In vitro anthelmintic potential of selected essential oils against gastrointestinal nematodes of sheep

Background Gastrointestinal nematodes (GINs) represent major obstacle to modern small ruminant farming due to their various negative effects on animal health, welfare and productivity. The exclusive use of commercial, synthetic anthelmintics is no longer a sustainable option due to the development of resistance in GINs and the problem of residues of these drugs in animal products and the environment. The aim of this study was to investigate in vitro the phytotherapeutic properties of seven different plant essential oils (EOs): Citrus x sinensis (L.) Osbeck, Citrus medica (L.), Mentha spicata (L.), Ocimum basilicum (L.), Hyssopus officinalis (L.), Salvia officinalis (L.) and Lavandula angustifolia Mill. against GINs in sheep. Methods First, the chemical composition (qualitative and semi-quantitative characterization) of the tested EOs was determined by gas chromatography–mass spectrometry (GC–MS) analysis. Their anthelmintic potential was evaluated in vitro using the egg hatch test, which was performed at eight different concentrations of each oil: 50, 12.5, 3.125, 0.781, 0.195, 0.049, 0.025 and 0.0125 mg/ml. Finally, a coproculture examination was performed to determine the genera of GIN against which the oils were tested. Results Some of the most frequently identified compounds in GC-MS analyses were limonene, myrcene, α- and β- pinene, γ-terpinene, sabinene, linalool, eucalyptol, terpinen-4-ol, camphor, etc., with their total number per oil varying from 8 ( C. sinensis ) to 42 ( O. basilicum ). The tested EOs showed a dose-dependent (R ² close to 1) ovicidal activity, with O. basilicum (23.3–93.3%, IC ₅₀  = 0.08 mg/ml), M. spicata (13.0-92.7%, IC ₅₀  = 0.07 mg/ml) and H. officinalis (42.7–91.3%, IC ₅₀  = 0.19 mg/ml) being the most effective. The high activity of these oils could be attributed to the high content of the monoterpene alcohol linalool and the terpenoid ketones carvone and pinocamphone. The least effective were S. officinalis (15.0–89.0%, IC ₅₀  = 0.53 mg/ml) and C. medica 12.3–95.0% (IC ₅₀  = 2.52 mg/ml). Four genera of sheep GINs were identified on coproculture examination, i.e. Haemonchus 43%, Trichostrongylus 35%, Teladorsagia 17% and Chabertia 5%. Conclusion The results obtained suggest that EOs tested, especially wild mint, basil and hyssop, could represent a valuable alternative in the control of GINs in sheep, which could help to reduce the use of chemical drugs. Future field trials should be performed to confirm these findings.