Fleas can infest pets and home environments. Flea bites can cause skin irritation and may also transfer a variety of diseases to humans. Pet owners spend more than $15 billion (US) annually to protect their pets from fleas. Read more to learn about the causes of flea bites, the symptoms, risk factors, and treatments.

What Are Flea Bites?



Flea bites cause lesions on the skin. These insects may bite numerous times while exploring the skin surface, resulting in clusters of bites on the skin. The flea bite appears as a raised, reddened region on the skin approximately 5 to 30 minutes after the bite. Flea bites harden 12 to 24 hours after a bite and can persist for a week or more [R].

Most fleas do not come into contact with humans or pets and have little or no medical relevance.

Fleas can cause harm to humans and pets include the human flea (Pulex irritans), cat flea (Ctenocephalides felis felis), dog flea (Ctenocephalides canis), oriental rat flea (Xenopsylla cheopis), northern rat flea (Nosopsyllus fasciatus), sticktight flea (Echidnophaga gallinacea), and sand flea or jigger (Tunga penetrans) [R].

Symptoms of Flea Bites

Flea bites present as:

  • Itching (pulicosis)
  • Small, raised pimples that may occur on any part of the body.

Fleas can bite and infest any body part they come in contact with. Fleas feed on the blood vessels of humans and animals, which allows for easy entry of pathogens. Hence, flea bites can give rise to symptoms of the disease as well [R].

Causes of Flea Bites

Flea infestations of pets and homes may cause flea bites. Fleas usually extend 2 to 10 mm in length and can jump up to 150 times their body length. Their hind legs (largely composed of the protein resilin) allow fleas this jumping ability. Fleas can jump onto a person and bite them with relative ease [R].

Research indicates that cat fleas infest pets and homes most frequently. In Columbia, cat fleas made up 94.2% of fleas taken from cats and dogs. Cat fleas usually cause health problems in both pets and humans, unlike some other flea species [R, R].

Flea Life Cycle

flea life cycle

The flea life cycle includes: egg → larva → pupa → adult [R]

    • In general, female fleas require a blood meal from a host (human or pet) to complete ovary development. Fleas bite hosts as a part of their life cycle. After an adult flea has taken a blood meal from a host, the flea will mate. Females then lay eggs on a host or the surrounding area [R].
    • Eggs take 1 to 10 days to hatch, depending on humidity and temperature [R].
  • Larvae have biting mouthparts but lack eyes and legs. They often live deep in carpet or bedding of homes with flea infestations [R].
  • Pupae live in cocoons that have an oval shape and white coloration. Pupae can remain dormant for several weeks and up to a year until a host arrives [R].
  • Adults emerge from cocoons and immediately seek a blood meal. Certain features of the host attract adult flea to host. Examples include body heat, exhaled carbon dioxide, movement, and scent [R, R].
  • Fleas usually reach their peak potential to attack humans and pets in late summer (August through September). Temperature and humidity during late summer allow flea populations to increase in comparison to other times of the year [R].

Risk Factors for Flea Bites

The majority of flea species infest rodents. If rodents have nests in a home, pets can pick up fleas if they come into contact with rodents or their nests. It is important to limit or eliminate rodent infestations in homes with pets [R].

Temperature and humidity are crucial determinants of flea development. As such, warmer temperatures caused by climate change will likely present another risk factor. Warmer temperatures lead to the expansion of flea populations in the northern hemisphere. The impact of this is that more people may experience flea bites [R].

Human encroachment on wild habitat may also increase the number of illness-causing flea bites. In fact, the reemergence of diseases from fleas may become an epidemic due to human encroachment and climate change [R].

Evidence for this comes from the discovery of:

  • Flea-borne diseases found in new hosts [R]
  • Flea discovery in previously flea-less habitats [R]

Flea Bites Cause Diseases in Humans


  • Tungiasis

Tungiasis comes from the bite of a sand flea or jigger. A female penetrates the skin, which becomes irritated and painful within 24 hours. Scratching of the irritated area spreads eggs and may lead to bacterial infection [R].

  • Plague (bubonic, pneumonic, septicemic)

Scientists have found plague on all continents, except for Europe, since 1989. Plague infects rodents. Human fleas may bite infected rodents. Infected fleas can then spread this bacteria to humans through bites [R].

Without treatment, the plague has a high death rate: 40% for bubonic and 100% for pneumonic and septicemic. Human fleas may spread the plague. Some scientists also suspect that cat fleas spread plague [R, R].

  • Flea-borne spotted fever

Scientists first detected flea-borne spotted fever in cat fleas. Many other species of fleas have also been shown to transmit the disease. Although rare, the disease occurs worldwide [R].

Main symptoms include rash, headache, and fever. Symptoms may also include black crusted skin lesions, vomiting, and diarrhea [R].

  • Murine typhus

Murine typhus gets transferred from a rodent to a flea, often oriental rat fleas. The fleas can then pass the disease to humans through bites [R].

Symptoms of the disease include high fever and nausea, which constitute symptoms of many other illnesses. Thus, a laboratory must confirm the presence of the disease [R].

  • Bartonella henselae

The cat flea may transmit Bartonella henselae through scratches or bites. The disease is present worldwide [R].

Symptoms include a small, raised and reddened area that develops after one week. Enlarged lymph nodes appear at 2 to 3 weeks. Enlarged lymph nodes may persist for up to 3 months [R].

Most cases resolve without treatment after 2 to 4 months. Twenty-five percent of patients with heart valve complications die from the disease [R].

  • Bartonella Quintana

Cat fleas and human fleas transmit Bartonella Quintana to humans through bites. A recent re-emergence of the disease has occurred in Europe and the USA, especially among homeless people. No deaths have been reported so far [R].

Some people infected with the disease show no symptoms. Others with this illness have a fever with a headache and pain in bones of the legs that lasts for short time periods. Others with these disease show symptoms that persist for long periods of time and do not stop [R].

Skin Diseases

Skin itching

  • Papular urticaria

Papular urticaria entails increased sensitivity of flea-bitten regions. The increased sensitivity is similar to an allergic response. This disease does not show at first bite but occurs after repeated flea bites [R].

Identifying which insect may cause papular urticaria remains difficult. Researchers have suspected that fleas and/or mosquitos cause the disease [R].

  • Flea allergic dermatitis

Most often, bites from cat fleas cause flea allergic dermatitis. When fleas pierce the skin with highly specialized mouthparts, some people have allergic reactions to flea saliva. This leads to dermatitis [R].

Symptoms of flea allergic dermatitis include severe itching and swelling in the region of the bite. Usually, symptoms only last a few days. In extreme cases, itching and swelling can last up to 2 years. Secondary infections may also follow due to excessive itching [R].

Common Diseases in Pets

  • Flea allergic dermatitis

Flea allergic dermatitis consists of severe itching, swelling, and hair loss in flea-bitten regions of pets. Flea allergic dermatitis usually arises from cat flea bites. This condition is common in cats and dogs. For example, a survey of 163 cats and dogs found that 58.3% of cats and dogs exposed to fleas showed signs of flea allergic dermatitis [R, R].

  • Tapeworm

Cat fleas can get infected with tapeworm. When a dog or cat eats an infected flea, they get tapeworm. Although rare, pets may transmit tapeworm to humans. Infection rarely causes disease in dogs and cats. Animals with tapeworm should get treated to prevent a risk of human infection [R, R, R].

Prevention of Flea Bites

flea prevention

Flea control often prevents discomfort and spread of disease from flea bites. Strategies in flea control include:

  • Spreading of insecticidal dust in and around pet bedding [R]
  • Vacuuming carpets and areas with pet bedding [R]
  • Consultation with pest control management to reduce contact with rodents that may harbor flea-borne diseases [R]
  • Treating companion animals with agents that prevent or stop flea infestations upon consulting a veterinarian [R]

Successful Flea Control Agents

  • Imidacloprid is a successful treatment for cats and dogs. In cats, imidacloprid eliminated fleas 14 days after treatment and decreased fleas by 86.8%. In dogs 7.5 to 10 mg/kg imidacloprid rapidly killed about 97% of adult fleas before egg production [R, R, R, R, R, R].
  • Selamectin, a topical agent applied to dogs and cats, controls flea outbreaks. In one study, 6 mg/kg doses were applied every 30 days to dogs and cats. On day 90 of application, 98% of fleas were eliminated compared to initial flea counts [R].
  • Amitraz repels ticks really well in dogs. Four days after treatment, there was about a 42% decrease in flea population.. Amitraz had no side effects on dogs [R].
  • Mixtures and formulations of topical agents to treat dogs and cats also exist: Permethrin spot-on with propylene glycol monomethyl ether killed 93 to 99% of fleas on dogs 3 to 28 days after treatment. Cats treated with fipronil spot-on with dimethyl sulfoxide killed over 95% of fleas for up to 5 weeks [R].
  • Mixtures of oral medicines have also shown effectiveness in eliminating fleas. Dogs and cats took lufenuron orally once every 3 months. The pets also took oral tablets of nitenpyram at each or every other day. The combination of these medicines leads to 100% reductions in fleas at days 84 to 90 of the study [R].
  • Sarolaner and spinosad chewable tablets reduced flea infestation in dogs by 99% and 97.3% respectively, within 7 days of treatment. Hence, the study suggests that either sarolaner or spinosad given to dogs every 2 months can eliminate fleas and reduce skin lesions [R].
  • Deltamethrin shampoo killed 100% of cat fleas in dogs 24 hours after application. The protection from the shampoo decreased flea populations by over 95% for at least 17 days [R].
  • Pyriproxyfen regulates the growth of cat fleas in cats. Researchers have found that it strongly inhibits developmental stages of cat fleas. One dose of 1 mg/kg completely inhibited reproduction of cat fleas for 7 weeks [R].
  • Insect development inhibitors prevent flea larvae from hatching from eggs. After feeding insect development inhibitors to pets once monthly, the agent gets stored in fat and is slowly released to blood in pets. A female flea eats the insect development inhibitor after biting a pet, which then gets incorporated into the flea egg. The agents are very safe for mammals [R, R].

Mostly Successful Flea Control Agents


Examples of possibly useful insecticides include:

  • Pyrethrins often get combined with an agent (synergists), which inhibit their degradation. Ultraviolet light, moisture, and air can degrade pyrethrins. Pyrethrins do not harm animals [R].
  • Synthetic insecticides include pyrethroids, a class that shares the same mechanism of action with pyrethrins. Pyrethroids are a bit more toxic than pyrethrins. Pyrethroids are also more stable compared to pyrethrins [R].
  • Rotenone comes from an extract of the root of a plant (derris plant). It contains 2 active ingredients. Rotenone is slightly more toxic than pyrethrins but is safe for use with most animals. Rotenone has high toxicity to fish [R].
  • D-limonene and linalool come from citrus pulp (limes, lemons, oranges, grapefruit, etc.). Fleas get dissolved in D-limonene and linalool, resulting in the death of the flea. Though effective, they don’t work for long periods after application and some cats have had severe toxic reactions to them [R, R].
  • Carbamates and organophosphates kill adult fleas. Reports indicate, however, they have high toxicity to pets [R].
  • Sodium polyborate comes as a powder that one can spread in a home environment. It interrupts flea life cycles. Researchers believe that flea larvae consume the powder and then die. Elimination of fleas may take 3 to 6 weeks after application of this insecticide. Sodium polyborate is considered safe around mammals [R].
  • Insect growth regulators prevent flea eggs from hatching and larvae from turning into pupae. Insect growth regulators have no effects on pets or people. They are used as an insecticide spread through a home environment. They can also be used as topical agents applied to pets [R].

Sometimes Successful Flea Control Agents

  • Sprays containing 0.29% fipronil sprayed on cats gave a 77.3% adult kill and 87.3% egg reduction. The US EPA accepts a 90% kill standard. Hence, the study mentioned suggests that the 0.29% fipronil spray may not be highly effective [R].
  • Ultrasonic pest controllers for fleas and ticks include a pet-collar unit and a unit for households. After 24 hours of exposure to the units in a study, neither unit had any effect on the activity of fleas, ticks, or cockroaches. The study supports that ultrasound has no effect in household pest control [R].
  • A few studies suggest that lufenuron alone may not be highly effective at eliminating fleas. One of the studies using flea-infested cats compared lufenuron with imidacloprid and fipronil. Evidence from the study showed that imidacloprid and fipronil completely controlled flea outbreaks. Two treatments with lufenuron were necessary for flea control [RR].
  • Many organochlorines are banned for environmental reasons. Examples of organochlorines include DDT, dieldrin, and chlordecone [R].

Insecticide Resistance

Often, treatment failure gets mistaken for insecticide resistance. Treatment failure comes from not following treatment directions. Examples include not treating all pets in a household, not treating during winter, and improper application of a product to pets [R].

Resistance occurs when fleas become less sensitive to insecticides. Cross-resistance can happen when resistance to one insecticide gives resistance to another insecticide. Recent reports show that cat fleas and human fleas have resistance for carbamates, organophosphates, pyrethroids, and pyrethrins [R].

There is less flea resistance to organochlorines, imidacloprid, lufenuron, and fipronil. Some studies suggest resistance to fipronil. Usually, cat fleas get tested with these products for studies. Different degrees of resistance may exist in other species [R, R, R].

Flea Treatment Safety Issues for Pets

Permethrin-based spot-on flea treatments for dogs can have harmful effects in cats. Pet owners should not use permethrin-based flea treatments for cats [R].

Treatment of dogs (Scottish terriers) with spot-on flea control showed no increase in cancer (transitional cell carcinoma). The spot-on flea controls tested contained imidacloprid or fipronil [R].

Flea control collars do not have toxic effects on dogs. Dogs can eliminate chemicals from flea control collars quickly [R].

Genes Linked to Flea Bites

Bacteria on the skin produce odors. Bacterial odors from the human skin may influence interactions with fleas. Fleas may use a sense of smell as a primary cue to identify a host (human). People differ strongly in their skin bacteria, so some people attract fleas more than others [R].

Genetics partially determine human odor. For example, bacteria on the skin produce a volatile compound (3-methyl butanal), which is affected by genes of the immune system. Insects, like fleas, smell these compounds and may attract fleas [R, R, R].

Research on which genes affect skin bacteria is still very new [R].

Treatments for Flea Bites

stop fleas

For humans, washing the area that a flea has bitten with soap and water will reduce the spread of infection. Application of an ice pack to a bitten region for 10 minutes reduces swelling from a flea bite. A bitten individual should avoid scratching the irritated area to avoid a risk of infection [R].

Treatments for Humans


Topical ivermectin or metrifonate were effective at eliminating fleas on the seventh day of application in infected humans. These drugs do not kill the fleas in early stages of skin penetration [R].

Other agents kill skin-embedded fleas after early phases of penetration. These agents include:

  • Topical chlorophenothane [R]
  • Clofenotane [R]
  • 4% formaldehyde solution [R]
  • Chloroform [R]
  • Turpentine [R]
  • 20% salicylic vaseline [R]
  • Oral thiabendazole [R]


  • Streptomycin is the drug of choice [R]
  • Chloramphenicol [R]
  • Doxycycline [R]
  • Gentamicin [R]
  • Ciprofloxacin [R]

Flea-borne spotted fever

  • Patients have been successfully treated with doxycycline [R]

Murine typhus

  • Tetracycline [R]
  • Doxycycline [R]
  • Fluoroquinolone [R]

Bartonella henselae

  • Most cases of Bartonella henselae respond poorly to antibiotic therapy. Usually, the illness resolves within 4 months [R].
  • For Bartonella Quintana:
  • Gentamicin for at least 14 days with ceftriaxone and/or doxycycline for 6 weeks [R].

Papular urticaria

  • For mild reactions, topical corticosteroids and antihistamines are suggested [R].
  • For severe reactions, an oral dosage or an intramuscular injection of corticosteroids is suggested [R].

Flea allergic dermatitis

  • Moisturizers and avoiding allergens (flea bites) provide the best treatment options. Topical steroids and/or calcineurin inhibitor application (2 to 3 times per week) help alleviate flares in irritation [R].

Treatment for Pets

  • For flea allergic dermatitis in pets:
    • In a small-scale study of dogs, fluralaner improved and resolved symptoms of flea allergic dermatitis. Treatment consisted of a single oral dose of fluralaner. Fluralaner resolved all signs of flea allergic dermatitis after 12 weeks of treatment [R].
  • For tapeworm in pets:
    • A test of milbemycin oxime/praziquantel combination tablets in infected dogs and cats showed 100% efficacy. With recommended doses, the tablets eliminated all adult tapeworm in infected dogs and cats. Elimination of all immature tapeworm also occurred in cats [R].

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