Essential oils for the treatment of dust mites

. House mites, including D. pteronyssinus and D. farina , are common causes of allergic diseases, such as asthma, atopic dermatitis, eczema, conjunctivitis and perennial rhinitis in susceptible patients. Conventional acaricides, such as benzyl benzoate, bifenthrin, chlorpyrifos are effective chemicals in treating mites. However, they are usually with high drug resistance and high adverse effects on environment, human health, and non-target organisms. Recently, essential oils from different plants were found to be safe and effective alternatives for killing different types of mites. The present review summarized the essential oils from different plants. The crude essential oils from Melissa officinalis, S.lavandulifolia oil, cade oil, P. cattleianum, Patchouli oil, Horseradish oil, C. cassia, P. cablin, Manuka oil, Clove Bud oil, and the pure essential oils, such as eugenol, caryophyllene, and camphor might be good candidates for the dust mites. Further studies on biological mechanisms of the acaricidal effects of these active essential oils, and the structure-activity relationships are needed to make the functions of these drugs more clear.


Introduction
Mites are tiny arthropods belonging to the class of arachnids and the subclass of acari. They are spread across a wide range of habitats. At least 45,000 different species of mites have been described. Some of these mites damage food, cause skin irritation, itchiness and severe bacterial infections.
House mites, including D. pteronyssinus and D. farina, are common causes of allergic diseases, such as asthma, atopic dermatitis, eczema, conjunctivitis and perennial rhinitis in susceptible patients [1,2]. D. pteronyssinus and D. farina are both pyroglyph mites, made up of 60-90% domestic mites.
Conventional acaricides, such as benzyl benzoate, bifenthrin, chlorpyrifos, pirimipho-methyl and pyrethroids, are effective chemicals in treating mites [3,4]. However, they are usually with high drug resistance and high adverse effects on environment, human health, and non-target organisms [4,5]. Recently, essential oils from different plants were found to be safe and effective alternatives for killing different types of mites (Table 1).

Crude essential oils for dust mites
Pineapple balm, patchouli, horseradish and clove are rich in essential oil. The essential oil from Melissa officinalis, a plant cultivated in France, Ireland, and Serbia, showed high activity on killing both D. farina and D. pteronyssinus. The LD 50 for D. farina and D. pteronyssinus was 3.91 and 3.53 μg/cm 2 , respectively, representing approximately four to five folds of DEET, a negative control. The patchouli oil extracted from Pogostemon cablin killed the D. farina with the LD 50 of 6.11µg/cm 2 , which was 9.58 times of dibutyl phthalate (DBP) (58.52 μ g/cm 2 ) [6]. Horseradish oil extracted from Armouracia rusticana consisted of different types of isothiocyanates. In a contact plus fumigant mortality bioassay, the horseradish oil (LD 50 : 1.54 μg/cm 2 ) was more potent compared to DBP (24.49 μg/cm2) on killing D. farina [7]. The Clove bud oil killed D. farina and D. pteronyssinus with the LD 50 of 5.91 μg/cm 2 and 7.86 μg/cm 2 , respectively, in the contact toxicity bioassay[8]. In addition, Melissa officinalis oil changed the colour of the mites from colorless to golden brown, indicating the acaricide potential of the oil [9].
The essential oils of P. cattleianum and Leptospermum scoparium also demonstrated strong acaricide activity. The acaricidal activity of P. cattleianum oil was significant when examined by fumigant or contact toxicity bioassays. The Traditional Chinese Medicine (TCM) was considered to be the source of environmentally acceptable and alternative commercial acaricides. The acaricidal activity of petroleum ether, ethyl acetate, and methanol extracts from 22 kinds of TCM was screened. The petroleum ether extract of C. cassia, E. caryophyllata, and P. cablin were with highest activities (4.6, 5.0 and 6.0 μg/cm 2 respectively, 24 h, LD 50 ) than the extracts of ethyl acetate and methanol [20].

Essential oil compounds for dust mites
Besides the crude essential oils from different plants, many other active essential oil components or compounds were demonstrated to be effective on dust mites ( Table 2).

Caryophyllenes
α-caryophyllene, β-caryophyllene, and caryophyllene oxide were found in the components of many essential oils, such as Psidium cattleianum, Melissa officinalis, Basil oil, Clove oil, and Cade oil. Previous studies indicated the neuroprotective, anti-inflammation, and anti-oxidant effects of these essential oils. However, the acaricidal activity of caryophyllene and its derivatives were demonstrated only recently.
Other studies also discovered good anti-mites effect of β-caryophyllene with activities of more than 2 folds of DEET or benzyl benzoate for D. farina, and more than 1.8 folds for D. pteronyssinus [9,12]. However, some studies didn't find the acaricidal effects for βcaryophyllene or caryophyllene oxide [15,17,21]. This might due to the difference of method used in the evaluation. For example, when the fabric-circle contact bioassay was used, no effects were found for caryophyllene [15,21].

Linalool (+/-)
Linalool is a widely used terpene found in many plant species, such as Salvia Lavandulifolia, and basils. Linalool is used as an important ingredient in shampoos, soaps, and detergents. Studies also demonstrated its antiinflammation, sedation, anti-anxiety effects. Linalool was reported to be an effective killer of dust mites. The acaricidal activities of linalool on D. farina and D. pteronyssinus were better than benzyl benzoate (1.83 folds and 2.04 folds, respectively) [12]. But less than DEET or DBP [17,21].

Camphor and camphene
Camphor and camphene are important aromatic ingredient of the wood and bark of the camphor tree. They are used as mild topical analgesic and insect repellent.
Terpeneol is a monoterpene alcohol. It is one of the active components in tea tree essential oil, Salvia lavandulifolia oil, hiba oil, basil oil, and cinnamon oil. The LD 50 value of terpineol against D. farina was between 4.33-22.05 μg/cm 2 , which was more active than DEET (28.1-37.67 μg/cm 2 ). Similar effects were found for D. pteronyssinus.

Conclusion
Dust mites induced diseases have led to a lot of sufferings for humans. Many drugs have been used for the treatment of the skin infections and allergies diseases caused by the dust mites. However, the problem of resistance, environmental issues linked to chemical treatment urged us to seek for new approaches. Recently, essential oils from different plants were found to be safe and effective alternatives for killing different types of mites. The crude essential oils from Melissa officinalis, S.lavandulifolia oil, cade oil, P. cattleianum, Patchouli oil, Horseradish oil, C. cassia, P. cablin, Manuka oil, Clove Bud oil, and the pure essential oils, such as eugenol, caryophyllene, and camphor might be good candidates for the dust mites. Further studies on biological mechanisms of the acaricidal effects of these active essential oils, and the structure-activity relationships are needed to make the functions of these drugs more clear. The