Comparison of the efficacy of different methods to apply acaricides for control of Rhipicephalus (Boophilus) microplus

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.

Anaplasmosis in the Amazon: diagnostic challenges, persistence, and control of Anaplasma marginale and Anaplasma phagocytophilum

Anaplasmosis remains a significant threat to livestock production in tropical regions, particularly in the Amazon basin, where ecological complexity and limited veterinary infrastructure challenge effective disease management. This review focuses on Anaplasma marginale and Anaplasma phagocytophilum, the primary species associated with bovine and granulocytic anaplasmosis, respectively. We examine the current state of diagnostic tools, highlighting the limited accessibility of molecular techniques in rural settings and the emerging but underutilized potential of technologies. Persistent infection and antigenic variation are explored as major obstacles for disease eradication and vaccine development. Although live attenuated and inactivated vaccines are in use for A. marginale, none provide sterilizing immunity, and no commercial vaccines exist for A. phagocytophilum. The review evaluates recent advances in recombinant antigens, chimeric constructs, and genetically attenuated strains, as well as future directions involving multiepitope design, novel adjuvants, and next-generation vaccine platforms. Additionally, we assess the role of tick control in disease prevention and emphasize the importance of integrated strategies in regions like the Amazon. Together, these findings underscore the need for context-specific solutions that address the ecological and epidemiological complexity of anaplasmosis in the Amazon basin.

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.

First evaluation of the efficacy and operational cost of acaricidal bath combining different droplet sizes, pressures and spray patterns in dairy cows naturally infested with Rhipicephalus microplus

Rhipicephalus microplus poses a significant challenge to animal health. This study evaluated efficacy and the effective operational cost (EOC) of different acaricidal treatments varying droplet size, pressure and spray pattern using a power sprayer. Three experiments were conducted on dairy cows, with each experiment involving three groups of 10 animals: two treated groups and one untreated control group. In the first experiment, acaricide treatments with the same droplet size (medium) were compared at different pressures (50 and 100 psi) using a full cone spray pattern: E1-M50 and E1-M100. In the second experiment, different droplet sizes (fine and medium) were compared at the same pressure (100 psi) using a full cone spray pattern: E2-F100 and E2-M100. The third experiment compared droplet sizes and spray patterns (fine at full cone and ultra coarse at flat fan) at the same pressure (100 psi): E3-F100 and E3-UC100. All animals received acaricide treatment on day 0 with a product based on organophosphate and pyrethroid. Tick counts were performed on days + 7, + 14, and + 21 to determine treatment effectiveness. Data on bath time were collected to determine the EOC, besides cost with acaricide, labor, water and electricity. No statistical differences (p > 0.05) regarding the mean tick count were observed between treated groups in all experiments. However, the highest efficacy values were found in treatments with larger droplets (medium and ultra coarse) and higher pressure (100 psi). In experiment 1, the efficacy varied from 70.6 to 88.9% and 75.2 to 93.7% for the treated groups E1-M50 and E1-M100, respectively. In experiment 2, efficacy varied from 48.7 to 86.2% for E2-F100 and from 68 to 94.7% for E2-M100. In experiment 3, efficacy ranged from 62.9 to 76.9% for E3-F100 and from 73.4 to 86.5% for E3-UC100. The EOC of the acaricide bath varied between 0.42 and 0.46 USD per animal. The findings show that treatment efficacy and EOC vary depending on the spray parameters used for the control of R. microplus, but further studies should be conducted to determine better combinations of pressure, droplet size and spray pattern.

Assessing Antiparasitic Compounds Persistence in Cattle Hair by DART-MS

This study introduces an alternative strategy for evaluating antiparasitic persistence compounds in cattle hair by Direct Analysis in Real Time Mass Spectrometry (DART-MS). The developed DART-MS method aimed to determine fenthion, chlorpyrifos, and cypermethrin in cattle hair samples. DART-MS analyses were performed in positive ion mode, and parameters related to the DART source were evaluated. The analytical performance demonstrated the efficiency of the optimized DART-MS method for fenthion, chlorpyrifos, and cypermethrin quantification in the evaluated samples, meeting criteria for precision, accuracy and limits of detection. Overall, the DART-MS method provided a fast and efficient analysis for determination of antiparasitic agents in cattle hair, which contributes to the evaluation of drug administration protocols and dosage intervals, and aids the safety and advancement of the livestock sector.

Susceptibility of Amblyomma sculptum, Vector of Rickettsia rickettsii, Ticks from a National Park and an Experimental Farm to Different Synthetic Acaricides

Amblyomma sculptum is a relevant tick species from a One Health perspective, playing an important role as a vector of Rickettsia rickettsii, the main agent of spotted fever rickettsiosis in Brazil. In this study, we evaluated the susceptibility of two A. sculptum populations from Goiás state (midwestern Brazil) to different acaricides. The first tick population (GYN strain) originated from an experimental farm, where the ticks are annually exposed to acaricides. The second (PNE strain) was collected in a national park (Emas National Park), where the ticks had not been exposed to acaricides. Immersion tests were conducted with 21-day-old laboratory-reared larvae and nymphs originating from adult ticks collected in the areas mentioned above. The chosen acaricides were two synthetic pyrethroids (cypermethrin and deltamethrin), one organophosphate (chlorfenvinphos), one formamidine (amitraz), and two combinations of pyrethroids and organophosphates (cypermethrin, chlorpyrifos and citronellal; cypermethrin, fenthion and chlorpyrifos). Mortality data were used to determine the lethal concentration (LC) values at which 50%, 90%, and 99% of the ticks died (LC50, LC90, and LC99, respectively), and resistance ratios (RR) were calculated based on the LC values. The RR revealed differences between the acaricide-exposed (GYN) and unexposed (PNE) tick strains. The PNE strain larvae and nymphs were susceptible to all the tested acaricides. The GYN strain larvae were tolerant to cypermethrin, whereas the nymphs were tolerant to deltamethrin, chlorfenvinphos, and the combination of cypermethrin, chlorpyrifos, and citronellal (2 < RR ≤ 10). The GYN strain nymphs were resistant to amitraz (RR > 10). This is the first report of A. sculptum nymphs with resistance to amitraz and tolerance to deltamethrin, chlorfenvinphos, and the combination of cypermethrin, chlorpyrifos, and citronellal.

Selective versus strategic control against Rhipicephalus microplus in cattle: A comparative analysis of efficacy, animal health, productivity, cost, and resistance management

This study compared selective control versus strategic control against Rhipicephalus (Boophilus) microplus, under conditions of a real dairy farm. Strategic control requires that all animals be treated with acaricide at regular pre-determined intervals. Selective control only requires treatment of infested animals and only when they are at or above a pre-determined threshold. Tick counts on animals and in pasture were performed and the susceptibility of tick populations to the different treatment methods was evaluated at the beginning and end of the study using the Larval Packet Test, Larval Immersion Test, and Adult Immersion Test. Over the four years of the study strategic control was more advantageous than the selective control as the group experienced lower tick burden on animals and in pasture, absence of skin lesions and myiases, lower operating costs, treatments concentrated in fewer months of the year and lower resistance pressure.