Test systems for tick repellents

Developing attractants and repellents for ticks: promises and challenges

Historically, some of the most effective tools to counter vector-borne diseases have been those directed against the vectors. Ticks are undergoing a population explosion as evidenced by the recent expansion of their distribution range. Tick control has traditionally relied heavily on pesticides. However, sustained use of acaricides is resulting in resistant tick populations. Multipronged management strategies that build and expand upon innovative control methods are sorely needed. Behavior-modifying chemicals, referred to as semiochemicals, such as pheromones and repellents, offer a first line of personal protection against ticks. We review the current understanding of tick semiochemicals, and how such understanding is leading to the identification of novel chemistries that are effective and safe.

Natural Clerodendrum-derived tick repellent: learning from Nepali culture

Ticks attaching to ear canals of humans and animals are the cause of otoacariasis, common in rural areas of Nepal. The plant Clerodendrum viscosum is used in multiple indigenous systems of medicine by ethnic communities in the Indo-Nepali-Malaysian region. Visiting the Chitwan National Park, we learned that in indigenous medicine, flower extract of C. viscosum is utilized to treat digestive disorders and extracts from leaves as tick repellent to prevent ticks from invading or to remove them from the ear canal. The objective of our study was to provide support to indigenous medicine by characterizing the in vivo effect of leave extracts on ticks under laboratory conditions and its phytochemical composition. We collected plant parts of C. viscosum (leaves and flowers) and mango (Mangifera indica) leaves at the Chitwan National Park, previously associated with repellent activity to characterize their effect on Ixodes ricinus ticks by in vivo bioassays. A Q-ToF high-resolution analysis (HPLC-ESI-QToF) was conducted to elucidate phenolic compounds with potential repellent activity. Clerodendrum viscosum and M. indica leaf extracts had the highest tick repellent efficacy (%E = 80-100%) with significant differences when compared to C. viscosum flowers extracts (%E = 20-60%) and phosphate-buffered saline. Phytochemicals with tick repellent function as caffeic acid, fumaric acid and p-coumaric acid glucoside were identified in C. viscosum leaf extracts by HPLC-ESI-QToF, but not in non-repellent flower extracts. These results support the Nepali indigenous medicine application of C. viscosum leaf extracts to repel ticks. Additional research is needed for the development of natural and green repellent formulations to reduce the risks associated with ticks resistant to acaricides.

Repellency and toxicity of a CO2-derived cedarwood oil on hard tick species (Ixodidae)

The repellency and toxicity of a CO₂-derived cedarwood oil (CWO) was evaluated against actively questing unfed nymphs of four species of hard ticks: Amblyomma americanum (L.), Dermacentor variabilis (Say), Ixodes scapularis Say, and Rhipicephalus sanguineus (Latreille). Using a vertical climb bioassay for repellency, nymphs of these species avoided a CWO-treated filter paper in proportional responses to treatment concentrations. At 60 min of exposure, I. scapularis nymphs were most sensitive with 50% repellency concentration (RC₅₀) of 19.8 µg cm^-2, compared with RC₅₀ of 30.8, 83.8 and 89.6 µg cm^-2 for R. sanguineus, D. variabilis and A. americanum, respectively. Bioassays determined the lethal concentration for 50% (LC₅₀) and 90% (LC₉₀) mortality of nymphs exposed to CWO in treated vials after 24- and 48-h exposure. After 24 h exposure, the LC₅₀ values were 1.25, 3.45 and 1.42 µg cm^-2 and LC₉₀ values were 2.39, 7.59 and 4.14 µg cm^-2 for D. variabilis, I. scapularis and R. sanguineus, respectively, but had minimal effect on A. americanum. After 48 h exposure, the LC₅₀ values were 4.14, 0.78, 0.79 and 0.52 µg cm^-2, and LC₉₀ values were 8.06, 1.48, 1.54 and 1.22 µg cm^-2 for A. americanum, D. variabilis, I. scapularis and R. sanguineus, respectively. The repellency of CWO on tick species decreased with time. The repellency and toxicity bioassays demonstrated concentration-dependent responses of tick nymphs to the oil, indicating the potential of the CO₂-derived cedarwood oil be developed as an eco-friendly repellent and/or acaricide.

A novel Tick Carousel Assay for testing efficacy of repellents on Amblyomma americanum L

Ticks are important vectors of human and veterinary diseases. A primary way ticks gain access to human hosts is by engaging to clothing. Repellents or acaricides sprayed onto fabric are used to deter ticks’ access to human hosts. However, there are a limited amount of standardized laboratory assays that can determine the potency and efficacy of repellents. We present a novel fabric-engagement assay referred to as the ‘Tick Carousel Assay’. This assay utilizes fabric brushing past ticks located on an artificial grass patch and measures tick engagements to fabric over time. After screening a variety of tick species, we used the lone star tick (Amblyomma americanum) to test the efficacy of four commonly used active ingredients in repellents: DEET, Picaridin, IR3535, and Oil of Lemon Eucalyptus. Repellency was tested immediately, after three hours, and six hours post application to fabric. Our data show that each repellent we tested significantly reduced the number of tick engagements to fabric for at least 6 hours. We did not find significant differences in repellent efficacy between the four active ingredients tested directly and three hours after application. After six hours, Oil of Lemon Eucalyptus repelled ticks more than the other active ingredients. We show that our Tick Carousel Assay provides an affordable, repeatable, and standardized way to compare and test repellent efficacy on treated fabrics. Our results confirm that commonly used repellents applied to fabric are an effective way to reduce tick engagement.

Three molecules found in rosemary or nutmeg essential oils repel ticks (Dermacentor variabilis) more effectively than DEET in a no-human assay

BACKGROUND

Development of new personal repellents to prevent tick bites is progressing to keep up with the increasing threat of tick-borne diseases in humans. However, the market share of natural, plant-based repellents is still quite small. We tested all of the major (>1%) components found in the essential oils of rosemary (Spanish variety) and nutmeg. These were tested with Dermacentor variabilis ticks, a robust tick capable of vectoring human diseases, in a no-human horizontal Petri dish assay. For comparison, we also tested the active ingredients currently recommended by the CDC and EPA as personal repellents for use against ticks.

RESULTS

Of the 16 oil components tested, three performed significantly better than N,N-diethyl-meta-toluamide (DEET) in our assay. These compounds were myristicin and safrole found in the nutmeg oil, and terpinolene found in nutmeg oil and as a minor (<1%) constituent of rosemary oil. The current market natural product, 2-undecanone, also performed strongly in our assay.

CONCLUSIONS

Our assay focused on comparing active ingredients as potential repellents and found that many natural products can elicit a strong response from ticks. Terpinolene is the most promising active ingredient identified, owing to human health concerns regarding myristicin and safrole. Our results from examining individual terpenoids from two essential oils suggest that there remain unexploited natural compounds that could be further developed for new personal tick repellents. © 2020 Society of Chemical Industry.

Considerations for Human Blood-Feeding and Arthropod Exposure in Vector Biology Research: An Essential Tool for Investigations and Disease Control

Eventually there may be a broadly acceptable, even perfected, substitute for the human host requirement for direct feeding experiments by arthropods, most notably mosquitoes. However, for now, direct and indirect feeding on human volunteers is an important, if not essential, tool in vector biology research (VBR). This article builds on the foundational publication by Achee et al. (2015) covering considerations for the use of human participants in VBR pursuits. The authors introduced methods involving human participation in VBR, while detailing human-landing collections (catches) as a prime example. Benedict et al. (2018) continued this theme with an overview of human participation and considerations for research that involves release of mosquito vectors into the environment. In this study, we discuss another important aspect of human use in VBR activities: considerations addressing studies that require an arthropod to feed on a live human host. Using mosquito studies as our principal example, in this study, we discuss the tremendous importance and value of this approach to support and allow study of a wide variety of factors and interactions related to our understanding of vector-borne diseases and their control. This includes establishment of laboratory colonies for test populations, characterization of essential nutrients that contribute to mosquito fitness, characterization of blood-feeding (biting) behavior and pathogen transmission, parameterization for modeling transmission dynamics, evaluation of human host attraction and/or agents that repel, and the effectiveness of antivector or parasite therapeutic drug studies.

Chemosensory and Behavioural Responses of Ixodes scapularis to Natural Products: Role of Chemosensory Organs in Volatile Detection

Simple Summary

Ticks are responsible of transmitting serious disease agents of importance to human and veterinary health. Despite the importance of repellents, deterrents and acaricides in tick management, little is understood about the types of chemicals recognized and the mechanism behind chemoreception. Being almost totally blind, ticks rely on chemosensation to identify and locate hosts for a successful blood meal and to detect chemical signals in the environment. We explored the neurophysiology of tick chemosensory system in the context of behaviourally-relevant volatile stimuli, including essential oil components, to evaluate how the combination of attractants and plant volatile compounds is detected and processed. The observed inhibition (or deterrent effect) in tick electrophysiological response and behavioural activity, after the tick has been exposed to a binary mixture of attractant and volatile compound, represents an important advancement in understanding how tick olfaction works and what may be the mechanism behind detecting unpleasant odor stimuli and consequently been deterred. These information will provide more insights in developing new natural product-based deterrents for self-protection.

Abstract

Blacklegged ticks, Ixodes scapularis, represent a significant public health concern due to their vectoring of tick-borne disease. Despite their medical importance, there is still limited knowledge of the chemosensory system of this species, and thus a poor understanding of host-seeking behaviour and chemical ecology. We investigated the electrophysiological sensitivity of adult female blacklegged ticks to attractants and plant-derived compounds via an electrode inserted into the scutum. The response of female ticks to binary mixtures with a constant concentration of a selected attractant (butyric acid) and increasing concentration of volatile organic compounds (VOCs) (geraniol, phenethyl alcohol, β-citronellol, and citral) was recorded. A strict relationship between increasing volatile concentration and a decreasing response was observed for each VOC. Y-tube bioassays confirmed that tick attraction towards butyric acid decreased with the presence of a VOC, which exerted a deterrent effect. To determine the specific role of sensory appendages involved in the detection of attractant chemical stimuli, we tested tick electrophysiological response after removing appendages that house chemosensory sensilla (foretarsi, pedipalps, or both). The chemosensory response was related to the molecular structure of attractant odorant, and the lack of pedipalps significantly reduced olfactory responses, suggesting they play an important role in detecting attractants. This study provides new insight into the neurophysiological mechanisms underlying tick olfaction and the potential for interactions between attractant and deterrent chemical detection.

Solvent, drying time, and substrate affect the responses of lone star ticks (Acari: Ixodidae) to the repellents deet and picaridin

Behavioral bioassays remain a standard tool in the discovery, development, and registration of arthropod repellents. Tick repellent bioassays are generally uncomplicated, but their results can be affected by basic variables (e.g., dimensions of testing materials, substrate, timing, temperature) of the assay. Using lone star tick, Amblyomma americanum (L.), nymphs in climbing bioassays, we tested for the effects of substrate, solvent, and drying time on tick responses. In dose-response tests, the widely used repellents N,N-diethyl-3-methyl benzamide (deet) and 1-methyl-propyl-2-(hydroxyethyl)-1-piperidinecarboxylate (picaridin) were applied to filter paper strips and challenged by ticks at 10, 20, 30, 40, and 120 min after application. At 10-min drying time, repellency at the intermediate concentration 500 nmol repellent/cm2 filter paper was significantly lower for ethanol solutions of deet and picaridin (0 and 10% ticks repelled, respectively) than for solutions of deet and picaridin in acetone (96.7 and 76.7% ticks repelled, respectively). Repellency was greatest for both the acetone and ethanol solutions of deet and picaridin when challenged 120 min after application, and at shorter drying times at the highest concentration tested (2,000 nmol compound/ cm2). The repellency of picaridin relative to deet differed at some combinations of solvent and drying time but not others. In dose-response tests using different paper substrates and a drying time of 10 min, both ethanol and acetone solutions of deet differed in repellency, depending on both the paper substrate and the solvent. However, there were no differences in repellency between ethanol and acetone solutions of deet applied to nylon organdy in an in vitro and in an in vivo (fingertip) bioassay. When deet in solution with various proportions of ethanol:water was applied at 2,000 nmol deet/cm2 filter paper, the proportion of ticks repelled decreased as the proportion of water in the test solutions increased. Somewhat similar results were seen for solutions of deet in an acetone solvent. Water absorbed from the atmosphere may affect the efficacy of repellents in solution with anhydrous ethanol. Overall, results obtained from bioassays that differ in seemingly minor ways can be surprisingly different, diminishing the value of comparing studies that used similar, but not identical, methods. Nylon organdy or another similar thin cloth may be preferable to filter papers and copier paper for minimizing solvent-related differences. When a paper substrate is used, acetone may be the more suitable solvent if the solubility of the test compound and other factors allow.

Prevention of infectious tick-borne diseases in humans: Comparative studies of the repellency of different dodecanoic acid-formulations against Ixodes ricinus ticks (Acari: Ixodidae)

Background

Ticks of the species Ixodes ricinus are the main vectors of Lyme Borreliosis and Tick-borne Encephalitis – two rapidly emerging diseases in Europe. Repellents provide a practical means of protection against tick bites and can therefore minimize the transmission of tick-borne diseases. We developed and tested seven different dodecanoic acid (DDA)-formulations for their efficacy in repelling host-seeking nymphs of I. ricinus by laboratory screening. The ultimately selected formulation was then used for comparative investigations of commercially available tick repellents in humans.

Methods

Laboratory screening tests were performed using the Moving-object (MO) bioassay. All test formulations contained 10% of the naturally occurring active substance DDA and differed only in terms of the quantitative and qualitative composition of inactive ingredients and fragrances. The test procedure used in the human bioassays is a modification of an assay described by the U.S. Environmental Protection Agency and recommended for regulatory affairs. Repellency was computed using the equation: R = 100 - N_R/N × 100, where N_R is the number of non-repelled ticks, and N is the respective number of control ticks. All investigations were conducted in a controlled laboratory environment offering standardized test conditions.

Results

All test formulations strongly repelled nymphs of I. ricinus (100-81% protection) as shown by the MO-bioassay. The majority of ticks dropped off the treated surface of the heated rotating drum that served as the attractant (1 mg/cm² repellent applied). The 10% DDA-based formulation, that produced the best results in laboratory screening, was as effective as the coconut oil-based reference product. The mean protection time of both preparations was generally similar and averaged 8 hours.

Repellency investigations in humans showed that the most effective 10% DDA-based formulation (~1.67 mg/cm² applied) strongly avoided the attachment of I. ricinus nymphs and adults for at least 6 hours. The test repellent always provided protection (83-63%) against I. ricinus nymphs equivalent to the natural coconut oil based reference product and a better protection (88-75%) against adult ticks than the synthetic Icaridin-containing reference repellent.

Conclusion

We found that the 10% DDA-based formulation (ContraZeck^®) is an easily applied and very effective natural repellent against I. ricinus ticks. By reducing the human-vector contact the product minimises the risk of transmission of tick-borne diseases in humans.

Repellent activity of fractioned compounds from Chamaecyparis nootkatensis essential oil against nymphal Ixodes scapularis (Acari: Ixodidae)

Preliminary repellent activity of 14 natural products isolated from essential oil components extracted from the heartwood of Alaska yellow cedar, Chamaecyparis nootkatensis (D. Don) Spach., were evaluated against nymphal Ixodes scapularis Say in a laboratory bioassay and compared with technical grade N,N-diethyl-3-methylbenzamide (deet). Four hours after treatment, nootkatone and valencene-13-ol had repellent concentration (RC)50 values of 0.0458 and 0.0712% (wt:vol), respectively; two additional Alaska yellow cedar compounds, nootkatone 1 --> 10 epoxide and carvacrol had reported RC50 values of 0.0858 and 0.112%, respectively. The observed RC50 value for deet was 0.0728% (wt:vol). Although not statistically significantly more active than deet, the ability of these natural products to repel ticks at relatively low concentrations may represent a potential alternative to synthetic commercial repellents.

Repellency of deet and SS220 applied to skin involves olfactory sensing by two species of ticks

Responses of host-seeking nymphs of the blacklegged tick, Ixodes scapularis Say and lone star tick, Amblyomma americanum (Linnaeus) (Acari: Ixodidae) to the repellents N,N-diethyl-3-methylbenzamide (deet) and (1S, 2'S)-2-methylpiperidinyl-3-cyclohexene-1-carboxamide (SS220) were studied using fingertip laboratory bioassays. Ethanol solutions of both compounds applied to the skin strongly repelled both species of ticks at 0.8 and 1.6 micromole of compound/cm2 skin. The ticks were also repelled when two layers of organdie cloth covered the portion of a finger treated with either deet or SS220. Gas chromatographic analyses of the outer layer of cloth that had covered skin treated with 1.6 micromole compound/cm2 skin revealed only 0.1 nmole SS220/cm2 cloth and 2.8 nmole deet/cm2 cloth. However, in bioassays in which a single layer of cloth was treated with a dose of deet or SS220 equivalent to the amount found in the outer layer of cloth, ticks were not repelled. Results unequivocally demonstrated that these ticks responded to the repellents in the vapour phase when repellent treated skin was covered with cloth to obviate tactile contact with them, and made it clear that the ticks detect the repellents by olfactory sensing. Heretofore, the mode of action of deet and SS220 was unclear.