Unlocking the Antiparasitic Potential of Apiaceae: The Promise of Geraniol for Scabies Treatment

doi:10.21203/rs.3.rs-4813219/v1

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Unlocking the Antiparasitic Potential of Apiaceae: The Promise of Geraniol for Scabies Treatment

https://doi.org/10.21203/rs.3.rs-4813219/v1

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Sarcoptic mange, also referred to as scabies in humans, is a highly contagious skin condition caused by the burrowing mite Sarcoptes scabiei (Astigmata: Sarcoptinae). This parasitic disease significantly affects livestock production and poses a substantial threat to both animal and human health. The current treatment methods primarily involve synthetic acaricides like permethrin and ivermectin. However, these medications have several limitations, including a lack of ovicidal activity, concerns about toxicity, potential parasite resistance, residues in animal products, and environmental contamination, making them less than ideal. Volatile constituents derived from Apiaceae plants, especially essential oils, present a promising alternative. In this study, we have, first, reviewed all previously (until June 2024) reported volatile constituent from Apiaceae plants. Second, we carried out a comprehensive in silico-based investigations on the collected compounds to suggest a new potential antiparasitic or insecticidal agent derived from Apiaceae volatile oils. Geraniol, among other possible candidates, was found to be a potential acaricidal agent that might be used as a natural safe alternative. Furthermore, it showed a promising inhibitory activity against S. scabiei, both in vitro and in vivo. Geraniol demonstrated acaricidal activity, achieving a 100% mortality rate at high concentrations and significant effectiveness even at a lower concentration of 6.25%. In vivo studies on rabbits confirmed the efficacy of geraniol, showing complete clinical recovery, including the disappearance of scabies lesions and an improved skin health. Histopathological analysis indicated skin regeneration in the geraniol-treated group compared to the control group. These results suggest that geraniol could be a viable natural option for scabies treatment, likely due to its ability to disrupt mite nerve function by targeting acetylcholinesterase (AChE). Further research is necessary to determine the optimal formulation and delivery methods for geraniol-based treatments for scabies.

Biological sciences/Biochemistry

Biological sciences/Computational biology and bioinformatics

Biological sciences/Drug discovery

Acaricidal activity

Acetylcholinesterase

Apiaceae

Geraniol

In silico analysis

Parasitic diseases are considered as a widespread health problem in rural tropical areas where humidity and high temperatures promote the growth of parasites. Many parasites affect populations with low standards of living and poor hygiene and sanitation [1-3]. Human parasitic diseases can be caused by endoparasites (protozoa and helminth worms) and ectoparasites (fleas, lice, ticks, and mites) [1]. The Sarcoptes scabiei var. hominis itch mite is an ectoparasite causing a contagious parasitic skin infestation in humans called scabies [4]. It develops intense pruritus and skin rash as a result of the immune reaction caused by burrowing the mites into the skin epidermis and laying eggs [5]. Scabies causes impetigo (secondary bacterial skin infection), which leads to many complications such as streptococcal septicemia, rheumatic fever, and chronic heart and renal diseases [6, 7]. It is estimated by the World Health Organization (WHO) that 200 million people worldwide are currently infested [8] with high prevalence in overcrowded, poor regions and during wars [9]. WHO has recognized scabies as a neglected tropical disease in 2017 [8, 10].

Ivermectin and permethrin are commonly used for the treatment of scabies in addition to sulfur compounds, γ-benzene hexachloride (lindane), and benzyl benzoate. However, these drugs show adverse effects and many studies have reported resistance to them [11, 12]. This prompted the development of effective and safe alternative treatments to appropriately manage scabies. Nature provides humanity with several metabolites for treatment of parasitic activities [13-15]. Several studies have shown that essential oils (EOs) derived from plants in addition to some plant extracts are being used in the treatment of scabies, such as tea tree oil, Lippia multiflora oil, Azadirachta indica oil, Curcuma long oil and coconut seed extract [11, 12, 16].

EOs are natural complex mixtures of volatile organic substances; they are extracted by hydro- or steam-distillation from different parts of the plants [17]. EOs of many plants contain small lipophilic compounds which show antiparasitic properties by disturbing their biomembranes [18]. In addition, the hydrophobicity of the EO components and their low density promote their intracellular targeting into parasites. The antiparasitic activity of plant EOs may be due to their immunomodulatory effects through enhancing nitric oxide (NO) production in macrophages. NO protects macrophages against parasitic infections. Moreover, EOs have anti-inflammatory activity, which protects the host from NO damaging effects [19]. Another mode of action of antiparasitic activity of EOs has been suggested. The hydrophobic EOs and their components bind to the hydrophobic site of cruzain, a cysteine protease enzyme, inhibiting it and blocking the life cycle of the parasite [16, 20, 21].

The plant family Apiaceae, formerly known as Umbelliferae, comprises approximately 434 genera and 3780 species, distributed in the tropical areas. Many studies have reported biological activities of Apiaceae plants such as antimicrobial, antitumor, analgesic, radical scavenging, diuretic, anti-inflammatory, anti-obesity, and insecticidal properties. Besides, most of these plants are used traditionally to treat respiratory, reproductive, and gastrointestinal tract disorders [22, 23]. Plants belonging to this family are rich in different bioactive compounds such as coumarins, polyacetylenes, terpenoids and triterpenoid saponins, flavonoids, and steroids. In addition, Apiaceae plants are characterized by the presence of EOs secreted by schizogenous oil ducts located in their different organs and responsible for their distinctive flavors. The volatile oil components isolated from different Apiaceae plants are listed in Table S1. These EOs exhibit insecticidal, insect repellents, antifeedant and growth regulators activities [22-24]. In addition, EOs found in members of family Apiaceae display interesting antiparasitic activity. As an example, the EO of Anethum graveolens showed anthelmintic activity against Haemonchus contortus, when compared with thybendazole and levamisole as positive controls [25]. The EO of Pimpinella anisum was found to possess acaricidal activity on adult Dermanyssus gallinae, the poultry red mite, with an LC₅₀ value of 47.5 μg/mL when compared with permethrin as the positive control and against Tetranychus urticae [26, 27]. Apium graveolens EO showed antiparasitic activity against the larvae of Aedes aegypti in addition to repelling mosquitoes when mixed with 5% vanillin [28]. The EO of Ferula macrecolea had antiparasitic activity against protoscoleces of Echinococcus granulosus [29]. The EO of Foeniculum vulgare exerted moderate anthelmintic activity against adult worms of Schistosoma mansoni [30]. In addition, it showed acaricidal activity against Rhipeciphalus annulatus, Dermatophagoides pteronyssinus, and Dermatophagoides farina and it exerted larvicidal activity against the Asian vector of malaria, Anopheles stephensi [31-33]. The EO of Petroselinum crispum exhibit

ted insecticidal activity against Trialeurodes vaporariorum adult pest [34]. The EO of Carum carvi showed antiprotozoal activity against Plasmodium falciparum [35], while the EO of Cuminum cyminum possessed antiparasitic activity against the insects Anopheles gambiae, Tribolium confusum, and Ephestia kuehniella [35, 36]. Coriandrum sativum EO exerted antiparasitic activity against the insects Tribolium confusum, Callosobruchus maculatus, Aedes albopictus, and Aedes aegypti and against the nematodes Bursaphelenchus xylophilus, Haemonchus contortus, Trichostrongylus colubriformis, Trichostrongylus axei, Trichostrongylus vitrines, and Teladorsagia circumcincta [35-37]. Different antiparasitic activities of screened compounds are listed in Table S2.

Linalool, the major constituent of Coriandrum sativum oil, showed strong anthelmintic activity against Trichostrongylus colubriformis nematodes, Trichostrongylus axei, Teladorsagia circumcincta, Trichostrongylus vitrines, and Haemonchus contortus and exhibited a synergistic anthelmintic effect when combined with coriander EO [37]. (R)-Carvone and D-limonene, the components of EO of Carum carvy showed strong fumigant toxicity against Sitophilus zeamais and Tribolium castaneum adults, with LC₅₀ values of 2.76 and 48.18 mg/L for S. zeamais and LC₅₀ = 1.96 and 19.10 mg/L for T. castaneum, respectively. This makes the EO of Carum carvi to have antiparasitic activity against both S. zeamais and T. castaneum adults (LD₅₀ values of 3.07 and 3.29 μg/adult respectively) [35, 38]. (7,10) (+)-Limonene epoxide exerted antischistosomal activity against Schistosoma mansoni adult worms [39]. The main component of Foeniculum vulgare, trans-anethole, exhibited acaricidal activity against Rhipeciphalus annulatus [31].

Several studies reported the activity of EOs and their components against scabies (Table S3), but only few are found dealing with members of the Apiaceae family, which initiated us to carry out this work.

We examined the main volatile oil components identified in Apiaceae plants over the past years (until June 2024). From this research, we compiled a list of 122 key components, which can be found in Table S1. Chemically, these components are classified into terpene hydrocarbons (38 compounds), terpene oxygenated compounds (78 compounds) and miscellaneous compounds (six compounds), hence representing 31, 64, and 4.9% respectively, of the total number of compounds (Figure 1). The terpene hydrocarbon compounds are classified into monoterpenes (19 compounds, 15.6%) and sesquiterpene hydrocarbons (19 compounds, 15.6%). On the other hand, oxygenated terpene compounds are classified into terpene alcohols (25 compounds, 20.5%), aldehydes (14 compounds, 11.5%), ketones (12 compounds, 9.8%), phenol and phenolic ether components (ten compounds, 8.2 %), oxides (nine compounds, 7.4%), esters and lactones (six compounds, 4.9%) and sulfur-containing compounds (two compounds, 1.6 %). Miscellaneous compounds are (six compounds, 4.9%) (Figures 1 and 2).

Among all reported compounds, only 44 compounds were reported to exhibit antiparasitic activity (Table S2). These compounds are: β-myrcene (V-1), limonene (V-4), γ-terpinene (V-5), citronellal (V-6), cis-dihydrocarvone (V-8), α-pinene (V-11), β-pinene (V-12), camphene (V-13), sabinene (V-14), δ-3-carene (V-15), p-cymene (V-19), α-humulene(V-23), δ-cadinene (V-29), β-caryophyllene (V-30), linalool (V-39), citronellol (V-40), geraniol (V-41), E-nerolidol (V-43), α- terpineol (V-48), borneol (V-51), α-bisabolol (V-55), guaiol (V-58), β-eudesmol (V-62), citral (V-64), citronellal (V-65), p-anisaldehyde (V-72), cuminaldehyde (V-73), cis-dihydrocarvone (V-80), carvone (V-82), fenchone (V-85), camphor (V-86), trans-anethole (V-90), estragole (V-91), thymol (V-92), eugenol (V-94), elemicin (V-97), dillapiole (V-98), cis-limonene oxide (V-102), caryophyllene oxide (V-105),

cineole (V-106), E-limonene-1,2-oxide (V-107), geranyl acetate (V-109), neryl acetate (V-110), and n-hexadecanoic acid (V-118). They were found to exhibit a wide spectrum of antiparasitic activities against human, fish, animal, plants, stored food parasites, and miscellaneous groups. For example, compounds were reported for their antitrypanosomal (17 compounds), antischistosomal (four compounds), antimalarial (13 compounds), antileishmanial (22 compounds), anti-taxoplasmosis (two compounds) activities. Also, compounds were reported for their activities against parasites of stored grains (28 compounds), parasites of plants (18 compounds), parasites of animals (cattle and sheep) (11 compounds) and fish parasitic activity (nine compounds). In addition, several compounds were reported for their activities against: Musca domestica (housefly) (25 compounds), Culex pipiens (house mosquito) (eight compounds), Pediculus humanus capitis (five compounds) (Figure 3).

The reviewed compounds were analyzed using a virtual screening approach on the PASS (www.way2drug.com) platform, which incorporates sophisticated machine learning algorithms to predict the biological activities of specific structures. This analysis produced scores and a list of predicted biological activities, which were scrutinized for antiparasitic properties, including specific anti-leishmanial, anthelmintic, anti-trypanosomal, anti-coccidial, anti-plasmodial, acaricidal and insecticide activities. The scores and their associated antiparasitic activities were cataloged in an Excel spreadsheet (Table S3) for subsequent statistical evaluation.

To differentiate these compounds based on their scores and the range of predicted antiparasitic activities, the data underwent Principle Component Analysis (PCA) based clustering. As depicted in Figures 4 and 5, most of the screened compounds displayed low predicted antiparasitic activities (i.e., Pa scores below 0.5) and were grouped together, represented as black-colored dots in the analysis. Conversely, 24 oxygenated terpenes exhibited probable activity against at least one parasite, achieving Pa scores of 0.5 or higher.

Among these potentially active compounds, six were identified with Pa scores ranging from 0.5 to 0.6, displaying activity against only one or two parasites, as indicated by orange-colored dots and their structures in Figures 4 and 5.

The second category of potentially active compounds comprised 11 substances with Pa scores ranging from 0.5 to 0.6, which showed activity against three parasites, as indicated by blue-colored dots and structures. The third category included seven compounds with Pa scores from 0.5 to 0.6 or higher, effective against more than three parasites, depicted as green-colored dots and structures.

These compounds, represented as green in Figure 5, were then subjected to further analysis to uncover their potential mechanisms of action through inverse docking-based screening.

This investigation was conducted using the PharmMapper virtual screening platform (https://www.lilab-ecust.cn/pharmmapper/), which facilitates rapid and accurate docking of structures against a comprehensive database of protein structures from the RCSB PDB. The affinity scores, or FitScores, for each structure were arranged in an Excel sheet from the highest to the lowest.

Following this, scores and protein interactions specifically related to insects or parasites were manually selected, filtering out protein hits with FitScores above 7. Notably, Trypanosoma brucei target pteridine reductase-1 (PTR1, PDB ID: 4WCF) and Drosophila melanogaster acetylcholinesterase (AChE; PDB ID: 6XYY) emerged as optimal targets for daucol and geraniol, respectively (with FitScores of 10.23 and 9.49, respectively). Re-docking of these compounds into the active sites of each enzyme revealed that daucol achieved good docking scores of -7.16 and -5.02 kcal/mol with PTR1 and AChE, respectively, while geraniol showed scores of -4.33 kcal/mol and -7.85 kcal/mol. The interaction of geraniol with AChE was notably comparable to that of a co-crystallized inhibitor, with a docking score of -7.97 kcal/mol.

Further analysis involved 100-ns long molecular dynamics (MD) simulations to assess the binding stability of these compounds with their target proteins. Daucol demonstrated significant instability, detaching rapidly from the binding sites of both enzymes at early stages of the simulation (at 5.76 and 9.67 ns for PTR1 and AChE, respectively). The same results were shown for geraniol with PTR1, where its modeled structure was detached from the active site of the enzyme at 12.22 ns. These results might be attributed to the small molecular sizes of both structures and lacking multiple stabilizing hydrophilic functional groups, and hence, their calculated absolute binding free energy (ΔG_Bind) were -2.66 and -1.67 kcal/mol for daucol with PTR1 and AChE, respectively, and -2.78 kcal/mol for geraniol with AChE.

Conversely, geraniol showed significant binding stability inside the binding tunnel of AChE over a 100-ns long MD simulation, which was comparable to that of the co-crystallized inhibitor (i.e., tacrine). As shown in Figure 6, the modelled structure of geraniol was able to stabilize its binding inside the binding tunnel of AChE by the establishment of four hydrophobic interactions with TRP-80, TYR-370, TRP-472, and HIS-480 via its hydrophobic hydrocarbon structure. Additionally, it formed strong and durable H-bonds with both TYR-162 and GLU-237 via its terminal hydroxy group. Similarly, tacrine, as the co-crystallized inhibitor, was able to establish stable hydrophobic interactions with the same amino acid residues, while it formed a single stable H-bond with HIS-480. Ultimately, the calculated ΔG_Bind values for both compounds were closely aligned (-7.88, and -8.15 kcal/mol, respectively), suggesting that geraniol could be an effective AChE inhibitor and thus a potential broad-spectrum insecticide. Previous studies had also indicated that geraniol is a moderate to potent inhibitor of various AChE variants, supporting its potential utility in this capacity. For this reason, it was selected for further experimental validation against Sarcoptes scabeii [40-43].

3.1. Materials and methods

3.1.1. Geraniol

Geraniol (98%) was purchased from Sigma-Aldrich, Co. (Germany).

3.1.2. Scabicidal activity testing

3.1.2.1. In vitro testing

A total of 200 individuals from adult active Sarcoptes scabeii were collected by skin scrabbing from experimentally infected rabbits, which used for this study. The collected S. scabeii mites were transferred to 20 petri dishes (ten S. scabeii in each petri dish) using featherweight forceps and micropipette under the dissecting microscope. For each concentration, four petri dishes were used, one of them for the control containing paraffin oil and three replicates of each concentration, the used concentrations from geraniol were 100%, 50%, 25%, 12.5%, and 6.25%, the paraffin oil was used for dilution.

3.1.2.2. In vivo testing:

In this study, 20 experimentally infected New Zealand Dutch strain albino rabbits were used. The age of these rabbits was six months and their weights was between 1 to 1.2 kg. They were divided into four groups (5 rabbits/each) as follows: positive control (infected and not treated), negative control (not infected not treated), geraniol-treated (paraffin oil-diluted geraniol was applied gently to the infected regions in rabbits day by day), and ivermectin-treated (commercial ivermectin -1% w/v, AVICO, Jordan- was subcutaneously injected at a dose of 0.3 mg/kg body weight/rabbit). All rabbits were examined clinically to reveal the improvement along the study period, which lasted for 2 weeks till complete recovery of treated group. An intraperitoneal injection of a ketamine-xylazine mixture was used to anesthetize the animals. (90 mg/kg body wt. ketamine and 10 mg/kg body wt. xylazine) before wound creation. The rat's dorsal surface was shaved using an electric fur clipper, and 70% ethanol was sprayed over it to clean the underlying skin. A full-thickness excision wound (88 mm) was created using a sterile surgical blade and scissors under aseptic conditions, followed by topical application of a pain reliever (Buprenorphine, 0.5 mg/kg body wt.) To mitigate the pain threshold and all efforts were made to minimize suffering. By the end of the experiment, all rabbits were euthanatized after an intraperitoneal injection of a ketamine-xylazine mixture, mentioned above, as an anesthetizing agent. After that, the treated regions were histopathologically examined. All methods were performed in accordance with the guidelines of Research Ethics Committee, Assiut Veterinary Medicine, Assiut, Egypt. The ethical approval number is 06/2024/0204 (issued on June 13, 2024). All animal-based studies were conducted according to ARRIVE guidelines.

3.1.2.3. Histopathological examination

Fresh specimens from the noses and ears of all tested rabbits were formalin-fixed and processed for histopathological analysis. Hematoxylin and eosin (H&E) staining was applied to 5-µm thick paraffin sections. The microscopic examination was conducted by light microscopy (Olympus CX31, Tokyo, Japan) and photographed using a digital camera (Olympus, Camedia C-5060, Tokyo, Japan) in the Photomicrograph Lab of the Department of Parasitology, Faculty of Veterinary Medicine, Assiut University, Assiut, Egypt.

Firstly, an in vitro study was done to reveal the effect of geraniol on mites at different concentrations to select the most effective and least corrosive concentration for the proceeding in vivo study. As shown in Table 1, the mortality rate was investigated at the different concentrations (100%, 50%, 25%, 12.5%, and 6.25%), in three replicates. Geraniol showed very strong acaricidal activity in its full strength (100% mortality rate from the first minutes). A mite considered dead when it stopped its mobility for 1 min even with continuous stimulation with needle, showing no more reaction. The mortality rate of adult mites decreased gradually by dilution of the oil. At 6.25% concentration the total mortality rate was obtained after 15 min treatment, so it was chosen as the concentration for a further in vivo study, as it was highly effective against mites but may have less -corrosive properties on the skin of the rabbits.

The survival curves (Figure 7) showed that all concentrations were lethal within 15 min, the average lethal time after exposure to 100%, 50%, 25%, 12.5%, and 6.25% was 1 min, 1.1 min, 1.3 min, 5.4 min., 9.5 min, respectively. The LT₅₀ values are shown in Table 2.

The LC₅₀ value (%) was 0.54 and the LC₉₀ value (%) was 0.67, representing very strong miticidal activities (Table 3). The negative control in paraffin oil showing no miticidal activity against adult S. scabiei mites, all the adults were still alive for the following 24 h.

Table 1: The mortality rates of Sarcoptes scabeii mites exposed to different concentrations of geraniol in each plate within 15 min.

Mortality rate (%)
Control	15 min	10 min	5 min	3 min	1 min	Plate	Concentration
0%	100%	100%	100%	100%	100%	P1	100%
0%	100%	100%	100%	100%	100%	P2
0%	100%	100%	100%	100%	100%	P3
0%	100%	100%	100%	90%	90%	P1	50%
0%	100%	100%	100%	100%	100%	P2
0%	100%	100%	100%	100%	90%	P3
0%	100%	100%	100%	80%	80%	P1	25%
0%	100%	100%	100%	100%	90%	P2
0%	100%	100%	100%	90%	80%	P3
0%	100%	100%	80%	80%	60%	P1	12.5%
0%	100%	90%	90%	80%	70%	P2
0%	100%	100%	100%	90%	50%	P3
0%	100%	100%	70%	70%	50%	P1	6.25%
0%	100%	90%	80%	60%	40%	P2
0%	100%	90%	80%	70%	60%	P3

Table 2. Probit regression of the toxicity (LT₅₀) of geraniol against Sarcoptes mites in vitro

Compound	*LT₅₀ (95% CL)**
	100%	50%	25%	12.5%	6.25%
Geraniol	1 min	1.1 min (1-1.4)	1.3 min (1.1-1.5)	5.4 min (2.6-6.7)	9.5 min (3.2-14.1)

* 95% Confidence Limits

Table 3. Concentrations of geraniol required to kill 50% and 90% of Sarcoptes mites at 15 minutes after exposure

Compound	LC₅₀ (%)	95% CI	LC₉₀ (%)	95% CI
Geraniol	0.54	0.45–0.62	0.67	0.61–0.74

3.2. Results

3.2.1. Clinical observations

The untreated control rabbits exhibited signs of active mange (restlessness, scratching, and multifocal alopecia containing hard dried crusts and scars spread on external ears, nose, face, eyelid, and lips). Throughout the study, two out of the five rabbits (40%) developed complete recumbency and dullness in the last week of the study period (Figure 8), one rabbit was dead 2 weeks after the start of the study.

The geraniol-treated group expressed complete recovery from clinical mange 2 weeks post-treatment. This recovery was expressed by the gradual absence of scratching and itching, crusting, restlessness, and skin thickening in addition to the improvement of the general condition of the animals - the treated rabbits exhibited no recumbency and completely normal behavior and activity, in a sharp contrast to the rabbits of the positive control group which showed signs of dullness, recumbency, and general illness. The gross lesions prominently regressed. Smoother skin with new hair growth was observed within the first week following treatment. Complete gross recovery developed at the end of the experiment (2 weeks post-treatment), as a complete absence of crusts, sound skin, and hair growth was observed (Figure 9). The ivermectin-treated rabbits didn’t develop any clinical healing to the affected skin areas within the experimental period.

3.2.2. Histopathological observations

After 2 weeks, the geraniol-treated group showed a marked regeneration in the skin histology and apparent enhancement in its epidermis and dermis with few infiltrations of inflammatory cells, mild edema, and absence of mites (Figures 10 A and B).

The positive control group and ivermectin-treated rabbits showed the mite developmental stages implanted within the epidermal hyperkeratotic layer. Also, the parasite was found intermixed with the ulcerated necrotic debris and inflammatory cells on the skin surface and it occasionally made its way deeper into the underlying tissues of the ear. The hyperplasia of the adjacent epidermal cells with marked acanthosis, extensive subepithelial inflammatory cell accumulation and dermal edema, congestion, thrombosis, and extravasation were observed (Figure 10C-D).

3.3. Discussion

Several essential oils have demonstrated promising result when used against Sarcoptes scabiei mites, however, until now not applied commercially, because the commercial manufacturing of these products requires determining the efficacy of each component of the respective essential oils, individually or in combination. These natural substances have been suggested as an alternative to conventional acaricides because of their low toxicity, repellent properties, and hindering capacity to the activities of many insect species.

In the present work, geraniol showed a strong acaricidal activity, with a very high mortality rate reaching 100% within 15 min at a concentration of 6.25%. The geraniol-treated rabbits showed obvious clinical and histological improvement in skin lesions, expressed by the absence of crusts and mite stages and tissue regeneration. These results agreed with those obtained by Buitrago et al. [44]. Sparagano et al. [45] demonstrated acaricidal activity using undiluted geraniol, obtaining 100% mortality of Dermanyssus gallinae. However, when it is used at a 10% concentration, mortality decreased to 20.7%. This behavior was associated with a higher concentration of the geraniol exerting a greater acaricidal effect. This is presumably attributed to the various biological properties of geraniol as a volatile monoterpene damaging the membrane and the ion channels of the parasite, altering the action of the membrane-bound proteins and the intracellular signaling cascades [46, 47]. The significant lethality that geraniol has demonstrated, even at low concentrations, may be related to the modes of action that it exhibits and the presence of OH group in its structure [48]. The structural OH group affects the binding activity to the neuronal receptors of the insect.

Geraniol, like other naturally occurring volatile oils, may possess neurotoxic properties that impact the central nervous system of the insect by inhibiting nicotinic acetylcholine receptors, octopamine, tyramine, and acetylcholinesterase activity dependent on γ-aminobutyric acid (GABA) sodium and chloride channels [49]. Many studies were done testing the efficacy of volatile oils. A previous study demonstrated that carvacrol at a concentration of 0.63% was able to kill Psoroptes ovis after 12 h [16]. This inconsistency in miticidal efficacy could be attributed to the differences in the tested mite species [22]. Eugenol, the main component in clove and cinnamon oil, showed strong miticidal activity [16, 18]. Eugenol has been demonstrated to possess antibacterial, antiviral, antifungal, anticancer, anti-inflammatory, and antioxidant properties [23]. A previous study had revealed that the LC₅₀ value of eugenol against Sarcoptes mites was 0.79%, at 30 min, while at 1 h, the value was about 0.2% [17]. The less pronounced effect of eugenol may be due to the fact that the mites produce some resistance as a reaction to the regular treatment with ivermectin. It was proven that the drug-resistant mites are less susceptible to essential oils treatment than the native ones [17]. Geraniol, which is a widely used fragrance terpene found in citronella, rose, and palmarosa oils, has a number of pharmacological properties, including antitumor, anti-inflammatory, antioxidative, and antimicrobial activities, as well as hepatoprotective, cardioprotective, and neuroprotective effects [25]. Citral, the major component of lemongrass oil, was lethal to two-spotted spider mites (Tetranychus urticae) [29]. Perrucci et al. [30] showed that 3% linalool applied twice a week for 2 weeks resulted in 80% of rabbits being free of Psoroptes mites.

Geraniol had previously been tested in numerous earlier studies against different mite species (Dermanyssus gallinae and Psoroptes cuniculi), ticks, and head lice [20, 26-28]. Concerning Sarcoptes scabiei, geraniol at concentration of 1% killed all mites in 24 min; these results are consistent with our findings [20]. The mode of mortality of essential oils to mites exhibited neurotoxicity by targeting the nervous systems of arthropods, including the enzyme acetylcholinesterase (AChE) [31, 32], ionotropic GABA receptors [33], and octopamine receptors [34]. Carvacrol, thymol, and eugenol exert neuroinhibitory effects, while linalool produces excitatory effects [35]. Also, the lipophilic nature of essential oils plays an important role in penetrating the cuticle of the arthropods [35].

This study has explored the potential of geraniol, a naturally occurring monoterpene alcohol, as an acaricidal agent against Sarcoptes scabiei, the mite responsible for scabies. Our findings demonstrate that geraniol exhibits promising acaricidal activity: In vitro studies revealed high mortality rates of S. scabiei mites at geraniol concentrations as low as 6.25%. In vivo experiments on rabbits infected with S. scabiei showed significant clinical and histological improvement in geraniol-treated animals compared to the control groups. These results warrant further investigations with geraniol as a potential therapeutic option for scabies treatment.

Based on these promising findings, future research should explore several key areas:

Formulation development: Developing a user-friendly and effective topical formulation containing geraniol is crucial for practical application. Factors like concentration, penetration enhancers, and stability need to be optimized.

Mechanism of action: Elucidating the precise mechanisms by which geraniol exerts its acaricidal effect against S. scabiei would provide valuable insight for the optimization of its use and for potential combination therapies.

Combination therapy: Investigating potential synergies between geraniol and other acaricides could enhance efficacy and might possibly reduce the risk of resistance development in mite populations.

Clinical trials: Conducting well-designed clinical trials is essential to assess the safety and efficacy of geraniol-based formulations for scabies treatment in humans. This would involve determining optimal dosage, treatment duration, and minimizing possible side effects.

Resistance monitoring: Long-term monitoring of S. scabiei populations for potential development of resistance to geraniol is crucial for ensuring the sustainability of this approach. By addressing these future directions, geraniol has the potential to become a valuable addition to the scabies treatment armamentarium, offering a potentially safe, effective, and natural alternative to conventional acaricides.

Availability of data and materials

All data generated or analyzed during this study are included in this published article (and its Supplementary Information files).

All animal-based studies were conducted according to ARRIVE guidelines.

Funding

Not applicable.

Competing interests

The authors declare no competing interests.

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Unlocking the Antiparasitic Potential of Apiaceae: The Promise of Geraniol for Scabies Treatment

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