Essential oils and isolated compounds for tick control: advances beyond the laboratory

BACKGROUND

Tick control is a worldwide challenge due to its resistance to acaricides. Essential oils (EOs) and isolated compounds (EOCs) are potential alternatives for tick control technologies.

METHODS

A review with EOs and EOCs, under field and semi-field conditions, was performed based on Scopus, Web of Science and PubMed databases. Thirty-one studies published between 1991 and 2022 were selected. The search was performed using the following keywords: "essential oil" combined with "tick," "Ixodes," "Argas," "Rhipicephalus," "Amblyomma," "Hyalomma," "Dermacentor," "Haemaphysalis" and "Ornithodoros." The words "essential oil" and "tick" were searched in the singular and plural.

RESULTS

The number of studies increased over the years. Brazil stands out with the largest number (51.6%) of publications. The most studied tick species were Rhipicephalus microplus (48.4%), Ixodes scapularis (19.4%), Amblyomma americanum and R. sanguineus sensu lato (9.7% each). Cattle (70%) and dogs (13%) were the main target animal species. Regarding the application of EOs/EOCs formulations, 74% of the studies were conducted with topical application (spray, pour-on, foam, drop) and 26% with environmental treatment (spray). Efficacy results are difficult to evaluate because of the lack of information on the methodology and standardization. The nanotechnology and combination with synthetic acaricides were reported as an alternative to enhance the efficacy of EOs/EOCs. No adverse reactions were observed in 86.6% of the studies evaluating EOs/EOCs clinical safety. Studies regarding toxicity in non-target species and residues are scarce.

CONCLUSIONS

This article provides a comprehensive review on the use of EOs and EOCs to reduce tick infestations, in both the hosts and the environment. As future directions, we recommend the chemical characterization of EOs, methodology standardization, combination of EOs/EOCs with potential synergists, nanotechnology for new formulations and safety studies for target and non-target organisms, also considering the environmental friendliness.

Combinations of amitraz with essential oils from Lippia sidoides and Thymus vulgaris, thymol and thymol acetate for Rhipicephalus microplus control: studies under laboratory and field conditions

This study aimed to assess the effect of combining amitraz with essential oils (EOs) from Thymus vulgaris and Lippia sidoides, as well as the monoterpenes thymol and thymol acetate, on Rhipicephalus microplus in laboratory conditions, and to select the most effective combination for testing in field conditions. The chemical analysis showed that EOs were mainly composed of monoterpenes, with thymol and p-cymene as the major compounds. In larval (LIT) and adult (AIT) immersion tests using different concentrations of the oils and terpenes mixed with amitraz, the results showed that both EOs and thymol improved the efficacy of amitraz against larvae and engorged females of R. microplus, whereas thymol acetate only enhanced activity against larvae. The most favorable outcome was obtained with the EO of L. sidoides combined with amitraz, resulting in 99 % and 100 % efficacy against larvae and engorged females, respectively. Furthermore, the combination of amitraz with thymol showed presented an efficacy of 94 % and 91 % against larvae and engorged females, respectively. Thus, for the other tests, the combination of thymol + amitraz was chosen due to the ease of working with pure thymol in bioassays, and easier standardization. The immersion test (thymol + amitraz) with semi-engorged females showed 100 % efficacy for the combination of thymol + amitraz, while in tests with different solvents (thymol + amitraz), ethanol being the most effective solvent among those tested (ethanol, Triton, and Tween), resulting in 95 % efficacy on engorged females. In the field test, in treatments with amitraz and thymol + amitraz, efficacy of 54 % and 74 % was observed on day + 3 and 33 % and 43 % on day + 7, respectively. Assessing the reproductive biology of females recovered from animals treated with amitraz or amitraz + thymol, in day + 7, efficacies of 33 % and 52 %, respectively, were observed. EOs from T. vulgaris and L. sidoides and thymol improved the acaricidal activity of amitraz on larvae and engorged females of R. microplus under laboratory conditions, while thymol acetate only enhanced activity against larvae. Thymol increased the efficacy of amitraz under field conditions, however for the development of a commercially available acaricide to R. microplus control, additional studies are needed to increase the efficacy. Further research is needed (by changing concentrations, adding other compounds and/or developing formulations) to increase acaricidal efficacy and develop new effective products to combat R. microplus infestations in cattle.

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

The present study compared the efficacy of different methods to apply an acaricide formulation to control Rhipicephalus (Boophilus) microplus. To compare the methods, an acaricide blend containing three active ingredients (a pyrethroid and two organophosphates) was used. In experiment 1 (farm 1: Goiânia, GO, Brazil), three methods were tested: a backpack sprayer (BS), power sprayer (PS) and spray race (SR). In experiment 2 (farm 2: São José do Rio Pardo, SP, Brazil), two methods were tested: BS and PS. In both experiments, 10 cattle with similar tick burdens were used. On day 0 in both experiments, the animals were treated with the acaricide. On day +1 (only in experiment 1), +3, +7, +14, +21, +28 and +35 (only in experiment 2), tick counts were performed to determine the control efficacy. The time application, pressure (KPa), volume applied (L) and ergonomic aspects of each spraying system were also evaluated. The adult immersion test (AIT) using three different acaricide blends (combinations of pyrethroid + organophosphate) was performed to compare the susceptibility of strains of each farm. In experiment 1, all treatments significantly reduced (p < 0.05) the number of ticks on the animals, and PS resulted in the greatest acaricide efficacy since day +1. In experiment 2, both treatments (PS and BS) reduced (p < 0.05) the tick burden, and as observed in experiment 1, PS resulted in the best reduction. The application times were 4.5, 150 and 330 s, while pressures were 306.8, 4,826.3 and 220.6 KPa for SR, PS and BS, respectively. In the AIT, the efficacy values were between 99.8 and 100% for the tick strain form farm 1 (Goiânia), while for tick strain from farm 2 (São José do Rio Pardo), the efficacy was between 67.2 and 80.9%. We conclude that the sprayer methods chosen influences the efficacy of the acaricide. All sprayer methods were efficient for acaricide application; the best efficacy was obtained using the PS, while the SR resulted in good efficacy and lower application time. The strain from farm 2 was less susceptible to all acaricides tested.

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.