores energy just like a bowstring.

A tendon holds the bent leg in place. When the flea releases this tendon, the leg straightens almost instantly, and the flea accelerates like an arrow from a crossbow. This anatomy gives fleas the ability to jump about 7 inches (17.8 centimeters) vertically or 13 inches (33 centimeters) horizontally. In human proportions, that's a 250-foot (76-meter) vertical jump or a 450-foot (137-meter) horizontal jump. As it lands, the flea uses tiny claws on the ends of its legs to grasp the surface underneath.

Aside from these adaptations, fleas look a lot like most other insects, and their reproductive cycles are similar as well.

The Flea Life Cycle

The life cycle of a flea is similar to the life cycle of a butterfly. HowStuffWorks

Fleas reproduce a lot like butterflies. Females lay eggs that hatch into worm-like larvae. The larvae then spin cocoons and become pupae, and an adult flea emerges from the cocoon. In a population of fleas, about half are eggs and 5 percent adults.

Female fleas can lay eggs only if they've had a meal. If a female emerges from her cocoon and isn't able to find food, she'll die without reproducing. But once she's eaten, a flea can lay close to 20 eggs at a time, for a total of 500 eggs during her lifetime.


Most of the time, fleas lay their eggs on their host. The eggs are completely smooth, so they slide off the host and land in its environment. In people's homes, the eggs sink deep into carpet fibers and floor cracks. Outside, flea eggs settle into the soil. Flea eggs are white — the black specks you see on flea-infested animals and their bedding are particles of dried blood and flea droppings.

Flea eggs need a warm, moist environment with a temperature of about 70 degrees Fahrenheit (21 degrees Celsius) and 70 to 85 percent humidity to develop. In these conditions, the eggs hatch in about 12 days. This 12-day window is one of the reasons why it can be hard to get rid of fleas. Some insecticides kill adult fleas but not eggs, which means a whole new set of fleas can emerge after the adults have died.

Flea larvae are about 0.5 inches (1.5 millimeters) long and look like white, segmented worms. They avoid light and migrate toward cracks in the floor, where they remain for their development. Unlike their parents, the larvae don't eat blood. Instead, they eat skin cells, flea droppings and other debris. Larvae develop through three stages, or instars, molting after each.

After a week or two, flea larvae spin silk cocoons. They attach pieces of dirt and debris to their cocoons as camouflage. If food is plentiful, the adult flea emerges after about a week. Otherwise, the flea may stay in its cocoon for up to a year.

Although flea larvae and adults have completely different eating patterns, what they eat has a big impact on the diseases fleas can carry. Next, we'll take a look at flea-borne illnesses and complications from flea bites.

Fleas and Disease

Fleas can spread disease. These tiny parasites are actually to blame for spreading bubonic plague. Carol Yepes/Getty Images

Most people think of fleas as a nuisance — they bite people and animals, causing itchy, red bumps. This reaction comes from substances in fleas' saliva, and some people and animals react to these more strongly than others. Fleas often draw blood from two or three tightly spaced punctures, causing two or three itchy bumps.

Animals that develop a sensitivity to flea bites can scratch or groom themselves excessively. This can lead to rough skin, baldness and skin infections. Sometimes, these infections can be mistaken for a parasitic infection called mange. Itchy, uncomfortable pets also can be ill-tempered or restless.


In addition to being annoying, fleas can transmit diseases to other parasites, people and animals. Flea larvae feed on the eggs of tapeworms. Once swallowed, the tapeworms begin to develop in the gut of the flea. If an animal swallows an infected flea while grooming, the animal also becomes infected with the tapeworm.

The tapeworm grows in the intestine of the animal, shedding its egg sacs from the animal's rectum as it sleeps. The sacs land in areas near the animal's bedding — where flea larvae are likely to live — and the cycle starts all over again. People, especially children, also can become infected with tapeworms if they swallow fleas while petting animals, or if they come in contact with infected waste and don't wash their hands before eating.

Fleas also can transmit serious diseases, such as the bubonic plague. Fleas are vectors, which means they transport plague bacteria from rodents — the natural carriers of the disease — to people. A particular species of flea, the Oriental rat flea, usually is the culprit. In addition to preferring to feed on rats, the Oriental rat flea's gut can become blocked by plague bacteria. Then, when it bites its next host, it may regurgitate infected blood into the wound. Symptoms of plague include swollen lymph nodes called buboes, fever, headache and exhaustion.

Fleas also can carry murine typhus, particularly in the Southern and Southwestern parts of the U.S. Murine typhus is caused by the bacteria Rickettsia typhi. The Oriental rat flea and cat flea can carry these bacteria to people. The infection typically comes from the flea's waste rather than its mouth. The fleas defecate while they're eating, and their hosts scratch the infected waste into the bite. People also may break the skin while scratching, and the bacteria can enter the body through the scratch. Murine typhus causes symptoms like fever, headaches and nausea. Both it and bubonic plague are treated with antibiotics.

Flea Bites on Humans

Fleas bite humans, too, and those tiny little bites itch like crazy. HeikeKampe/Getty Images

Since fleas can clearly spread disease, and owning a pet increases the likelihood of a flea infestation, you want to be sure to prevent fleas from setting up in your home via soft goods like fabric (or fur) because once they're inside, their quick breeding cycle means they can immediately get out of hand.

But if you have a flea infestation, how do you know if you've been bitten by a flea? An allergic reaction can appear, with swelling, itching and minuscule red spots in groups of three or four (and sometimes in a line), typically around your feet, ankles or legs. If uncontrolled, fleas can spread across your body and bite anywhere, especially if you have dense areas of hair on your legs and chest. Sometimes, hives and a rash also will show up, and scratching can cause an infection.


The best way to control the spread of fleas is to regularly clean and vacuum flooring, furniture, bedding and skirting boards. Typically, however, humans often are secondary options when it comes to flea bites because they don't make good hosts.

Instead, people tend to become targets for hungry adult fleas who haven't yet located the family pet or another, more suitable, host. An adult flea that recently emerged from its cocoon also may feed on an adult or child who just happens to walk or crawl by.

If you are bitten by a flea, be sure to:

  • Wash the bite.
  • Use an antiseptic and apply an icepack.
  • Resist the urge to scratch.
  • Use lotions to treat the itch.
  • Consult a pharmacist about over-the-counter anti-itch creams and antihistamines.

If you have been bitten and have difficulty breathing, nausea, and swelling of the lips or face, you should seek medical attention immediately. You might have an infection if you experience swollen glands, extreme pain around the site of the bite, excessive redness, or a white pocket or rash. In some rare cases, fleas carry diseases that can be transmitted through bites, including flea-borne spotted fever, plague, typhus and cat scratch fever.

Getting Rid of Fleas

Getting rid of fleas can sometimes be tough, but there are a lot of different ways to control them in your home and on your pets. deimagine/Getty Images

For many people, seeing fleas prompts an immediate trip to a pet-supply store or vet for some type of chemical that will get rid of them. These chemicals can fall into a few basic categories:

  • Insect growth regulators (IGRs) keep flea eggs from hatching by mimicking flea hormones. They may or may not affect pupae, and they have no effect on adult fleas. Some IGRs are topical — pets absorb them through their skin, and fleas ingest them with the hosts' blood. One IGR, methoprene, is often used as a spray. The World Health Organization also has approved methoprene for use in controlling mosquito larvae in malaria-prone areas.
  • Chitin synthesis inhibitors, also known as insect development inhibitors, keep young fleas from developing properly by disrupting the formation of their exoskeletons. Lufenuron, also known as Program, is a chitin synthesis inhibitor.
  • Pesticides kill adult fleas. Imidacloprid, sold under the brand name Advantage, is an example of a topical pesticide.
  • Repellents like DEET discourage fleas from entering areas.

Some chemicals are intended for use in a particular environment, like a room or yard. Most of the time, you should remove pets and children before applying the chemical, and everyone should stay away from the area until it dries.


Other flea treatments are topical or oral medications for pets. Some of these treatments require fleas to bite pets to work, while others do not. Typically, you can purchase these flea treatments from a veterinarian. If you plan on using chemicals to treat your pets or home, use them sparingly, and follow all accompanying instructions to the letter.

Today, many flea treatments are exceptionally powerful, killing fleas and their eggs within a few days. But sometimes a simple on-the-spot treatment isn't enough to rid your home and pets of fleas. Here are some simple steps for preventing and dealing with flea infestations:

  • Treat your pets and their environment at the same time.
  • Bathe your pet and use a flea comb to remove fleas. If you plan on using a topical flea treatment on your pet, follow the instructions regarding how long to wait before or after bathing your pet.
  • Vacuum your home thoroughly at least every other day. Immediately empty the canister or replace the bag, and discard the debris in an outdoor garbage can.
  • Launder or replace pets' bedding frequently.
  • Mow your lawn regularly. Rake and discard any leaves or other lawn debris, and keep piles of sand away from your home.
  • Whenever possible, discourage wild animals from entering your yard.

There are a few home remedies for fleas that don't work. While flea collars can help keep fleas off your pets, they generally don't have much of an effect on established infestations. Feeding pets brewer's yeast, garlic and vitamin supplements also is unlikely to do much good against fleas. Ultrasonic collars also don't appear to act as a flea deterrent or repellent.

Most of the time, if you treat your pet, lawn and home for fleas at the same time, you can get rid of an infestation on your own. If not, you may need to contact a professional exterminator.

Originally Published: Aug 24, 2007