Efficacy of essential oils from plants cultivated in the Amazonian Biome against gastrointestinal nematodes in sheep

The excessive use of anthelmintics to control nematodes has resulted in anthelminthic resistance. Essential oils (EOs) are a rich source of bioactive molecules that can be assessed for their ability to control resistant parasite populations. The aims of this study were to screen EOs from 10 plant species in vitro for anthelmintic activity against Haemonchus contortus, evaluate the cytotoxicity of those EOs in a human immortalized keratinocyte cell line (HaCaT), and test the most promising EO candidate in vivo in Santa Inês sheep. The efficacy was investigated in vitro using an egg hatch test (EHT) and a larval development test (LDT). EO cytotoxicity was evaluated with the sulforhodamine-B assay. In the in vivo experiment, 28 Santa Inês sheep naturally infected were distributed into groups: G1—Mentha arvensis (EO₅), 200 mg kg⁻¹; G2—menthol, 160 mg kg⁻¹; G3—negative control; and G4—positive control (monepantel). EO₅, from M. arvensis (86.7% menthol), had the lowest LC₅₀ and LC₉₀ values in the EHT (0.10, 0.27 mg mL⁻¹, respectively), good performance in the LDT (0.015, 0.072 mg mL⁻¹, respectively), and the lowest cytotoxicity (190.9 µg mL⁻¹) in HaCaT cells. In the in vivo test, a single dose of the EO₅ (200 mg kg⁻¹ BW) had an efficacy of approximately 50% on days 1, 14, and 21; however, values were not significantly from day 0. Conversely, pure menthol at a dose of 160 mg kg⁻¹ BW showed no in vivo efficacy. This can be attributed to key factors related to bioavailability and pharmacology of terpenes in the host organism, as well as to the fact that menthol is mainly excreted as glucuronides in urine. Thus, further studies should be conducted with formulation systems that deliver bioactives directly to the abomasum, focusing on terpenes, whose excretion route is mainly via faeces.