Most of the reports on the antiparasitic activity of halophyte species include an in vitro screening, followed by the determination of the chemical composition of raw extracts, and less frequently of purified fractions or pure compounds. Sixteen species belonging to 14 different families have been described with in vitro antiprotozoal activity, and those reports are summarized in Table 1.

Essential oils are described as a composite mixture of volatile molecules obtained from aromatic plants, mostly by hydrodistillation, and display highly relevant biological properties, including antiparasitic activities. Essential oils were the main target to evaluate the potential antiprotozoal properties of halophyte species, mostly against Leishmania and Trypanosoma parasites. For example, the essential oil of flowering aerial parts of Crithmum maritimum was highly effective towards T. brucei parasites (EC₅₀ = 5.0 µg/mL), which was linked to its mono-terpene hydrocarbon content, such as in limonene (EC₅₀ = 5.6 µM; Figure 1) and sabinene (EC₅₀ = 6.0 µM; Figure 1) ^[1][43]. However, they were less effective against L. infantum promastigotes (IC₅₀ = 122 and 205 µg/mL, respectively) ^[2][44]. The antiparasitic features of major compounds, including monoterpene hydrocarbons, sesquiterpene hydrocarbons, oxygen-containing sesquiterpenoids, and diterpenoids, are well described ^[2][44]. In turn, the essential oil of leaves and fruits of Pistacia lentiscus exerted high inhibitory effects against promastigotes of Leishmania major, L. tropica, and L. infantum, with IC₅₀ values varying between 8 and 26.2 µg/mL ^[3][45]. The major volatile components were myrcene and α-pinene in leaves, and α-pinene and limonene, in fruits, all reported with antileishmanial activities ^[4][5][46,47]. In another work, essential oils from leaves of P. lentiscus collected from two areas in Tunisia were tested against L. major intramacrophage and axenic amastigote forms ^[6][48], displaying moderate activities against intracellular amastigote (IC₅₀ = 12.5–35.6 µg/mL), and high activity against L. major axenic amastigote forms (IC₅₀ = 0.5 µg/mL) ^[6][48]. The main compounds were identified as pinene, β-myrcene, d-limonene, O-cymene, terpinen-4-ol, β-pinene, and α-phellandrene, which may disrupt parasite intracellular metabolic pathways ^[6][48].

Some essential oils from other species have been described with antiparasitic activities, but with higher IC₅₀ values, such as those from P. oleracea leaves and stems (IC₅₀ = 360 and 680 µg/mL) on L. major promastigotes ^[31][67], or from F. vulgare seeds against T. vaginalis (MLC = 1600 µg/mL) ^[18][54]. The essential oil of D. ambrosioides aerial organs have been investigated for its in vitro activity against L. amazonensis and L. donovani, being highly active towards their epimastigotes (IC₅₀ = 21.3 µg/mL) and trypomastigotes (IC₅₀ = 28.1 µg/mL), as well as towards T. cruzi amastigotes (IC₅₀ = 50.2 µg/mL) ^[7][49]. Terpinolene was the major active component ^[7][49]. In addition, ascaridole, identified as the main component of D. ambrosioides leaves’ essential oil, also exhibited in vitro activity against E. histolytica parasites, which are responsible for amebiasis, a parasitic disease considered a public health problem in developing countries ^[16][52].

Extraction with organic solvents has less expression than the extraction of essential oils, but even so, it also proves to be very effective in the extraction of compounds with antiprotozoal activity from salt-tolerant species. In this context, Oliveira and colleagues ^[24][60] performed an in vitro screening of T. cruzi trypomastigotes on 94 samples belonging to 31 halophytes species from Southern Portugal. From those, the dichloromethane extract of Juncus acutus roots was the most active (IC₅₀ < 20 µg/mL), which was further fractionated, affording one active fraction with an IC₅₀ of 4.1 µg/mL and selectivity index (SI) of 1.5. The active constituents were identified as phenanthrenes, dihydrophenanthrenes, and benzocoumarins ^[24][60]. C. maritimum flower decoction also presented anti-T. cruzi activity with an EC₅₀ value of 17.7 µg/mL and SI of 5.65, and a fraction rich in falcarindiol has an increased activity (EC₅₀ = 0.47 µg/mL) and selectivity (SI = 59.6) ^[17][53].

The same research group also screened 25 salt-tolerant plant species from southern Portugal for promastigotes and intracellular amastigotes of L. infantum, and the highest activity was obtained with the dichloromethane extract of S. rubra and I. crithmoides aerial organs ^[20][56]. The active extracts from I. crithmoides were rich in phenolic acids (gallic, syringic, salicylic caffeic, coumaric, and rosmarinic acids) and flavonoids (epicatechin, epigallocatechin gallate, catechin hydrate, quercetin, and apigenin), while catechin hydrate was detected in S. rubra ^[20][56].

In addition, hexane and methanol extracts of F. vulgare seeds were highly active against T. vaginalis (MLC = 360 µg/mL) ^[18][54], whereas aqueous extracts exhibit anti-Blastocystis activity with IC₅₀ values ranging between 223.8 and 174.9 µg/mL ^[19][55]. The predominant compounds in F. vulgare were hesperidin, ferulic, and chlorogenic acid ^[19][55]. Several authors reported the in vitro antimalarial properties of G. glabra, particularly of root and aerial part extracts ^[29][33][34][65,70,71]. Furthermore, Licochalcone A (Figure 1) was isolated from its root water extract, with activity against L. major amastigotes (0% infected cells at 5 and 10 µg/mL) and promastigotes (0.4% infected cells at 1:100). Moreover, major components of a methanol:chloroform fraction obtained from the leaves of the tea mangrove Pelliciera rhizophorae, showed high antiprotozoal activity against L. donovani (Oleanolic acid: IC₅₀ = 5.3 μM; Kaempferol: IC₅₀ = 22.9 μM; Quercetin: IC₅₀ = 3.4 μM), T. cruzi (α-amyrin: IC₅₀ = 19.0 μM), as well as P. falciparum (Betulinic acid: IC₅₀ = 18.0 μM) (Figure 1) ^[32][68].

Other authors reported the in vitro anti-Plasmodium efficacy of ethanolic extracts of Plantago major seeds (P. falciparum: IC₅₀ = 40.0 µg/mL) ^[29][65] and C. rotundus tuber root ethyl acetate extract (P. falciparum IC₅₀ = 5.1 µg/mL and 4 µg/mL for sensitive and resistant strains, respectively) ^[21][57]. Peganum harmala seeds and aerial parts extracts have been extensively studied for their antileishmanial properties against L. major, L. donovani, and L. tropica as sustained by different authors ^{[25][26][27][28]}[61,62,63,64].

Overall, some authors defined that extracts with IC₅₀ values below 15 µg/mL and selectivity above 3 can be considered promising for further development as drug leads ^[35][69]. Following this guideline, the fraction of flower decoction of C. maritimum can be considered the most promising sample with anti-T. cruzi activity by coupling both criteria (EC₅₀ = 0.47 µg/mL; SI = 59.6) ^[17][53].

Only a few authors evaluated the in vivo antiprotozoal potential of halophytes, including only three species and using mainly rodents as models. These reports are summarized in Table 2.

The most studied species was D. ambrosioides, investigated by five different authors. For instance, essential oil from its aerial parts was more effective against experimental cutaneous leishmaniasis by L. amazonensis in BALB/c mice than its pure main components, namely ascaridole, carvacol, and caryophyllene oxide ^[36][72]. In turn, hydroalcoholic extracts of its leaves displayed in vivo antimalarial properties against BALB/c mice infected with P. berghei intraperitoneally ^[15][51], and in vivo effects on C3H/HePas mice infected with L. amazonensis promastigotes ^[37][73]. Moreover, ascaridole was the main component in D. ambrosioides leaves’ essential oil and exhibited in vitro and in vivo activity against E. histolytica parasites ^[16][52]. Besides, other authors reported the anti-Plasmodium efficacy of ethanolic extracts of A. officinalis flowers (P. falciparum IC₅₀ = 62.7 µg/mL; P. berghei suppression of parasitemia in vivo [400 mg/kg] = 62.86 %), and of P. major seeds (P. falciparum: IC₅₀ = 40.0 µg/mL; P. berghei suppression of parasitemia in vivo [400 mg/kg] = 22.5%) ^[29][65].