Susceptibility of cat fleas (Siphonaptera: Pulicidae) to fipronil and imidacloprid using adult and larval bioassays

Resistance to Fipronil in the Common Bed Bug (Hemiptera: Cimicidae)

Cimex lectularius L. populations have been documented worldwide to be resistant to pyrethroids and neonicotinoids, insecticides that have been widely used to control bed bugs. There is an urgent need to discover new active ingredients with different modes of action to control bed bug populations. Fipronil, a phenylpyrazole that targets the GABA receptor, has been shown to be highly effective on bed bugs. However, because fipronil shares the same target site with dieldrin, we investigated the potential of fipronil resistance in bed bugs. Resistance ratios in eight North American populations and one European population ranged from 1.4- to >985-fold, with highly resistant populations on both continents. We evaluated metabolic resistance mechanisms mediated by cytochrome P450s, esterases, carboxylesterases, and glutathione S-transferases using synergists and a combination of synergists. All four detoxification enzyme classes play significant but variable roles in bed bug resistance to fipronil. Suppression of P450s and esterases with synergists eliminated resistance to fipronil in highly resistant bed bugs. Target-site insensitivity was evaluated by sequencing a fragment of the Rdl gene to detect the A302S mutation, known to confer resistance to dieldrin and fipronil in other species. All nine populations were homozygous for the wild-type genotype (susceptible phenotype). Highly resistant populations were also highly resistant to deltamethrin, suggesting that metabolic enzymes that are responsible for pyrethroid detoxification might also metabolize fipronil. It is imperative to understand the origins of fipronil resistance in the development or adoption of new active ingredients and implementation of integrated pest management programs.

Visceral leishmaniasis on the Indian Subcontinent: Efficacy of fipronil-based cattle treatment in controlling sand fly populations is dependent on specific aspects of sand fly ecology

BACKGROUND

Visceral leishmaniasis (VL) is a deadly disease transmitted by the sand fly Phlebotomus argentipes on the Indian subcontinent, with a promising means of vector control being orally treating cattle with fipronil-based drugs. While prior research investigating the dynamic relationship between timing of fipronil-based control schemes and the seasonality of sand flies provides insights into potential of treatment on a large scale, ecological uncertainties remain. We investigated how uncertainties associated with sand fly ecology might affect our ability to assess efficacy of fipronil-based control schemes. To do this, we used a previously-described, individual-based, stochastic sand fly model to quantify how uncertainties associated with 1) the percentage of female sand flies taking blood meals from cattle, and 2) the percentage of female sand flies ovipositing in organic matter containing feces from treated cattle might impact the efficacy of fipronil-based sand fly control schemes.

PRINCIPAL FINDINGS

Assuming no prior knowledge of sand fly blood meal and oviposition sites, the probabilities of achieving effective sand fly population reduction with treatments performed 3, 6 and 12 times per year were ≈5-22%, ≈27-36%, and ≈46-54%, respectively. Assuming ≥50% of sand flies feed on cattle, probabilities of achieving efficacious control increased to ≈8-31%, ≈15-42%, and ≈52-65%. Assuming also that ≥50% of sand flies oviposit in cattle feces, the above probabilities increased further to ≈14-53%, ≈31-81%, and ≈89-97%.

CONCLUSIONS

Our assessments of the efficacy of fipronil-based cattle treatments in controlling sand fly populations depend on our assumptions regarding key aspects of sand fly ecology. Assessments are most sensitive to assumptions concerning the percentage of sand flies ovipositing in feces of treated cattle, thus emphasizing the importance of identifying sand fly oviposition sites. Our results place the evaluation of fipronil-based cattle treatment within a broader ecological context, which could aid in the planning and execution of a largescale field trial.

Synergism of Adulticides and Insect Growth Regulators Against Larval Cat Fleas (Siphonaptera: Pulicidae)

The use of topical and oral therapies on pets has revolutionized the control of cat fleas, Ctenocephalides felis felis (Bouché). Herein, we tested the biological activity of two adulticides, fipronil and imidacloprid, and the insect growth regulators (IGRs), methoprene and pyriproxyfen. The LC50's of fipronil, imidacloprid, methoprene, and pyriproxyfen in larval rearing medium for second and third instars were 1.13, 0.73, 0.35, and 0.23 ppm, respectively. Combinations of imidacloprid and methoprene and pyriproxyfen were synergistic. The combination indices (CIs) at an effective dose (ED95) of imidacloprid:methoprene (Im:Meth) were 0.54, 0.44, 0.66, 0.73, and 0.62 for Im1:Meth1, Im5:Meth1, Im10:Meth1, Im20:Meth1, and Im40:Meth1, respectively. Similarly, the CIs of imidacloprid:pyriproxyfen (Im:Pyri) at an ED95 were 0.73 and 0.50 for Im1:Pyri1 and Im5:Pyri1, respectively. Combinations of fipronil:methoprene (Fip:Meth) provided variable results with Fip1:Meth1 being antagonistic (CI = 1.61). Combinations at 5:1, 10:1, and 20:1 at an ED95 were moderately synergistic. Combinations of Fip:Pyri at 1:1 were antagonistic at an ED95 with a CI of 2.87. When the combinations were reversed, neither the imidacloprid nor fipronil synergized either IGR. The dose response indices (DRI) for both Im:Meth and Im:Pyri indicate that the concentrations of the combinations could be significantly reduced and still be as effective as imidacloprid alone. Certain combinations of adulticides and IGRs were synergistic against immature fleas.

International Program to Monitor Cat Flea Populations for Susceptibility to Imidacloprid

An international team of scientists and veterinarians was assembled in 1999 to develop a monitoring program to determine the susceptibility of cat fleas, Ctenocephalides felis felis (Bouché) (Siphonaptera: Pulicidae), to imidacloprid. Cat flea eggs were collected, shipped to laboratories, and tested for their susceptibility to imidacloprid. Over 3,000 C. felis populations were collected from 2002 to 2017 from 10 different countries. Of these, 66.3% were collected from cats and 33.7% from dogs. C. f. felis populations (n = 2,200) were bioassayed by exposing cat flea eggs and the emerging larvae to a Diagnostic Dose (DD) of 3 ppm imidacloprid in larval rearing medium. Flea eggs hatched and developed in the untreated controls in 1,837 of the isolates (83.5%) bioassayed. Flea isolates (n = 61) that had ≥5% survival at the DD of 3 ppm were retested with a second DD of 3 ppm. None of them had ≥5% survival to the second dose of 3 ppm. Of the 1,837 valid C. felis isolates tested, there has been no evidence of a decreased susceptibility to imidacloprid over the past 17 yr. The methods outlined in this article should provide an acceptable protocol for testing many of the new active ingredients that have been registered for cat flea control.

The Biology and Ecology of Cat Fleas and Advancements in Their Pest Management: A Review

The cat flea Ctenocephalides felis felis (Bouché) is the most important ectoparasite of domestic cats and dogs worldwide. It has been two decades since the last comprehensive review concerning the biology and ecology of C. f. felis and its management. Since then there have been major advances in our understanding of the diseases associated with C. f. felis and their implications for humans and their pets. Two rickettsial diseases, flea-borne spotted fever and murine typhus, have been identified in domestic animal populations and cat fleas. Cat fleas are the primary vector of Bartonella henselae (cat scratch fever) with the spread of the bacteria when flea feces are scratched in to bites or wounds. Flea allergic dermatitis (FAD) common in dogs and cats has been successfully treated and tapeworm infestations prevented with a number of new products being used to control fleas. There has been a continuous development of new products with novel chemistries that have focused on increased convenience and the control of fleas and other arthropod ectoparasites. The possibility of feral animals serving as potential reservoirs for flea infestations has taken on additional importance because of the lack of effective environmental controls in recent years. Physiological insecticide resistance in C. f. felis continues to be of concern, especially because pyrethroid resistance now appears to be more widespread. In spite of their broad use since 1994, there is little evidence that resistance has developed to many of the on-animal or oral treatments such as fipronil, imidacloprid or lufenuron. Reports of the perceived lack of performance of some of the new on-animal therapies have been attributed to compliance issues and their misuse. Consequentially, there is a continuing need for consumer awareness of products registered for cats and dogs and their safety.

Intrinsic Activity of IGRs Against Larval Cat Fleas

Insect growth regulators (IGRs) such as lufenuron, methoprene, and pyriproxyfen have been important tools in the integrated pest management of cat fleas, Ctenocephalides felis (Bouché), for the past two decades. Other IGRs have been registered for the control of termite, dipteran, and lepidopterous pests including chlorfluazuron, cyromazine, dicyclanil, and precocene I, but have been not tested against C. felis. The intrinsic activity of IGRs was determined by exposing larvae to treated larval rearing media. The LC50s of chlorfluazuron, cyromazine, dicyclanil, lufenuron, and precocene I against cat fleas were 0.19, 2.66, 0.04, 0.20, and 10.97 ppm, respectively. The LC95s of chlorfluazuron, cyromazine, dicyclanil, lufenuron, and precocene I were 0.78, 51.24, 0.30, 0.62, and 175.05, respectively. The regression slopes of chlorfluazuron and lufenuron were 2.65 ± 0.24 and 3.40 ± 0.45 (SEM), respectively, and considerably steeper than the other IGRs tested (1.51 to 1.74). The intrinsic activity of IGRs tested is summarized as dicyclanil > chlorfluazuron = lufenuron > cyromazine > precocene I. The responses of the laboratory UCR strain to these IGRs can serve as a baseline of susceptibility until a more susceptible cat flea strain is found. Chlorfluazuron and dicyclanil look like promising candidates against cat fleas.

Insecticide Resistance in Fleas

Fleas are the major ectoparasite of cats, dogs, and rodents worldwide and potential vectors of animal diseases. In the past two decades the majority of new control treatments have been either topically applied or orally administered to the host. Most reports concerning the development of insecticide resistance deal with the cat flea, Ctenocephalides felis felis. Historically, insecticide resistance has developed to many of the insecticides used to control fleas in the environment including carbamates, organophosphates, and pyrethroids. Product failures have been reported with some of the new topical treatments, but actual resistance has not yet been demonstrated. Failures have often been attributed to operational factors such as failure to adequately treat the pet and follow label directions. With the addition of so many new chemistries additional monitoring of flea populations is needed.

Efficacy of a fixed combination of permethrin 54.5% and fipronil 6.1% (Effitix) in dogs experimentally infested with Ixodes ricinus

Background

Ticks are the most important vectors of disease-causing pathogens in domestic animals and are considered to be second worldwide to mosquitoes as vectors of human diseases. In Europe, Ixodes ricinus, the sheep tick, plays an important role as companion animal parasite but is also the primary vector of medically important diseases such as tick-borne encephalitis and Lyme borreliosis.

The present study was designed to evaluate the efficacy under laboratory conditions of a new fixed spot-on combination of fipronil and permethrin (Effitix®, Virbac) in treating and preventing tick infestations of Ixodes ricinus in dogs.

Methods

Twelve dogs were included in this randomized, controlled, blinded laboratory study. They were randomly allocated to two groups of six dogs each according to their pre-treatment live attached Ixodes ricinus tick count. On day 0, the dogs from Group 2 were treated with the recommended dose of Effitix®, the dogs from Group 1 remained untreated. On days −2, 7, 14, 21, 28 and 35, all dogs were infested with 50 (±4) viable unfed adult Ixodes ricinus (20 ± 2 males, 30 ± 2 females). Ticks were removed and counted at 48 ± 2 hours post product administration or tick infestations.

Results

Through the study, the tick attachment rates for the untreated group were greater than 25% demonstrating that adequate levels of infestation were reached on the control dogs. Based on both arithmetic and geometric means (AM and GM), Effitix® was deemed to be effective against Ixodes ricinus on days 2, 9, 16, 23, 30 and 37 with a percentage of efficacy of 98%, 100%, 100%, 100%, 93% and 95% respectively (AM). No clinical abnormalities were detected during the study.

Conclusions

The study has shown under laboratory conditions, that Effitix® is a safe and an effective combination to treat and protect dogs from Ixodes ricinus up to 37 days after administration. The high immediate efficacy of 98% evaluated at 48 hours post-treatment was particularly interesting, meaning that Effitix has a curative effect against ticks (Ixodes ricinus) and provides a rapid control of existing Ixodes ricinus infestation on a dog at the time of treatment.