Farmers in developing countries are faced with many diseases that limit the productivity of their animals, several of which are caused by tick infestations. To date, treatment of host animals with synthetic, chemical tick repellents and acaricides remains the method used to reduce the influence of the parasites on animal and human health. Awareness of the environmental health hazards posed by these acaricides, development of tick resistance leading to recurrent ectoparasitism, danger of misuse and presence of toxic residues in food, water and animal by-products has led to the search for safe and environmentally-friendly alternatives, one of which is the use of medicinal plants.
Because there appears to be a need and to contribute to research in this field, extensive literature surveys of published scientific articles were conducted. The following aspects were addressed: the role of ticks in animal health, problems encountered in using synthetic, chemical acaricides, medicinal plants with in vitro acaricidal or tick repellent activities against immature and adult stages of ticks and bioassays employed. Veterinary databases (All Databases, CAB Abstracts and Global Health, Medline, Pubmed, Web of Science, BIOSIS Citation Index, Science Direct, Current Content Connect and Google Scholar) were searched. The search words included “acaricidal”, “tick repellent”, “medicinal plants”, “isolated compounds” and “antitick assays”. Meta-analysis was conducted using the Fixed-effect model in an Excel programme to compare the results.
The tick climbing repellency and adult immersion tests were the most commonly used assays to test for repellency and acaricidal activity respectively. Ethanol was the most commonly used extractant and Rhipicephalus (Boophilus) microplus was the most commonly studied tick across all the reviewed papers. More than 200 plant species from several countries globally have tick repellent or acaricidal properties discovered using in vitro assays. A large proportion of the published work was done in tropical countries such as Brazil, India and South Africa where ticks cause major problems. The median efficiency values for acaricidal, larvicidal, egg hatching inhibition, inhibition of oviposition, repellency, acaricidal effects of the Lamiaceae and Asteraceae family using a total of 1428, 1924, 574, 281 and 68 events were 80.12 (CI95%: 79.20 - 81.04), 86.05 (CI95%: 85.13 - 86.97), 83.39 (CI95%: 82.47 - 84.31), 53.01 (CI95%: 52.08 - 53.93), 92.00 (CI95%: 91.08 - 92.93), 80.79 (CI95%: 79.87 - 81.71) and 48.34% (CI95%: 47.42 - 49.26) respectively. Extracts of some species including Azadirachta indica, Gynandropsis gynandra, Lavandula angustifolia, Pelargonium roseum and Cymbopogon species have good acaricidal and larvicidal activities with 90-100% efficacy, comparable to those of currently used acaricides, although, usually at higher dosages. Compounds with acaricidal activity such as azadirachtin, carvacrol, linalool, geraniol and citronellal were listed.
As a country, South Africa is rich in vascular plant flora, possessing over 10% of the world’s vascular floral species. Only a fraction of its plants have been rigorously studied and analyzed for their biological activity against ticks and seventeen plant species based on their ethnoveterinary use in tick control were selected for this study. The plants are Aloe rupestris Baker, Antizoma angustifolia (Burch.) Miers ex Harv., Calpurnia aurea subsp. aurea (Aiton) Benth., Cissus quadrangularis L., Clematis brachiata Thunb., Cleome gynandra L., Ficus sycomorus L., Gnidia deserticola Gilg., Hypoxis rigidula Baker var. rigidula, Maerua angolensis DC., Monsonia angustifolia E. Mey. ex A.Rich., Pelargonium luridum (Andrews) Sweet, Ptaeroxylon obliquum (Thunb.) Radlk, Schkuhria pinnata (Lam.) Kuntze ex Thell., Sclerocarya birrea (A.Rich.) Hochst., Senna italica subsp. arachoides (Burch.) Lock. and Tabernaemontana elegans Stapf.
Crude extracts of the above mentioned plants were prepared using four different solvents (acetone, ethanol, ethanol/water and hot water). The extracts at a concentration of 200 mg/ml were screened for their acaricidal efficacy against adult Rhipicephalus turanicus ticks using the contact assay. The plant species with the highest acaricidal efficacies for their acetone and ethanol extracts were C. aurea, S. pinnata and S. italica with mortality of 97, 93, 90% and 93, 93, 87% respectively. The ethanol/water and hot water extracts of many of the plants had low acaricidal activities (<60%). An acaricidal dose-response bioassay of two-fold graded decreasing concentrations (100 to 3 mg/ml) of the acetone and ethanol extracts of S. pinnata, C. aurea and S. italica was determined using the adult immersion tests. The LC50 values for the acetone extracts were 35.75, 111.24 and 42.05 mg/ml respectively and for the ethanol extracts were 37.07, 98.69 and 37.50 mg/ml respectively compared with the positive control (cypermethrin) with LC50 of 2.41 mg/ml. In order to evaluate the potential safety of these plants, cytotoxicity against Vero and HepG2 cells was determined. Most of the plant extracts were non-cytotoxic to the two cell lines (LC50>100 μg/ml) and there was a statistically significant higher toxicity to HepG2 cells compared with Vero cells. The ethanol/water and hot water extracts of most of the plants were less toxic to the cells (LC50>1000 μg/ml) compared with their acetone and ethanol extracts. The selectivity indices of S. pinnata, C. aurea and S. italica were low. Particularly good acaricidal activities were displayed by C. aurea subsp. aurea extracted using four different solvents on R. turanicus ticks. The plant extract also had lower cytotoxicity against the cell lines tested and was selected as the most promising plant species, based on its efficacy and potential safety for further studies.