Modern Flea Eradication: The Best Of The Old And The New

Terese C. DeManuelle, BSc, DVM, Diplomate ACVD
Allergy & Dermatology Veterinary Referral Center, Milwaukie, OR
Adjunct Professor of Dermatology, Washington State
University College of Veterinary Medicine, Pullman, WA


            Throughout most of the world, flea infestation is the most common ectoparasitic condition of dogs and cats.  Flea allergy dermatitis is the most common small animal dermatologic disease. 

Efforts to control fleas have advanced in recent years because of the availability of topically applied flea control products and have dramatically altered attaining the goal of flea eradication.


            The recent development of effective topical and systemic flea control products has decreased the necessity of treating the environment.  Environmental flea control may still be necessary at the onset of flea control when a heavy flea burden exists or when a multiple pet household is treated or when flea allergy dermatitis exists in pets or ownersand rapid eradication of fleas is essential.  The treatment of choice for the environment is a residual insect growth regulator (methoprene, pyriproxifen) or other larvicide in combination with an adulticide.  


Knowledge of flea biology is necessary in order to determine the class of flea control necessary for the individual pet and/or the environment.  The cat flea, Ctenocephalides felis felis, is most common flea infesting dogs, cats, pet rabbits, guinea pigs and ferrets in United States.  The cat flea has a variable length of developmental cycle depending on temperature and humidity.  Under conditions of  75 F and 78% relative humidity, the developmental cycle varies from 17 to 26 days depending on the insect’s gender.  Dryness, high heat and extreme cold will prolong the life cycle and may be deleterious to larvae.

Flea eggs are laid on the host within 24-36 hours after flea’s first blood meal.  Approximately seventy percent of eggs fall from haircoat within eight hours and are distributed in the any portion of the environment to which the  pet gains access.  Flea larvae are susceptible to heat and dessication.  Flea pupae are resistant to dessication.  Adult fleas commence feeding and mating within the first twenty-four hours.  The cat flea is a permanent ectoparasite, which means that once on the host, the flea is removed only by grooming activity.


Flea saliva contains a histamine-like compound, enzymes that have cytolytic, anticoagulant, and proteolytic properties, and complete protein antigens of 14-90 kilodaltons.  Flea allergy dermatitis is characterized as Type I, Type IV, late phase (IgE), and cutaneous basophil hypersensitivity.  The immunologic mechanism of flea allergy dermatitis that develops in a specific animal is contingent upon the length and degree of exposure to fleas and genetic predisposition.  Animals with atopic dermatitis are predisposed to development of flea hypersensitivity.  

            There is no breed or sex predilection for flea allergy dermatitis.  The development of flea allergy dermatitis may occur at any age.  The disease initally may be seasonal.  Flea allergy dermatitis may occur year-round contingent on the environmental parameters conducive to flea development.             The lesions of flea allergy dermatis are multiple and consist of licking, chewing, excessive grooming, especially involving the rear half of the body, dorsal lumbosacral area and groin.  Fibropruritic nodules are uncommon lesions seen in dogs with chronic flea allergy dermatitis.  The lesions are usually located in the dorsal lumbosacral region of dogs and are firm alopecic nodules of 0.5 to 2 cm in diameter.

Commonly noted signs of flea allergy dermatitis in cats include symmetric alopecia (self induced) and miliary dermatitis (generalized or “collar” pattern).  Feline eosinophilic disease associated with flea allergy dermatitis include eosinophilic plaques, indolent ulcers, and eosinophilic granulomas.  A peripheral eosinophilia has been documented in approximately 20% of cats diagnosed with flea allergy dermatitis.  

Diagnosis and treatment of secondary pyoderma in dogs with flea allergy dermatitis is an important component of therapy.  Dogs with flea allergy dermatitis may have accompanying lesions of pyotraumatic dermatitis, superficial pyoderma, or deep pyoderma.(Figure 3)  The lesions of deep pyoderma with accompanying cellulitis seen in German Shepherd Dog pyoderma are commonly associated with flea allergy dermatitis.   

            Antihistamine and fatty acid therapy is rarely effective in dogs and cats with flea allergy dermatitis.  Fatty acid therapy has been proposed for the treatment of cats with flea-induced miliary dermatitis.             Immunodiagnostic tests available for diagnosis of flea allergy include intradermal skin testing with total flea extract (1:1000 wt/vol).  An immediate reaction (20 minutes) will be observed in 80% of patients with flea allergy dermatitis; 20% of patients will exhibit a delayed reaction (48-72 hours).   Serum in vitro testing has variable accuracy in the diagnosis of flea allergy dermatitis.  Multiple hyposensitization studies have shown no significant benefit for therapy of canine or feline flea allergy dermatitis.4-6


There are several important concepts to consider for effective flea control.  The most important concept is complete flea eradication.  Treatment of all “in contact” animals and possibly environment may be necessary.  It is important to focus on “source points” which are areas of most preferential flea development.  Individualized “personalized” protocols are the most effective form of flea management.

The ideal treatment is safe, non-toxic, non-irritating, immediate effect, repellant effect, long duration of action, and activity against multiple stages of the flea cycle



-           Advantages: convenience of use, residual activity, activity on flea cycle (inhibitors of chitin synthesis (lufenuron, cyromazine, diflubenzuron), repellent activity (fenthion=Pro-Spot )

-           Disadvantages: incomplete efficacy for flea allergic animals, limited efficacy on male fleas (cythioate=Proban ) due to insufficient absorption during blood meal



-           Advantages: good dispersion of products, focal treatment of source points

-           Disadvantages: expensive cost, cumbersome application

-           Active ingredients: pyrethroids, organophosphates, insect growth regulators (pyriproxifen, methoprene)


-           Advantages: convenience of use

-           Disadvantages: expensive cost, insufficient concentration at source points, poor dispersion around corners and under furniture


-           Mechanism of action: toxic effect on flea larvae by ingestion (Dryden, unpublished data, 1993), blockage of flea digestive tract?, abrasion of cuticle and dessicating effect?

-           Advantages: possible use of inert, non-insecticide compounds (e.g. sodium polyborate)

-           Disadvantages: cumbersome application, poor aesthetic effect, variable expense, efficacy decreased in presence of water-cannot shampoo carpets


-           Mechanism of action: entomopholic nematode (Steinernema carpocapsae) larvae is parasite of insect larvae (including flea larvae) and infested with pathogenic bacteria; nematodes penetrate the insect cuticle and release the pathogenic bacteria

-           Advantages: non insecticide agents, specific, active, targeting of the flea larvae, absence of mammalian toxicity

-           Disadvantages: limited to outdoor use, destruction of some saprophyte insect larvae, will not colonize, requires monthly application, outdoor use only, 20% soil moisture requirement, cost?



Imidacloprid is a nitroguanidine compound that binds to postsynaptic portion on nicotinic receptors preventing impulse transmission.  Advantage® is available as a 9.1% solution for dogs and cats.  Advantage® is applied as a spot treatment every 28 days; more optimal response (flea allergic animals) requires therapy every 21 days and in some cats every 14 days.  Ease of application greatly enhances owner compliance.  Manufacturer literature shows no evidence of toxicity in animals receiving exaggerated dosages.  There is concern for breakdown after bathing or water exposure, therefore, limit shampooing and swimming between applications or reapply.


Fipronil is a phenylpyrazole compound that disrupts CNS activity by interference with chloride ion passage through GABA-regulated chloride channels of invertebrates; selective neurotoxicity for insects.  Frontline® is available in spray and spot-on formulations, is acaracidal and has been labeled for ticks.  There have been anecdotal reports of clinical success for treatment of sarcoptic acariasis with aggressive application once every 2 weeks for 3 treatments.  There have also been reports of improvement in treatment of Otodectes (ear mites).  Use of Frontline® for sarcoptic acariasis and Otodectes is extralabel usage.

Frontline® has an affinity for sebaceous secretions.  The chemical is found in hair follicles and sebaceous glands and is redistributed to skin surface for residual activity.  Studies using Frontline spray monthly with weekly benzoyl peroxide, anti- seborrheic or emollient shampoos demonstrate that a reduced residual efficacy may occur.  To control fleas in flea allergic cats, it may be necessary to reapply Frontline® every 2-3 weeks.  Frontline® should be used every 30 days to control fleas on dogs. Multiple reports of toxicity with use in rabbits indicate that Frontline Spray or Topspot usage is contraindicated.  Toxicity may be related to the vehicle.  


            Lufenuron is a benzoylphenyl urea.  Program ® is considered an extremely safe flea medication with no mammalian toxicity since mammals do not produce chitin There is no direct toxic effect on adult fleas and the time factor to disrupt the flea life cycle is significant (60-90 days).  The adult flea must feed on animal to ingest; thereby inducing an allergic reaction.  There is a significant potential for reinfestation by ova from untreated animals with access to environment.  Injectable lufenuron is available for cats and is administered subcutaneously once every 6 months.  Report of possible fibrosarcoma associated with injectable form.  Program® should be administered with a meal or decreased absorption may be noted.  Program® has limitations as sole therapy in a flea allergic animal but may be used adjunctive therapy or as a component of an integrated pest management program.


Knockout® is a 2% permethrin (mechanism of action: activity on sodium channels) and pyriproxifen (insect growth regulator) at 0.05% concentration.  Knockout® has efficacy against fleas and ticks as well as a repellent effect.  It is one of the lesser expensive new forms of canine flea control.  For enhanced efficacy in flea allergic dogs, use Knockout® twice weekly for first month, then on a weekly basis.  The product is intended for usage in adult dogs only.  Reapplication is necessary after swimming or bathing.  The alcohol base provides “knockdown” effect to fleas.

The product is not intended for use in cats.  The Knockout® product is also available in an environmental spray and may be used outdoors as pyriproxifen is not inactivated by sunlight.  


Selamectin was released in September 1999.  Selamectin (Revolution®), an avermectin, kills fleas by inducing neuromuscular paralysis.  Revolution® is distributed into sebaceous glands.  Revolution® kills adult fleas and manufacturer literature reports prevention of eggs from hatching.  Revolution® is also labeled for treatment of ear mites, sarcoptic acariasis, intestinal nematodes, and for use as a heartworm preventative.  A concern exists for efficacy with frequent shampooing with sebum-stripping products, therefore, Revolution® may need to be reapplied for optimal efficacy. Treatment is once monthly. Revolution® has FDA labeling; Advantage and Frontline are EPA labeled.


Integrated pest management (IPM) has been recommended to decrease the incidence of resistance to the newer flea control products.  The cat flea, of all of the species of fleas tested, is resistant to the greatest number of categories of insecticides. 

Dryden has shown that even with the newer insecticides (fipronil and imidacloprid), insecticide selection pressure can produce resistant fleas.  The resistance concentration significantly reduces the residual efficacy of the products. 

A program has recently been initiated by the Bayer Corporation to monitor the susceptibility of field flea populations to imidacloprid (Advantage®).  The program will develop susceptibility assays, identify flea strains with reduced susceptibility, determine the mechanisms of reduced susceptibility, and develop strategies to counter any decrease in efficacy.  The initiative involves international researchers in flea biology and control.