Volatile Compounds as Upcoming Antigiardial Agents: In Vitro Action of Carvacrol, Thymol and p-Cymene on Giardia lamblia Trophozoites

<b>Background/Objectives:</b> Carvacrol and thymol are monoterpenes present in phenolic-rich essential oils extracted from aromatic plants that exhibit antimicrobial activity. This study evaluates the antiprotozoal effect of carvacrol, thymol and their precursor, p-Cymene, against <i>Giardia lamblia</i> and investigates their mechanism of action and cytotoxicity profile. <b>Methods:</b><i>G. lamblia</i> susceptibility, cell viability, swelling and adhesion abilities following application of carvacrol, thymol and p-Cymene were assessed. Ultrastructural changes were evaluated using electron microscopy. Cytotoxicity was determined in mammalian cell lines (murine macrophages RAW 264.7 and bovine aortic endothelial cells) exposed to the same IC₅₀ concentrations effective against <i>G. lamblia</i>. <b>Results:</b> Carvacrol and thymol led to significant inhibition of <i>G. lamblia</i> trophozoite proliferation (IC₅₀ ≅ 50 µg/mL). After 7 h of incubation, total cell number decreased by 30% (<i>p</i> < 0.01) with carvacrol and by 50% (<i>p</i> < 0.001) with thymol, accompanied by reduced motility and adhesion (<20% attached cells). At IC₅₀ concentrations, <i>G. lamblia</i> trophozoites exposed to carvacrol and thymol underwent considerable ultrastructural alterations (e.g., aberrant-shaped cells, mitochondrial swelling and autophagosomal structures). Reduced trophozoite motility and adhesion capacity were also observed. In mammalian cells, thymol showed no significant cytotoxicity, whereas carvacrol significantly reduced viability in both cell lines. In contrast, p-Cymene showed no antigiardial activity. <b>Conclusions:</b> Our data suggests that carvacrol and thymol disrupt <i>G. lamblia</i> trophozoite integrity, possibly through alterations in membrane permeability and osmoregulatory processes. In conclusion, these compounds reveal in vitro antigiardial activity, supporting their potential as antigiardial drugs.