the inhibitory properties of and mode of action of plant essential oils ...

diseases caused by parasites. Properties such as low density (around 0.94 g ml -1 ) and
ready diffusion across cell membranes (due to membrane lipid solubility) may
enhance the targeting of active constituents within the oil to intracellular parasites
(Boyom et al. 2003). The oils and their constituents have been well characterised by
high performance liquid chromatography (HPLC) and the suppliers of these oils have
good quality control in regard to consistent composition and purity, due to the
requirements of the food and aromatherapy industries. As the oils lack contaminants
and because they are ‘clean’, then it is easy to separate the compounds within the oils
on HPLC or gas liquid chromatography (GLC). A major advantage of using oils,
especially those which are commercially sourced, is that it is possible to get the same
batch from suppliers over long periods of time. It is now possible to make synthetic
oils, with many of the constituents being characterised and synthesised in the
laboratory.
Although the greatest amount of research concerning the effects of plants on parasite
infections has been carried using aqueous or alcoholic extractions some, work has
been undertaken using plant oils.
1:2 Antiparasitic effects of plant essential oils (PEOs)
Generalised parasiticidal actions
Of the plant essential oils available and currently used only one has been investigated
for actions against multiple prokaryotic or eukaryotic organisms. Garlic oil has a
multitude of effects including antibacterial, antifungal, antiviral and antiparasitic
(Ankri & Mirelman 1999). As a parasiticidal compound, it affects the growth of over
12 different parasites by cysteine proteinase inhibition and parasite proliferation by
the inhibition of phosphatidyl choline (PC) synthesis. The allicin constituent of garlic
(Figure 1.2) inhibits the growth of Entamoeba histolytica at concentrations of 30 µg
ml -1 in vitro (Ankri & Mirelman 1999; Mirelman, Monheit and Varon 1987) with
lower concentrations able to inhibit the virulence of trophozoites by 90% as
measured by the ability to destroy baby hamster kidney (BHK) cell monolayers in
vitro (Ankri et al. 1997). This and other garlic oil constituents inhibit the growth of
other protozoan parasites including: G. lamblia (syn. intestinalis and duodenalis),
4