Welcome, friends. We’re here today to discuss everybody’s favorite topic: bloodsucking arthropods! And not just any old arthropod. I’m talking about those loathsome emissaries of infectious disease — and the heebie geebies in general – ticks.
Infectious disease has been on the rise in the last century. Indeed, an astonishing 335 human diseases have materialized since 1940. Something like 60% of those diseases are zoonotic, otherwise known as “pathogens that normally thrive within other animals, but have somehow managed to infect humans, too.” 72% are transmitted to humans from wildlife, with the remainder bequeathed by domestic animals. A significant portion of this transmission occurs thanks to parasitic vectors like mosquitoes and ticks (Ostfeld, 2011).
As if the idea of little monsters thrusting their heads into your flesh and drinking your blood wasn’t grim enough. Ticks jostle for the reputation of “nastiest parasitic vector.” They’re second only to the odious mosquito as bearers of human infectious disease worldwide, but in North America, ticks are unrivaled.
Let’s talk about Lyme disease. It was first described in the United States in the mid-1970s, though reports surfaced as early as the late 1800s. These days, the US is infested with Lyme. Cases pop up all over the Northeast, mid-Atlantic, and upper Midwest, with lower-risk pockets on the West Coast. As environmental conditions change, that range increases every year. According to the Center for Disease Control, 30,000 cases of Lyme disease are reported annually in the US alone, but studies suggest that the actual number of cases is likely ten times that.
It gets better. Lyme may be the most notorious tick-borne pathogen, but it’s not the only one. There’s a whole circus of colorful characters: Rocky Mountain spotted fever, anaplasmosis, ehrlichiosis, Powassan virus, and babesiosis, to name a few, each with their own symptom profile and geographic distribution. Babesiosis and anaplasmosis frequent the same regions as Lyme disease, that is to say, primarily the Northeast and upper Midwest. Rocky Mountain spotted fever is a bit more picky, with over 60% of cases documented in just five states: Arkansas, Missouri, North Carolina, Oklahoma, and Tennessee. (CDC, 2018).
The Life and Times of the Tick
Ticks are diminutive arachnids in the same class as spiders and scorpions. They share the subclassification Acari with mites which, like ticks, have lost the abdominal segmentation of their ancestral arthropods as a result of the fusion of abdomen and cephalothorax.
Known as external, or “ecto” parasites, ticks feed on the blood of marsupial and placental mammals, birds, reptiles, and amphibians. The 899 estimated species of tick are divided into three families: the Nuttalliellidae and two sister families, the Ixodidae (hard ticks) and Argasidae (soft ticks). The first consists of a single South African species Nuttalliella namaqua, while Ixodidae contains over 700, and Argasidae approximately 200, globally-distributed species.
Though they share a common ancestor, there are numerous notable differences between soft and hard ticks. Hard ticks are characterized by a tough exterior known as a “scutum,” a feature absent in soft ticks. The mouthparts of hard ticks are readily visible from above, whereas the mouthparts of soft ticks are concealed beneath the body. You know it’s good when “mouthparts” is one word.
The differences between hard and soft ticks theoretically emerged during the late Cretaceous period (146-66 million years ago) when ticks transitioned from free-wheeling scavenger to blood-feeding parasite. It’s impressive, actually. As Mans and Neitz write in their enchanting article “Adaptation of ticks to a blood-feeding environment: evolution from a functional perspective” published in the journal Insect Biochemistry and Molecular Biology, “The vertebrate hemostatic system originated approximately 400 million years ago (MYA) and is a formidable barrier for hematophagous parasites.” Indeed, hemostasis is the efficient defense mechanism that prevents blood loss through an open wound. Essentially, ticks figured out how to overcome the complex vertebrate hemostatic system with their own suite of slick counter-measures, and, in doing so, differentiated into two separate families with distinct anatomy, behavior, and life cycles (Mans and Neitz, 2004).
For our purposes we’ll focus on Ixodidae, the hard tick, as they present a greater threat to human well-being than soft ticks.
Hard ticks possess three visible mouthpart components. First, there’s the dextrous jointed palps which maneuver laterally during feeding and do not penetrate the skin of the host. So far so good. Next come the paired and toothy chelicerae that snag into the host’s skin with a breaststroke-like motion. The flexion of the chelicerae ratchets the final mouthpart, a sword-shaped central structure called the hypostome, deep into the skin. The hypostome boasts recurved barbs that lock it in the host’s flesh for the duration of feeding. There’s a groove running down the middle that shunts saliva into the host, and the host’s blood into the tick (Yong, 2013).
What a nightmare.
Hard ticks feed on their hosts long-term, anything from days to weeks depending on their species, life stage, and host type. They thus have developed a suite of evolutionary tricks so they can finish their meal undisturbed. To avoid being dislodged, they drool a salivary cement that glues them in place, a substance which dissolves once they’re swollen with blood. The rigid cuticle of the hard tick undergoes cell division to accommodate the massive quantity of blood ingested. Adult hard ticks can swell to 200-600 times their original body weight in the course of one feeding.
To snag passing hosts, hard ticks use a sly maneuver known as “questing.” They scurry up the stems of grass or squat on low-lying leaf edges, clinging to the perch with their third and fourth pairs of legs while extending their front legs, much like a hitchhiker thumbing a ride from, in this case unsuspecting, traffic. Questing behavior is triggered by indicators of potential host proximity. Ticks detect their prey by sensing biochemicals such as carbon dioxide, as well as vibration, heat, and movement. Once something brushes their extended front legs, they swing aboard (Vredevoe, 2018).
As you may have guessed, the lifecycle of the tick is not for the faint of heart. It’s a drama of blood, sex, and molting, equal parts revolting and riveting. We’ll start with a mini-case study of Ixodes scapularis, the black-legged tick. In the northeastern United States this particular tick is the primary vector of Borrealia burgdorferi, the spirochete bacterium responsible for Lyme disease. The Ixodes is a three-host tick, meaning that it feeds on a different host at each of its three life stages — larva, nymph, adult — after graduating from egghood. At each stage, the tick extracts a single blood meal from a vertebrate host. Sated, the tick drops from the host and molts; larva to nymph, nymph to adult. After a final blood meal during which adult male and female ticks copulate, they disengage. The female lays eggs, and both sexes die. The life cycle is complete (Ostfeld, 2011).
Ticks are what’s known as “obligate hematophages,” meaning that they require blood meals to survive and achieve each subsequent life state. In a way, ticks are simple creatures. They just have two needs from their environment: First, a sufficiently high host population density to feed on, and second, a high enough humidity; the poor dears need to stay moisturized to achieve metamorphosis (Wall and Shearer, 2001).
Lyme disease: Bacteriology, Vectors, and Reservoirs
Ticks distribute the widest variety of pathogens of any blood-sucking arthropod. These include bacteria, rickettsiae (obligately intracellular Gram-negative bacteria), protozoa, and viruses. Since Lyme is so notorious right now, let’s take a closer look at the Lyme-causing bacteria of the Borrelia genus.
Borrelia species are gram-negative microaerophilic mobile spirochetes. In other words, a genus of bacteria known for its swanky corkscrew shape. Ticks pick up species like Borrealia burgdorferi while feeding on infected reservoir hosts. Once inside the tick, spirochetes lurk in the midguts. As ticks molt and enter their next developmental stage, spirochetes rapidly disperse to all body organs, including the salivary glands. During feeding, ticks may transmit Borrelia to susceptible hosts.
Numerous mammals, rodents in particular, have been incriminated as reservoir hosts for B. burgdorferi. In epidemiology, a reservoir is defined as a population of organisms in which an infectious pathogen naturally lives and reproduces, or upon which the pathogen primarily depends for its survival. In the northeastern United States the white-footed mouse (P. leucopus) is considered to be the preeminent reservoir of B. burgdorferi sensu stricto. Interestingly, white-tailed deer (O. virginianus) are a significant host for adult ticks (I. scapularis) but do not act as a reservoir for B. burgdorferi (Parola and Raoult, 2001).
It’s rare for ticks to transmit the bacteria to their offspring, and thus ticks themselves are not considered to be reservoirs of B. burgdorferi. Due to the lack of transovarial transmission, larval ticks hatch uninfected with Lyme disease spirochetes. Once hatched, however, they feed indiscriminately on any warm-blooded vertebrate they meet while questing on the forest floor. Larval ticks that feed on a host infected with Lyme disease spirochetes may become infected. They will subsequently molt into an infected nymph capable of transmitting the infection to its next host.
It’s during the nymphal stage that Lyme and other tick-borne diseases are most likely to be transmitted to humans. This is due to the high prevalence of infection among nymphal ticks, their small size which makes them less likely to be detected and removed, and the coincidence of peak nymphal activity and peak outdoor human activity in late spring and early summer (Ostfeld, 2011).
Modern Tick Tactics
There are many possible strategies for contending with ticks and tick-borne pathogens, from concerted environmental intervention targeting ticks, hosts, and pathogens, to human behaviors such as the use of repellents and protective clothing. Vaccines are in development, but not yet available.
An article from the archives of the New York Times illustrates a curious experiment on Martha’s Vineyard intended to halt the spread of Lyme disease. In this project, funded partly by the United States Department of Agriculture, tiny parasitic chalcid wasps were released in an attempt to reduce the population of Lyme-carrying deer ticks. According to the article, “The wasp, roughly the size of a typewritten dot, deposits its eggs in tick larvae. The wasp eggs are believed to hatch during the second, or nymph, stage of the tick, killing it before it reaches adulthood.”
A clever, self-perpetuating plan. Especially considering the concerns at that time about the widespread use of synthetic pesticides, for both environmental and economic reasons. As the article reads:
[Edman’s] colleagues at the agricultural cooperative extension affiliated with the University also worry about the effectiveness and expense of attacking the tick with insecticides. “Pesticides are really not an option,” said Michael Zoll, director of the Island’s cooperative extension unit. “Damminix is feasible in some small areas but the present cost seems to be a limiting factor for wide-scale application.”
The parasitic wasps were a promising project, but for some reason it fizzled out in the ‘90s. The success rate in first year was “pretty amazing,” according to presiding entomologist John Edman, so it’s not entirely clear where things fell apart. Maybe it has something to do with the weird “coincidence” that parasitic wasps only targeted ticks not infected with Lyme disease spirochetes. Or maybe those concerns about the cost and toxicity of synthetic pesticides fizzled out, too (NYT, 1990).
Regardless, the battle against ticks rages on, and synthetic pesticides have become the #1 weapon. An excellent 2001 article published in Clinical Infectious Diseases entitled “Ticks and Tickborne Bacterial Diseases in Humans: An Emerging Infectious Threat” enumerates the various modern strategies of tick control.
“Reducing and controlling tick populations is difficult,” the authors begin. The predicament established, they forge on ahead:
Habitat modifications, including vegetation management by cutting, burning, and herbicide treatment, and drainage of wet areas are one strategy for tick control, but their effects are often short-lived and they can cause severe ecological damage. In some areas, host exclusion or depopulation may result in a reduction in the density of ticks, but this is mostly impractical and is also not ecologically sound. The use of organophosphates or pyrethroids, which may be combined with pheromones to control ticks, may cause environmental contamination and toxicity for animals and humans, even when applied only to selected habitats. Acaricides, however, can be applied directly to wild or domestic hosts to kill attached ticks and disrupt tick feeding.
Biological control methods for ticks are also available, and these include the promotion of natural predators (including beetles, spiders, and ants), parasites (insects, mites, and nematodes), and bacterial pathogens of ticks; the mass release of males sterilized by irradiation or hybridization; and the immunization of hosts against ticks. At the present time, tick control is best based on the concept of integrated pest management, in which different control methods are adapted to one area or against one tick species with due consideration to their environmental effects.
In terms of personal lifestyle interventions for prevention, they recommend an “integrated approach.” This involves the use of protective clothing and tick repellents, checking the entire body daily if exposed to ticks, and “prompt removal of attached ticks before transmission of infection may occur.”
If a tick does attach, they acknowledge that “removing these ticks may not be easy.” The authors offer thorough a thorough tick-removal protocol:
It is best to use blunt, rounded forceps, and a magnifying glass may be helpful if immature ticks are found. The forceps are used to grasp the mouthparts of the ticks as close as possible to the skin, and the tick is then pulled upward, perpendicular to the skin, with a continuous and steady action. Specific instruments are commercially available and may be particularly useful for removing nymphal stages. Usually any mouthparts of the ticks retained in the skin are eliminated uneventfully by the body. Shave incisions close to the skin may also be used. After removal of the ticks, a disinfectant should be applied to the bite site and the tick stored at −20°C in case the patient subsequently develops a disease that requires the tick for detection or isolation of the causative agent. Other methods of removing ticks, such as using the fingers instead of forceps or using lighted cigarettes, petroleum jelly, or suntan oil to kill the ticks in situ, should be avoided, because they may increase the risk of regurgitation by the tick and, consequently, the transmission of infectious agents (Parola and Raoult, 2001).
So… remove the tick with forceps, disinfect the site, and stick the tick in a freezer for safekeeping. Got it.
But what about this “disinfectant?” Seems like an opportunity to make use of antiseptic essential oils. Certainly, a cursory Google search coughs up a load of pages recommending the topical application of essential oils to prevent infection post-tick bite. But don’t just believe everything you read on the internet… Let’s dig a little deeper.
A Pubmed search with the terms “Essential oil + antiseptic” (as of September 29, 2018) disgorges a whopping 1463 results. Indeed, history is littered with corroborating anecdotes. French surgeon Dr. Jean Valnet used essential oils to treat septic wounds during the Indo-China war when antibiotics were scarce. Thymol, a monoterpenoid phenol found in thyme oil, was famously included in the proprietary formula for Listerine (Buckle, 2001). Buckle erroneously attributes the discovery of thymol to 19th century pioneer of antiseptic surgery Joseph Lister, but the compound was in fact first isolated by German chemist Caspar Neumann in 1719 (Greenwood, 2008). Originally intended for use as a powerful surgical antiseptic, with some dilettante dalliances as a floor cleaner and gonorrhea treatment, Listerine achieved commercial success when marketed to the unhappily unwed as a cure for halitosis (Clark, 2015). Needless to say, antiseptic essential oils have a motley history in which “tick bite treatment” seems hardly out of place.
A pubmed search with the terms “essential oil + borrelia” narrows it down to two recent studies of the antimicrobial activity of essential oils against Lyme-causing spirochetes. One 2017 study published in Frontiers in Medicine tested the antimicrobial activity of essential oils derived from spice or culinary herbs in vitro against stationary phase and biofilm Borrelia burgdorferi. In this experiment the researchers investigated the hypothesis that post-treatment Lyme syndrome is caused by “persister bacteria,” or spirochetes tolerant to antibiotics that prevail in the system despite treatment. Remarkably, many of the tested essential oils were effective against both stationary phase (a model of persister bacteria) and biofilm bacteria, which are notoriously immune to antibiotics.
23 of the 34 essential oils tested, at a 1% concentration, were more effective than the known persister drug daptomycin (Dap). Oregano, cinnamon bark, clove bud, citronella, and wintergreen exhibited significant antimicrobial activity, stronger than that of Dap. Oregano, cinnamon bark, and clove bud essential oils distinguished themselves as remarkably active against B. burgdorferi in the stationary phase model, even at the low concentration of 0.125%. Oregano and cinnamon bark essential oils were the most active, completely eradicating B. burgdorferi at a concentration of 0.05%. These oils permanently damaged B. burgdorferi during treatment, such that even in fresh medium residual cells could not regrow.
In biofilm models, oregano oil at a concentration of 0.25% dramatically reduced the size of aggregated biofilm-like microcolonies compared to antibiotic controls. At concentrations of 0.5% and 1% it eradicated them entirely. Monoterpenoid phenol carvacrol, a constituent of oregano oil, is considered to be the “active” ingredient, and has been previously shown to disrupt microbial cell membranes. This compound alone exhibited strong antimicrobial activity against B. burgdorferi in a stationary phase model. In sum, oregano, cinnamon bark, and clove bud essential oils demonstrated significant antimicrobial activity as demonstrated by their complete eradication of all stationary phase cells with no regrowth in vitro, and remarkable biofilm-dissolving capability (Feng, et al., 2017).
Similarly, a 2010 paper published in Pharmazie reports that the volatile oil of Cistus creticus L. (pink rock rose) inhibits the growth of Borrelia burgdorferi s.s.. Researchers examined the effects of C. creticus on B. burgdorferi in vitro in response to borreliosis patients from self-help groups reporting considerable pain relief as a result of the ingestion of Cistus creticus leaf preparations. The volatile fraction of C. creticus at a concentration of 0.2% (w/v) in the cultivation media led to a “complete stagnation of bacteria growth,” while a concentration of 0.02% reduced the total amount of bacteria to approximately 2% (w/v) compared to control. The volatile oil was found to be significantly more effective in inhibiting bacterial growth than aqueous, ethyl acetate, and hexane preparations of the same plant (Hutschenreuther, 2010).
Based on the available research so far, it looks like oregano, cinnamon, clove, and pink rock rose essential oils are a solid bet for topical application following a tick bite. But let’s not forget: just because something hasn’t been scientifically validated (yet) doesn’t mean it’s not effective.
There are significant precedents for using plants to manage tick populations and complications. A 2015 study published in Veterinary World investigated the ethnoecological knowledge of ticks and treatments among Maasai people in Northern Tanzania, and found that 25 plant species belonging to 18 families were used to treat 8 different tick-borne diseases in livestock. The plant species used were principally members of the Fabaceae and Burseraceae families. The most commonly used plant species were Aloe volkensii, Cissus grandifolia, and Terminalia brownii. Stems and bark were the parts more frequently used, and most treatments were taken orally (Kioko, et al., 2015).
Now let’s launch into modern research on the use of essential oils for managing tick populations.
Essential Oils vs. Ticks: the Research
Scientists have conducted significant research in the domain of tick-borne disease prevention and treatment. Vaccines, management strategies for tick vector populations, genomic-based tools to govern tick-host-pathogen interactions, Integrated Pest Management (IPM) practices to reduce tick involvement with livestock, and pheromone-based tools are under investigation. Despite these developments, the prevention of tick exposure, at least in the context of livestock, is chiefly achieved through the application of chemical repellents and/or acaricidal (killers of ticks and mites) agents.
But there’s a problem. The extant acaricides used for livestock can be expensive, contaminate meat and milk, and exert toxic effects on “non-target” species.
And the biggest concern? Ticks are building up a resistance to them (Benelli and Pavela, 2018).
As a result, researchers are seeking novel chemicals with acaricidal and repellent activities. Or perhaps not so “novel;” there’s nothing new about plant-based tick treatments. That being said, with the advent of scientific research on the efficacy of essential oils against ticks, botanical interventions are making a comeback.
Tick Repellency
The topic of repellency is an unexpectedly complicated one. Ticks have a long-lasting host-parasite association, thus the term “repellency” can acquire myriad meanings. According to Goode et al. in their paper “Preventing Tick Attachment to Dogs Using Essential Oils” published in Ticks and Tick-Borne Diseases, “the term ‘repellency’ commonly subsumes a range of effects, including avoiding or leaving the host, failing to attach, to bite, or to feed” (Goode, et al., 2018).
Plants routinely mentioned in the scientific literature for their antagonistic(k) merits include lemongrass (Cymbopogon nardus, C. excavatus martinii), cedar (Chamaecyparis nootkatensis and Juniper virginiana), eucalyptus (Eucalyptus maculata), geranium (Pelargonium reniforme), mint (Mentha piperita), lavender (Lavandula augustifolia), lemon-scented gum (Corymbia citriodora), soybeans (Neonotonia wightii), and wild tomato (Lycopersicon hirsutum). Vanillin is commonly added to formulations of essential oils in order to increase the duration of repellency by reducing evaporation from the skin. Fixatives such as genapol (10%) and polyethylene glycol (10%) have also been used (Pages, et al., 2014).
A 2018 study published in the Saudi Pharmaceutical Journal investigated the repellency of two Anthemis essential oils from Saudi Arabia against the lone star tick (Amblyomma americanum L). Anthemis is a genus of aromatic flowering plants in the Asteraceae family, a close relation to Chamaemelum and consequently, like that genus, known by the common name “chamomile.” In this study, the essential oils of Anthemis melampodina and Anthemis scrobicularis were extracted from the aerial parts of the plants via hydrodistillation. Constituents of the oils were assessed using GC-FID and GC-MS. In the A. melampodina, 56 components comprising 85.5% of the oil were identified, with the monoterpene hydrocarbon α-pinene (17.1%) and oxygenated sesquiterpene β-eudesmol (13.8%) representing the primary components. In the A. scrobicularis, 41 constituents representing 86% of the oil were identified, β-eudesmol (12.8%) being the most abundant.
In addition to these more abundant constituents, Anthemis species are known to contain flavonoids, sesquiterpene lactones, fatty acids, sterols, essential oils and polyacetylenes. Anthemis species have been widely used in European folk medicine in the form of tinctures, extracts, tisanes, salves, decoctions, infusions, and other traditional formulations for the treatment of dysmenorrhea, inflammation, hemorrhoids, hepatotoxicity, abdominal pain, and various inflammatory conditions of the skin.
Researchers found in tick bioassays that A. melampodina essential oil demonstrated 80.0% repellency against the lone star tick, while A. scrobicularis essential oil achieved 96.7% repellency at a concentration of 2.5%. The positive control DEET exhibited 95% repellency at a concentration of 0.625%. A. scrobicularis at that same concentration produced 80% repellency.
The authors posit that the tick repellent properties of A. scrobicularis are the result of its high sesquiterpene content (59.4%). Previous studies suggest that terpenoids containing two functional groups, one which is negatively charged (either ester/ether bonds or an ethanol hydroxyl group) and the other which is positively charged (an alkyl group) is bioactive against mosquitoes. Whether this mechanism is at work in tick repellency remains unknown (Yusufoglu, et al., 2018).
The journal of Ticks and Tick-Borne Diseases published a 2018 article on the efficacy of essential oils in preventing tick attachment to dogs. Researchers performed a bioassay using sebum extracted from canine hair as a tick attractant, testing the following essential oils for their repellency: bog myrtle (Myrica gale), cajeput (Melaleuca cajeputi), geranium (Pelargonium gravolens), ginger (Zingiber officinale), grapefruit (Citrus paradisi), lavender (Lavendula angustifolia), niaouli (Melaleuca viridiflora), orange (Citrus sinensis), peppermint (Mentha arvensis), spearmint (Mentha spicata), thyme (Thymus vulgaris) and turmeric root (Curcuma longa), and the carrier oils blackseed (Nigella sativa) and soya (Soja hispida) at a concentration of 5% (v/v). These were compared with DEET and PMD (a botanical insect repellent with DEET-like efficacy found in the essential oil of the lemon-scented gum tree).
In this bioassay repellency was evaluated by “the ability of an essential oil to abolish the climbing behaviour of a tick.” Substances demonstrating positive geotaxis (repellence) in the assay were spearmint, turmeric, thyme, geranium, ginger, DEET, and PMT. Turmeric was the most promising due to the longevity of its effect; repellency persisted four hours post-application. Turmeric essential oil was found to be as repellent as DEET at concentrations as low as 2.5% in the laboratory bioassays as well as in field blanket drag trials.
In vivo experiments revealed that dogs sprayed on the legs and belly with a suspension of turmeric essential oil were significantly less likely to have a tick attach or feed than those untreated or treated with a negative control. Indeed, only two dogs out of the 30 in the turmeric test group were found to have any tick attached whatsoever.
The longevity of turmeric’s repellency is of particular interest. The volatility of essential oils tends to limit the duration of their effect. Why did turmeric essential oil exhibit superior endurace over the others? The authors have a theory: Turmerone, the principle component of turmeric essential oil, has significantly lower vapor pressure than thymol and carvone, the primary constituent of thyme and spearmint oils, respectively (Goode, et al., 2018).
A 2017 study published in Experimental and Applied Acarology explored the repellency of the essential oil of 11 Egyptian aromatic plants on the common tick Ixodes ricinus. This tick is a potential vector for both Babesia divergens, the cause of babesiosis, and Borrelia burgdorferi sensu lato, the source of Lyme disease. The authors reference a 2015 study in which researchers in northern Norway found that the overall prevalence of B. burgdorferi s.l. infection in I. ricinus nymphs and adult ticks was 21 and 46% respectively.
Of the 11 essential oils studied, the strongest tick repellency in laboratory bioassays was observed with C. dioscoridis (94%), A. herba alba (84.2%), and C. officinalis (82%) at a concentration of 1 mg/ml. C. dioscoridis, demonstrating the strongest repellency, was tested in a field trial. A 1×1 m white flannel cloth was dragged across 10 square meters of vegetation 20 times on two consecutive days, and any attached ticks were noted. The cloth was treated with 6.5 µg essential oil per cm2 of cloth. The essential oil test cloths demonstrated statistically significant tick repellency compared with a hexane control cloth.
As in the previous study, essential oils containing high volumes of oxygenated sesquiterpenes were found to exhibit higher degrees of repellency. Both C. dioscoridis and C. officinalis contain equally high oxygenated sesquiterpene content. Interestingly, they produced differing degrees of repellency, leading the authors to theorize a synergistic effect between various other compounds in the oil. Another constituent only detected (of those tested) in these two plants, is α-cadinol. Previous studies have found α-Cadinol to be highly effective in controlling certain species of house mite (El-Seedi, et al., 2017).
A 2018 systematic review published in Acta Tropica reports on the problems of standard acaricidal treatments in managing tick populations amidst livestock, and the promise of essential oils and their repellent properties. In this study, 83 plant species from 35 families were investigated. Strikingly, all tested essential oils exhibited repellence independent of the species or developmental stage of the tick.
Even relatively low concentrations of essential oils were effective in repelling ticks. In short-term tests, where the repellent effect was observed within 15 minutes of application, essential oil concentrations from 1-9% generally induced 70% repellence. EC50, the concentration of a drug that gives half-maximal response (a common measure of a drug’s potency), ranged from 0.1-0.3 mg/cm2. Essential oils applied in concentrations greater than 10% produced 100% repellence in the majority of cases. The most effective essential oils were catnip (Nepeta cataria L.), with estimated EC50 0.005 mg/cm2 evaluated one hour from application on Rhipicephalus appendiculatus Neum., and “Stinking Roger” (Tagetes minuta L.), an invasive and troublesome annual weed, with EC50 0.07 mg/cm2, evaluated one hour from application, on Hyalomma marginatum rufipes Koch.
The authors noted that the longer the observation period (>1 h), the higher the dose required to achieve repellence. No surprise there, considering the volatility of essential oils. Other possible methods of delivery have been suggested to prolong the effects of essential oils, such as encapsulation or nanoformulation.
In addition to whole essential oils, the authors investigated 41 essential oil isolates and their repellency against ticks. The most effective were nootkatone (the primary aromatic in grapefruit) and valencene-13-ol (a sesquiterpene present in yellow-cedar heartwood essential oil). Nootkatone demonstrated the highest repellency, with estimated EC50 0.04% on Ixodes scapularis Say (the blacklegged tick) in assays lasting four hours, while valencene-13-ol came in at EC50 0.07%. The phenylpropene eugenol, common in clove oil, nutmeg, cinnamon, basil and bay leaf, demonstrated 100% repellency six hours from application against cattle tick R. microplus larvae, and 80% repellency 12 hours from application, at a concentration of 5%.
As is often the case, whole essential oils were found to be more effective than their extracted constituents alone. Lippia alba essential oil provided significantly higher efficacy against R. microplus larvae when compared to its predominant constituents — limonene and carvone — alone. Five hours after application, the EC50 of the whole essential oil was 2.2 mg/cm2, while the EC50 values of (S)-(−)-limonene, (R)-(+)-limonene, (S)-(+)-carvone and (R)-(−)-carvone were 19.2, 85.8, 48.8 and 22.7 mg/cm2 respectively (Benelli and Pavela, 2018).
A handful of older studies zeroed in on geranium (Pelargonium graveolens) essential oils as effective repellents. A 2013 study published in the Journal of Agricultural and Food Chemistry compared the repellency of 10 different commercial geranium essential oil products against nymphs of the Lone Star Tick (Amblyomma americanum), with DEET as a positive control. No significant differences were found between the products, and all ten oils were significantly more repellent than controls. The essential oils and DEET both repelled over 50% of ticks at a concentration of 0.026 mg/cm2.
The major constituents of one tested essential oil sample were identified by GC/MS analysis as citronellol (27%), geraniol (11%), citronellyl formate (7%), and the sesquiterpene alcohol 10-epi-γ-eudesmol (6%). Five constituents of the geranium essential oil [geraniol, citronellol, geranyl formate, citronellyl formate, and (−)-10-epi-γ-eudesmol] were tested individually for their repellence against ticks, and all were found to exert strong repellent activity.
At a concentration of 0.206 mg/cm2 citronellol repelled 100% of ticks, geranyl formate 95%, geraniol 90%, and citronellyl formate 86.7%. The most repellent single constituent was (−)-10-epi-γ-eudesmol, which at concentrations of 0.103 and 0.052 mg/cm2 repelled 90 and 73% of ticks respectively. The efficacy of (−)-10-epi-γ-eudesmol was comparable to DEET at concentrations of ≥0.052 mg/cm2, howeber (−)-10-epi-γ-eudesmol lost much of its activity at lower concentrations, whereas DEET remained active above ≥0.013 mg/cm2.
The authors propose that (−)-10-epi-γ-eudesmol would be a beneficial addition to natural repellent-based formulations. They note that many commercial repellent products contain “well in excess” of 5% of active ingredients, and that (−)-10-epiγ-eudesmol is effective at lower concentrations than that. A repellent product containing ≥20% (−)-10-epi-γeudesmol, they conjecture, “should provide good protection against tick bites” (Tabanca et al., 2013).
Another study investigating the constituents of geranium as a tick repellent performed a field trial on two farms near Rabat, Morocco. This 2009 paper, published in Parasite, reports that 1% geraniol (FULLTEC®) exerted repellent effects against Hyalomma sp. (hard) ticks when sprayed on naturally infested grazing cattle. Following application of the product, the number of ticks on cattle was reduced by 98.4%, 97.3% and 91.3% on days 7, 14, and 21 respectively. Treatment was well-tolerated by cattle, with no adverse events reported.
Geraniol is not only obtained from plants of the Pelargonium (geranium) genus, but also from such genera as Eucalyptus and Cymbopogon (lemongrass). This particular batch of geraniol had been extracted from Palmarosa essential oil. The authors conclude that 1% geraniol is an effective treatment for the management of tick infestation, and will help avoid the development of chemoresistance in ticks (Khallaayoune et al., 2009).
Essential oils thus offer an effective alternative to standard, and problematic, acaricidal treatments. Importantly, some of the most effective oils are distilled from plants that are abundant and otherwise considered an ecological and economic nuisance, such as Stinking Roger.
Looks like an opportunity to turn straw into gold.
Acaricidal Activity and Obstructed Reproduction
Thus far we’ve honed in on the repellent properties of essential oils. Now we’ll turn to those essential oils that don’t just rebuff pests, but actually kill them.
A 2018 study published in Veterinary Parasitology evaluated the acaricidal and repellent activity of Ocotea elegans essential oil on Rhipicephalus microplus. The authors explain the motivation for their research: “The application of chemical acaricides has become indispensable for cattle production and is the main method used for tick control. However, scientific reports of resistance of R. (B.) microplus to the chemical acaricides available in the market are alarming.” The authors propose that by adding essential oils to extant commercial products, they might “prolong the efficacy by reducing the selective pressure for development of resistant tick strains” and, at least in the Brazilian cattle market, stave off the US$ 3.24 billion lost per year from reduced productivity and the expense of treating tick-borne diseases.
It’s not just an economic concern; there are significant environmental and human health perks as well. “Active molecules from plant species have interesting characteristics for parasite control in animal production systems,” the authors explain, “such as reduced environmental impact, reduced residues in food, lower cost, and delayed parasite resistance.”
Ocotea is a genus of flowering plants in the Lauraceae family known to contain terpenoids, alkaloids, neolignans, allyl phenols, coumarins, and sesquiterpene lactones. Notably, the present study confirmed the presence of sesquiterpenes in the essential oil of O. elegans leaves. Sesquiterpenes, abundant in the Ocotea genus, are the compounds cited to be key in exerting biological activities against ticks and other pests. Indeed, the authors specify the sesquiterpene sesquirosefuran, the major constituent of the essential oil, as responsible for the acaricidal and repellent activities of O. elegans against R. (B.) microplus.
At concentrations of 50 and 100 mg/mL O. elegans essential oil was 100% effective at impairing the reproductive parameters of engorged adult female ticks. Both oviposition (egg laying) and hatching were hampered, a result that manifest 63.9% efficacy even at the relatively low concentration of 6.2 mg/mL.
The researchers also observed larval susceptibility. A concentration of 12.5 mg/mL repelled 99% of the larvae, an effect which persisted 6 hours post-application. High repellency rates were observed even at the lowest concentrations. This study did not, however, demonstrate powerful larvicidal performance. After 24 hours of being enfolded in an essential-oil impregnated filter sheet packet (the “larval packet test”), the mortality rate was 34.5% at a concentration of 100 mg/mL. The 48 hour tests were more promising, with a mortality rate of 76% at 100 mg/mL.
While the authors were not wowed by this activity, they do note that other studies achieved superior larvicidal results using ethanol extracts of other species of the Ocotea genus. They offer the example of extracts of O. diospyrifolia, which elicited 95% mortality in a larval immersion test at a concentration of 40%. In another study O. aciphylla achieved over 90% mortality in a larval packet test at a concentration of 50 mg/mL. The authors conclude that “These findings may be related to the type of extraction from the plant material, as well as the difference of composition among species, since this can vary widely in the same genus.”
In a typical move, the researchers zero in on the “active” constituent, sesquirosefuran. Their plan? Step 1: Maximize its production via genetic selection in order “to obtain genotypes suitable for commercial production.” Or find a way to synthesize it in the lab. Step 2: Incorporate it into available synthetic acaricides to “add repellent action” and check the burgeoning crisis of tick resistance to commercial pesticides (Figueiredo et al, 2018).
A 2017 study published in the journal of Experimental Applied Acarology investigated the capacity of cinnamon essential oil, in both pure and nanostructured forms, to kill ticks and interfere with their reproduction. Cinnamaldehyde, the primary constituent of cinnamon oil, has been described as bactericidal, antifungal, repellent, and antiparasitic by numerous sources. In this study, both pure and nanostructured cinnamon essential oil were found to impair oviposition and inhibit larva in Rhipicephalus microplus ticks on dairy cows.
Nanostructured essential oils are considered particularly promising for essential oil applications against ticks. A nanocapsule is a vesicle made from a nontoxic polymer that encapsulates liquids at the nanoscale. Nanostructures are advantageous because they protect the substance of interest from an adverse environment, and allow for controlled release and precise targeting. They are of particular interest in medical applications for nutraceutical and drug delivery.
In in vitro tests the nanostructured forms (nanocapsules and nanoemulsions) demonstrated acaricidal outcomes at remarkably low concentrations. Cinnamon oil in its pure form was 100% effective in achieving reproductive inhibition at a concentration of 10%, as measured by the reduction in the number of ticks that performed oviposition, weight of deposited eggs, and impaired hatchability. 5 and 1% concentrations resulted in an efficacy of 99 and 62%, respectively. Nanocapsules demonstrated 73, 95, and 95% efficacy at concentrations of 0.5, 1.0, and 5.0%, while nanoemulsions achieved 83, 85, and 97% efficacy at those concentrations. In comparison, treatment with 10% cypermethrin (a conventional acaricide) showed 54.2% efficacy, which indicates pesticide resistance in the tick population.
The authors also tested cinnamon oil in vivo on tick-infested dairy cattle. One group of cows was sprayed with the commercial pesticide Triton as a positive control, a second group was sprayed with 5% cinnamon oil, a third group was sprayed with cinnamon oil (0.5%) nanocapsules, and a fourth with cinnamon oil (0.5%) nanoemulsion. Each animal was counted for ticks on days 0, 1, 4 and 20 after spraying. Researchers found that those animals sprayed with pure and nanoencapsulated cinnamon oil carried significantly fewer ticks on days 1 and 4 post-treatment, and were completely free of ticks 20 post-treatment. Ticks collected from these cows 24 h after application of cinnamon oil demonstrated impaired oviposition and larval inhibition. Pure cinnamon oil was found to be 90.5% effective in obstructing reproductive parameters, while nanoencapsulated cinnamon oil was 100% effective. Nanoemulsions performed the least well, with only 63.5% efficacy (Dos Santos et al., 2017).
So, to recap, we have essential oils that repel ticks, kill them, prevent them from reproducing, and kill the pathogens they transmit. All in all, volatile distillates present a promising tactic for controlling ticks and tick-borne diseases.
The vegetable kingdom’s got some tricks up its sleeves.
Looks like ticks are in trouble!
Tick Repellent Spray
| Essential Oil | Drops |
|---|---|
| Grapefruit | Citrus paradisi | 15 drops |
| Geranium | Pelargonium graveolens x. asperum | 12 drops |
| Patchouli* | Pogostemun cablin | 7 drops |
| *Can substitute Patchouli with Sandalwood if you wish |
Add above oils to a 2 ounce spray top bottle, then fill bottle with one of the following:
100% distilled water
50% distilled water plus 50% vodka
50% distilled water plus 50% aloe vera gel
Shake before using. Apply to clothing and exposed areas of skin as needed.
References
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