Successive treatments with ivermectin (3.15%) to control the tick Rhipicephalus (Boophilus) microplus in cattle: Pharmacokinetic and efficacy assessment

Resistance of Rhipicephalus microplus to different acaricides in tropical climates: Are the laboratory and field results related?

The aim of this study was therefore to investigate the relationship between laboratory (Larval Packet Test -LPT; Larval Immersion Test - LIT; Adult Immersion Test - AIT) and field studies using active ingredients or commercial formulations to control R. microplus in a tropical region. This comparative study was carried out with four populations of R. microplus from four Brazilian farms. For laboratory assays, technical grade compounds of cypermethrin and chlorpyrifos were used in the LPT, while fipronil and ivermectin were used in the LIT. The AIT was conducted using commercial spray formulations containing pyrethroid and organophosphate: cypermethrin 187.5 ppm + chlorpyrifos 375 ppm + fenthion 187.5 ppm and chlorpyrifos 825 ppm + High-Cis cypermethrin 99 ppm. For the field assays, the same commercial products of the AIT used, a pour-on formulation of fipronil 1 mg/kg, injectable 200 µg/kg ivermectin, injectable 630 µg/kg ivermectin, injectable 200 µg/kg doramectin and injectable 200 µg/kg moxidectin. For field studies, populations of R. microplus with the mean therapeutic efficacy ≤89 %, on days 7 up to 21 post-treatment, were classified as resistant to such compounds. To standardize the comparative analysis between laboratory and field results, this same value of larval efficacy or mortality (≤89 %) was used to classify the population as resistant by laboratory tests (LPT, LIT, and AIT). Of the 16 laboratory tests conducted using R. microplus larvae (LPT and LIT), 66.6 % showed no relation with field study results. Inconsistencies were observed in 100 % of cases for spray formulations, 25 % for fipronil, and 75 % for macrocyclic lactones. Although the efficacy results of the AIT with commercial formulations were slightly higher than the therapeutic efficacy observed in the field, it is important to note that there was a 100 % positive relation in the classification of the status of the four populations, which were all considered susceptible in both laboratory and field analyses. These findings demonstrate that, in situations like this study, it is essential to calibrate laboratory tests using larvae, particularly against field results, for each formulation. This approach will allow for more accurate recommendations regarding the use of a chemical formulation for a specific tick population. Furthermore, it reduces the risk of incorrectly identifying R. microplus population as resistant or susceptible and helps clarify the practical implications of resistance.

Acaricidal efficacy of fluralaner against Rhipicephalus microplus ticks under laboratory and field conditions in Brazil

BACKGROUND

The first isoxazoline-based acaricide (fluralaner) for the control of Rhipicephalus microplus was introduced onto the market in 2022, initially in Brazil, followed by other Latin American countries. Therefore, it is important to establish laboratory methods to monitor the susceptibility of populations of R. microplus to this molecule and to determine the relationship between the results of laboratory tests and those from field trials.

METHODS

A larval immersion test (LIT) was performed on 18 populations of R. microplus. The lethal concentration 50 (concentration causing 50% mortality [LC50]) values were calculated to determine the resistance ratios (RRs) of the populations. The lethal concentration 99 (concentration causing 99% mortality [LC99]) values were calculated to determine the discriminating doses (DDs = 2 × LC99). The DDs were applied in tests with the POA (susceptible) and GYN (resistant) strains, as well as in tests with the population that presented the lowest LC50 value (population 14) and with the two populations that presented the highest LC50 value (populations 10 and 16). Finally, we performed field trials with the population that presented the lowest and two highest LC50 values.

RESULTS

In the LIT with fluralaner, the LC50 values ranged from 0.144 to 0.481 µg/mL for the 18 R. microplus populations. The mortality rate was 100% in the tests of the DDs in the five populations tested. In the field trials, the efficacy of fluralaner was similar for the three populations of R. microplus tested (populations 14, 10 and 16), with therapeutic efficacy (until day 21) of 100% and persistent efficacy (between days 28 and 42) > 95%.

CONCLUSION

We observed natural variability in the susceptibility of larvae from the different populations of R. microplus that had never been treated with this compound. Despite the observed variability in the in vitro results (LC50), a comparable efficacy of > 90% lasting until day 42 was observed in the field trials. Also, based on the results of the laboratory testing (LC50 and DD) and field trials, we can conclude that there was no resistance to fluralaner in the 18 studied tick populations.

Macrocyclic lactones and ectoparasites control in livestock: Efficacy, drug resistance and therapeutic challenges

Macrocyclic lactones (MLs) are the cornerstone of parasite control in livestock due to their broad-spectrum activity against endo (nematodes) and ecto (lice, ticks, mites) parasites. These molecules, introduced into the veterinary pharmaceutical market 40 years ago, have substantially improved animal welfare and productivity by offering extended high efficacy, reducing treatment frequency, and displaying a favorable safety profile. However, their widespread and intensive use has led to a significant challenge nowadays: the development of parasite resistance. This review focuses on the critical link between drug pharmacokinetics (variation in concentration profiles and exposure over time) and pharmacodynamics (drug efficacy) and the ability of both avermectin and milbemycin MLs families to control livestock ectoparasites. This review discusses the integrated assessment of drug behavior in the host, its diffusion into target parasites, and the impact of different pharmaceutical formulations on enhancing drug delivery to infection sites. These are considered critical research/development areas to optimize the use of MLs, preventing treatment failures and finally extending the lifespan of these essential pharmaceutical ingredients. Finally, the importance of the rational use of MLs, guided by parasite epidemiology and pharmacological knowledge, is emphasized as a key strategy to preserve the antiparasitic efficacy of these still very useful molecules.

Comparative effect of ivermectin and amitraz on cellular architecture of ovaries, synganglion and Gené's organ of Rhipicephalus microplus (Acari: Ixodidae)

The present study investigated the effect of ivermectin and amitraz on the cellular architecture of vital organs of Rhipicephalus microplus. Adult female ticks were treated with lethal concentrations (LC95) of ivermectin and amitraz, and the ovaries, synganglion, and Gené's organ were processed 48 h post treatment. In both the treatment groups, the ultra-thin sections of ovary exhibited deformed oocytes, irregular plasmic membrane and chorion layer, extensive vacuolation in the cytoplasm mainly at periphery of the cell and oocyte-pedicel junction. Marked vacuolations in the cortex and neuropile region with significant structural disorganization of the neural fibers were common alterations observed in the synganglion of ticks exposed to ivermectin and amitraz. The tissue sections of Gené's organ revealed deformed tubular glands with severe loss of cellular limit of secretory epithelium and cytoplasmic vacuolations in the ivermectin treated ticks whereas, the alterations were comparatively less severe in amitraz exposed ticks. The cellular deformities in these vital organs probably impaired reproductive function, nerve signal transmission and metabolic activities and thus affected fecundity and survivability of the treated ticks. The findings suggested that the action of ivermectin and amitraz are not restricted to the nervous system of ticks, but also on other vital organs, ovary and Gené's organ affecting the oviposition. The study provided insights into the development of targeted interventions for tick control strategies.

Hard Ticks as Vectors: The Emerging Threat of Tick-Borne Diseases in India

Hard ticks (Ixodidae) play a critical role in transmitting various tick-borne diseases (TBDs), posing significant global threats to human and animal health. Climatic factors influence the abundance, diversity, and vectorial capacity of tick vectors. It is imperative to have a comprehensive understanding of hard ticks, pathogens, eco-epidemiology, and the impact of climatic changes on the transmission dynamics of TBDs. The distribution and life cycle patterns of hard ticks are influenced by diverse ecological factors that, in turn, can be impacted by changes in climate, leading to the expansion of the tick vector's range and geographical distribution. Vector competence, a pivotal aspect of vectorial capacity, involves the tick's ability to acquire, maintain, and transmit pathogens. Hard ticks, by efficiently feeding on diverse hosts and manipulating their immunity through their saliva, emerge as competent vectors for various pathogens, such as viruses, parasites and bacteria. This ability significantly influences the success of pathogen transmission. Further exploration of genetic diversity, population structure, and hybrid tick vectors is crucial, as they play a substantial role in influencing vector competence and complicating the dynamics of TBDs. This comprehensive review deals with important TBDs in India and delves into a profound understanding of hard ticks as vectors, their biology, and the factors influencing their vector competence. Given that TBDs continue to pose a substantial threat to global health, the review emphasizes the urgency of investigating tick control strategies and advancing vaccine development. Special attention is given to the pivotal role of population genetics in comprehending the genetic diversity of tick populations and providing essential insights into their adaptability to environmental changes.

Comparative Pharmacokinetics and Bioequivalence of Pour-On Ivermectin Formulations in Korean Hanwoo Cattle

This study aimed to conduct a bioequivalence study of applying three pour-on ivermectin formulations at a dose of 1 mg/kg on the back of Korean native beef cattle (Hanwoo). To conduct bioequivalence testing, the pharmacokinetics of three groups (control Innovator, test Generic A, and test Generic B) of five clinically healthy Korean Hanwoo cattle (average weight 500 kg) were studied. After topical application to the skin, blood samples were drawn at the indicated times. These blood samples were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The time required to reach the maximum concentration (Tmax), the maximum concentration (Cmax), and the area under the curve (AUClast) of each pharmacokinetic parameter were compared for bioequivalence. The results showed that the control had a Tmax of 41 ± 1.24 h, a Cmax of 0.11 ± 0.01 μg/mL, and an AUClast of 9.33 ± 0 h*μg/mL). The comparator Generic A had a Tmax of 40 ± 1.14 h, a Cmax of 0.10 ± 0.01 (μg/mL, and an AUClast of 9.41 ± 0.57 h*μg/mL, while Generic B had a Tmax of 40 ± 2.21 h, a Cmax of 0.10 ± 0.01 μg/mL, and an AUClast of 9 h*μg/mL. The values of the bioequivalence indicators Cmax, Tmax, and AUC were all within the range of 80% to 120%, confirming that all three tested formulations were bioequivalent. In conclusion, the study showed that the two generic products were bioequivalent to the original product in Hanwoo cattle.

Evaluation of the impact of successive acaricide treatments on resistance evolution in Rhipicephalus microplus populations: Monodrugs versus drug combinations

The aim of this work was to comparatively evaluate the evolution of resistance in Rhipicephalus microplus tick populations exposed to successive treatments with monodrug-based formulations and combinations of them in the same commercial formulation. Thirty-six heifers, naturally infested with R. microplus, were divided into three groups (G) and subjected to three successive treatments, on days 0 (Nov-2021), 43 (Jan-2022) and 78 (Feb-2022), with the following formulations: I) ivermectin 3.15% (Ivomec Gold®) (GI), II) fipronil 1% (Ectoline®) (GII) and III) a combination of fipronil 2% and ivermectin 1% (Vaquero®) (GIII). From Nov-2021 to Dec-2022, counts of R. microplus were periodically performed to evaluate the tick infestation. Engorged females were collected at the beginning and end of the trial to determine the evolution of tick resistance to ivermectin and fipronil by in vitro bioassays. At the end of trial, GII and GIII had higher tick counts (39.18 ± 11.88 and 38.33 ± 14.31, respectively) than group I (5.11 ± 6.24) (P<0.05). The in vitro results shows that the resistance ratio (RR50) values after the treatments were higher for fipronil (5.584 and 5.649 for GII and GIII, respectively) than for ivermectin (1.165 and 1.088 for GI and GIII, respectively). In the group treated with the combination (GIII), the RR50 increased for both drugs simultaneously. These results suggest that the successive use of drug combinations could exacerbate the problem of multi-resistance of R. microplus to chemical acaricides.

Pharmacokinetics of tenvermectin in swine, a novel antiparasitic drug candidate-comparison with ivermectin

Tenvermectin (TVM) is a novel 16-membered macrolide compound isolated and purified from the fermentation broth of genetically engineered Streptomyces avermitilis strain MHJ1011. TVM and ivermectin were administered at the dose of 0.3 mg/kg body weight through a single subcutaneous injection route followed by plasma collectiom and analysis at different time intervals. Plasma concentrations of TVM and IVM were determined by high-performance liquid chromatography with fluorescence detector. Pharmacokinetic analysis was completed using the non-compartmental method with WinNonlin™ 6.4 software. TVM is rapidly absorbed after administration with peak plasma concentrations (C_max , 9.78 ± 2.34 ng/ml) obtained within 6-22 h. AUC_0-last was 586.86 h·ng/ml ± 121.24 h·ng/ml. The mean elimination half-life of TVM (T_1/2λz ) was 97.99 h ± 46.41 h. The T_1/2λz of IVM was 146.59 h ± 22.26 h in the study. The present study showed that subcutaneous administration of TVM at 0.3 mg/kg body weight (BW) in swine is absorbed more rapidly than IVM in swine. Compared to the pharmacokinetic characteristics of IVM, there was little difference in the half-life of TVM among different individuals. The data will contribute to refining the formulation and dosage regime for TVM administration.