Gastrointestinal nematodes are an important problem in the small ruminant production and their control
nearly exclusively depends on commercial anthelmintics. However, parasite resistance to those chemical
formulations is an increasing global problem, so new plant-derived compounds are being studied for their
potential use against gastrointestinal nematodes. Citrus sinensis and Melaleuca quinquenervia essential oils
were evaluated against Haemonchus contortus Embrapa2010 resistant isolate, through the egg hatch test
(EHT) and larval development test (LDT) at concentrations ranging from 0.02 mg/mL to 50 mg/mL and
from 0.04 mg/mL to 3.12 mg/mL, respectively. All concentrations, positive controls (thiabendazole in EHT
or ivermectin in LDT), and negative controls (2% Tween 80 in EHT or 0.5% DMSO in LDT) were performed in
six replicates and in three independent experiments, for a total of 18 repetitions involving approximately
1800 parasites per treatment. Significant differences (P ≤ 0.01) of inhibition percentages were identified
by one-way analysis of variance followed by the Tukey test. Inhibitory concentration values (IC50 and
IC90) were analyzed by the Probit procedure of SAS. The essential oils evaluated showed that C. sinensis
presented limonene as major component (96.0%), while M. quinquenervia contained longifolene (32.95%)
and 1,8-cineole (25.43%) as major components. In the EHT, the IC50 and IC90 of the essential oils were
respectively 0.27 and 0.99 mg/mL for C. sinensis and, 1.52 and 5.63 mg/mL for M. quinquenervia. In the LDT,
the IC50 and IC90 were 0.97 and 2.32 mg/mL for C. sinensis and, 0.44 and 0.94 mg/mL for M. quinquenervia.
C. sinensis was five times more effective on eggs than M. quinquenervia. However, it was twice more
effective on larvae than C. sinensis, indicating that bioactive essential oils have different modes of action.
The results suggestthatthese compounds are goodcandidates fornematode control.However, alternative
anthelmintics have to be safe to the host, promote parasite control in vivo and not generate residues. So,
all these key points need to be elucidated before using any plant extract and/or components.