Do animal fleas live on humans?

Understanding Fleas

What are Fleas?

General Characteristics

Animal fleas are small, wing‑less insects belonging to the order Siphonaptera. Adults measure 1–4 mm, have laterally compressed bodies, and possess powerful hind legs adapted for jumping. Their exoskeleton is covered with hardened cuticle that resists desiccation, allowing survival in dry environments.

The flea life cycle includes egg, larva, pupa, and adult stages. Eggs are deposited on the host or in the surrounding habitat. Larvae are blind, feed on organic debris, and develop within a protective cocoon. Pupation occurs in a silken cocoon; emergence is triggered by vibrations, carbon dioxide, or heat—signals associated with a potential host. Adults emerge ready to locate a blood source.

Host specificity varies among species. Some fleas, such as Ctenocephalides felis (cat flea) and Ctenocephalides canis (dog flea), preferentially infest mammals but can bite humans when animal hosts are unavailable. Other species, like Pulex irritans (human flea), are adapted primarily to humans. Feeding behavior involves piercing the skin with a proboscis, injecting saliva that contains anticoagulants, and ingesting blood.

Environmental preferences include warm, humid microhabitats found in animal nests, bedding, or carpeting. Fleas can survive several weeks without a host, but reproductive success depends on regular blood meals. Their capacity to infest humans hinges on host availability, proximity to infested animals, and the presence of suitable shelter.

In summary, fleas are ectoparasites with a multi‑stage life cycle, morphological adaptations for jumping and blood feeding, and varying degrees of host specificity. While many species favor animal hosts, they are capable of temporary habitation on humans under conditions that bring them into contact with infested mammals.

Lifecycle Stages

Fleas are obligate blood‑feeding ectoparasites that can infest humans as well as a wide range of mammals. Human infestations occur when adult fleas transfer from animal hosts or from contaminated environments to a person’s skin, where they feed and reproduce.

Egg – The adult female deposits up to 50 eggs on the host or in the surrounding environment. Eggs are light‑colored, oval, and hatch within 2–5 days under suitable temperature and humidity.
Larva – Emerging larvae are blind, legless, and feed on organic debris, including adult flea feces (blood‑rich “flea dirt”). Development lasts 5–11 days, during which the larva undergoes three molts.
Pupa – Larvae spin silken cocoons and enter the pupal stage. Within the cocoon, metamorphosis to the adult form takes 5–10 days, but emergence may be delayed for weeks if conditions are unfavorable.
Adult – Fully formed adults emerge, seek a blood meal, and begin reproducing within 24–48 hours. Adults live up to several weeks, feeding repeatedly on the host’s blood.

Human infestation is possible because adult fleas are not host‑specific; they will bite humans when preferred animal hosts are absent or when environmental exposure is high. However, humans are generally less suitable than rodents, dogs, or cats, so flea populations on people are typically transient and dependent on proximity to infested animals or contaminated bedding. Effective control requires eliminating eggs, larvae, and pupae from the environment and treating any animal hosts that serve as primary reservoirs.

Types of Fleas

Common Animal Fleas

Common animal fleas are small, wing‑less insects that feed on the blood of mammals and birds. Their life cycle includes egg, larva, pupa, and adult stages, each requiring a suitable environment for development. Adult fleas locate a host by sensing heat, carbon‑dioxide, and movement, then attach to the skin and begin blood feeding.

The most frequently encountered species include:

Cat flea (Ctenocephalides felis) – primarily infests cats and dogs; readily bites humans, especially in indoor environments where pets reside.
Dog flea (Ctenocephalides canis) – prefers dogs but also attacks cats and humans; less common than the cat flea.
Human flea (Pulex irritans) – historically associated with humans; now rare, it can still bite people but prefers other mammals when available.
Rodent flea (Xenopsylla cheopis) – vector of plague; mainly parasitizes rats; occasional human bites occur in infested dwellings.
Pigeon flea (Ceratophyllus columbae) – lives on birds; may bite humans when bird nests are close to living spaces.

Fleas generally require a warm‑blooded host to survive. While their primary hosts are animals, most species will bite humans if their preferred hosts are absent or if humans provide easy access. Human infestations often arise from close contact with infested pets or from environmental contamination such as carpets, bedding, or pet habitats where flea eggs and larvae develop.

Control measures focus on treating the animal host, cleaning the environment, and using insecticidal products approved for flea management. Regular grooming, vacuuming, and laundering of bedding reduce the risk of secondary human bites. In the absence of proper control, flea populations can persist, leading to repeated human exposure and potential transmission of flea‑borne pathogens.

Human Flea (Pulex irritans)

The human flea, Pulex irritans, belongs to the order Siphonaptera and is the only flea species that regularly infests people as a primary host. Unlike the cat flea (Ctenocephalides felis) and the dog flea (Ctenocephalides canis), which prefer companion animals, P. irritans feeds on a broad range of mammals, but its life cycle is completed most often on human skin when conditions allow.

Adult fleas are wing‑less, laterally flattened insects measuring 2–4 mm. They locate hosts by detecting body heat, carbon dioxide, and movement. After a blood meal, females lay 30–50 eggs on the host’s clothing or bedding; eggs fall to the environment, where larvae develop in organic debris. The pupal stage can remain dormant for months, emerging when vibrations or increased carbon dioxide indicate a nearby host.

P. irritans is cosmopolitan, found in temperate and subtropical regions worldwide. It thrives in human dwellings with poor sanitation, especially where occupants wear infrequently washed clothing or use infested bedding. Outdoor habitats such as rodent burrows and bird nests also support populations that may spill over onto humans.

Medical impact includes irritation, pruritic papules, and secondary bacterial infection from scratching. The flea can mechanically transmit Yersinia pestis (plague) and, rarely, Rickettsia spp., though human-to-human transmission is uncommon.

Key points:

Primary host: humans, though occasional feeding on other mammals occurs.
Size: 2–4 mm; wing‑less, laterally compressed.
Life cycle: egg → larva → pupa → adult; pupal dormancy responsive to host cues.
Habitat: indoor environments with organic debris; also rodent and bird nests.
Health concerns: skin irritation, potential vector for plague and rickettsial diseases.

Fleas and Host Specificity

Can Animal Fleas Bite Humans?

Accidental Hosts

Animal fleas are obligate ectoparasites that normally complete their life cycle on specific mammalian or avian hosts. An accidental host is any organism that a flea contacts unintentionally, providing a temporary feeding opportunity without supporting the parasite’s full development.

Humans can become accidental hosts when fleas from pets, wildlife, or environmental reservoirs transfer onto the skin. The insects may feed briefly, reproduce poorly, and die without establishing a stable colony. This transient association does not indicate that humans are suitable primary hosts.

Factors that increase accidental human infestation include:

High flea density in the environment
Close contact with infested animals
Poor sanitation or clutter that shelters flea larvae
Seasonal temperature spikes that drive fleas from preferred hosts

Accidental feeding may cause itching, dermatitis, or allergic reactions, but it rarely leads to long‑term colonization. Control measures focus on eliminating flea populations from primary hosts and their habitats, thereby reducing the chance of incidental human exposure.

Factors Influencing Biting

Fleas that normally infest animals sometimes bite humans when certain conditions align. The likelihood of a bite depends on multiple biological and environmental variables.

Species specificity – Some flea species, such as the cat flea (Ctenocephalides felis), readily feed on human blood, while others prefer strict animal hosts.
Host availability – High density of domestic animals creates a reservoir of fleas; proximity increases the chance of accidental human contact.
Temperature and humidity – Warm, moist environments accelerate flea development and stimulate feeding activity, raising bite incidence.
Chemical cues – Carbon‑dioxide exhalation, sweat composition, and skin odors attract fleas; variations in these signals influence host selection.
Movement and vibration – Activity levels generate cues that trigger flea jumping and host engagement.
Blood type and immunological factors – Certain blood group antigens and immune responses affect flea attachment success.
Personal hygiene and grooming – Regular bathing and clothing changes reduce surface cues that attract fleas, lowering bite risk.
Previous exposure – Prior infestations can lead to residual populations that persist in the environment, maintaining a biting threat.

Understanding these determinants clarifies why human bites are occasional rather than constant, despite the presence of animal‑associated fleas in shared habitats.

Why Animal Fleas Prefer Animals

Physiological Differences

Animal fleas are adapted to the physiological environment of their typical hosts, which differs markedly from that of humans. Their cuticular enzymes efficiently digest the specific proteins and lipids found in the blood of mammals such as cats, dogs, and rodents, while human blood presents a distinct protein composition and lower levels of certain nutrients. This mismatch reduces the flea’s ability to obtain sufficient nourishment for development.

Temperature regulation also separates the two groups. Fleas that specialize in small mammals thrive at body temperatures around 36–38 °C, matching the thermal profile of those hosts. Human skin temperature averages 33–35 °C, creating a cooler microenvironment that slows flea metabolism and impedes egg production.

Respiratory adaptations further limit cross‑infestation. Host‑specific fleas possess tracheal systems tuned to the humidity and carbon‑dioxide concentrations typical of their usual hosts’ burrows or nests. Human skin emits lower CO₂ levels and maintains higher humidity than many animal habitats, challenging the flea’s respiratory efficiency.

These physiological distinctions explain why animal fleas rarely establish permanent colonies on humans, although temporary contact may occur. The combination of incompatible blood chemistry, suboptimal temperature, and mismatched respiratory conditions prevents sustained survival and reproduction on human hosts.

Environmental Factors

Environmental conditions determine the likelihood that animal‑derived fleas will infest humans. Fleas require a narrow temperature band, typically between 20 °C and 30 °C, to maintain metabolic activity and reproduce. Temperatures below this range slow development; temperatures above it increase mortality.

Relative humidity influences flea survival as well. Humidity levels of 70 %–85 % prevent desiccation of eggs and larvae, supporting population growth. Dry environments reduce egg viability and accelerate adult death.

Seasonal patterns reflect the combined effect of temperature and humidity. Warm, moist months see peak flea activity, while colder, drier periods suppress populations. Geographic regions with mild climates sustain continuous flea presence, whereas areas with harsh winters experience seasonal gaps.

Human environments affect flea transfer from animals to people. Factors include:

Presence of domestic animals that host fleas (dogs, cats, rodents)
Indoor heating or cooling that stabilizes temperature within the optimal range
High indoor humidity from poor ventilation or excessive moisture
Accumulation of organic debris (fur, skin flakes) that serves as food for flea larvae
Use of insecticidal treatments that lower flea survival rates

Sanitation practices modify these variables. Regular cleaning reduces organic material and humidity, limiting larval development. Conversely, cluttered, poorly maintained spaces create microhabitats favorable to fleas.

Overall, temperature, humidity, seasonality, host availability, and indoor conditions interact to shape the probability that animal fleas will establish on humans. Adjusting these environmental factors can substantially reduce infestation risk.

Flea Infestations in Humans

Signs of Flea Bites

Appearance of Bites

Fleas that normally infest pets or wildlife occasionally bite people when they encounter a suitable host. The insects deposit saliva while feeding, which triggers a localized skin response.

Typical flea bite lesions are small, round, red papules ranging from 2 mm to 5 mm in diameter. The center often appears slightly raised, surrounded by a faint halo of erythema. Multiple bites frequently cluster in groups of three to five, forming a “breakfast‑scrambled” pattern on exposed skin such as ankles, calves, waistline, and arms.

The reaction usually begins within minutes of the bite and peaks after 12–24 hours. Itching intensity varies, but most individuals experience moderate to severe pruritus that may lead to excoriation. In sensitized hosts, secondary inflammation can persist for several days, sometimes accompanied by swelling of the surrounding tissue.

Key points for distinguishing flea bites from other arthropod bites:

Grouped arrangement of 2–5 lesions, often linear or triangular.
Central punctum with a peripheral erythematous ring.
Preference for lower extremities and clothing‑covered areas.
Rapid onset of itching, followed by a possible delayed secondary flare.

If bites appear atypically or are accompanied by fever, rash spreading beyond typical sites, or signs of infection, medical evaluation is warranted.

Common Locations

Fleas that occasionally bite humans tend to settle on body regions that provide warmth, moisture, and protection from clothing friction. The most frequently infested sites are:

Scalp and hairline, where hair traps the insects and the skin is thin.
Neck and shoulder area, offering a concealed surface near the head.
Axillary folds, where sweat and reduced airflow create a favorable microclimate.
Groin and inner thigh, providing warmth and limited disturbance from movement.
Waistline and belt region, where clothing seams and elastic bands create crevices for concealment.

These locations allow fleas to feed efficiently while remaining hidden from the host’s immediate awareness. The distribution reflects the parasite’s need for a stable environment and easy access to blood vessels.

Symptoms Associated with Bites

Itching and Irritation

Animal fleas can bite humans, delivering saliva that contains anticoagulants and enzymes. The immediate reaction is a localized itch, often described as a sharp, pricking sensation that intensifies after a few minutes. Repeated bites produce clusters of red papules, sometimes forming a line known as a “breakfast‑scramble” pattern.

The irritation results from histamine release triggered by flea saliva. Histamine dilates blood vessels, increases permeability, and stimulates nerve endings, creating the characteristic redness and swelling. In susceptible individuals, the response may extend to:

Larger wheals lasting several hours
Secondary bacterial infection from scratching
Persistent dermatitis after multiple exposures

Continuous scratching can breach the epidermal barrier, allowing skin flora such as Staphylococcus aureus to invade. This secondary infection manifests as pus‑filled lesions, increased pain, and prolonged healing times.

Flea saliva also contains proteases that degrade skin proteins, prolonging inflammation. The inflammatory cascade involves cytokines (IL‑1, TNF‑α) that recruit additional immune cells, sustaining the itching cycle. Antihistamines, topical corticosteroids, and proper flea control interrupt this process by reducing histamine activity and inflammation.

Effective management requires:

Immediate cleansing of bite sites with mild antiseptic
Application of anti‑itch agents (e.g., hydrocortisone 1%)
Environmental treatment to eliminate flea reservoirs on pets and in the home

By addressing both the bite reaction and the source of infestation, itching and irritation can be minimized, preventing complications and reducing discomfort.

Allergic Reactions

Fleas normally infest dogs, cats, and other mammals, yet they readily bite humans when animal hosts are unavailable. During a blood meal the insect injects saliva that contains proteins capable of provoking an IgE‑mediated hypersensitivity reaction. The resulting condition, known as flea allergy dermatitis (FAD), manifests as an acute inflammatory response on the skin.

Typical manifestations include:

Intense pruritus at bite sites
Small, raised papules or pustules
Redness and swelling surrounding the puncture
Secondary bacterial infection if scratching persists

Diagnosis relies on a combination of patient history, detection of fleas or flea debris in the environment, and, when necessary, skin‑prick or serum IgE testing for flea antigens. Absence of alternative causes strengthens the attribution to flea‑related allergy.

Therapeutic measures focus on eliminating the source of exposure and controlling the inflammatory reaction:

Apply topical corticosteroids or calcineurin inhibitors to reduce local inflammation
Administer oral antihistamines for systemic itch relief
Treat the primary animal host with veterinary‑approved flea control products
Conduct thorough vacuuming, washing of bedding, and use of environmental insecticides to eradicate residual fleas

Preventive strategy consists of maintaining continuous flea control on pets, regular cleaning of living areas, and prompt removal of stray animals that could introduce new infestations. Consistent application of these measures suppresses flea populations and minimizes the risk of allergic skin reactions in humans.

Potential Health Risks

Secondary Infections

Animal fleas occasionally bite humans when their preferred hosts are unavailable, creating puncture wounds that serve as portals for additional pathogens. The immediate reaction is often a localized papule or hive, but the more serious concern lies in secondary infections that develop after the initial bite.

Common complications include:

Bacterial cellulitis – invasion of streptococci or staphylococci into the dermal tissue, presenting with redness, swelling, and pain; requires prompt antimicrobial therapy.
Impetigo – superficial pyoderma caused by Staphylococcus aureus or Streptococcus pyogenes, producing honey‑colored crusts that spread rapidly across adjacent skin.
Allergic dermatitis – hypersensitivity to flea saliva, leading to intense pruritus and excoriation, which can become secondarily infected with skin flora.
Vector‑borne diseases – fleas can transmit Yersinia pestis (plague), Rickettsia typhi (murine typhus), and Bartonella henselae (cat‑scratch disease); these infections manifest systemically and demand specific diagnostic and therapeutic measures.

Effective management begins with thorough cleansing of bite sites, application of antiseptic agents, and monitoring for signs of infection. When erythema expands, purulence appears, or systemic symptoms such as fever develop, medical evaluation is essential to initiate appropriate antibiotic regimens and, if necessary, address any underlying vector‑borne illness.

Disease Transmission (Rare)

Animal fleas primarily parasitize mammals such as dogs, cats, rodents, and livestock. Occasionally they bite humans, but sustained colonization on human hosts is uncommon. When human infestation occurs, the principal health concern is the rare transmission of pathogens that fleas can carry.

Fleas implicated in zoonotic disease transmission include:

Yersinia pestis – the bacterium responsible for plague; transmitted by the oriental rat flea (Xenopsylla cheopis) after feeding on infected rodents. Human cases are sporadic, limited to endemic regions.
Rickettsia typhi – agent of murine typhus; spread by the cat flea (Ctenocephalides felis) and rat flea. Infection rates remain low in temperate zones but can rise in areas with poor rodent control.
Bartonella henselae – causes cat‑scratch disease; cat fleas serve as vectors, transferring the bacterium to humans through flea feces that contaminate scratches or bites. Cases are infrequent but documented.
Dipylidium caninum – a tapeworm whose larval cysticercoid develops inside flea larvae. Humans acquire infection by ingesting an infected flea, a rare event linked to close contact with pets.

Transmission pathways share key characteristics: fleas must first feed on an infected animal reservoir, acquire the pathogen, and then contaminate the human host either through a bite, contact with flea feces, or accidental ingestion. Human infestation seldom leads to large‑scale outbreaks because fleas lack adaptations for long‑term survival on people, and effective veterinary ectoparasite control reduces reservoir exposure.

Risk mitigation focuses on eliminating flea infestations in domestic animals, maintaining rodent control, and applying approved insecticidal treatments in homes. Prompt removal of fleas from the skin and proper hygiene lower the probability of pathogen transfer. Although the diseases listed are uncommon, they illustrate the potential medical relevance of animal fleas when they temporarily occupy human hosts.

Managing Flea Exposure

Preventing Flea Infestations

Pet Flea Control

Pet fleas frequently bite people, transferring from dogs or cats to human skin. While they cannot complete their life cycle on a human host, temporary infestations cause itching and possible allergic reactions. Effective control of pet fleas therefore reduces the risk of accidental human exposure.

Key components of a comprehensive flea‑management program include:

Regular veterinary‑prescribed treatments – topical spot‑on products, oral medications, or collars that kill adult fleas and interrupt development stages.
Environmental sanitation – frequent vacuuming of carpets, rugs, and upholstery; washing pet bedding in hot water; and disposing of vacuum bags or contents immediately.
Insect growth regulators (IGRs) – chemicals applied to indoor areas that prevent eggs and larvae from maturing, breaking the infestation cycle.
Seasonal monitoring – intensified treatment during warm months when flea reproduction accelerates, with continued vigilance through cooler periods to eliminate residual eggs and pupae.

Consistent application of these measures limits flea populations on pets, thereby minimizing the likelihood of human bites. Monitoring pets for signs of flea activity and responding promptly with veterinary‑approved products sustains a flea‑free environment for both animals and people.

Home Environment Management

Fleas are ectoparasites that prefer mammals such as dogs, cats, and rodents. Humans can be bitten when fleas encounter a person, but humans are not a suitable long‑term host; fleas rarely complete their life cycle on human skin.

Preventing accidental human exposure requires managing the home environment where fleas develop. Effective control focuses on interrupting the flea life cycle—egg, larva, pupa, adult—within the residence.

Regularly vacuum carpets, rugs, and upholstery; discard vacuum bags or clean canisters immediately.
Wash bedding, pet blankets, and removable covers in hot water (≥ 60 °C) weekly.
Treat pets with veterinarian‑recommended flea preventatives; maintain consistent monthly dosing.
Apply an insect growth regulator (IGR) to indoor areas where pets rest; IGRs inhibit development of immature stages.
Keep indoor humidity below 50 % to discourage larval survival; use dehumidifiers if necessary.
Seal cracks and gaps around doors, windows, and baseboards to limit entry of wild rodents and flea carriers.

Monitor for signs of infestation by inspecting pets for fleas, checking for small dark specks (flea dirt) on fabric, and noting unexplained bites on occupants. Prompt elimination of detected fleas prevents escalation to a situation where humans may experience repeated bites.

A systematic approach—cleaning, pet treatment, environmental chemicals, and humidity control—reduces the likelihood that fleas will encounter and bite people, ensuring a healthier home environment.

What to Do if Bitten

First Aid for Bites

Fleas primarily infest animals, yet they can bite people and, in rare cases, remain on human skin. Bites cause itching, redness, and possible irritation, requiring prompt care to reduce discomfort and prevent infection.

Wash the area with mild soap and lukewarm water.
Pat dry with a clean towel; avoid rubbing.
Apply a cold compress for 5–10 minutes to lessen swelling.
Use an over‑the‑counter antihistamine cream or oral antihistamine if itching is intense.
Cover the bite with a sterile adhesive bandage only if the skin is broken.

Monitor the site for signs of infection: increasing warmth, pus, expanding redness, or fever. Seek professional medical attention if any of these symptoms appear, or if an allergic reaction develops (e.g., hives, difficulty breathing).

Prevent further bites by maintaining personal hygiene, regularly washing bedding, and treating pets with approved flea control products. Keeping living spaces clean and vacuumed reduces the likelihood of flea exposure.

When to Seek Medical Advice

Animal fleas primarily infest pets and wildlife, yet they can bite humans when host animals are unavailable. Human exposure usually results in localized skin irritation, but certain circumstances demand professional evaluation.

Seek medical advice if any of the following occur:

Persistent or worsening redness, swelling, or pain beyond a few days.
Development of a rash, blistering, or ulceration at bite sites.
Fever, chills, or flu‑like symptoms accompanying the bites.
Signs of secondary infection such as pus, increased warmth, or spreading redness.
Allergic reactions, including hives, difficulty breathing, or swelling of the face and throat.
Exposure to fleas in a household with infants, elderly individuals, or immunocompromised patients, especially if the bite count is high.

A healthcare provider can confirm whether an allergic response, infection, or a vector‑borne disease such as murine typhus or plague is present, and prescribe appropriate treatment. Prompt consultation reduces the risk of complications and ensures effective management of flea‑related health issues.

Distinguishing Fleas

Animal Flea vs. Human Flea

Key Differences in Appearance

Animal fleas that typically infest pets, rodents, or wildlife differ visibly from the species that occasionally bite humans. These distinctions aid identification and clarify whether a flea found on a person originated from an animal host.

Size: Pet‑associated fleas (e.g., Ctenocephalides felis or C. canis) measure 2–4 mm when unfed; the human flea (Pulex irritans) averages 1.5–2 mm, slightly smaller but overlapping in range.
Body shape: Dog and cat fleas possess a laterally flattened body adapted for moving through dense fur, while the human flea exhibits a more rounded, less flattened thorax suited for sparse hair.
Coloration: Pet fleas display a reddish‑brown hue that darkens after a blood meal; the human flea is generally lighter, ranging from tan to pale brown, and retains a consistent shade regardless of feeding.
Jumping ability: Leg musculature in pet fleas enables jumps up to 150 mm, considerably farther than the human flea’s maximum of about 70 mm.
Antennae and setae: Pet fleas have longer, more pronounced sensory hairs on the head and abdomen, whereas the human flea’s setae are shorter and less dense.

These morphological traits allow entomologists and clinicians to differentiate between fleas that have transferred from animals and those that are native to human hosts. Recognizing the visual cues prevents misdiagnosis and informs appropriate control measures.

Behavioral Distinctions

Fleas that primarily infest mammals such as dogs, cats, and rodents exhibit distinct host‑selection behavior compared to species that regularly bite humans. The former rely on scent cues from fur, body heat, and carbon‑dioxide levels typical of their preferred animals, while human‑associated fleas respond to different chemical signals and often encounter a smoother skin surface that affects attachment.

Feeding patterns differ markedly. Animal‑specific fleas initiate blood meals after a brief period of crawling on the host’s coat, then drop off to complete digestion within a protected environment like the host’s bedding. Human‑biting fleas tend to remain on the skin longer, feeding repeatedly throughout the night and producing faster‑growing larvae in household debris.

Environmental tolerance also separates the groups. Fleas adapted to pets thrive in warm, humid microhabitats such as carpets, pet beds, and outdoor shelters. Human‑targeted fleas survive better on dry indoor surfaces, tolerating lower humidity and persisting in cracks, upholstery, and clothing.

Key behavioral distinctions:

Host‑cue preference: fur odor vs. human skin odor
Attachment method: claws on hair vs. claws on bare skin
Feeding duration: short, intermittent vs. prolonged nightly bouts
Development site: pet bedding vs. household dust and fabrics

These differences explain why most animal fleas rarely establish permanent colonies on people, while a minority of species have evolved habits that allow them to exploit human hosts effectively.