The different infections caused by protozoan parasites, such as malaria,
leishmaniasis, toxoplasmosis, balantidiasis, trichomoniasis, giardiasis, Chagas disease,
amoebic dysentery, are responsible for considerable morbidity and mortality worldwide
with desolating social and economic consequences. These protozoan diseases occur all
over the world. For the treatment of these diseases, there is a lack of effective, safe, and
affordable therapies. Due to the lack of vaccines in most instances and the development
of resistant strains to the available synthetic therapeutics, it is important to search for
alternative sources of anti-parasitic drugs. Since ancient times, natural products have
been used as sources of potential drugs to cure diseases. It has been reported that 80%
of drug molecules are natural products. The diversity of natural products can vary, i.e.,
it may be found in plants, fungi, algae and marine organisms. The plant-based natural
products (secondary metabolites), i.e., alkaloids, phenolics, terpenes, and lipids, are
potent anti-protozoal molecules. The natural products (secondary metabolites) obtained
from microbial origin showed promising anti-protozoal activity. These bio-active
molecules 2-(hept-1-enyl)-3-(hydroxymethyl)- 5-(3-methyl but-2-enyl)benzene--
,4-diol, flavoglaucin, tetrahydroauroglaucin, auroglaucin, 2-(20,3-epoxy-10-
30-heptadienyl)-6-hydroxy-5-(3-methyl-2-butenyl)benzaldehyde, obtained from the
fungus Eurotium repens, showed anti-malarial activities even chloroquine-sensitive and
chloroquine-resistant strains of Plasmodium falciparum. Some of the flavonoid
compounds, i.e., eupatilin, jaceosidin and nepetin, isolated from the plant Eupatorium
arnottianum, showed the highest activity against Chagas disease. The three most
important flavonoids, namely kaempferol, (–)-epicatechin and tiliroside showed
promising activity against Entamoeba histolytica. The isoquinoline alkaloid berberine
is found in several medicinal plants. Berberine salts have profound inhibitory activity
against Giardia trophozoites. Two flavonoids, i.e., luteolin and quercetin, isolated from
Vitex negunsdo and Fagopyrum esculentum, showed anti-leishmanial activity. An
aclerodane diterpene isolated from an ethanolic extract of Polyalthia longifolia
displayed anti-leishmanial activity against Leishmania donovani. A novel triterpene
Astrakurkurone isolated from the wild edible mushroom, Astraeus hygrometricus, has a
definitive effect on promastigote and amastigote form both in vitro and in vivo against
L. donovani. Natural products have displayed promising activity against different protozoan infections, but most of these studies on natural products have been
performed in vitro only. The transitions from in vitro study to in vivo trials and also the
clinical trials of the new compounds are urgently required to prove their efficacy and
safety.
Keywords: Alkaloids, Amoebic dysentery, Anti-malarial, Anti-amoebic, Antigiardial, Antileishmanial, Flavonoids, Giardiasis, Leishmaniasis, Malaria, Marine organisms, Microbial origin, Natural products, Phenolics, Plant origin, Secondary metabolites, Terpenes.
