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Veterinary Therapeutics Winter 2007 (Vol 8, No 4)

Efficacy of Selamectin and Fipronil-(S)-Methoprene Spot-On Formulations Applied to Cats against Adult Cat Fleas (Ctenocephalides felis), Flea Eggs, and Adult Flea Emergence

by Michael W. Dryden, DVM, MS, PhD, Patricia Payne, DVM, PhD, V. Smith, RVT

    Clinical Relevance

    A study was conducted to evaluate the efficacy of selamectin and fipronil-(S)-methoprene against adult cat fleas (Ctenocephalides felis), flea egg production, and the viability of flea eggs collected from treated cats. Cats were infested with approximately 50 adult fleas 2 days before treatment and weekly thereafter; flea eggs were collected and counted on days 0, 1, 2, and 3 and 48 and 72 hours after each weekly flea infestation. Live fleas were collected approximately 72 hours after treatment or infestation. Compared with fipronil-(S)-methoprene, selamectin provided significantly greater control of adult fleas from days 24 to 31 and significantly greater reduction in egg production from days 16 to 45. For the most part, both products significantly impacted larval and adult emergence for the entire 6-week study, with fipronil-(S)-methoprene providing significantly greater reduction in larval and adult emergence at week 6.


    Elimination of fleas on dogs and cats and from their surrounding environments can be achieved by killing adult fleas currently residing on the host and eradicating existing environmental life stages. Several insecticides, such as fipronil, imidacloprid, nitenpyram, and selamectin, do an excellent job of killing existing adult fleas on pets.1-6 After treatment, however, pets continue to reside in their home environments, where flea eggs, larvae, pupae, and emerging fleas remain. These life stages provide an ongoing source of new adult fleas that will continually reinfest pets treated only with adulticidal compounds.7 Classically, insecticides and insect growth regulators were repeatedly applied into the environment in an attempt to eliminate these life stages.7,8 Client compliance with environmental treatment protocols has been disappointing, and therefore ineffective, with resulting recurrent infestations.

    A different approach that combines topical and/or systemic adulticides with ovicidal compounds will ultimately eliminate flea infestation by controlling reproduction.9-11 This approach essentially turns the treated pet into a "living flea vacuum."

    Residual flea adulticides such as dinotefuran, fipronil, imidacloprid, metaflumizone, selamectin, and spinosad provide prolonged activity, but for an adulticide to stop flea reproduction, it must either kill or render toxic newly acquired fleas within 24 hours.9,12 A previous investigation determined that while fipronil, imidacloprid, and selamectin have pronounced residual activity, the speed at which they kill fleas on cats decreases throughout the month following application.2 In addition, although the study determined that selamectin provided the highest level of residual activity on cats against the Kansas 1 (KS1) flea strain, it did not kill all fleas within 24 hours for 30 days, allowing for the possibility of egg production between monthly applications.

    Flea eggs can be killed by administration of topical or systemic insect growth regulators such as methoprene, pyriproxyfen, or lufenuron.6,13-15 Insect growth regulators can be used in flea control to provide prolonged residual ovicidal activity; thus, if fleas survive the activity of an insecticide and produce eggs, these compounds will prevent the development of larvae within the laid eggs.

    The following study was conducted at Kansas State University College of Veterinary Medicine to evaluate the effectiveness of selamectin (Revolution, Pfizer Animal Health) and fipronil-(S)-methoprene (Frontline Plus, Merial) spot-on formulations to kill adult cat fleas (Ctenocephalides felis), reduce egg production, reduce egg viability, and inhibit adult flea emergence.

    Materials and Methods


    Twenty-one purpose-bred domestic shorthaired cats, 4 to 6 months of age and weighing 1.9 to 3.2 kg, were used in this study. The cats used had no exposure to ectoparasiticides before treatment and were in good health throughout the study.

    On day -6, each cat was infested with approximately 50 C. felis from the KS1 strain established and maintained as a closed colony at Kansas State University. On day -3, flea comb counts were conducted to assess the ability of cats to maintain infestations. Cats were combed with a fine-toothed flea comb having 12 to 13 teeth/cm. Flea removal was achieved by combing each cat thoroughly for 10 minutes. If five or more fleas were recovered during this period, the cat was combed for an additional 5 minutes; if any fleas were recovered during the second combing period, the cats were again combed for an additional 5 minutes. The 18 cats used in the study were selected based on highest pretreatment flea counts. These cats were ranked in descending order by flea count and assigned to the three treatment groups using a randomized block design.

    All cats were housed individually in indoor cages in the same room. Each cage was approximately 2 cu ft and constructed of stainless steel with solid sides and back and a steel-barred door. Each cage was identified by animal number and color code only and was not identified by treatment. Cats were physically separated by the solid stainless-steel cage walls. Cats were fed a commercial dry cat food ration, and water was available ad libitum. Other than what is described in the protocol, no baths, shampoos, or pesticides were administered to the cats during the preconditioning phase or the course of the study. All animal care procedures conformed to guidelines established by the Institutional Animal Care and Use Committee at Kansas State University (Approval No. 1910).

    Experimental Design and Methods

    • Cats in Group A remained untreated and served as controls.
    • Cats in Group B were treated with 6% w/v selamectin (6-12 mg/kg; Revolution).
    • Cats in Group C received 10% w/v fipronil and 12% w/v (S)-methoprene (7.5-15 mg/kg fipronil and 10-20 mg/kg (S)-methoprene; Frontline Plus).

    Both products were applied topically to the skin at the base of the neck in front of the scapulae according to label directions. Treatment was applied on study day 0.

    Each cat was infested with approximately 50 unfed cat fleas (C. felis) on days -2, 7, 14, 21, 28, 35, and 42. Flea eggs were collected from a pan under each cat cage on days 1, 2, and 3 after treatment and at 24 and 48 hours after each posttreatment reinfestation. Before eggs were collected, cats were massaged or brushed vigorously by hand in their cages for approximately 20 seconds to dislodge any flea eggs remaining in the cat's haircoat, allowing the eggs to fall into the drop pan below the cage. Collected eggs were counted by placing them in a glass Petri dish that was then observed under a dissecting microscope. Approximately 72 hours after treatment or infestation, each cat was combed to remove and count live fleas.

    Viability of eggs was determined by attaching up to 100 flea eggs from each collection (two replicates of about 50 eggs) to the lids of glass Petri dishes using water-based, nontoxic glue (Glue Stic, Avery, Brea, CA). The lid was inverted and placed over a corresponding lower dish containing growth medium and held for 24 hours in the dark in a rearing chamber (I-30B, Percival Manufacturing, Boone, IA) at 27°C to 28°C and 70% to 80% relative humidity; to determine the egg hatch, eggs and hatched larvae were examined using a dissecting microscope 5 days after eggs were placed in the Petri dish. Hatched larvae were allowed to continue development in the growth medium. At 10 to 12 days after egg collection, pupae (and any larvae that had not completed cocoon formation) were sifted from the medium and placed in plastic vials with lids. Adult emergence was determined by counting adult fleas about 28 days after egg collection. Personnel conducting comb counts, egg collections, and viability assessments were blinded to the cats' treatment group allocation.

    Data Analysis

    Comb flea counts and number of eggs recovered were transformed using natural log (count + 1) before analyses and were back transformed to a geometric mean for presentation. Percent egg hatchability and percent adults emerged were transformed using the arcsine of the square root (percent/100) before analyses and were back-transformed to percent for presentation.

    All response variables (flea egg counts, comb flea counts, percent egg hatchability, percent larvae emerged, and percent adults emerged) were analyzed with a general linear repeated measures mixed model that included fixed effect terms for treatment, day of study, and treatment by day of study interaction and random effect terms for block and block by treatment interaction. The variance-covariance structure among the repeated measures was chosen from a set of preselected structures based on the best fit according to Akaike's information criterion. One degree of freedom contrasts among treatments were made at each day of study after detecting a significant (P ≤ .05) treatment effect or treatment by day of study interaction effect. Percent efficacy was calculated for all Group B and Group C (treated) response variables as shown in the above equation.


    Nontreated control cats maintained adequate flea infestations throughout the study, with geometric mean flea counts ranging from 36.1 to 47.8 (Table 1). Cats treated with selamectin or fipronil-(S)-methoprene had significantly lower geometric mean flea counts versus nontreated control cats for 45 days or 38 days, respectively (P < .05; Table 1). Both treatments provided similar levels of control of adult fleas through day 17; on days 24 and 31, selamectin-treated cats had significantly (P ≤.05; Table 1) fewer fleas than did the cats treated with fipronil-(S)-methoprene. The efficacy of selamectin against adult fleas was >97.3% through 31 days, whereas the efficacy of fipronil-(S)-methoprene against adult fleas was >97.9% through 24 days and then dropped to 79.3% on day 31 (Table 1).

    Geometric mean flea egg counts from fleas infesting untreated control cats also remained high throughout the study, ranging from 171.5 to 627.4 (Table 2). Geometric mean flea egg counts for fleas infesting cats treated with selamectin or fipronil-(S)-methoprene were significantly reduced within 48 hours of treatment (P < .05; Table 2). Egg production on selamectin-treated cats was reduced by 95.4% within 48 hours of product application and was still reduced by greater than 99% and greater than 89.9% after flea reinfestations at 4 and 6 weeks after treatment, respectively (Table 2). Egg production on cats treated with fipronil-(S)-methoprene was reduced by 89.2% within 48 hours of product application and was reduced by greater than 83.7% and greater than 35.6% after flea reinfestations at 4 and 6 weeks after treatment, respectively (Table 2). Treatment of cats with fipronil-(S)-methoprene reduced egg production significantly compared with untreated controls from day 2 through day 38 after treatment (P < .05; Table 2). Treatment of cats with selamectin significantly reduced egg production compared with untreated controls from day 2 through day 45 after treatment and, compared with fipronil-(S)-methoprene treatment, significantly reduced egg numbers at day 3 and from week 2 throughout the remainder of the 6-week study (P < .05; Table 2).

    Viability of eggs produced from fleas infesting untreated control cats remained high throughout the study, with larval emergence rates ranging from 67.3% to 86.9% (Table 3). Similarly, adult emergence ranged from 57.2% to 85.1% (Table 4). Both selamectin and fipronil-(S)-methoprene produced a significant effect on larval emergence and correspondingly on adult flea emergence from day 1 throughout the 6-week study (P < .05; Table 3 and Table 4) except for day 38 larval emergence in the selamectin group. Within 24 hours of application, fipronil-(S)-methoprene completely inhibited larval emergence from eggs and, for the next 6 weeks, egg hatch and subsequently adult flea emergence were negligible (Table 3 and Table 4). Similarly, within 48 hours of application of selamectin to cats, flea egg viability was reduced to 1.2%, and both larval emergence and adult flea emergence were minimal for the next 4 weeks. While still significantly reduced compared with untreated controls (P < .05, Table 4), adult flea emergence from selamectin-treated cats rose slightly during the fifth and sixth weeks to 9.7% and 21.3% on days 38 and 45, respectively. The 13.1% adult flea emergence on day 24 in the selamectin-treated group was not statistically different from the 0% emergence in the fipronil-(S)-methoprene group. This was a result of the low number of eggs (geometric mean, 0.7) produced from selamectin-treated cats, so that the 13.1% represents only one adult flea.


    This study demonstrated that both selamectin and fipronil-(S)-methoprene can provide excellent control of an existing flea infestation on cats. Selamectin provided 97.3% control of adult fleas at 31 days after treatment, and fipronil-(S)-methoprene killed 97.9% at 3 weeks after treatment. Similar to a previous investigation in this laboratory using the KS1 flea strain,2 selamectin provided significantly greater control of fleas on cats than did fipronil-(S)-methoprene at 4 weeks after treatment.

    In addition to controlling adult flea infestations, fipronil-(S)-methoprene and selamectin administration significantly reduced the numbers of flea eggs produced. In a previously mentioned investigation2 using the same KS1 flea strain, it was shown that selamectin applied to cats killed 98.3% and 90.1% of newly acquired adult fleas within 24 hours of infestation on days 21 and 28 after treatment, respectively. In this current study, it was determined that such a rapid rate of adult flea kill corresponded to a reduction in egg production of at least 99.5% for up to 31 days after treatment. Similarly, in the previous study, fipronil-(S)-methoprene applied to cats killed 98.1% and 81.4% of the adult fleas within 24 hours on days 21 and 28 after treatment, respectively.2 Those 24-hour kill rates correspond in this study to a reduction in egg production of at least 83.7% for up to 31 days after application of fipronil-(S)-methoprene.

    Not only did these formulations dramatically reduce flea egg production, the viability of the few eggs produced was also markedly impaired as evidenced by the minimal number of larvae and ultimately adult fleas emerging for up to 6 weeks after treatment. Methoprene is well established as a potent ovicide,6,13,14 and its effects on egg viability when administered to cats in the fipronil-(S)-methoprene formulation was further evidenced in this study. Within 24 hours of application of fipronil-(S)-methoprene, not a single adult flea developed from the flea eggs deposited from treated cats. Even though the adulticidal activity of fipronil against the KS1 flea waned slightly after the third week, allowing for increased egg production, egg viability and adult flea emergence were negligible for the entire 6 weeks of the study. As previously mentioned, the selamectin formulation also displayed prolonged effects on egg viability and adult flea emergence. The exact nature of how this compound exerts its ovicidal activity is unknown at this time.


    This study demonstrated that selamectin and fipronil-(S)-methoprene applied to cats can have a significant impact on established flea populations and almost completely prevent future flea generations through their action on egg production and egg viability.


    We thank John Thompson, PhD, biometrician, for statistical assistance.

    Downloadable PDF

    This study was funded in part by a grant from Pfizer Animal Health.

    1. Dryden MW, McCoy CM, Payne PA: Rate of kill of nitenpyram tablets, imidacloprid spot-on and fipronil spot-on against flea infestations on dogs. Compend Contin Educ Pract Vet 23(3A):24-27, 2001.

    2. Dryden MW, Smith V, Payne PA, McTier TL: Comparative speed of kill of selamectin, imidacloprid, and fipronil-(S)-methoprene spot-on formulations against fleas on cats. Vet Ther 6(3):28-236, 2005.

    3. Hopkins TJ, Kerwick C, Gyr P, Woodley I: Efficacy of imidacloprid to remove and prevent Ctenocephalides felis infestations on dogs and cats. Compend Contin Educ Pract Vet 19:11-16, 1997.

    4. Jacobs DE, Hutchinson MJ, Krieger KJ: Duration of activity of imidacloprid, a novel adulticide for flea control, against Ctenocephalides felis on cats. Vet Rec 140:259-260, 1997.

    5. Ritzhaupt LK, Rowan TG, Jones RL: Evaluation of efficacy of selamectin and fipronil against Ctenocephalides felis in cats. JAVMA 217:1666-1668, 2000.

    6. Young DR, Jeannin PC, Boeckh A: Efficacy of fipronil/(S)-methoprene combination spot-on for dogs against shed eggs, emerging and existing adult cat fleas (Ctenocephalides felis, Bouché). Vet Parasitol 125(3-4):397-407, 2004.

    7. Rust MK, Dryden MW: The biology, ecology, and management of the cat flea. Ann Rev Entomol 42: 451-473, 1997.

    8. Dryden M, Bennett G, Neal J: Concepts of flea control. Comp Anim Pract 19(4-5):11-22, 1989.

    9. Dryden MW, Broce AB: Integrated flea control for the 21st century. Compend Contin Educ Pract Vet 24(1 suppl):36-39, 2002.

    10. Chin A, Lunn P, Dryden M: Persistent flea infestations in dogs and cats controlled with monthly topical applications of fipronil and methoprene. Aust Vet Pract 35(3):89-96, 2005.

    11. Dryden M, Payne P, Lowe A, et al: "Why am I still seeing fleas?" Troubleshooting client issues with flea control. Vet Tech 28(5A)2-, 2007.

    12. Dryden M: Host association, on-host longevity and egg production of Ctenocephalides felis. Vet Parasitol 34:117-122, 1989.

    13. Olsen A: Ovicidal effect on the cat flea, Ctenocephalides felis (Bouché), of treating fur of cats and dogs with methoprene. Int Pest Control 27:10-13, 1985.

    14. Palma KG, Meola SM, Meola RW: Mode of action of pyriproxyfen and methoprene on eggs of Ctenocephalides felis (Siphonaptera: Pulicidae). J Med Entomol 30:421-426, 1993.

    15. Blagburn BL, Vaughan JL, Lindsay DS, Tebbitt GL: Efficacy dosage titration of lufenuron against developmental stages of fleas (Ctenocephalides felis felis) in cats. Am J Vet Res 55:98-101, 1994.

    (Correspondence should be sent to Dr. Payne: phone, 785-532-4604; fax, 785-532-4039; email, payne@vet.k-state.edu.)

    References »

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