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Parasitology, Preventive Medicine

Conquering Fleas: Preventing Infestations & Limiting Disease Transmission

Conquering Fleas: Preventing Infestations & Limiting Disease Transmission


Susan Little, DVM, PhD, Diplomate ACVM (Parasitology), and Lindsay Starkey, DVM

The cat flea, Ctenocephalides felis, readily feeds on cats, dogs, humans, and other animals, causing local pruritus, allergic reactions and, in severe infestations, anemia.1

Fleas are particularly troublesome because populations are quickly established in homes and kennels where constant sources of nutrition—pets—are readily available.

Eggs, larvae, or pupae comprise the majority of fleas in an infestation (Figure).2 Once immature stages are present, they may continue to emerge for weeks to months, despite control of adult fleas.2 Flea populations can persist indefinitely when untreated pets or other animals are present, creating a constant source of infestation to pets and humans.

Figure. Immature stages of fleas, including eggs (A), larvae (B), and pupae (C), can provide a continuous, long-term source of reinfestation for pets in the home.


Figure A


Figure B

Figure 1C

Figure C


Fleas are also a major source of disease. Pets infested with cat fleas have been associated with transmission of an increasing number of infections, including those that cause zoonotic disease.3 Infections may be transmitted by:

  • Flea bites
  • Flea frass (feces)
  • Fleas ingested during grooming.

Infectious agents transmitted by cat fleas include:

  • Bartonellosis (Bartonella henselae and other Bartonella species)
  • Murine typhus and other rickettsial infections (Rickettsia typhi and R felis)
  • Hemotropic mycoplasmosis (Mycoplasma haemofelis and Candidatus M haemominutum)
  • Tapeworm (Dipylid ium caninum).


The most well-known form of bartonellosis in humans is Bartonella henselae infection (cat scratch disease). Infected individuals, usually children, develop fever and regional lymphadenopathy after a cat scratch or bite.

  • The role of fleas in transmitting B henselae between cats is well established; transmission of other Bartonella species to cats, dogs, and wildlife hosts is also likely.4,5
  • Fleas feeding on bacteremic animals shed the organisms in their feces; the bacteria-laden frass is transferred to claws and teeth during grooming, resulting in infection if a human is bitten or scratched.6
  • Other disease manifestations have been associated with bartonellosis in recent years, including arthropathies, myalgia, and granulomatous hepatitis.7-9

Rickettsial Infections

While rickettsial infections are commonly considered tick borne, some pathogens are transmitted by fleas.

  • Rickettsia typhi is responsible for cases of murine (flea-borne) typhus reported each year in humans from Texas, California, and Hawaii.10
    • Infection involves opossum reservoir hosts and C felis infestations in pets.11,12
    • Both flea bites and inhalation of flea frass are considered likely sources of human infection13; affected humans develop rash, fever, headache, and muscle pain.12
  • Rickettsia felis is a more recently identified organism that has been found worldwide in C felis populations and it is associated with murine typhus-like illnesses in humans.14,15

Hemotropic Mycoplasmosis

  • Mycoplasma haemofelis is another agent widely thought to be transmitted by cat fleas.16
    • Infections can cause feline infectious anemia—characterized by severe anemia, anorexia, and lethargy.
    • The disease is more common in immunosuppressed cats.17,18
  • Candidatus Mycoplasma haemominutum”, a related agent, appears less pathogenic.
    • Anemia in cats with “Candidatus Mycoplasma haemominutum” is usually mild, if it occurs at all, even when co-infections are present.
    • A number of cases have been reported in cats infected with feline leukemia virus (FeLV).18,19

Infection with feline hemoplasmas is more common in male cats and cats with concurrent feline immunodeficiency virus (FIV) or FeLV.20 Fleas collected from infected cats harbor the bacterium, but experimental transmission studies have not yet been successful, leading some to suspect that transmission may occur through another route, such as bite wounds from fighting.18,21,22

Dipylidium caninum Infection

Dipylidium caninum (tapeworm) is another common infection vectored by fleas. Dogs and cats are readily infected upon ingestion of fleas containing cysticercoids while grooming, with prevalence rates as high as 62% of dogs and 22% of cats.23

  • While not considered pathogenic, D caninum infections are aesthetically unpleasant for the owner as proglottids are often found on the perianal region or in the home environment.
  • Effective treatment of D caninum infection requires:
    1. Cestode treatment
    2. Diligent flea control.
    Treatment of both fleas and cestodes prevents re-infection of pets and reappearance of proglottids.1
  • Human infection with D caninum (due to ingestion of fleas containing cysticercoids) is commonly reported, most often in children, and may be confused with pinworms.24-26


Untreated Pets

One reason flea infestations continue despite attempted control efforts is the presence of untreated pets in the home environment.

  • Some clients treat a favorite indoor dog, but another dog that resides primarily outdoors or only visits occasionally remains untreated.
  • Similarly, owners may treat their dogs but fail to implement flea control for their cats, especially those that only occasionally go outdoors.
  • Pets of roommates, visiting family or friends, or those living in adjacent condominiums or apartments can continually expose treated pets to fleas.

Untreated pets seed the environment with eggs, creating a constant source of newly-emerged fleas that leads to failure of flea control.27

Wildlife, Strays, & Roaming Pets

Although many home owners are unaware of their presence, flea-infested wildlife, strays, and roaming pets contribute to flea burdens around homes.27,28 These flea-bearers include:

  • Peridomestic wildlife, such as opossums, often take refuge in crawl spaces or beneath porches and decks.
  • Raccoons or foxes investigate the trash on a nightly basis.
  • Stray or feral cats hunt around the neighborhood.
  • Neighbors’ unruly dogs visit other pet owners’ lawns.

Areas where many animal populations mingle—neighbors’ homes, dog parks, boarding kennels, and camp sites—provide an excellent opportunity for newly-emerged adult fleas to find food sources, such as pet dogs, and ultimately create an infestation in the home.

Simply put: fleas are ubiquitous. However, measures can be taken to protect pets from these voracious insects.


The vast majority of a home’s fleas—eggs, larvae, and pupae—resides in carpets, pet beds, and furniture.2 Adult fleas on the pet account only for about 5% of the overall flea population.

Stages of Fleas

  • Eggs and larvae are usually concentrated in areas where pets spend most of their time. While these stages can be killed, they may be difficult to reach as they are often buried in carpet, upholstered furniture, or cracks and crevices, such as grooves in a hardwood floor.
  • Pupal stages are very difficult to destroy with insecticides and can last for months in the environment, waiting for an opportunity to start an infestation.29
  • Newly emerged adult fleas feed on pets and produce more offspring, creating or maintaining an infestation, when flea control is not consistently applied every month.

Flea Control Products

A number of highly effective flea control products are available in the U.S.

  • Adulticides are compounds that kill adult fleas on treated pets; some compounds also kill developing larvae within the egg. Adulticides include:
    • Deltamethrin
    • Dinotefuran
    • Fipronil
    • Imidacloprid
    • Indoxacarb
    • Nitenpyram
    • Permethrin
    • Pyrethins
    • Selamectin
    • Spinosad.
  • Insect growth regulators and insect development inhibitors prevent normal development of eggs and larvae, killing developing stages.30 Examples include:
    • Lufenuron
    • Methoprene
    • Pyriproxyfen.
  • Combining adulticides with compounds that kill developing flea stages is recommended to ensure environmental infestations do not become established.29

However, the most important component of any flea control strategy is consistent use of control products.

  • In ideal conditions of warmth and humidity, a fairly moderate flea infestation that continues for days to weeks can lead to environmental contamination with eggs, resulting in emergence of hundreds of fleas each day.
  • When conditions are less than ideal, such as clean, dry home environments, or in the winter months when air is drier and colder, most larvae are killed by desiccation and starvation; however, brief breaks in flea control still allow continued infestations.31

Year-Round Control

Many clients believe that flea control is not necessary during winter. It is true that all life stages of fleas are killed when temperatures fall below freezing (30.2°F or -1°C) for > 5 days.32

However, adult fleas within the hair coat or immature stages inside climate controlled homes or kennels may survive even when outdoor temperatures plummet. Newly developed adult C felis can survive indoors within the pupal case for 4 to 5 months, and delayed emergence of adult C felis has been suggested as an overwintering strategy.31,33

Mild winters often result in unexpected early flea infestations in pets that haven’t been consistently treated during what has been historically considered the “off-season.” Pets must be treated every month, all year long to keep flea populations in check.

Treating Pets & the Environment

Once a flea infestation has developed in a home, it often takes several weeks to months to eliminate. Owners should be aware that:

  • A single treatment will not eliminate infestations in pets (and the home).
  • While most fleas are seen on pets, pets host only a small fraction of the entire flea population.
  • If significant numbers of developing stages are present, the infestation will take several weeks to months to eliminate.

Treatment for flea infestations often involves multiple steps:

  1. Treating the pet: In most cases, treating the pet with insecticides + compounds effective against immature stages of fleas is adequate to eliminate mild to moderate infestations as long as every pet is treated every month.
  2. Investigating flea sources: When infestations persist despite consistent use of effective compounds for at least 3 months, sources of flea introduction should be investigated, including:
    • Untreated animals (domestic and wild)
    • Pet access to contaminated areas, such as under decks or in bushes
    • Highly contaminated home environment.
  3. Applying premise sprays: Severe flea infestations may take several months to eradicate. Application of premise sprays, either by a licensed pest control operator or the pet owner, will likely accelerate flea elimination. Treatment of premises should only be used in combination with consistent, routine treatment of all pets.
  4. Habitat management: These strategies can remove a significant number of developing stages of fleas:
    • Washing rugs, beds, and blankets
    • Thorough vacuuming of wood floors, carpets, and upholstered furniture, including under cushions
    • Disposing of vacuum cleaner bags immediately with municipal waste outdoors.


  1. Administer insecticides to all pets in the household, including:
    • Both dogs and cats
    • Indoor and outdoor pets
  2. Use compounds effective against immature stages of fleas:
    • On all pets in the household
    • In the environment
  3. Apply preventive products to pets:
    • Consistently
    • Year round
  4. Limit encounters, if possible, with potentially infested animals, such as:
    • Pets of friends and neighbors
    • Wildlife and feral animals
    • Areas where pets congregate—dog parks, camp sites, boarding kennels, etc.
  5. Protect the home environment, both indoors and outdoors:
    • Apply premise sprays:
      • By licensed pest control operator or pet owner
      • In combination with consistent, routine treatment of pets
    • Wash pet bedding and accessories
    • Thoroughly vacuum floors, carpets, and furniture; dispose of vacuum bags immediately outdoors


Dogs and cats have ample opportunities to become infested with limitless supplies of fleas.

  • When fleas feed on pets, they breed, contaminating the environment with eggs, which will establish a home infestation that can take several weeks to months to eliminate.
  • Consistently treating all pets in the home with quality flea adulticides limits feeding of adult fleas, reducing their overall reproductive capacity; incorporating a product that prevents maturation of immature stages further hinders establishment of infestations.
  • Flea control prevents transmission of diseases to pets and people.
    Consistent use of products—in every pet, every month, all year long—kills adult fleas and limits development of immature fleas. This approach is a critical aspect of basic wellness care and responsible pet ownership. Every pet deserves to live flea-free.


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f04_LittleSusan Little, DVM, PhD, Diplomate ACVM (Parasitology), is the Director of the National Center for Veterinary Parasitology at Oklahoma State University’s Center for Veterinary Health Sciences, where she serves as Regents Professor and the Krull-Ewing Chair in Veterinary Parasitology. Her research focuses on zoonotic parasites and tick-borne diseases. She has been recognized for teaching excellence through the Pfizer (Norden) Distinguished Teaching Award (1999 and 2010) and the national Student AVMA Excellence in Teaching Award (1998 and 2005), and for her outstanding research through the Pfizer Award for Research Excellence (2012). Dr. Little received her DVM from Virginia Tech.

f04_StarkeyLindsay Starkey, DVM, currently serves as the Bayer Resident in Veterinary Parasitology through the National Center for Veterinary Parasitology at Oklahoma State University. In 2012, Dr. Starkey received the AAVP/CAPC Graduate Student Award in Zoonotic Disease from the American Association of Veterinary Parasitologists, for her dissertational research, which focuses on ticks and tick-borne diseases. She received her DVM from Oklahoma State University.