Acaricide resistance mechanisms in Rhipicephalus (Boophilus) microplus

Current profile of phenotypic pyrethroid resistance in Rhipicephalus microplus (Acari: Ixodidae) populations sampled from Marathwada region of Maharashtra state, India

This study examined the pattern of resistance to widely applied synthetic pyrethroids, i.e., cypermethrin and deltamethrin, against larvae of Rhipicephalus microplus ticks sampled from Marathwada region in Maharashtra, India. The study also examined the role of α- and β-esterases and glutathione-S-transferase (GST) in resistance development. All eight R. microplus isolates tested were resistant to deltamethrin (RL IV), having RR50 values from 6.88 to 131.26. LPT analysis exhibited the resistance level II deltamethrin resistance in Beed and Hingoli, III in Dharashiv, and IV in Sambhajinagar, Parbhani, Latur, Jalna, and Nanded isolates. The LIT analysis showed that Dharashiv field isolates had the lowest LC50 value of 229.09 ppm against cypermethrin, while Sambhajinagar field isolates had the highest at 489.78 ppm. The RR50 ranged from 1145.45 to 2448.9. Seven isolates were level I resistant to cypermethrin while the Jalna isolate was level II resistant. In larvae treated with deltamethrin and cypermethrin, the activity of α- and β-esterase enzymes increased significantly compared to control groups. The enzyme ratios in treated larvae ranged from 0.7533 to 1.7023 for α-esterase and 0.7434 to 3.2054 for β-esterase. The Hingoli isolate treated with cypermethrin exhibited the highest α-esterase activity (903.261), whereas Sambhajinagar isolate had the highest GST enzyme ratio (2.8224) after deltamethrin exposure. When exposed to cypermethrin, the Hingoli isolate showed the highest GST enzyme ratio, 2.0832. The present study provides the current resistance status in tick populations from Marathwada region indicating deltamethrin and cypermethrin to be ineffective for tick control. The results also suggest that SP compounds should be regulated in this region and alternative control strategies should be introduced.

Molecular assessment of voltage-gated sodium channel (VGSC) gene mutations in Rhipicephalus microplus from Guangxi, China

BACKGROUND

Pyrethroid chemicals are one of the main acaricides used against ticks. Resistance to these chemicals has been reported to be associated with mutations in the voltage-gated sodium channel (VGSC) gene of the Rhipicephalus microplus. This study investigates R. microplus resistance to pyrethroids in Guangxi region of China, marking one of the first research efforts in this area. The findings are intended to provide vital baseline for the effective implementation of localized tick control strategies.

METHODS

From March to July 2021, 447 R. microplus tick samples were collected from five prefecture-level cities in Guangxi. Allele-specific polymerase chain reaction (AS-PCR) was used to amplify segments C190A and G215T of the domain II S4-5 linker and T2134A of domain III S6 in the VGSC, to detect nucleotide mutations associated with resistance to pyrethroid acaricides. Subsequent analyses were conducted to ascertain the prevalence, types of mutations, and genotypic distributions within the sampled populations.

RESULTS

Mutations within VGSC gene were identified across all five studied populations of R. microplus, although the mutation rates remained generally low. Specifically, the most prevalent mutation was C190A, observed in 4.9% of the samples (22/447), followed by G215T at 4.0% (18/447), and T2134A at 1.3% (6/447). The distribution of mutations across three critical sites of the VGSC gene revealed four distinct mutation types: C190A, G215T, C190A + G215T, and T2134A. Notably, the single mutation C190A had the highest mutation frequency, accounting for 4.3%, and the C190A + G215T combination had the lowest, at only 0.7%. The analysis further identified seven genotypic combinations, with the wild-type combination C/C + G/G + T/T predominating at a frequency of 90.4%. Subsequently, the C/A + G/G + T/T combination was observed at a frequency of 4.3%, whereas the C/C + T/T + T/T combination exhibited the lowest frequency (0.2%). Additionally, no instances of simultaneous mutations at all three sites were detected. Geographical differences in mutation types were apparent. Both samples from Hechi to Chongzuo cities exhibited the same three mutation types; however, C190A was the most prevalent in Hechi, while G215T dominated in Chongzuo. In contrast, samples from Beihai to Guilin each exhibited only one mutation type: G215T occurred in 12.5% (4/32) of Beihai samples, and C190A in 7.5% (4/53) of Guilin samples.

CONCLUSIONS

These findings underscore the relatively low frequency of VGSC gene mutations in R. microplus associated with pyrethroid resistance in the Guangxi, China. Moreover, the variation in mutation types and genotypic distributions across different locales highlights the need for regionalized strategies in monitoring and managing pyrethroid resistance in tick populations. This molecular surveillance is crucial for informing targeted control measures and mitigating the risk of widespread resistance emergence.

Genetic Diversity of Rhipicephalus (Boophilus) microplus for a Global Scenario: A Comprehensive Review

Rhipicephalus microplus poses a substantial threat to livestock health and agricultural economies worldwide. Its remarkable adaptability to diverse environments and hosts is a testament to its extensive genetic diversity. This review delves into the genetic diversity of R. microplus, employing three pivotal genetic markers: the cytochrome c oxidase I (COX1) gene, ribosomal genes, and microsatellites. The COX1 gene, a crucial tool for genetic characterization and phylogenetic clustering, provides insights into the adaptability of ticks. Ribosomal genes, such as internal transcribed spacer regions (ITS-1 and2) as well as 18S and 28S, are routinely utilized for species differentiation. However, their use is limited due to indels (insertions and deletions). Microsatellites and minisatellites, known for their high polymorphism, have been successfully employed to study populations and genetic diversity across various tick species. Despite their effectiveness, challenges such as null alleles and marker variations warrant careful consideration. Bm86, a well-studied vaccine candidate, exhibits substantial genetic diversity. This diversity directly influences vaccine efficacy, posing challenges for developing a universally effective Bm86-based vaccine. Moreover, the review emphasizes the prevalence of genes associated with synthetic pyrethroid resistance. Identifying single nucleotide polymorphisms in the acaricide-resistant genes of R. microplus has facilitated the development of molecular markers for detecting and monitoring resistance against synthetic pyrethroids. However, mutations in sodium channels, the target site for synthetic pyrethroid, correlate well with the resistance status of R. microplus, which is not the case with other acaricide target genes. This study underscores the importance of understanding genetic diversity in developing effective tick management strategies. The choice of genetic marker should be tailored based on the level of taxonomic resolution and the group of ticks under investigation. A holistic approach combining multiple markers and integrating additional molecular and morphological data may offer a more comprehensive understanding of tick diversity and relationships. This research has far-reaching implications in formulating breeding programs and the development of vaccine against ticks and tick-borne diseases (TTBDs) as well as strategies for the management of resistant ticks.

Strategic control of the cattle tick Rhipicephlaus microplus applied to rotational and silvopastoral grazing systems in subtropical areas

This work evaluated if strategic control based on no more than three or four annual treatments is useful to control Rhipicephalus microplus infestations on cattle when it is applied to intensive rotational grazing and silvopastoral systems with high stocking rates in subtropical areas. In the intensive rotational grazing system, three annual treatments with chemical acaricides were applied on cattle in two different schemes: between spring and early summer and from late winter and late spring. Strategic control based on three treatments with chemical acaricides from late winter to late spring plus an additional fourth treatment in summer was tested in the silvopastoral system. In the intensive rotational grazing systems, the control schemes allow to reach a significant reduction in the tick load on cattle considering a time interval from spring to autumn. However, the efficacy levels were not high enough in some specific moments, namely, the tick counts of summer and autumn (there were not significant differences between treated and control groups). The scheme evaluated in the silvopastoral grazing system yielded better results than those tested for the intensive rotational system, because significant differences in tick load between treated and control groups were observed in all post-treatment counts and when the analysis was performed for the whole study period. However, values of efficacy in the count-by-count comparison were disparate, ranging from 64.1 to 99.7. Although the efficacy values obtained in the silvopastoral system were better than those of the rotational grazing systems, the total tick load on treated cattle in autumn was not low enough (mean abundance values 25.14 and 38.14). Ticks were more evenly distributed among hosts in late summer and autumn than in spring or early summer, where few hosts carry most of the ticks. Some management strategies as intensive rotational systems or silvopastoral structures can lead to a more efficient forage use, but they imply greater tick challenge than in extensive grazing systems. In these situations, the schemes of strategic control bases on three or four annual treatments should be complemented with additional tactical treatments in late summer or autumn.

Comparative analysis of Rhipicephalus microplus (Canestrini, 1888) infestation in pure breed (Hereford) and cross breed (Braford) cattle herds subjected to the same chemical treatments

The aim of this work was to analyze the R. microplus (Canestrini, 1888) infestation in two bovine herds with different degrees of natural resistance (i.e., Hereford and Braford) to ticks subjected to an identical chemical treatment scheme to ticks at the same farm, to demonstrate the impact on tick control of the incorporation of a more resistant bovine breed. Two groups of ten Hereford and Braford cows each were subjected to eleven chemical treatments between August 2022 and October 2023 (four fluazuron, two fipronil 1%, one ivermectin 3.15% and four immersion in a dipping vat with a combination of cypermethrin 10% and ethion 40%). Tick population was shown to be susceptible to ivermectin, fluazuron and the mix cypermethrin 10%-ethion 40% and resistant to fipronil according to in vitro tests. Tick infestation was significantly greater in the Hereford cows than in the Braford cows. Tick infestation in both Hereford and Braford breeds was similar when treatment with functional drugs was applied, but when a block of the treatments was done with drugs with decreased functionality due to resistance (i.e. fipronil), treatment failure was manifested more strongly in the most susceptible breed. The incorporation of cattle breeds with moderate or high resistance to R. microplus is instrumental to optimize the efficacy and sustainability of chemical control of ticks in a scenario where resistance to one or more chemical groups is almost ubiquitous, because it favors the biological control of this parasite.

Chemical Composition and Acaricidal Activity of Essential Oil of Lavandula dentata L. on Engorged Females of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae)

PURPOSE

Rhipicephalus (Boophilus) microplus is one the most significant ectoparasite in cattle farming in tropical and subtropical regions, causing problems to livestock health worldwide. The control of this ectoparasite primarily relies on the use of synthetic acaricides. However, the emergence of acaricide resistance has stimulated the search for new control alternatives, including phytocompounds with acaricidal and insecticidal potential. The aim of this study was to evaluate the acaricidal potential of Lavandula dentata essential oil against the engorged females of R. (B.) microplus.

METHODS

Engorged females were obtained from infested bovines in dairy farms in Pernambuco, Brazil. L. dentata essential oil was extracted, and adult immersion test assays were performed using the following oil concentrations: 0.2%, 0.4%, 0.6%, 0.8%, and 1%.

RESULTS

L. dentata essential oil at a concentration of 1% was lethal to all engorged females, and concentrations of 0.6% and 0.8% caused mortality of 98.6% and 99.1%, respectively. These concentrations disrupted the reproductive capacity of engorged females, reducing oviposition by more than 90% and preventing egg hatching by over 87%.

CONCLUSION

The data revealed that L. dentata essential oil possesses effective pharmacological properties against R. (B.) microplus and could be used for tick control following in vivo evaluation, thus contributing to mitigating the negative impacts of synthetic acaricide use.

Acaricide resistance status of deltamethrin and coumaphos in Hyalomma anatolicum ticks collected from different districts of Haryana

To study the acaricide resistance status and possible mechanisms of action in conferring resistance to commonly used acaricides (deltamethrin and coumaphos), Hyalomma anatolicum ticks were collected from 6 dairy farms of Hisar and Charkhi Dadri districts of Haryana. By using standard larval packet test, H. anatolicum tick larvae of Charkhi Dadri isolates were found to be susceptible (100% mortality) to both the acaricides. Level-I resistance against coumaphos was recorded from four isolates, whereas, level-II was observed in only one isolate, collected from Hisar. One isolates (Kaimri) from Hisar also showed level-I resistance against deltamethrin. Biochemically, the ticks having higher values of resistance factor (RF) against coumaphos were found to possess increased enzymatic activity of α-esterase, β-esterase, glutathione-S-transferase (GST) and mono-oxygenase enzymes, whereas, the monoamine oxidase did not show any constant trend. However, the RF showed a statistical significant correlation with GST only. Native PAGE analysis of H. anatolicum ticks revealed the presence of nine types of esterases (EST-1 h to EST-9 h) by using napthyl acetate as substrate. In the inhibitory assay, esterases were found to be inhibited by PMSF, indicating the presence of serine residue at catalytic triad. The partial cds of carboxylesterase and domain II of sodium channel genes were sequenced to determine any proposed mutations in resistant isolates of H. anatolicum ticks, however, no mutations were observed in either gene, indicating that increased expression of detoxification enzymes as a possible mechanism for resistance development, in the current study.

Repeated Cross-Sectional Survey of Ectoparasites in Sheep from Central Tunisia: Does Low Prevalence Indicate Good Hygiene or Resistance to Ectoparasites?

Sheep ectoparasites such as chewing lice, fleas and ticks are serious constraints to sheep productivity and are the cause of skin lesions in animals that decrease their market value. This study aims at investigating the ectoparasite fauna infesting small ruminants in the district of Sidi Bouzid (central Tunisia). A total of 1243 Barbarine and Queue Fine de l'Ouest (QFO) sheep were examined every two months for one year. Of the total animals examined, 74 were infested by at least 1 parasite group (5.95%). Three ectoparasite groups were identified as Psoroptes ovis (0.48%; 6/1243), ticks (5.3%; n = 66/1243) and one specimen of Ctenocephalides canis (0.08%; n = 1/1243). The most abundant tick among the 358 specimens was Rhipicephalus sanguineus sensu lato (n = 337; 94.1%), followed by Hyalomma impeltatum (n = 7/358; 1.9%), H. dromedarii (n = 7/358; 1.9%), H. excavatum (n = 5/358; 1.4%) and only two specimens of H. scupense (n = 2/358; 0.55%). The sheep herds showed low infestation prevalence by ectoparasite over the year, with a significant difference according to the seasons (p < 0.05). A higher infestation prevalence was recorded in March (14.36%). Barbarine sheep breed showed significantly higher infestation prevalence (16.8%) compared to QFO (0.8%) (p < 0.01). There were no differences in infestation prevalence according to sex of the animal or age groups. Knowledge of the ectoparasite population harboured by sheep, its activity dynamics and risk factors is required to develop effective ectoparasite control options. The low prevalence of ectoparasite infestation in sheep reported here may be due to possible genetic resistance or simply to successful hygiene measures implemented by farmers.

Daylight photodynamic inactivation of cattle tick Rhipicephalus microplus by porphyrins: An alternative for the ectoparasite control

The bovine tick Rhipicephalus microplus, a primary ectoparasite of veterinary concern, contributes significantly to disease transmission and reduced cattle productivity, resulting in substantial economic losses. The overuse of chemical acaricides has led to the emergence of resistant strains, posing a considerable challenge to veterinary medicine. Consequently, the development of alternative parasite control methods is essential to ensure livestock quality and enhance food safety worldwide. Our study introduces an innovative approach to photodynamic inactivation (PDI) of the bovine tick, harnessing natural daylight for a potential field application. Reproductive parameters (female and egg mass, egg production index, and larval hatch) were evaluated in engorged female ticks under photodynamic action using the hematoporphyrin (HP) and tetra-cationic porphyrins free-base meso-tetra-ruthenated (4-pyridyl) (RuTPyP) and its zinc(II) complex (ZnRuTPyP) as photosensitizers (PS). The results showed that there was no significant difference between the groups treated with tetra‑ruthenium porphyrins and the control group. However, HP exhibits a control percentage of 97.9% at a concentration of 2.5 μmol.L-1, aligning with the expected control rates achieved by conventional chemical acaricides. Photophysical and physicochemical parameters such as the number of singlet oxygen produced and lipophilicity were discussed for each PS and related to tick control percentages. Furthermore, the interaction between HP and chitin, an important macromolecule presents in the tick's cuticle, considered as the primary target tick structure during PDI was observed by the absorption and fluorescence emission spectroscopic techniques. Therefore, the results presented here extend the potential for controlling R. microplus through photodynamic inactivation while utilizing sunlight as a source of natural irradiation.

Stochastic modelling to predict a priori the potential outcomes of different control schemes against the cattle tick Rhipicephalus microplus

Stochastic models are valuable tools to describe and analyze tick population dynamic in a given area, and to evaluate different control schemes. The objective of this study was to provide a tool to predict a priori how a control scheme could affect the abundance of Rhipicephalus microplus in an area highly favourable for its development, through the building of a stochastic model of the population dynamic of this tick. The dynamic was stochastically modelled using field data of the parasitic and non-parasitic phases of R. microplus. The host susceptibility to tick infestations was also considered by including representative values of natural resistance of three different breeds (Bos indicus, B. taurus and B. indicus x B. taurus). Two different control schemes (strategic and threshold methods) using three annual applications of synthetic acaricides in different moments were evaluated. Furthermore, we have evaluated the impact of different moments of pasture spelling as a tool for tick control on the dynamic of the non-parasitic phases of R. microplus. The results showed that the model appropriately fits to field data and can simulate the annual variability in female ticks count in animals from different ecological characteristics. The model has also captured the variability that host susceptibility has on tick abundance. This fact was expressed in the different levels of tick abundance obtained for the three breeds of bovines. According to the model, strategic control applications of chemical acaricides have more efficacy than the threshold control method. The model indicates that a pasture spelling started in early or mid-summer generates a higher reduction in the number of available eggs in pastures. Analysis of the results allows to conclude that the model developed in this study was useful to predict a priori the performance of different control methods based on the strategic application of chemical acaricides or by applying pasture spelling.

Two protocols using fluralaner for Rhipicephalus microplus strategic control on taurine cattle in a tropical region

BACKGROUND

The present study aimed to evaluate the effects of different treatment strategies using a new commercial formulation containing pour-on fluralaner on Rhipicephalus microplus infestation in cattle and in pastures in a tropical climate region where up to five generations of this tick species can occur per year.

METHODS

Forty-five naturally infested cattle were divided into three experimental groups: T01, treated with fluralaner (2.5 mg/kg) pour-on every 42 days; T02, the cattle received the first treatment with fluralaner on Day 0 but the next treatment involved a weekly visual evaluation; T03, control, received palliative treatment with a spray formulation when the group mean was ≥ 30 ticks. Counts of female R. microplus were performed weekly until day 343, and larval counts on pasture were performed on Days 0, 30, and 60 and every 30 days until Day 330.

RESULTS

Using fluralaner, six applications were performed in Group T01, and four were performed in Group T02. In the control group (T03), it was necessary to perform eight palliative acaricide treatments with the spray formulation. The animals in T01 and T02 showed lower mean tick counts (p ≤ 0.05) than the control group (T03) on 28 and 27 of the 49 evaluated dates, respectively. In the paddock where the animals were kept as controls, the R. microplus larvae counts increased to 1458. In the paddocks where the animals were treated with fluralaner, the number was ≤ 19 per paddock during the study.

CONCLUSIONS

The different strategic treatment protocols performed with pour-on fluralaner (2.5 mg/kg) over a year in taurine cattle in a tropical region with a history of up to five annual generations of cattle ticks were effective, maintaining levels of R. microplus infestations in animals and in pastures close to zero in most of the study. Depending on the retreatment criterion adopted, the number of applications per year may be lower, resulting in a reduction in the mean cost of acaricide treatment per year and lower exposure of R. microplus populations to the active ingredient, resulting in lower resistance and selection pressure.

Efficacy of ethyl-4-bromophenyl carbamate on different Rhipicephalus microplus stages implanted in cattle

The effect of ethyl-4-bromophenyl carbamate on different Rhipicephalus microplus stages implanted in cattle was evaluated using the pen test with infestation chambers. Twelve steers were distributed into four groups (n = 3), each with four chambers (12 chambers per group), where approximately 1,000 R. microplus larvae were placed in each chamber. The chambers of the first group were sprayed with a solution of ethyl-4-bromophenyl carbamate (0.668 mg/mL) on day 2 post-infestation (PI) (exposed larvae). The chambers of the second group were sprayed with the same solution on day 8 PI (exposed nymphs), and the chambers of the third group were sprayed on day 16 PI (exposed adults) with the same solution. The chambers of the fourth group were used as controls. The percentages of engorged females, egg laying, egg production and egg hatching were evaluated in all groups. The percentage of cumulative reduction of hatched larvae was 98.3, 96.1 and 94.4% when larvae, nymph and adult stages were treated, respectively. The average cumulative reduction of hatched larvae, considering the three treated stages, was 96.3%, whereby the reproductive potential of this tick was drastically reduced. In conclusion, ethyl-4-bromophenyl carbamate acted as an ixodicide (lethal effect) when larval stages were sprayed and as a growth regulator when nymphal and adult stages were sprayed. The sum of these effects had a direct impact on the efficacy of the product in the pen test, and future studies will indicate the potential use of this product for tick control.

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.

Elucidating structure and dynamics of glutathione S-transferase from Rhipicephalus (Boophilus) microplus

Rhipicephalus (Boophilus) microplus is tick parasite that affects the cattle industry worldwide. In R. (B.) microplus, acaricide resistance develops rapidly against many commercial acaricides. One of main resistance strategies is to enhance the metabolic detoxification mediated by R. (B.) microplus glutathione-S-transferase (RmGST). RmGST detoxifies acaricides by catalyzing the conjugation of glutathione to acaricides. Although structural and dynamic details of RmGST are expected to elucidate the biologic activity of this molecule, these data have not been available to date. Thus, Molecular Dynamics simulations were employed to study ligand-free RmGST at an atomic level. Like other m-class GSTs, the flexible m loop (m1) of RmGST was observed. M1 seems to shield the active sites from the bulk. A RmGST dimer is stabilized by the lock-and-key motif (F57 as "key") and hydrogen bonds of R82-E91 and R82-D98 at the dimer interface. Without substrates, conserved catalytic Y116 and N209 can interact with V112, G210 (for Y116) and F215 (for N209). Overall, most residues involving in RmGST function and stability are similar to other m-class GSTs. This implies similar structural stability and catalytic activity of RmGST to other GSTs. An insight obtained here will be useful for management of acaricide resistance and tick control.Communicated by Ramaswamy H. Sarma.

Investigating volatile semiochemical production from Bos taurus and Bos indicus as a novel phenotype for breeding host resistance to ixodid ticks

Ticks and tick-borne diseases cause significant loss in livestock production with about 80% world's cattle at risk. The cost of chemical control is high and there is an ever-increasing tick resistance to chemical acaricides. Genetic selection as alternative long-term control strategy is constrained by laborious phenotyping using tick counts or scores. This study explored the use of host volatile semiochemicals that may be attractants or repellents to ticks as a phenotype for new tick resistance, with potential to be used as a proxy in selection programmes. Approximately 100 young cattle composed of Bos indicus and Bos taurus were artificially infested with 2,500 African blue tick, Rhipicephalus decoloratus larvae, with daily female tick (4.5 mm) counts taken from day 20 post-infestation. Volatile organic compounds were sampled from cattle before and after tick infestation by dynamic headspace collection, analysed by high-resolution gas chromatography (GC) and subjected to multivariate statistical analysis. Using 6-day repeated measure analysis, three pre-infestation GC peaks (BI938 - unknown, BI966 - 6-methyl-5-hepten-2-one and BI995 - hexyl acetate) and one post-infestation GC peak (AI933 - benzaldehyde / (E)-2-heptenal) were associated with tick resistance (P < 0.01 and P < 0.05 respectively). The high correlation coefficients (r = 0.66) between repeated records with all volatile compounds support the potential predictive value for volatile compounds in selective breeding programmes for tick resistance in cattle.

Acaricides resistance in Rhipicephalus microplus and expression profile of ABC-transporter genes in the sampled populations

Currently, livestock owners manage tick infestations using chemicals, but the method is increasingly losing effectiveness as resistant tick populations have established in the field conditions. Thus, to develop effective tick management strategies, monitoring of resistance in most predominant tick species, Rhipicephalus microplus was targeted. The ticks were collected from eleven districts of Madhya Pradesh and one district of Punjab and tested against deltamethrin (DLM), cypermethrin (CYP), coumaphos (CMP), ivermectin (IVM) and fipronil (FIP), through adult immersion and larval packet tests. The field isolates were highly resistant to DLM [Resistance factor (RF) = 3.98-38.84]. Against CYP, resistance was observed in BWN (Barwani; RF = 2.81) and MND (Mandsaur; RF = 3.23) isolates. Surprisingly, most of the isolates were susceptible to CMP (0.34-1.58). Emerging level of resistance against IVM (1.05-4.98) and FIP (0.40-2.18) was also observed in all the isolates. Significantly elevated production of esterases (p < 0.01) was 90% correlated with RF of DLM while no positive correlation between production of monooxygenase and Glutathione S-transferase with RF to DLM was noted. Multiple sequence analysis of S4-5 linker region of the sodium channel gene of all the isolates revealed a point mutation at 190th position (C190A) which is associated with DLM resistance. Treatment of resistant LDH (Ludhiana) isolate with IVM resulted in upregulation of RmABCC2 gene and insignificant upregulation of RmABCC1 and RmABCB10 genes indicating the probability of linking IVM resistance with over-expression of RmABCC2 gene. The possible tick management strategies are discussed.

Control of ticks and tick-borne diseases in Africa through improved diagnosis and utilisation of data on acaricide resistance

A meeting, sponsored by the Bill and Melinda Gates Foundation (BMGF) and organised by Clinglobal, was held at The International Livestock Research Institute (ILRI) in Nairobi, Kenya, from 19th - to 21st October 2022. The meeting assembled a unique group of experts on tick control in Africa. Academia, international agencies (FAO and ILRI), the private Animal Health sector and government veterinary services were represented. The significant outcomes included: (i) a shared commitment to standardisation and improvement of acaricide resistance bioassay protocols, particularly the widely used larval packet test (LPT); (ii) development of novel molecular assays for detecting acaricide resistance; (3) creation of platforms for disseminating acaricide resistance data to farmers, veterinary service providers and veterinary authorities to enable more rational evidence-based control of livestock ticks. Implementation of enhanced control will be facilitated by several recently established networks focused on control of parasites in Africa and globally, whose activities were presented at the meeting. These include a newly launched community of practice on management of livestock ticks, coordinated by FAO, an African module of the World Association for the Advancement of Veterinary Parasitology (WAAVP-AN) and the MAHABA (Managing Animal Health and Acaricides for a Better Africa) initiative of Elanco Animal Health.

Transcriptome Profiling of Rhipicephalus annulatus Reveals Differential Gene Expression of Metabolic Detoxifying Enzymes in Response to Acaricide Treatment

Ticks are hematophagous ectoparasites of economic consequence by virtue of being carriers of infectious diseases that affect livestock and other sectors of the agricultural industry. A widely prevalent tick species, Rhipicephalus (Boophilus) annulatus, has been recognized as a prime vector of tick-borne diseases in South Indian regions. Over time, the use of chemical acaricides for tick control has promoted the evolution of resistance to these widely used compounds through metabolic detoxification. Identifying the genes related to this detoxification is extremely important, as it could help detect valid insecticide targets and develop novel strategies for effective insect control. We performed an RNA-sequencing analysis of acaricide-treated and untreated R. (B.) annulatus and mapped the detoxification genes expressed due to acaricide exposure. Our results provided high-quality RNA-sequenced data of untreated and amitraz-treated R. (B.) annulatus, and then the data were assembled into contigs and clustered into 50,591 and 71,711 uni-gene sequences, respectively. The expression levels of the detoxification genes across different developmental stages of R. (B.) annulatu identified 16,635 transcripts as upregulated and 15,539 transcripts as downregulated. The annotations of the differentially expressed genes (DEGs) revealed the significant expression of 70 detoxification genes in response to the amitraz treatment. The qRT-PCR revealed significant differences in the gene expression levels across different life stages of R. (B.) annulatus.

Putative target sites in synganglion for novel ixodid tick control strategies

Acaricide resistance is a global problem that has impacts worldwide. Tick populations with broad resistance to all commercially available acaricides have been reported. Since resistance selection in ticks and their role in pathogen transmission to animals and humans result in important economic and public health burden, it is essential to develop new strategies for their control (i.e., novel chemical compounds, vaccines, biological control). The synganglion is the tick central nervous system and it is responsible for synthesizing and releasing signaling molecules with different physiological functions. Synganglion proteins are the targets of the majority of available acaricides. In this review we provide an overview of the mode-of-action and resistance mechanisms against neurotoxic acaricides in ticks, as well as putative target sites in synganglion, as a supporting tool to identify new target proteins and to develop new strategies for tick control.

Thrombospondin-Related Anonymous Protein (TRAP) Family Expression by Babesia bovis Life Stages within the Mammalian Host and Tick Vector

The tick-transmitted disease bovine babesiosis causes significant economic losses in many countries around the world. Current control methods include modified live-attenuated vaccines that have limited efficacy. Recombinant proteins could provide effective, safe, and low-cost alternative vaccines. We compared the expression of the Babesia bovis thrombospondin-related anonymous protein (TRAP) family from parasites in bovine blood, in vitro induced sexual stages, and kinetes from tick hemolymph. Quantitative PCR showed that in blood and sexual stages, TRAP3 was highly transcribed as compared to the other TRAPs. In contrast, the TRAP1 gene was highly transcribed in kinetes as compared to the other TRAPs. Fixed immunofluorescence assays showed that TRAP2, 3, and 4 proteins were expressed by both blood and sexual stages. Conversely, TRAP1 protein, undetected on blood and induced sexual stages, was the only family member expressed by kinetes. Live IFA revealed that TRAP2, 3, and 4 proteins were expressed on the surface of both B. bovis blood and sexual stages. Modeling of B. bovis TRAP1 and TRAP4 tertiary structure demonstrated both proteins folded the metal-ion-dependent adhesion site (MIDAS) domain structure of Plasmodium TRAP. In conclusion, TRAP proteins may serve as potential vaccine targets to prevent infection of bovine and ticks with B. bovis essential for controlling the spread of bovine babesiosis.

Analysis of sequence variations in the GABA gated chloride gene associated with resistance to fipronil in Rhipicephalus (Boophilus) microplus ticks from Argentina

This study aimed to evaluate the sequence variations of the GABA-Cl gene of Rhipicephalus (Boophilus) microplus ticks from Argentina with different resistance levels to fipronil. Genomic DNA was obtained from engorged females (n = 50) of fipronil-susceptible and resistant tick field populations from five localities in northern Argentina. Tick populations were tested for fipronil resistance by in vitro larval packet test. DNA encoding amino acid residues 264-360 of the GABA-Cl was amplified by PCR. PCR products were sequenced and the translated amino acid sequences were aligned and compared among each other and with sequences of the GABA-Cl belonging to the Brazilian and Uruguayan reference strains fipronil-susceptible (Mozo) and resistant (RFSan and Juarez). All the GABA-Cl amino acid sequences obtained from the R. microplus specimens from Argentina were identical, regardless of the fipronil resistance-susceptibility status of each population. Mutations present in GABA-Cl sequence of RFSan and Juarez strains were not present in any of the populations from Argentina analyzed. These results indicate the failure of this gene as a molecular marker of resistance to fipronil, at least in the R. microplus populations from northern Argentina.

Synergistic property of piperonyl butoxide, diethyl maleate, triphenyl phosphate and verapamil hydrochloride with deltamethrin and ivermectin against Rhipicephalus microplus ticks

The present study was taken up to evaluate the synergistic properties of piperonyl butoxide (PBO), diethyl maleate (DEM), triphenyl phosphate (TPP) and verapamil (VER) with deltamethrin (DLM) and ivermectin (IVM) against DLM and IVM resistant tick populations collected from Madhya Pradesh and Punjab states of India. The collected field tick populations were resistant to DLM (Resistance Factor [RF] in the range of 21.71-32.98) and IVM (RF in the range of 1.89-4.98). A strong synergism between DLM and, IVM with PBO and IVM with VER was noticed. The synergistic efficacy of PBO and VER with IVM in reducing the lethal concentration 50 (LC50) value (1.69-5.72 times for PBO and 3.00-10.62 times for VER) of IVM in resistant ticks suggest that a combination of these synergists with IVM can significantly enhance the effectiveness of IVM against IVM-resistant Rhipicephlaus microplus populations gradually establishing in the different parts of the country. The synergistic efficiency of PBO with DLM in reducing the LC50 value was 2.65 and 18.01 times, respectively, against DLM- resistant two R. microplus populations (KTN and LDH). The study revealed the gradual establishment of DLM and IVM resistant populations in the surveyed states suggesting the need to adopt required resistance management strategies. The use of synergists with DLM and IVM has emerged as an effective approach for controlling the acaricide-resistant ticks.

Combination of synthetic acaricides with (E)-cinnamaldehyde to control Rhipicephalus microplus

This work had the objectives to (1) evaluate the susceptibility of various Rhipicephalus microplus populations to commercial acaricides, and (2) select commercial acaricides (50-80% effective) and evaluate the effects of binary combinations of the phenylpropanoid (E)-cinnamaldehyde with selected commercial acaricides to control R. microplus under laboratory and field conditions. Using adult immersion tests with 116 populations and 14 commercial acaricides, products showing 50-80% effectiveness (percent control) with the lowest number of active ingredients were selected. Acaricides containing amitraz or chlorfenvinphos were tested in combination with (E)-cinnamaldehyde on a field population (strain CM). We found that (E)-cinnamaldehyde enhanced the activity of both commercial acaricides against R. microplus larvae; however, the enhancement was more accentuated when using amitraz. Experiments combining (E)-cinnamaldehyde + amitraz on unfed larvae and engorged females from another population (strain Gyn) were performed, verifying (E)-cinnamaldehyde enhanced the activity of amitraz. In the field experiment, the application of (E)-cinnamaldehyde appeared toxic to the tick hosts (cattle). We concluded that (E)-cinnamaldehyde enhanced the activity of amitraz against unfed larvae and engorged females of R. microplus; however, in the field test this phenylpropanoid caused intoxication in the cattle. Studies searching for new combinations of compounds from essential oils with amitraz deserve attention, as well as studies to develop formulations using amitraz + (E)-cinnamaldehyde that will be efficient and will not have toxic effects in cattle.

Acaricides Resistance in Ticks: Selection, Diagnosis, Mechanisms, and Mitigation

Ticks are blood-feeding ecto-parasites that have a cosmopolitan distribution in tropical and subtropical regions of the world. Ticks cause economic losses in the form of reduced blood, meat and dairy products, as well as pathogen transmission. Different acaricides such as organochlorines, organophosphates, formamidines (e.g. amitraz), synthetic pyrethroids, macrocyclic lactones, fipronil, and fluazuron are currently used sequentially or simultaneously to control tick infestations. Most acaricide treatments now face increasingly high chances of failure, due to the resistance selection in different tick populations against these drugs. Acaricide resistance in ticks can be developed in different ways, including amino acid substitutions that result in morphological changes in the acaricide target, metabolic detoxification, and reduced acaricide entry through the outer layer of the tick body. The current literature brings a plethora of information regarding the use of different acaricides for tick control, resistance selection, analysis of mutations in target sites, and resistance mitigation. Alternatives such as synergistic use of different acaricides, plant-derived phytochemicals, fungi as biological control agents, and anti-tick vaccines have been recommended to avoid and mitigate acaricide resistance. The purpose of this review was to summarize and discuss different acaricides applied for tick control, their mechanisms of action and resistance selection, genetic polymorphisms in their target molecules, as well as the approaches used for diagnosis and mitigation of acaricide resistance, specifically in Rhipicephalus microplus ticks.

Neuropeptides in Rhipicephalus microplus and other hard ticks

The synganglion is the central nervous system of ticks and, as such, controls tick physiology. It does so through the production and release of signaling molecules, many of which are neuropeptides. These peptides can function as neurotransmitters, neuromodulators and/or neurohormones, although in most cases their functions remain to be established. We identified and performed in silico characterization of neuropeptides present in different life stages and organs of Rhipicephalus microplus, generating transcriptomes from ovary, salivary glands, fat body, midgut and embryo. Annotation of synganglion transcripts led to the identification of 32 functional categories of proteins, of which the most abundant were: secreted, energetic metabolism and oxidant metabolism/detoxification. Neuropeptide precursors are among the sequences over-represented in R. microplus synganglion, with at least 5-fold higher transcription compared with other stages/organs. A total of 52 neuropeptide precursors were identified: ACP, achatin, allatostatins A, CC and CCC, allatotropin, bursicon A/B, calcitonin A and B, CCAP, CCHamide, CCRFamide, CCH/ITP, corazonin, DH31, DH44, eclosion hormone, EFLamide, EFLGGPamide, elevenin, ETH, FMRFamide myosuppressin-like, glycoprotein A2/B5, gonadulin, IGF, inotocin, insulin-like peptides, iPTH, leucokinin, myoinhibitory peptide, NPF 1 and 2, orcokinin, proctolin, pyrokinin/periviscerokinin, relaxin, RYamide, SIFamide, sNPF, sulfakinin, tachykinin and trissin. Several of these neuropeptides have not been previously reported in ticks, as the presence of ETH that was first clearly identified in Parasitiformes, which include ticks and mites. Prediction of the mature neuropeptides from precursor sequences was performed using available information about these peptides from other species, conserved domains and motifs. Almost all neuropeptides identified are also present in other tick species. Characterizing the role of neuropeptides and their respective receptors in tick physiology can aid the evaluation of their potential as drug targets.

Resistance of the cattle tick Rhipicephalus (Boophilus) microplus to fluazuron in Argentina

The aim of this work is to report the presence of resistance to fluazuron in a population of Rhipicephalus microplus in Argentina. The evidence was obtained from field and in vitro trials. In the field trial, cattle infested with ticks was treated with two commercial formulations of fluazuron. The in vitro trial (adult immersion test, AIT) was performed by using technical grade fluazuron. In the field trial, there were no significant differences between the treated and control groups between days 2 and 34 post-treatment. The only exceptions (treated group I in day 14 post-treatment, treated group II in days 23 and 29 post-treatment) had a significantly lower tick load than the untreated group, but the efficacy was not higher than 70%. Viable engorged females were collected on both groups of treated bovines in all counts, and the production of viable larvae was not precluded with the application of the two commercial formulations of fluazuron evaluated in this study. The results obtained with the in vitro assay (AIT) also indicate that the R. microplus population tested in this work has a higher level of resistance to fluazuron than another susceptible field strain. The integrated analysis of the field and in vitro trials clearly reveals the emergence of resistance to fluazuron in a R. microplus population from Argentina. This diagnosis of resistance does not imply that the fluazuron has lost its functionality at a regional scale, but it highlights the need to establish control strategies that minimize the use of this drug in order to preserve its functionality as an acaricide.

Evaluation of acaricidal resistance status of Rhipicephalus microplus ticks from the hilly state (Uttarakhand) of India and evaluation of efficacy of a natural formulation for the management of resistant ticks

The resistance status against deltamethrin, cypermethrin, coumaphos and ivermectin was assessed of Rhipicephalus microplus from five districts of Uttarakhand, through adult immersion test and larval packet test. The field isolates were highly resistant to deltamethrin (median resistance ratio [RR₅₀] = 9.10-29.13-fold) followed by cypermethrin (2.23-3.55). Surprisingly the isolates were susceptible to coumaphos (0.34-3.17). Emerging resistance against ivermectin (1.55-3.27) was also observed in all the isolates. Elevated levels of esterases (enzyme ratio = 2.93-5.84-fold), glutathione S-transferases (5.10-10.06) and monooxygenases (1.68-4.02) in resistant fields isolates were highly correlated (47.4-86.0%) with the resistant factor (RR₅₀) determined by bioassay. All the isolates except Uttarkashi possess mutation at the 190th position in domain II of the sodium channel gene. As a mitigation strategy an Ageratum conyzoides-based characterized natural formulation was tested against all the isolates and was found effective at the concentration of 10.1-11.5%. The possibility of using the natural formulation for the management of multi-acaricide resistant ticks is discussed.

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

This study aimed to evaluate the pharmacokinetics, the potential accumulation in the body of treated animals and the efficacy of ivermectin long-acting formulation (3.15%) against the cattle tick Rhipicephalus (Boophilus) microplus in a scheme of three successive treatments. Fifteen 12-month-old heifers, naturally infested with R. microplus, were divided into two groups (G). Cattle from GI (n = 10) were subjected to three treatments with ivermectin 3.15% (IVOMEC GOLD®, Merial Argentina S.A.) at a rate of 1 mL/50 kg on days 0, 35, and 70. Cattle from GII (n = 5) were not treated. From day 1 to 202 post-treatment blood samples were taken to measure ivermectin concentrations by HPLC and female ticks (4.5-8 mm) were counted to evaluate the efficacy of the treatment. The level of tick resistance to ivermectin was evaluated before and after finishing the scheme of successive treatments by larval immersion test (LIT) bioassay from engorged females collected from GI. The area under the concentration vs. time curves (AUC_0-35d) obtained post-second treatment was 1.51 ± 0.39-fold higher than those observed post-first treatment (P<0.05). The mean plasma concentrations of ivermectin 3.15% at 20 days after the first, second and third treatment were 17.0, 27.5 and 37.8 ng/mL, respectively (P<0.01). The elimination half-life of ivermectin post-third treatment was significantly longer than that was previously reported after a single dose (P<0.01). Values of therapeutic efficacy percentage reached 75.6% post-first treatment and between 95.9 and 100% after the second treatment. Ticks evaluated by LIT showed a significant increase in lethal concentrations after treatments. Although the efficacy level was high, the successive treatments with long-acting ivermectin formulation generate a significant accumulation of drug in plasma and could increase the levels of resistance to this drug in the tick population.

Effects of acaricidal essential oils from Lippia sidoides and Lippia gracilis and their main components on vitellogenesis in Rhipicephalus microplus (Canestrini, 1888) (Acari: Ixodidae)

Rhipicephalus microplus is an important cattle tick, and resistant strains to synthetic compounds have been widespread. The combined effects of different essential oil compounds enhance biological activity and reduce selection for the development of target organism resistance. Essential oils of two different genotypes of each of Lippia sidoides and Lippia gracilis and their main components, the isomers thymol and carvacrol, have acted as acaricides against R. microplus. Little is known about the effects of the essential oils of L. sidoides and L. gracilis and thymol and carvacrol on the morphophysiology of R. microplus ovaries. This study aimed to identify the morphological changes in the ovaries of R. microplus females treated with essential oils from two different genotypes of each of L. sidoides (102 and 103) and L. gracilis (106 and 201) and the terpenes thymol and carvacrol through histological techniques. The LC₅₀ and LC₇₅ of essential oils and thymol and carvacrol were used for Adult Immersion Test (AIT) with groups of five fully engorged females of R. microplus. A negative control (DMSO 3% solution) was performed. Seven days after the AIT, the ticks were dissected to collect ovaries and their histologic analysis. Only the group treated with the essential oil of L. gracilis genotype 106 at the LC₅₀ had no change compared with the control. The other groups showed the following changes in oocytes I to V: vacuolation, chorion deformation, disorganization of yolk granules, and irregularities at the cell periphery, causing incomplete process of vitellogenesis. Thus, the essential oils tested in this study may be potent products for the control of cattle ticks and thereby preventing further life cycles.

Characterization of deltamethrin, cypermethrin, coumaphos and ivermectin resistance in populations of Rhipicephalus microplus in India and efficacy of an antitick natural formulation prepared from Ageratum conyzoides

Rhipicephalus microplus is posing a serious threat to productive animal husbandry. Excessive use of synthetic chemicals in tick management has led to the development of resistant tick populations. Characterization of resistance to deltamethrin, cypermethrin, coumaphos and ivermectin in ticks is necessary to develop a suitable and sustainable control strategy. Based on adult immersion test and larval packet test, the resistance ratios (RR₅₀) for adults and larvae of R. microplus populations from two Indian states ranged from 3.8 to 19.4 and 1.35-25.0 against deltamethrin, 0.061-26.3 and 0.22-19.2 against cypermethrin, and 0.2-9.5 and 0.01-3.1 against coumaphos, respectively, were recorded. Moreover, the RR₅₀ for adults ranged from 0.212 to 3.87 against ivermectin. The RR₅₀ for different acaricides was significantly (p<0.01) correlated with esterases, Glutathione S-transferase and monooxygenase activity. A point mutation at the 190th position of the domain II S4-5 linker region of the sodium channel gene in synthetic pyrethroids (SP) resistant populations was also detected. An antitick natural formulation prepared from the plant Azeratum conyzoides and containing two major compounds, Precocene-I (7‑methoxy-2, 2-dimethyl 2H-chromene) and Precocene II (6, 7-dimethoxy-2, 2-dimethyl- 3-chromene), was developed and tested against the resistant ticks. The LC₅₀ values of the natural formulation against the resistant populations were in the range of 4.31-5.33% irrespective of their RR₅₀ values. Multi-acaricide resistant populations of R. microplus are established in India and the A. conyzoides based natural formulation can be used for its management.

Comparative susceptibility of Rhipicephalus microplus collected from the northern state of India to coumaphos, malathion, deltamethrin, ivermectin, and fipronil

The chemical-based tick management method is gradually losing its clutch due to the establishment of resistant ticks. For development of region-specific tick management strategies, the present study was aimed to evaluate the comparative resistance profile of Rhipicephalus microplus isolates collected from seven districts of Uttar Pradesh, a northern state of India. Comparative analysis of the dose-response data using adult immersion test (AIT) against coumaphos, malathion, deltamethrin, ivermectin, and fipronil revealed that all the isolates were resistant to discriminating concentration of deltamethrin having LC₅₀ of 295.12-436.52 ppm with a resistance ratio of 22.02-32.58. An emerging low level of ivermectin resistance (resistance ratio, RR₅₀ = 1.03-2.26) with LC₅₀ in the range of 22.39-48.98 ppm was found across the isolates. The coumaphos was highly effective against all except Amethi (AMT) isolate. Similarly, malathion was efficacious against most of the isolates except Pratapgarh (PRT) and Sultanpur (SUL) isolates showing LC₅₀ of 5128.61 and 5623.41 ppm, respectively. All the isolates were responsive to fipronil. Comparative detoxifying enzymes profiles revealed a significant correlation between the increased activity of esterase and deltamethrin resistance. The GST activity was 51.2% correlated with RR₅₀ of malathion while esterase activity was significantly correlated (68.9%) with RR₅₀ of coumaphos. No correlation between the ivermectin resistance and enzyme activity was established. Multiple sequence analysis of S4-5 linker region of the sodium channel gene of all the isolates revealed a point mutation at 190th position (C190A) which is associated with deltamethrin resistance. The possible tick management strategies in this part of the country are discussed.

Genotyping of Deltamethrin Resistance in Rhipicephalus (Boophilus) microplus Population in Kerala, South India

PURPOSE

Genotyping of Rhipicephalus (Boophilus) microplus for polymorphisms in deltamethrin-resistant loci of sodium channel gene by allele-specific PCR (AS-PCR).

METHODS

Adult R. (B.) microplus ticks were collected from naturally infested cattle in Kerala. The larval packet test (LPT) was performed with deltamethrin and dose response data were analysed by probit method. Adult and larval tick DNA were amplified by PCR and later sequenced to identify mutations, if any, in the domain II S4-5 linker and domain III S6 regions in para voltage-gated sodium channel gene, at loci that were previously documented to be associated with deltamethrin resistance. Allele-specific PCR was performed for the amplification of target gene locus (C190A and T2134C) to genotype 1000 larvae each, at these loci. Genotype frequency was statistically analysed by Chi-square test.

RESULTS

Bioassay using LPT revealed that LC₅₀ and LC₉₅ values of all the R. (B.) microplus isolates in this study were more than double the reported values of reference susceptible strain. Sequence analysis of the PCR amplicons of domain II S4-5 linker of voltage-gated sodium channel gene revealed C190A mutation, A271G mutation as well as A-G mutation at 217th position. AS-PCR done to genotype C190A mutations revealed a frequency of 6%, 15% and 64%, respectively for homozygous-susceptible (SS), heterozygous (RS) and homozygous-resistant (RR) genotypes. In domain III S6 region of the gene, C2121T and A2102T mutations were observed. AS-PCR to genotype the previously reported T2134C mutation revealed 100% SS genotype in R. (B.) microplus isolates of Kerala.

CONCLUSIONS

Genotyping of R. (B.) microplus isolates of Kerala for target site mutations reportedly associated with deltamethrin resistance revealed a significantly high frequency of resistant genotypes at II S4-5 linker of voltage-gated sodium channel gene. This study forms the first report of such mutations in Kerala, south India and demands serious attention in the light of increased prevalence of tick-borne haemoparasitic infection.

Efficacy of two commercial synthetic pyrethroids (cypermethrin and deltamethrin) on Amblyomma variegatum and Rhipicephalus microplus strains of the south-western region of Burkina Faso

Since 2011, period of the livestock invasion by the cattle tick Rhipicephalus microplus in Burkina Faso (BF), tick-control problems were exacerbated. Based on farmer’s reports, most commonly used commercial acaricides were found to be ineffective in Western South part of the country. To investigate the occurrence and extent of such acaricidal ineffectiveness, we performed the standardized larval packet test (LPT) with commercial deltamethrin (vectocid) and cypermethrin (cypertop), on two cattle tick species, the native Amblyomma variegatum and the invasive R. microplus. The resistance ratios (RR) were computed with susceptible Hounde strain of Rhipicephalus geigyi as reference. The R. microplus population showed resistance to the two acaricides tested with the highest lethal concentration (LC) values, and different resistance ratios higher than 4 (deltamethrin: RR₅₀ = 28.18 and RR₉₀ = 32.41; cypermethrin: RR₅₀ = 8.79 and RR₉₀ = 23.15). In the contrary, A. variegatum population was found to be highly susceptible to acaricides tested with low lethal concentrations and resistance ratio values (deltamethrin: RR₅₀ = 0.5 and RR₉₀ = 0.48; cypermethrin: RR₅₀ = 0.68 and RR₉₀ = 0.79). These data demonstrate high synthetic pyrethroid resistance in R. microplus strain, leading to conclude that the acaricide ineffectiveness in tick populations control remains a concern in BF.

Mechanisms of amitraz resistance in a Rhipicephalus microplus strain from southern Brazil

Amitraz is one of the most used acaricides for the control of ticks of domestic animals, however, extensive use of this active ingredient has favored the development of resistant populations of Rhipicephalus microplus worldwide. The possible mechanisms of metabolic and/or target-site alterations mechanisms of amitraz resistance were investigated in a Brazilian field population of R. microplus (São Gabriel strain). Bioassays with the synergists piperonylbutoxide, triphenylphosphate and diethyl-maleate were used to evaluate the metabolic mechanisms involved. Target-site insensitivity was investigated by amplification and sequencing of a fragment of the octopamine/tyramine (OCT/TYR) receptor gene. Piperonylbutoxide synergism (synergism ratio = 2.8) indicated the participation of the P450 pathway in the detoxification of amitraz. Previously reported single nucleotide polymorphisms that confer amino acid changes in the OCT/TYR receptor, threonine to proline (T8P) and leucine to serine (L22S), were found in the amitraz-resistant strain but not in the susceptible reference strain. The results suggest that amitraz resistance in the studied strain is multi-factorial and may result from cytochrome P450 detoxification and mutations in octopamine receptors.

Deltamethrin resistant alleles predominate in Rhipicephalus sanguineus sensu lato in South India

Brown dog tick, Rhipicephalus sanguineus sensu lato, an important ectoparasite transmitting several pathogens, is the most common tick species infesting dogs. Control of ticks being central to the control of fatal tick-borne diseases, this study attempted to assess the susceptibility/resistance of brown dog ticks to synthetic pyrethroids, the commonly used acaricides against ticks. Larval packet assay revealed 60% of isolates tested to be resistant and tolerant to deltamethrin as per the resistance factor that ranged from 1 to 53.7. Sequence analysis of the PCR amplified product of domain II S4-5 linker of sodium channel gene in R. sanguineus revealed novel polymorphisms, viz., C190A, G215T and T270C. In domain III S6 region of the gene, a T2134C mutation was observed. Genotyping with allele-specific PCR targeting domain II S4-5 linker region using single larvae revealed that most R. sanguineus larvae in the study population were homozygous resistant (RR) genotypes, followed by heterozygous (RS) and homozygous susceptible (SS) genotypes. A higher proportion of RS genotypes was also observed in domain III S6 region. This first report of genotyping of Indian R. sanguineus to analyse synthetic pyrethroid resistance highlights the need to devise alternate control strategies to reduce the brown dog tick population.

Entomopathogenic Fungi and Bacteria in a Veterinary Perspective

Simple Summary

Several fungal species are well suited to control arthropods, being able to cause epizootic infection among them and most of them infect their host by direct penetration through the arthropod’s tegument. Most of organisms are related to the biological control of crop pests, but, more recently, have been applied to combat some livestock ectoparasites. Among the entomopathogenic bacteria, Bacillus thuringiensis, innocuous for humans, animals, and plants and isolated from different environments, showed the most relevant activity against arthropods. Its entomopathogenic property is related to the production of highly biodegradable proteins. Entomopathogenic fungi and bacteria are usually employed against agricultural pests, and some studies have focused on their use to control animal arthropods. However, risks of infections in animals and humans are possible; thus, further studies about their activity are necessary.

Abstract

The present study aimed to review the papers dealing with the biological activity of fungi and bacteria against some mites and ticks of veterinary interest. In particular, the attention was turned to the research regarding acarid species, Dermanyssus gallinae and Psoroptes sp., which are the cause of severe threat in farm animals and, regarding ticks, also pets. Their impact on animal and human health has been stressed, examining the weaknesses and strengths of conventional treatments. Bacillus thuringiensis, Beauveria bassiana and Metarhizium anisopliae are the most widely employed agents. Their activities have been reviewed, considering the feasibility of an in-field application and the effectiveness of the administration alone or combined with conventional and alternative drugs is reported.

A retrospective review on ixodid tick resistance against synthetic acaricides: implications and perspectives for future resistance prevention and mitigation

The ending of the nineteenth-century was characterized by an escalation of ticks and tick-borne diseases that resulted in the death of many cattle. This necessitated the search for an effective means of tick control. Arsenicals were introduced in Australia in 1895, and arsenic-based dipping vats went on to be used for about 40 years until resistance was found in ticks and more effective alternatives - chemical acaricides - were developed after World War II. However, the development of resistance by ticks, environmental persistence, and mammalian toxicity militated against the sustained use of subsequent chemical acaricides. Furthermore, the development of resistance is a phenomenon that would always evolve, and the multiple mechanisms underlying the synthetic acaricides resistance are of great importance for future integrated control of ticks and tick-borne diseases. Hence, this study retrospectively reviewed the development of synthetic acaricides and the underlying mechanisms of tick resistance against synthetic acaricides in the hope of providing the implications and perspectives for resistance prevention and mitigation for future tick control.

Acaricidal activity of the essential oil from Senecio cannabifolius and its constituents eucalyptol and camphor on engorged females and larvae of Rhipicephalus microplus (Acari: Ixodidae)

The objective of this experiment was to evaluate the acaricidal activity of Senecio cannabifolius essential oil, and two of its constituents, eucalyptol and camphor. Efficacy against females and larvae of Rhipicephalus (Boophilus) microplus was assessed by the adult immersion test (AIT) and the larval immersion test (LIT). The oil was analyzed by gas chromatography-flame ionization detection (GC-FID) and GC-mass spectrometry (MS) and in total 68 components were identified representing 99.2% of the essential oil. AIT revealed that the oil, eucalyptol, and camphor at the highest concentration presented efficacy of 68.9, 57.1, and 71.9%, respectively. LIT revealed that the essential oil and camphor achieved 100% mortality at concentration of 1.6% wt/vol, whereas eucalyptol showed moderate inhibitory activity. Biochemical assays indicated that the essential oil and camphor can reduce significantly overall detoxification enzyme activities in engorged females and larvae at high concentration (≥ 0.4% wt/vol), whereas the inhibitory effect of eucalyptol is weaker than that of the oil and camphor. Taken together our results indicated that the S. cannabifolius essential oil and its isolated constituent had potential for the development of a new and safe acaricide for the control of R. microplus ticks.

Tick saliva-induced programmed death-1 and PD-ligand 1 and its related host immunosuppression

The tick Rhipicephalus microplus is a harmful parasite of cattle that causes considerable economic losses to the cattle breeding industry. Although R. microplus saliva (Rm-saliva) contains several immunosuppressants, any association between Rm-saliva and the expression of immunoinhibitory molecules, such as programmed death (PD)-1 and PD-ligand 1 (PD-L1), has not been described. In this study, flow cytometric analyses revealed that Rm-saliva upregulated PD-1 expression in T cells and PD-L1 expression in CD14⁺ and CD11c⁺ cells in cattle. Additionally, Rm-saliva decreased CD69 expression in T cells and Th1 cytokine production from peripheral blood mononuclear cells. Furthermore, PD-L1 blockade increased IFN-γ production in the presence of Rm-saliva, suggesting that Rm-saliva suppresses Th1 responses via the PD-1/PD-L1 pathway. To reveal the upregulation mechanism of PD-1/PD-L1 by Rm-saliva, we analyzed the function of prostaglandin E₂ (PGE₂), which is known as an inducer of PD-L1 expression, in Rm-saliva. We found that Rm-saliva contained a high concentration of PGE₂, and PGE₂ treatment induced PD-L1 expression in CD14⁺ cells in vitro. Immunohistochemical analyses revealed that PGE₂ and PD-L1 expression was upregulated in tick-attached skin in cattle. These data suggest that PGE₂ in Rm-saliva has the potential to induce the expression of immunoinhibitory molecules in host immune cells.

Cytotoxic effects of Satureja montana L. essential oil on oocytes of engorged Rhipicephalus microplus female ticks (Acari: Ixodidae)

In addition to pesticidal activity, the capacity of natural compounds to inhibit the reproduction of parasites emerge as an important alternative tick control method. In this context, Satureja spp. stand out due to their recognized pesticidal properties. Among parasites of veterinary importance, the cattle tick, Rhipicephalus microplus, is responsible for great economic losses in livestock and transmission of relevant pathogens. Thus, the aim of this study was to evaluate the effects of different concentrations of the essential oil of Satureja montana L. on the ovary morphology of R. microplus engorged females through histological and histochemical techniques. The most remarkable morphological changes found were: cytoplasmic vacuolation of germ cells, irregular and thicker chorion, irregular oocyte shape, ring-shaped nucleolus, decrease in protein and carbohydrate content in oocytes, in addition to cellular changes in the oviduct and pedicel. All morphological changes were assessed using a semiquantitative method already established in the literature. Ticks exposed to 5.0 μl/ml of essential oil showed the most significant changes when compared to control groups. Thus, the essential oil of S. montana L. damaged the reproductive system of R. microplus, which may impair ticks' offspring production and promote a long-term control of this species. HIGHLIGHTS: The essential oil of Satureja montana L. affects the ovary morphology of the cattle tick. The main morphological alterations found were cytoplasmic vacuolation, irregular and thicker chorion and irregular oocyte shape. These alterations may impair the development of eggs.

In vitro and in silico studies of the larvicidal and anticholinesterase activities of berberine and piperine alkaloids on Rhipicephalus microplus

Rhipicephalus microplus is responsible for high economic losses in livestock and its control has become difficult due to the establishment of tick populations resistant to commercial acaricides. This study aimed to evaluate the in vitro larvicidal effect of the alkaloids berberine and piperine, and also to investigate their inhibitory mechanisms against the acetylcholinesterase enzyme. The effects of the alkaloids on larvae were observed through the immersion test at the following concentrations: 1.5; 3; 6; 12; 16 and 24 mM. Berberine and piperine presented larvicidal activity greater than 95 %, not differing from 100 % for the positive fipronil control (p > 0.05). Of the two alkaloids, piperine had a lower effective concentration (EC), with an EC₅₀ of 6.04 mM. The acetylcholinesterase enzyme used in the study was obtained from R. microplus larvae (RmAChE) and the anticholinesterase activity was determined spectrophotometrically. The highest anticholinesterase activity, measured as inhibition concentration (IC), was observed for berberine (IC50 = 88.13 μM), while piperine showed lower activity (IC50 > 200 μM). Docking studies in RmAChE, followed by 10 ns molecular dynamics simulation, suggest that berberine stabilizes the RmAChE at lower Root-Mean-Square Deviation (RMSD) than Apo protein. Few hydrogen-bond interactions between berberine and RmAChE residues were balanced by hydrophobic and π-type interactions. Berberine fills preferentially the peripheral anionic site (PAS), which correlates with its non-competitive mechanism. These results suggest that berberine and piperine alkaloids have an in vitro acaricidal action on R. microplus larvae, and the likely mechanism of action of berberine is related to RmAChE inhibition when accessing the PAS residues. These findings could help the study of new natural products that could inhibit RmAChE and aid in the development of new acaricides.

Chemoprophylaxis for babesiosis and anaplasmosis in cattle: case report

Cattle tick fever (CTF) causes significant economic losses in the livestock sector. The pathogenic action of the hemoparasites is associated with anemia, weight loss, abortion and reduced productivity, which result with animal death. Programs to prevent CTF involve several procedures, including immunization, chemoprophylaxis and use of ectoparasiticides, together with the vector control in the environment. The objective of this study was to report an acute outbreak of CTF in a group of 157 Hereford cattle from a farm without presence of the vector, that were moved to a farm in the same state with a high tick infestation (Rhipicephalus microplus). On the day before the transportation, the animals received a chemoprophylaxis with imidocarb dipropionate (3 mg/kg, SC), which was repeated 21 days after the first application. After 42 days, some animals showed signs compatible with CTF, which was confirmed through clinical examination, necropsy, histopathological and hemoparasitological analyses. The morbidity rate was 37.6% and the mortality rate was 24.8%. Calves that were recently weaned were the group most affected with the tick fever, morbidity (100% and mortality (73%). Chemoprophylaxis in association with use of ectoparasiticides was not sufficient to control the outbreak of the disease.

A genome-wide screening for RNAi pathway proteins in Acari

BACKGROUND

RNA interference (RNAi) is a highly conserved, sequence-specific gene silencing mechanism present in Eukaryotes. Three RNAi pathways are known, namely micro-RNA (miRNA), piwi-interacting RNA (piRNA) and short interfering RNA (siRNA). However, little knowledge exists about the proteins involved in these pathways in Acari. Moreover, variable successes has been obtained in gene knockdown via siRNA pathway in their functional genomics and management. We hypothesized that the clue may be in the variability of the composition and the efficacy of siRNA machinery among Acari.

RESULTS

Both comparative genomic analyses and domain annotation suggest that all the analyzed species have homologs of putative core proteins that mediate cleaving of targeted genes via the three RNAi pathways. We identified putative homologs of Caenorhabditis elegans RNA-dependent RNA polymerase (RdRP) protein in all species though no secondary Argonaute homologs that operate with this protein in siRNA amplification mechanism were found, suggesting that the siRNA amplification mechanism present in Acari may be distinct from that described in C. elegans. Moreover, the genomes of these species do not encode homologs of C. elegans systemic RNAi defective-1 (Sid-1) protein that mediate silencing of the mRNA target throughout the treated organisms suggesting that the phenomena of systemic RNAi that has been reported in some Acari species probably occur through a different mechanism. However, homologs of putative RNAi spreading defective-3 (Rsd-3) protein and scavenger receptors namely Eater and SR-CI that mediate endocytosis cellular update of dsRNA in C. elegans and Drosophila melanogaster were found in Acari genomes. This result suggests that cellular dsRNA uptake in Acari is endocytosis-dependent. Detailed phylogenetic analyses of core RNAi pathway proteins in the studied species revealed that their evolution is compatible with the proposed monophyletic evolution of this group.

CONCLUSIONS

Our analyses have revealed the potential activity of all three pathways in Acari. Still, much experimental work remains to be done to confirm the mechanisms behind these pathways in particular those that govern systemic/parental RNAi and siRNA amplification in Acari. Disclosure of these mechanisms will facilitate the development of new and specific management tools for the harmful species and enrichment of the beneficial species.

In vitro diagnosis of resistance of the cattle tick Rhipicephalus (Boophilus) microplus to fipronil in Argentina

The aim of this work was to adapt and apply the in vitro larval bioassay packet test (LPT) to diagnose resistance of Rhipicephalus microplus ticks to fipronil in Argentina. Ten serial dilutions of fipronil were included in the LPT bioassay. Seven populations of R. microplus from six Argentinean Provinces were evaluated. Samples from Colonia Benítez, Quimilí and Intiyaco were unambiguously classified in the category 'resistant'. In all these cases the 95% confidence intervals did not overlap with that of the susceptible reference strain and the slope values were lower than the slope value obtained for the susceptible reference strain. The resistance ratio (RR₅₀) values of the populations from Colonia Benítez I and II, Quimilí and Intiyaco were 2.23, 3.49, 6.84 and 3.39, respectively. Populations from El Colorado I and II, Acambuco, Vera and Feliciano were classified as 'susceptible' because the 95% confidence intervals overlapped with that of the susceptible reference strain and the RR₅₀ values never exceeded 1.32. The results of these LPT bioassays indicate the presence of fipronil resistant R. microplus populations in Argentina.

In vivo and in vitro apoptosis induced by new acaricidal ethyl-carbamates in Rhipicephalus microplus

The ability of ethyl-4-bromophenylcarbamate (LQM 919) and ethyl-4-chlorophenylcarbamate (LQM 996) to induce in vivo apoptosis of Rhipicephalus microplus ovarian cells and in vitro apoptosis of tick and mammalian cell culture was evaluated. The ovaries of engorged females treated with 1 mg mL^-1 LQM 919 or LQM 996 presented more (p < 0.001) peroxidase-TUNEL-positive labeled cells (apoptotic cells) in situ than their respective control groups, and this increase was time-dependent (p < 0.001). The majority of apoptotic cells were observed in the epithelium and ovarian pedicel. HepG2, Vero and Rm-sus cells, as well as cells from primary cultures of R. microplus salivary glands, intestine and ovaries were exposed to different concentrations of the ethyl-carbamates. Both ethyl-carbamates induced a concentration-dependent reduction in the viability of all cell types (p < 0.001). Exposure to the ethyl-carbamates increased caspase 3 activity (p < 0.01) in primary cultures and cell lines, except in HepG2 cells. Fluorescent TUNEL-positive cells were observed in all cell types treated with 600 μM LQM 919 or LQM 996. These results indicate that both ethyl-carbamates induce apoptosis of the ovarian, intestinal and salivary glands cells in R. microplus and strongly suggest that this is their main mechanism of acaricidal action.

A physiologic overview of the organ-specific transcriptome of the cattle tick Rhipicephalus microplus

To further obtain insights into the Rhipicephalus microplus transcriptome, we used RNA-seq to carry out a study of expression in (i) embryos; (ii) ovaries from partially and fully engorged females; (iii) salivary glands from partially engorged females; (iv) fat body from partially and fully engorged females; and (v) digestive cells from partially, and (vi) fully engorged females. We obtained > 500 million Illumina reads which were assembled de novo, producing > 190,000 contigs, identifying 18,857 coding sequences (CDS). Reads from each library were mapped back into the assembled transcriptome giving a view of gene expression in different tissues. Transcriptomic expression and pathway analysis showed that several genes related in blood digestion and host-parasite interaction were overexpressed in digestive cells compared with other tissues. Furthermore, essential genes for the cell development and embryogenesis were overexpressed in ovaries. Taken altogether, these data offer novel insights into the physiology of production and role of saliva, blood digestion, energy metabolism, and development with submission of 10,932 novel tissue/cell specific CDS to the NCBI database for this important tick species.

A physiologic overview of the organ-specific transcriptome of the cattle tick Rhipicephalus microplus

To further obtain insights into the Rhipicephalus microplus transcriptome, we used RNA-seq to carry out a study of expression in (i) embryos; (ii) ovaries from partially and fully engorged females; (iii) salivary glands from partially engorged females; (iv) fat body from partially and fully engorged females; and (v) digestive cells from partially, and (vi) fully engorged females. We obtained > 500 million Illumina reads which were assembled de novo, producing > 190,000 contigs, identifying 18,857 coding sequences (CDS). Reads from each library were mapped back into the assembled transcriptome giving a view of gene expression in different tissues. Transcriptomic expression and pathway analysis showed that several genes related in blood digestion and host-parasite interaction were overexpressed in digestive cells compared with other tissues. Furthermore, essential genes for the cell development and embryogenesis were overexpressed in ovaries. Taken altogether, these data offer novel insights into the physiology of production and role of saliva, blood digestion, energy metabolism, and development with submission of 10,932 novel tissue/cell specific CDS to the NCBI database for this important tick species.