The Efficacy of a Topically Applied Combination of Imidacloprid and Permethrin Against Stomoxys Calcitrans on Dogs
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The Efficacy of a Topically Applied Combination of Imidacloprid and Permethrin Against Stomoxys calcitrans on Dogs Leon J. Fourie, MSc, PhD1 Dorothee Stanneck, Dr. Med. Vet2 Ivan G. Horak, DVSc, PhD1 1Department of Zoology and Entomology University of the Free State Bloemfontein, South Africa 2Bayer HealthCare AG Animal Health Division Leverkusen, Germany KEY WORDS: dogs, efficacy, imidaclo- implied by its common name, is a pest of prid/permethrin combination, repellency, domesticated animals, although severe Stomoxys calcitrans infestations have also been reported on wildlife.1 The fly has a cosmopolitan distri- ABSTRACT bution,2 and both sexes are hematophagous.3 This laboratory study was undertaken to Like many domestic flies, females lay their determine the efficacy of a combination of eggs in moist, rotting, or fermenting organic imidacloprid (10%) and permethrin (50%) material, such as marine grass, straw, grain applied topically as a spot-on, to prevent wastes, grass cuttings, and horse manure. blood feeding by stable flies (Stomoxys cal- One female lays up to 50 eggs at a time and citrans) on dogs. It was conducted in 3 can lay several batches in her lifetime. Eggs replicates, and a total of 52 dogs were used. hatch in 2–5 days, depending on air and They were challenged with fasted, laborato- substrate temperature. Larvae mature in ry-reared S. calcitrans 1 day after treatment 11–20 days and are followed by a pupal and at weekly intervals for 4 weeks there- period lasting 6–20 days. There is no proof after. The efficacy of the imidacloprid/per- that S. calcitrans is a biological vector of methrin combination always exceeded 82% any human disease, but it does transmit in preventing S. calcitrans from taking a Surra (Trypanosoma evansi) in horses and blood meal on dogs during a period of 29 mules and equine infectious anemia days after a single application, and the aver- (lentivirus) in horses and other single-hoofed age efficacy over the entire period was animals. Its main importance, however, is 85.6%. In addition, more than 77% of flies that it is a vicious biter and blood-sucker. that attempted to feed within the first 8 days The flies feed during the day and prefer after treatment died. warm, sunny, and windless weather. Daily activity appears to be bimodal with some INTRODUCTION activity during the morning and a peak in The stable fly, Stomoxys calcitrans, as the late afternoon.4 In urban and suburban Intern J Appl Res Vet Med • Vol. 4, No. 1. 2006 29 environments, S. calcitrans is often a seri- formulation suitable for topical spot-on ous nuisance problem on both humans and application on dogs (Advantix®, Bayer dogs. People are usually attacked on the HealthCare AG, Germany), and the purpose lower parts of their legs, and the insertion of of this investigation was to evaluate the the fly's rigid proboscis in search of blood immediate and sustained efficacy of this can be extremely painful. In dogs, the bites formulation in preventing S. calcitrans from of stable flies cause continuous irritation taking blood meals on dogs. and restlessness during the daylight hours. Because of this, the flies are frequently dis- MATERIALS AND METHODS turbed while feeding resulting in repeated Three replicates, consisting of 20, 16, and attempts to feed. They attack the dorsal 16 sub-adult to adult mixed breed dogs aspect of the muzzle, just posterior to the (mainly mongrels) of both sexes that had nose, and the ears in dogs, and the lesions not been treated with an ectoparasiticide they cause may vary from pinpoint hemor- during the preceding 8 weeks, were selected rhage to necrotic dermatitis on the tips of for the study. The dogs were individually the ears.5 In particular, stable fly bites give identified by means of implanted transpon- rise to so-called "ear bleeding," affecting ders and were housed separately, with no mainly the tips of erect ears and the bent contact between animals possible. The study edge of floppy ears. This may lead to the was performed in compliance with the 12 development of allergies and to fly-bite der- guidelines for good clinical practice. matitis, a local ulcerative inflammation On Day –7, all dogs in each replicate caused by bacterial infection.6,7 were challenged with a laboratory-reared Imidacloprid is a neonicitinoid insecti- strain of S. calcitrans and allocated to 2 cide and has been developed for the control equal groups. The allocation was done by of various insects of agricultural and veteri- randomization through minimization, using nary importance. Not only does a topically the Day –7 feeding success of the flies as a applied formulation have sustained insecti- primary criterion. By random draw, the 2 cidal activity against adult fleas on dogs and groups in each replicate were then allocated cats, but it also is effective against flea lar- to a treated group or an untreated control vae that come into contact with debris and group. hair from treated animals.8 It is also effec- Dogs in the treated groups weighing 10 tive against lice on dogs.9 Permethrin is a kg or less were treated topically with the synthetic pyrethroid that was first synthe- imidacloprid (10%)/permethrin (50%) com- sized in 1973. It has a broad spectrum of bination at a dosage rate of 0.1 mL/kg body activity against insects and acarines and is weight, applied as a single spot directly to well known for its repellent efficacy against the skin between the shoulder blades. various arthropods. Permethrin and imida- Heavier dogs received 4 equally spaced cloprid enhance each other's effect on the spots between the shoulder blades and the nervous system of the target species via sep- base of the tail. Each dog was restrained for arate and complementary activity along the about 1 minute after application to allow the axon and post-synaptic membrane, respec- medication to spread. tively.10 The repellent and insecticidal Approximately 14-day old S. calcitrans, effects of permethrin in combination with from which blood had been withheld and imidacloprid have been successfully test- replaced with a 10% sucrose solution for 3 ed against several mosquito species days before challenge, were fasted on the belonging to the genera Culex and day prior to challenge. To facilitate chal- Aedes.11 Imidacloprid 10% and perme- lenge and subsequent assessment, the dogs thrin 50% have been combined in a novel were fasted overnight and sedated. The 30 Intern J Appl Res Vet Med • Vol. 4, No. 1. 2006 sedated dogs were individually placed in The treated and untreated groups were fly-proof cages in environmentally con- also compared by an analysis of variance trolled rooms (temperature = 25 ± 3°C; rela- (ANOVA) with a treatment effect, based on tive humidity = 40%–70%) and each dog Table 1. Design of an Experiment to Evaluate the was challenged with approximately 50 Efficacy of an Imidacloprid (10%)/Permethrin (50%) flies. After 30 minutes, live and dead Combination in Preventing Stable Flies From Taking Blood Meals on Dogs. (including moribund) flies were collected with an aspirator and counted. These Daily Activities flies were stored at below –20°C and Ranking and Fly Challenge Initial Fly Allocation to Treatment and Efficacy subsequently squashed and the presence Challenge of blood in their intestines recorded. The Groups Assessments experimental design is summarized in +1, +8, +15, –7 –3 0 Table 1. +18* +22, +29 Because of the large variation in individual values, efficacy in preventing the total number of flies that had fed on blood feeding was calculated on each blood, and on the live fly counts on each assessment day using geometric means, and assessment day, as well as on the the following equation was used: AUC/(29–1) values. This ANOVA was per- Efficacy (%) = 100 × (mc – mt)/mc formed on the original untransformed data. mc = geometric mean number of flies that had taken a blood meal on dogs in the RESULTS control group The geometric mean numbers of flies that mt = geometric mean number of flies fed on untreated control dogs and on treated that had taken a blood meal on dogs in the animals, as well as the efficacy in prevent- treated group ing blood feeding on each assessment day, and over the entire assessment period of 29 Some of the counts were zero and, con- days, are graphically illustrated in Figure 1. sequently, geometric means were calculated Blood feeding by flies exposed to the after a natural logarithmic transformation of untreated control groups of dogs was con- (count +1), and the corresponding arith- sistently high (≥10.7), and an average of metic mean calculated. Thereafter a count of 12.6 flies took a blood meal over the entire 1 was subtracted from the antilog of the period of the study. The treated and control mean value to obtain the geometric mean. study groups differed significantly (P < Since treatment killed substantial num- 0.001) on all assessment days, as well as for bers of flies that attempted to feed, insectici- the overall average [AUC/(29–1)]. Efficacy dal efficacy based on mortality (dead and in preventing blood feeding was highest moribund flies) was also calculated using (90.2%) 1 day post-treatment and least the same method and equation as above. (82.1%) 4 weeks after treatment. In order to consolidate the results over The geometric mean numbers of flies the 29-day periods of the studies, the area that survived exposure to untreated and to under curve (AUC) for the total number of treated dogs, as well as the % efficacy on flies that had taken a blood meal and the each assessment day and over the entire AUC for the number of live flies versus assessment period of 29 days, are graphical- treatment day was calculated.