The speed of kill of selamectin, imidacloprid, and fipronil–(S)-methoprene against Ctenocephalides felis infestations on cats for 1 month following a single treatment was evaluated. Eighty cats were randomly allocated to four treatment groups of 20 cats each. On days –2, 7, 14, 21, and 28, each cat was infested with 100 adult C. felis of the Kansas 1 flea strain. Following initial application, only imidacloprid caused a significant reduction in adult fleas on treated cats within 6 hours; by 24 hours, however, all three formulations had killed at least 96.7% of the fleas. At 7 days after treatment, all three formulations reduced flea populations by at least 68.4% within 6 hours and by at least 99.4% within 24 hours. At 21 and 28 days after treatment, none of the formulations killed significant numbers of fleas (compared with controls) within 6 hours of infestation. At 28 days after treatment, selamectin, fipronil–(S)-methoprene, and imidacloprid killed 99.0%, 86.4%, and 72.6% of the fleas, respectively, within 48 hours of infestation. This study demonstrates that the speed of kill of residual flea products on cats decreases throughout the month following application. It also demonstrates that selamectin provides the highest level of residual activity on cats against the Kansas 1 flea strain.
Introduction
Flea control is achieved by killing adult fleas currently parasitizing pets, eliminating existing environmental infestations, and preventing future infestations. Several insecticides do an excellent job of killing existing fleas on pets, but the eggs, larvae, pupae, and emerging fleas remaining in the environment provide an ongoing source of new adult fleas to reinfest treated pets. Historically, control of these life stages was attempted through repeated application of short-acting insecticides on the pet and application of insecticides and insect growth regulators around the premises.1 It is possible that the repeated killing of fleas on the pets and reduction of developing and emerging adult fleas in the surrounding environment can control infestations. While this approach may be successful, the problem has been getting pet owners to consistently follow repeated on-animal and premises treatment protocols. Often, pet owner compliance is poor at best; pets repeatedly acquire new fleas from the premises, and infestations become a recurring problem. Over the past 10 to 15 years, insecticides and insect growth regulators have been developed with convenient dosage forms (spot-ons, collars, pills, oral suspensions, and injections) and prolonged residual activity, which have greatly improved pet owner compliance and altered the way flea control is conducted.2
One aspect of flea control is how rapidly fleas are killed after a product is applied to an infested pet or after they jump onto an already treated animal. Speed of kill can affect pet comfort, client satisfaction, severity of flea allergy dermatitis (FAD), and flea reproduction.
A study was conducted at Kansas State University College of Veterinary Medicine to evaluate the speed of kill of selamectin (Revolution, Pfizer Animal Health), imidacloprid (Advantage, Bayer Animal Health), and fipronil–(S)-methoprene (Frontline Plus, Merial) spot-on formulations against adult flea (Ctenocephalides felis) infestations on cats for 1 month following a single treatment.
Materials and Methods
Eighty-six domestic shorthair cats (43 males and 43 females 5 to 11 months of age and weighing 2.2 to 4.9 kg) were purchased from Liberty Research (Waverly, NY). Cats were housed individually in stainless-steel metabolic cages with expanded metal flooring. All cages were equipped with a litterbox and food and water dishes. Cats were fed a commercial dry diet and given water ad libitum. Other than what is described in the protocol, no drugs, 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. 1347).
Because of the large number of cats, the study was conducted in two batches of 43 cats each. On day –7, each cat was infested with approximately 100 C. felis (cat flea) from the Kansas 1 colony (KS1) established and maintained as a closed colony at Kansas State University since 1990.
On day –5, 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 (Safari Flea Comb, Whitco, Centereach, NY). 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 combed for an additional 5 minutes. Personnel conducting comb counts were blinded to treatment group allocation.
The 40 cats from each batch with the highest day –5 flea counts were selected for the study, for a total of 80 cats. The cats within each batch were then randomly allocated to a pen, treatment group (T01, T02, T03, or T04), and time posttreatment/postinfestation group (A, 6 hours; B, 12 hours; C, 24 hours; and D, 48 hours) within each treatment group according to a randomization plan produced by Pfizer Biometrics, Technology and Quality. Each treatment group and time posttreatment/postinfestation group consisted of five cats. In the randomization plan, two to three cats from each treatment group and each time posttreatment/postinfestation group were allocated to batch 1 (40 cats) and batch 2 (40 cats). This ensured that cats from all four treatment groups, including controls, were evaluated simultaneously.
Animals in batches 1 and 2 were weighed on day –1 for dosage selection. Cats in T2, T3, and T4 received treatment on day 0. Cats in T1 received no treatment. Cats in T2 received selamectin (6–12 mg/kg) applied topically to the skin at the base of the neck in front of the scapulae according to label directions. Cats in T3 received imidacloprid (10–20 mg/kg) and those in T4 received fipronil–(S)-methoprene (7.5–15 mg/kg) applied topically according to label instructions.
On day –2, all cats in batch 1 and 38 cats in batch 2 were infested with approximately 100 C. felis each; the other two cats in batch 2 received only 50 fleas each because of a shortage of available fleas. On days 7, 14, 21, and 28, approximately 100 C. felis were applied to each cat in the study.
Beginning on day 0, cats in each treatment group (T01, T02, T03, and T04) were assigned to a posttreatment or postinfestation combing group. Cats in group A were combed at 6 hours after treatment, cats in group B at 12 hours after treatment, cats in group C at 24 hours after treatment, and cats in group D at 48 hours after treatment. On days 7, 14, 21, and 28, cats were combed as described for day 0 except the combings were performed at 6, 12, 24, and 48 hours after flea infestation rather than after treatment. The number of live adult fleas was recorded.
Statistical Analysis
The number of live fleas was transformed before statistical analysis by taking the natural logarithm of the (count + 1). The transformed flea counts were analyzed using a general linear repeated measures mixed model. Least squares means, SE, and 95% CIs were calculated for each treatment and time posttreatment/postinfestation category. The least squares means were back-transformed to calculate the treatment geometric means. The CIs were also back-transformed for presentation. The treatments were compared at each time posttreatment/postinfestation and day of study, and the times posttreatment/postinfestation were compared within treatment and day of study. Percentage reduction was calculated for each time posttreatment/postinfestation at each study day using the equation.
The two-sided 5% level of significance (P ~ .05) was used to assess statistical differences.
Results
Control cats maintained ample flea burdens throughout the trials, with geometric mean flea counts for the control cats ranging from 54.1 on day 21 (12 hours) to 98.2 on day 0 (48 hours) (Table 1). At 6 hours after treatment, only imidacloprid produced a significant reduction (86.7%) in adult fleas on treated cats (Table 2). By 12 hours, however, all three formulations had significantly reduced flea populations (Table 1). At 12 hours after treatment, imidacloprid had killed significantly more fleas than either fipronil–(S)-methoprene or selamectin and fipronil–(S)-methoprene had killed significantly more fleas than selamectin; within 24 hours of application, imidacloprid, fipronil–(S)-methoprene, and selamectin had reduced flea populations by 99.8%, 97.8%, and 96.7%, respectively (Table 2).
When fleas were placed on cats 7 days after treatment, all three formulations reduced flea populations by at least 68.4% within 6 hours (Table 2). Within 12 hours of flea exposure, there were significantly fewer fleas (P < .05) on selamectin-treated cats than on imidacloprid-treated cats (Table 1). By 24 hours after infestation, all three products had reduced flea populations by at least 99.4%.
At 14 days after application, only the fipronil-based formulation reduced flea numbers significantly (73.7%) within 6 hours of infestation (Tables 1 and 2). At 12 hours after infestation, selamectin had produced the greatest level of flea kill (96.7%). By 24 hours, all three formulations had killed at least 94.0% of fleas on the cats.
At 21 days after treatment, none of the formulations killed significant numbers of fleas (compared with controls) within 6 hours of infestation (Table 1). At 12 hours after infestation, selamectin produced the greatest level of flea kill (91.3%). The fipronil–(S)-methoprene and imidacloprid formulations reduced flea numbers by 69.6% and 65.2%, respectively. By 24 hours after infestation, all formulations had reduced flea populations by at least 97.4%, but for the first time since treatment, none of the formulations was 100% efficacious at 48 hours (Tables 1 and 2).
At 28 days after treatment, none of the formulations killed significant numbers of fleas within 6 hours of infestation. Flea counts on the cats treated with the fipronil–based formulation were not significantly different from those of control cats at 12 hours (Table 1). At 24 hours, selamectin, fipronil–(S)-methoprene, and imidacloprid had killed 90.1%, 81.4%, and 79.7% of fleas, respectively. By 48 hours after infestation, selamectin had killed significantly more fleas (99.0%) than either the imidacloprid (72.6%) or fipronil-based (86.4%) formulations (Tables 1 and 2).
Discussion
All three formulations tested provided excellent initial flea kill after application. The imidacloprid formulation killed fleas the most rapidly after initial application; by 24 hours after treatment, however, at least 96.7% of the fleas on treated cats were killed regardless of the product applied. Veterinarians and pet owners should anticipate that any of these three formulations, if applied correctly, should eliminate the existing flea burden on a cat within 48 hours.
The newer topical spot-on insecticide formulations evaluated in this trial, fipronil–(S)-methoprene, imidacloprid, and selamectin, are marketed to provide at least 30 days of effective flea control. The speed at which these products kill newly acquired fleas should affect the management of FAD, reduce the number of eggs produced by female fleas, and improve client satisfaction. FAD occurs when fleas inject salivary antigens into a sensitized animal while feeding. It is recognized that these residual insecticides cannot completely stop salivary antigen injection because fleas begin feeding almost immediately after they acquire a host.3,4 However, a rapid reduction in the number of fleas feeding and a shortening of the duration that fleas feed may reduce the severity of the disease. Consumption of blood is necessary before cat fleas can initiate reproduction.4 Mating occurs after fleas have fed, and egg production does not begin until 24 hours after females take their first blood meal.5,6 If a residual insecticide can kill or produce toxicity in newly acquired fleas within 24 hours of when they jump on a treated host, egg production could be markedly reduced or halted. Older topical organophosphate and pyrethroid insecticide formulations often did not have sufficient residual activity to kill newly acquired fleas before egg production, which often resulted in client disatisfaction.7
The data gathered in this study reemphasize the difficulty of preventing a pet from being reinfested by developing life stages in the environment. When cats were exposed to fleas 7 days after treatment, all three formulations significantly reduced flea numbers within 6 hours and provided almost 100% elimination within 24 hours of infestation. However, 2 weeks after treatment, it was observed that only the fipronil-based formulation had significantly reduced flea counts (compared with controls) within 6 hours of infestation, whereas all three formulations significantly reduced flea numbers within the same time period on day 7. These data indicate that the speed of flea kill was beginning to slow within 2 weeks of application. Activity of all three formulations was still sufficient to produce significant kill within 12 hours, and all three formulations had reduced flea burdens by at least 94.0% by 24 hours.
Reduction in the speed of flea kill was even more evident when cats were exposed to fleas 3 weeks after treatment. None of the formulations provided significant flea kill over nontreated cats at 6 hours; by 12 hours after infestation, only the selamectin formulation was killing more than 90% of the fleas. The continued decline in residual activity was more pronounced at 4 weeks after treatment; again, none of the three formulations significantly reduced flea numbers within 6 hours of infestation, and by 48 hours after infestation, only the selamectin formulation had killed more than 90% of the fleas on cats.
Since fleas are killed at a slower rate as time progresses after treatment, clients may observe fleas more often and pets with FAD may start to show clinical signs. The numbers of fleas killed within 24 hours of infestation remained at or above 94% for all formulations at 7, 14, and 21 days. Therefore, egg production should be minimal during this period. By 4 weeks after treatment, only selamectin exceeded 90% efficacy within 24 hours after infestation. Because fleas on selamectin-treated cats were killed at a faster rate 4 weeks after treatment, flea egg production and FAD-associated pruritus might be more pronounced for cats treated with imidacloprid or fipronil–(S)-methoprene than for those treated with selamectin if cats were exposed to fleas at this time.
This study was conducted using the KS1 flea colony, a strain that has been documented to have some level of resistance to several insecticides, including pyrethroids, malathion, and fipronil.8–10 This may partially explain the reduced performance of the fipronil-based formulation in this study, particularly at 28 days after treatment. The efficacy of the fipronil–(S)-methoprene spot-on formulation against the KS1 flea colony at day 30 in this study (86.4%) is comparable to efficacy previously observed in this laboratory using a fipronil spray formulation (89.9%).11 While resistance to imidacloprid in flea populations has not been documented, some natural variability in susceptibility is to be expected and may account for the reduced performance of imidacloprid in this study. When four laboratory strains of C. felis were being used to develop a larval bioassay for imidacloprid, it was found that the KS1 strain had the highest LD95.12 Since insecticide susceptibility can vary between flea strains,8–12 different results may have been obtained if another cat flea strain had been used.
Conclusion
The controlled laboratory study described here allowed for the investigation of specific attributes of insecticide formulations, such as speed of kill under controlled conditions. In this study, it was observed that the residual speed of kill of the formulations does decline after application. This reduction in speed of kill may affect clinical disease and a client’s satisfaction with product performance. This study does not necessarily foretell how these products might compare under natural field conditions. In fact, field studies conducted in Tampa, Florida, where speed of kill was not evaluated, have shown that fipronil–(S)-methoprene, imidacloprid, and selamectin were all effective in eliminating natural flea infestations.13–15
References
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