Databases and Searching Criteria

To find the published literature, a systematic search was performed using different databases, including Google Scholar, PubMed, Scopus, and ScienceDirect. Research articles published in English language from 1980 to 2020 were gathered for this systematic review. Key words such as: anthelmintic activity, nematicidal activity of plants, medicinal plants used for H. contortus, in vitro/in vivo efficacy of plants against H. contortus, active compounds in plants, mechanism of plant extract inhibition and toxicity of plants. “Anthelmintic AND Hemonchus contortus”, “Natural nematicidal OR anti-haemonchiasis NOT synthetic”, “Natural in vitro OR in vivo anthelmintic”. Bibliographies of research articles were also searched and relevant references were extracted and downloaded. Moreover, to support the findings of the review further literature search was conducted and relevant articles were included.

Inclusion/Exclusion Criteria

Research articles describing (a) in vitro/in vivo efficacy of medicinal plants against H. contortus, (b) containing full information regarding plant name, country name, extract, concentration, inhibition, time exposure, and assay type, (c) original research articles, (d) and published in English language were included in this systematic review. While articles with (a) epidemiological and molecular dataset of H. contortus, (b) antiparasitic activities other than H. contortus, (c) synthetic drugs/chemicals tested against H. contortus, and (d) language other than English were excluded.

Data Extraction

Endnote (Thomson Reuters, San Francisco, CA, United States) was used to compile the articles. Researchers very carefully extracted all the data from the selected articles including author (s) name, country name, plant name, family name, plant part used, plants’ life form, extract used, concentration, time exposure, inhibition, and year of publication. Data were arranged into tables and figures. Chemical structures of the compounds were drawn using MarvinSketch (18.24.0) (https://chemaxon.com/products/marvinn) and Inkscape (0.92) (https://inkscape.org/) was used to further refine and improve the resolution of each chemical structure. PubChem (https://pubchem.ncbi.nlm.nih.gov) was also used to attain the IUPAC name (s) of pure compounds reported in this review.

Taxonomic Clarification

Plant scientific names, synonyms, and families were searched and corrected using “the plant list” (http://www.theplantlist.org), “tropicos” (http://www.tropicos.org), “world flora online” (http://www.worldfloraonline.org), and “Medicinal Plant Name Services-KEW” (https://mpns.science.kew.org/mpns-portal).

Taxonomic Clarification

According to the modern botanical nomenclature, 53 reported plant species have a synonym issue. Plant accepted names are mentioned in table (Table 1; Supplementary Tables S2, S3), while the synonym mentioned in the original articles were put into brackets. In addition, taxonomic corrections regarding the author of those plants and their family names were also revised.

Pharmacological Validation of Medicinal Plants Against H. contortus

Total of 187 plant species belonging to 59 families and 145 genera were tested against different life stages of H. contortus (Supplementary Tables S2, S3). Major contributed families with their species were Asteraceae (n = 29), Fabaceae (n = 19), Lamiaceae (n = 12), and Euphorbiaceae (n = 6). Different life forms of plants reported were herbs, trees, and shrubs 40, 31.4, and 27% in accordance of their order. Leaves were the most frequently used part (50%) followed by seeds (11.3%), roots (8%), and whole plants (7%). Among other plant parts stems, flowers, barks, shoots, fruits, bulbs, peel, fibers, pulp, and latex were included (Supplementary Table S2).

Different solvents including n-hexane, aqueous, ethanolic, hydro-alcoholic, methanolic, and others were used in extracts preparation (Figure 2). Among all, the methanol extract was the predominant one. Bioassays reported in this review included, egg hatching test (EHT), larval development test (LDT), larval motility test (LMT), adult worm motility test (AWMT), adult parasite mortality test (APMT), and larval artificial exsheathment assay (LAEA) in in vitro studies, while fecal egg count reduction (FECR), egg count per gram of feces (EPG), total warm count reduction (TWC/WCR) in in vivo studies. Furthermore, among the above mentioned assays, EHT was the most widely used assay for evaluation of medicinal plants against H. contortus in vitro, while FECR was common in vivo. Sheep were the commonly used experimental model in vivo (Supplementary Table S3).

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FIGURE 2

Selection and processing of plant material(s) for anthelmintic evaluation.

Out of the total reported plant species 171 plant species were evaluated in vitro and only 40 plant species in vivo against H. contortus. It is evident that in vitro studies were almost five times greater than in vivo studies. Mostly, in vitro studies were carried out in Brazil (n = 29 studies), then in Pakistan (n = 10 studies), and India (n = 9 studies) among others. Similarly, in vivo studies were also mostly reported from Brazil and India, 10 and 4 studies, respectively, (Figure 3). Most of the pharmacological studies were reported in the year 2019 (n = 15) and 2006, 2011, 2012 (n = 9) in each (Figure 4).

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FIGURE 3

Country-wise comparison of in vitro and in vivo studies.

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FIGURE 4

Year-wise comparison of in vitro and in vivo studies.

Comparative Analysis and Toxicity of Common Plant Species

Plant species, which were common in both in vitro and in vivo studies, were compared to discriminate their efficacy against H. contortus. A total of 24 plant species were found to be commonly used both in vitro and in vivo (Table 2).

After comparative analysis based on minimum concentration and maximum efficacy, the identified plant species with promising anthelmintic activity in vitro against H. contortus were Lachesiodendron viridiflorum (Kunth) P.G.Ribeiro, L.P.Queiroz and Luckow (syn. Piptadenia viridiflora (Kunth) Benth.), Cymbopogon schoenanthus (L.) Spreng., Allium sativum L., Lippia origanoides Kunth (syn. Lippia sidoides Chem.), Artemisia absinthium L., Cymbopogon citratus (DC.) Stapf, Eucalyptus staigeriana F. Muell. ex F.M. Bailey, Artemisia herba-alba Asso, and Corymbia citriodora (Hook.) K.D. Hill and L.A.S. Johnson in order of their appearance. While A. herba-alba, C. procera, C. citriodora, Lysiloma latisiliqum (L.) Benth., and Euphorbia helioscopia L. were reported with high efficacy in vivo. Among the plants, L. viridiflorum was found highly effective both in vitro and in vivo with no observed toxic effects. C. citriodora was moderately toxic in vivo, but with promising nematicidal activity in vitro and in vivo. Additionally, C. procera and A. herba-alba despite of their high anti-haemonchiasis activity were found to be highly toxic at the tested concentrations (Supplementary Table S4). However, five species namely; Annona squamosa L., Artemisia maritima L., Artemisia capillaris Thunb., Castela tortuosa Liebm., L. latisiliquum, and Seriphidium brevifolium (Wall. ex DC.) Ling and Y.R. Ling were not evaluated for their toxicological effects. Mostly non-toxic and low-toxic extracts were orally administered. The LC₅₀ value of most plant species was missing and was not calculated.

From the results, it is evident that the effects of different plant extracts were dose and time dependent. Moreover, the comparative analysis also revealed that plant species were more effective in vitro than in vivo against various stages of the parasite.

Phyto-Compounds With Anti-Haemonchiasis Activity

In this review, 19 compounds were reported to be assessed for in vitro activity against different life stages of H. contortus. While only 3 compounds were found to be evaluated for in vivo activity using gerbil (Meriones unguiculatus) as an animal model. Based on minimum concentration and maximum nematicidal activity, Cinnamaldehyde, anethole, and carvone were highly active and completely inhibited egg hatching of the parasite at the tested concentrations of 0.085, 0.085, and 0.366 mg/ml, respectively. Carvacrol inhibited the larval development at a minimum concentration of 1 mg/ml, though the effectiveness of thymol and anethole against larvae development was also significant, but relatively at high concentrations i.e., 10 and 20 mg/ml, correspondingly.

Carvacrol and carvacryl acetate at 2 mg/ml showed 100% larval motility inhibition as compared to other compounds. Similarly, carvacryl acetate (0.2 mg/ml) and citronellal (2 mg/ml) totally reduced the motility of adult parasites in vitro. However, citronellal was moderately toxic in mice. Anethole and carvone significantly reduced fecal egg count, decreased male length, and reproductive capacity of female at 50 mg/kg concentration in vivo (Table 3).

TABLE 3

Plant compounds efficacy against H. contortus.

Compound name	IUPAC name	Chemical structure	Plant name/family	Extract	Concentration (mg/ml)	Assays	Inhibition (%)	References
1,8-Cineole	1,3,3-Trimethyl-2-oxabicyclo [2.2.2] octan		Artemisia lancea Vaniot/Asteraceae	Essential oil	0.63	EHT	3.4	Zhu et al. (2013b)
					1.25		10.0
					2.5		26.6
					5.0		56.6
					10.0		74.8
					0.63	LDT	10.6
					1.25		23.4
					2.5		33.6
					5.0		49.2
					10.0		65.2
					0.63	LMT	5.5
					1.25		10.4
					2.5		30.2
					5.0		48.6
					10.0		60.3
			Produced synthetically	Essential oil	1,787	EHT	99	Katiki et al. (2017b)
Anethole	1-Methoxy-4-(prop-1-en-1-yl)benzene		Croton zehntneri Pax and K.Hoffm./Euphorbiaceae	Essential oil	0.31	EHT	6.7	Camurça-Vasconcelos et al. (2007)
					0.62		26.6
					1.25		99.9
					1.25	LDT	35.8
					2.5		52.1
					5.0		87.7
					10.0		96.7
					20.0		100
			Supplied by GRASP Ind. E com. (Curitibia-PR, Brazil)	Encapsulated oil	50	FEC*	Fecal egg count was significantly reduced, decreased male length, and reproductive capacity of female after 45 days in santa ines lambs	Katiki et al. (2019)
					20		The dose did not affect acquisition of parasites after pasture access and as FEC raised and body weight decreased of morada nova lambs
			Produced synthetically	Essential oil	0.085	EHT	99	Katiki et al. (2017b)
Borneol	1,7,7-Trimethylbicyclo [2.2.1]heptan-2-ol		Zanthoxylum bungeanum Maxim. (=Zanthoxylum simulans Hance)/Rutaceae	Essential oil	1.25	EHT	48.6	Qi et al. (2015)
					2.5		62.1
					5.0		80.0
					10.0		92.2
					20.0		98.8
					40.0		100
					1.25	LDT	38.2
					2.5		52.2
					5.0		80.2
					10.0		91.2
					20.0		98.0
					40.0		100
					1.25	LMT	52.8
					2.5		37.2
					5.0		23.2
					10.0		9.6
					20.0		2.6
					40.0		1.8
Camphor	1,7,7-trimethylbicyclo [2.2.1]heptan-2-one		Artemisia lancea Vaniot/Asteraceae	Essential oil	0.63	EHT	-	Zhu et al. (2013b)
					1.25		-
					2.5		2.8
					5.0		9.0
					10.0		12.8
					0.63	LDT	4.4
					1.25		15.8
					2.5		23.4
					5.0		37.2
					10.0		57.0
					0.63	LMT	5.7
					1.25		13.2
					2.5		17.4
					5.0		13.0
					10.0		18.1
Carvacrol	2-Methyl-5-(propan-2-yl)phenol		Arisaema franchetianum Engl., A. lobatum Engl./Araceae	Essential oil	0.32	EHT	52.6	Zhu et al. (2013a)
					0.63		65.6
					1.25		83.4
					2.5		93.2
					5.0		100
					10.0		100
					0.32	LDT	38.4
					0.63		54.2
					1.25		77.2
					2.5		89.2
					5.0		98.0
					10.0		100
			Produced synthetically	Essential oil	5.517	EHT	99	Katiki et al. (2017b)
				Essential oil	1	EHT	97.7	Andre et al. (2016)
					2	LMT	100
					0.2	AWMT	58.3
Carvacryl acetate*	Phenol, 2-methyl-5-(1-Methylethyl)-, Acetate		NA	NA	250	EGP	The compound reduced 57.7% of eggs per Gram of gastrointestinal parasites including H. contortus	André et al. (2020)
				Essential oil	8	EHT	89.3	Andre et al. (2016)
					2	LMT	100
					0.2	AWMT	100
Eugenol	4-Allyl-2-methoxyphenol		Ocimum gratissimum L./Lamiaceae	Essential oil	0.625	EHT	58.49	Pessoa et al. (2002)
					1.25		76.02
					2.5		94.56
					5		100
					10		100
			Produced synthetically	Essential oil	51.65	EHT	99	Katiki et al. (2017b)
Linalool	3,7-Dimethyl-1,6-octadien-3-ol		Arisaema franchetianum Engl., A. lobatum Engl./Araceae	Essential oil	0.32	EHT	2.0	Zhu et al. (2013a)
					0.63		6.4
					1.25		12.6
					2.5		29.6
					5.0		46.6
					10.0		65.8
					0.32	LDT	1.8
					0.63		4.4
					1.25		12.0
					2.5		25.2
					5.0		37.6
					10.0		48.2
			Produced synthetically	Essential oil	17.47	EHT	99	Katiki et al. (2017b)
Thymol	5-Methyl-2-(propan-2-yl)phenol		Lippia origanoides Kunth (=Lippia sidoides Cham.)/Verbenaceae	Essential oil	0.31	EHT	9.9	Camurça-Vasconcelos et al. (2007)
					0.62		93.6
					1.25		98.2
					1.25	LDT	33.0
					2.5		54.8
					5.0		73.9
					10.0		99.2
					20.0		99.7
			Produced synthetically	Essential oil	5.0	EHT	99	Katiki et al. (2017b)
Citronellal	3,7-Dimethyloct-6-enal		Corymbia citriodora (Hook.) K.D. Hill and L.A.S. Johnson (=Eucalyptus citriodora Hook.)/Myrtaceae	Essential oil	0.75	AWMT	37.50	Araújo-Filho et al. (2019)
					1		54.16
					1.25		70.83
					1.5		83.33
					1.75		95.83
					2		100
Lectin	9-Benzyl-3-methylidene-1,5-bis-(4-methylphenyl)sulfonyl-1,5,9-triazacyclododecane		Parkia platycephala Benth/Leguminosae	Protein	1.2	LDT	-	Silva et al. (2019)
					LET	-
					0.31	LDT	50
Goniothalamin	(2R)-2-[(E)-2-phenylethenyl]-2,3-dihydropyran-6-one		Cryptocarya massoy (Oken) Kosterm. (=Cryptocarya novoguineensis Teschner)/Lauraceae	NA	200–300 μM	LDT	IC50	Herath et al. (2019)
					6.25 μM	LMT	IC50
Dihydrokavain	(2 S)-4-methoxy-2-(2-phenylethyl)-2,3-dihydropyran-6-one		Piper methysticum G. Forst./Piperaceae	NA	207 μM	LDT	IC50	Herath et al. (2019)
						LMT	-
Desmethoxyyangonin	4-Methoxy-6-[(E)-2-phenylethenyl]pyran-2-one		P. methysticum G. Forst./Piperaceae	NA	31.7 μM	LDT	IC50	Herath et al. (2019)
						LMT	-
Yangonin	4-Methoxy-6-[(E)-2-(4-methoxyphenyl) ethenyl] pyran-2-one		P. methysticum G. Forst./Piperaceae	NA	23.7 μM	LDT	IC50	Herath et al. (2019)
						LMT	-
Carvone*	2-Methyl-5-prop-1-en-2-ylcyclohex-2-en-1-one		Supplied by GRASP Ind. E com. (Curitibia-PR, Brazil)	Encapsulated oil	50	FEC	Fecal egg count was significantly reduced, decreased male and reproductive capacity of female after 45 days in santa ines lambs	Katiki et al. (2019)
					20		The dose did not affect acquisition of parasites after pasture access and as FEC raised and body weight decreased of morada nova lambs
			Produced synthetically	Essential oil	0.366	EHT	99	Katiki et al. (2017b)
Cinnamaldehyde	(E)-3-phenylprop-2-enal		Produced synthetically	Essential oil	0.085	EHT	99	Katiki et al. (2017b)
Vanillin	4-Hydroxy-3-methoxybenzaldehyde		Produced synthetically	Essential oil	815.16	EHT	99	Katiki et al. (2017b)
Limonene	1-Methyl-4-prop-1-en-2-ylcyclohexene		Obtained from citrus peel	Essential oil	207.56	EHT	50	Katiki et al. (2017b)

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AWMT, Adult Worm Motility Test; LET, Larval Exsheathment Test; LMT, Larval Motility Test.

Pharmacological Action

Pharmacological activity of an extract/compounds/drug depends on how the candidate interacts with enzymes, proteins, nucleic acids, biomolecules, and different types of receptors (Roy, 2011). Pharmacological activity is an important phenomenon to know the precise target of the drug/extract/compound with anthelmintic efficacy against the parasite or other organism/pathogen under observation (Figure 6).

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FIGURE 6

Schematic representation of mechanism of action and different pharmacological targets of plant extracts/compounds (A) Essential oils alter the permeability and cause depolarization of cytoplasmic membrane by interacting and disrupting the chemical structures of lipids, polysaccharides, and phospholipids (B) Condensed tannins (CT) bind to the cuticle proteins thus, inducing chemical and physical damage (C) Artemisinin disrupts the mitochondrial membrane potential and releases cytochrome c into the cytoplasm leading to inhibition of electron transfer and oxidative phosphorylation (D) Allium sativum inhibits Acetylcholinesterase (AChE) that hydrolyzes the neurotransmitter acetylcholine. Inhibition of ACheE leads to the accumulation of acetylcholine at the synaptic junction and disrupting the neuromuscular transmission which causes muscle paralysis.

A. sativum ethanolic extract inhibits the motility of the H. contortus through a destructive and inhibitive effect on the enzyme acetylcholinesterase (AChE). The enzyme rapidly hydrolyzes acetylcholine (neurotransmitter) and thus, limits and terminates the cholinergic synaptic transmission (Taylor, 1990; Lee, 1996). Inhibition of ACheE leads to the accumulation of acetylcholine, thereby interrupting the neuromuscular transmission causes paralysis of musculature. Due to muscular discoordination/paralysis food swallowing and movement through the digestive system is stopped. The parasites enter the state of starvation and energy deprivation and thus, unable to survive inside the host (Kaur and Sood, 1982; Opperman and Chang, 1992). The antiparasitic activity of A. sativum may also be attributed to the sulfur containing compounds (e.g., ajoene and allicin) which can possibly form disulphide bonds with free thiol groups, and thus, inhibit enzymes or other proteins, which are important for survival of the parasite (Krstin et al., 2018). Crude aqueous extract of Achillea millefolium L. has profound anthelmintic activity, this could be due to the presence of several key chemical constituents one of which is eugenol. It is reported that eugenol can cause alteration of cell shape, membrane blebs (Machado et al., 2011), restrict cell growth, swelling, and collapsing cell membrane and arrest cell division (Ueda-Nakamura et al., 2006). The compound artemisinin of A. absinthium can inhibit vital enzymes of metabolic cascade by forming covalent bonds and resulting in irreversible inhibition of the enzyme (s) activities. The enzymes include S-adenosyl-methionine synthesase (SAMS), spermidine synthase (SpdSyn), L-lactate dehydrogenase (LDH), pyruvate kinase, and ornithine aminotransferase (OAT) (Wang et al., 2015). Artemisinin is also reported to disrupt mitochondrial membrane potential, cause cytochrome c release into the cytoplasm (Jia et al., 2016), and inhibit the electron transfer and oxidative phosphorylation of mitochondria, with final activation of caspase-3-mediated apoptosis (Li et al., 2005). Additionally, crude aqueous extract of A. absinthium can induce ultrastructural changes such as tegumental damage, nephridial canal epithelium lining and intrauterine eggs destruction, lipid accumulation, glycogen depletion, and finally worm paralysis and death (Beshay, 2018). Similar anthelmintic effects of tegumental disorganization/perforation and adult worm paralysis were also observed for C. procera aqueous and ethanolic extracts (Khalil et al., 2016). Bromelain extracted from Ananas comosus (L.) Merr. stem has been found as potent anthelmintic against gastrointestinal nematodes (Stepek et al., 2004; Domingues et al., 2013). Bromelain removes and digests the cuticle layer of nematodes resulting in immobility and death of the parasites (Stepek et al., 2004; Stepek et al., 2006).

Azadirachta indica A. Juss. leaves contain condensed tannins (CT) (Sakti et al., 2018), which facilitate diffusion of flavonoids by binding to the cuticle proteins (Kerboeuf et al., 2008). Flavonoids and CT inhibit secretion of key enzymes (e.g., esterase, tyrosin kinase, and nonspecific cholinesterase) that may cause fatal intracellular instability, neuromuscular disorganization, energy depletion, paralysis and death of parasites (Hoste et al., 2006; Kerboeuf et al., 2008).

Essential oils of Corriandrum sativum L., C. citratus, C. schoenanthus, E. staigeriana and L. origanoides were found highly effective against H. contortus in vitro. Essential oils may acquire this efficacy owing to a mixture of different chemical constituents whose interaction can result in compounds that inhibit or disorganize vital functions from the initial stages of development onward, interrupt with parasite metabolic activities, and interfere with drive mechanisms due to possible destructuring of the nervous system (Oka et al., 2000). Furthermore, essential oils can alter the permeability and cause depolarization of cytoplasmic membrane by interacting and disrupting the chemical structures of lipids, polysaccharides, and phospholipids (Bakkali et al., 2008). Phytochemical profile of Cocos nucifera L. revealed the presence of alkaloids, flavonoids, phenols, triterpenes, and condensed tannins among others (Lima et al., 2015). The compounds like flavonoids have antioxidant activities, while condensed tannins have shown antiparasitic activities by binding to proteins present in the cuticle, oral cavity, esophagus, and cloaca, thus inducing chemical and physical damage in the parasite (Costa et al., 2010). The efficacy of plant compounds may be attributed to the fact that they inhibit or retard the growth, maturation damage, suppress appetite or reduce procreative ability, which are all the causes of mortality. Moreover, the considerable activity of plants extracts may be due to the additive or synergistic relationship among different major components which can interact with multiple molecular targets in various developmental stages of the parasite to produce a pharmacological effect (Marie-Magdeleine et al., 2009).

H. contortus exposed to C. citriodora essential oils demonstrated ultrastructural changes, such as formations of vacuoles, disorganization of muscular layer, and changes in the mitochondrial profile. These changes suggest the loss of homeostasis and loss of motility due to muscular disorganization of the parasite (Araújo-Filho et al., 2019). E. helioscopia has monoterpens, which are lipophilic and can penetrate through cell membrane, induce expansion, increase permeability and disturb membrane structure and membrane embedded enzymes (Cox et al., 2000; Samy and Ignacimuthu, 2000; Cox et al., 2001). The nematicidal activity may also be attributed to the presence of tannins, which on the surface of nematodes can form complexes with proteins and result in alteration of metabolic pathways (Min et al., 2004; Ahmed, 2010) and enzymes (cysteine proteinases), which can damage the cuticle and kill the nematode parasites (Stepek et al., 2004; Stepek et al., 2006). Similarly, saponins, amino acids, and sterols can disturb proteins structure (Mabusela et al., 1990) therefore, affecting growth and reparation of nematode body. While compound muzigadial and ajoene has anti-feedant activity and inhibit proliferation of arterial smooth muscle cells and protein prenylation (Ferri et al., 2003; Mohanlall and Odhav, 2009). L. latisiliquum leaves forage utilization presented ultrastructural changes; for instance, disturbance of intestinal muscular cells/tissues and cytoplasmic vacuolization, suggesting that different secondary metabolites of leaves may provoke these changes. The alterations to the intestinal cells may be due to the ingestion of active compounds by the parasite, and the resulting direct contact between the bioactive compounds and the intestinal cells. The cytoplasmic vacuolization described can be interpreted as signs of disturbances in cellular functions, possibly due to imbalance of fluid exchanges between the intestinal and pseudocoelomic space and/or between the muscle and the pseudocoelomic space (Martínez-Ortiz-de-Montellano et al., 2019).

The high efficacy of Nicotiana tabacum L. against H. contortus could be attributed to the presence of nicotine, a ganglion stimulant (Bowman and Rand, 1980). Nematode muscles are known to contain excitatory neuromuscular junctions containing ganglion type nicotinic receptors with acetylcholine as their neurotransmitter (Neal, 2020). Any ganglion stimulant would tend to activate theses neuromuscular junctions causing a spastic paralysis in the worms leading to their death and expulsion from the host (Nouri et al., 2016). Among the reported plants with promising anti-haemonchiasis activity, L. viridiflorum leaves are rich in flavonoids and its anthelmintic action of egg hatching inhibition could be due to the effect on enzymatic activity and metabolic processes in helminthes (Kerboeuf et al., 2008).

Different protein contents e.g., proteases, ribosomal proteins, chitinases etc. isolated from Spigelia anthelmia L. were effective against different life stages of H. contortus (Araujo et al., 2017). Proteases can cause severe damage to the cuticle by disrupting cuticular proteins of the parasite (Liang et al., 2010), in larvae the proteases may hydrolyze/digest the proteins necessary for larval migration (Araujo et al., 2017), and also degrade the egg membrane during egg hatching (Mansfield et al., 1992). Chitinases were also identified which can degrade chitin present in the egg shell (Rogers and Brooks, 1977) and larva, inhibiting their development and leading to death (Rocha et al., 2015).

Comparative analysis also revealed that plant extracts/essential oils were more effective in vitro than in vivo against various stages of the parasite. Similar differences between in vitro and in vivo results with plant treatments have been previously reported (Peneluc et al., 2009; Nogueira et al., 2012) and might be related with bioavailability of plant chemical constituents in different parts of the ruminant gastrointestinal tract (Athanasiadou et al., 2007; Eguale et al., 2007b). Furthermore, adult nematodes may also be more resistant to the active components, or rumen microbiota may reduce the activity of metabolites (Nogueira et al., 2012), and other aspects, such as ruminal pH. Mostly, in gerbils the efficacy of plant extracts was reported to be comparatively low than the activity observed in sheep. Using rodents for evaluation of plants anthelmintic activity has some drawbacks, firstly the habitat for nematodes is quite different in rodents and small ruminants, hence, association between habitat and drug site absorption define the higher or lower drug activity (Hennessy, 1997). Secondly, different efficacy obtained in rodents and sheep can be described by mechanism of distribution and biotransformation of the drug in a monogastric and polygastric animal species. However, efficacy test on rodent nematodes can help researchers deduce the prescriptions to be used on sheep and goats (Camurça-Vasconcelos et al., 2007). The studies concerning isolation and purification of plant compounds responsible for antiparasitic acitivities are few and insufficient. Most of the studies reported the presence of key components of the plants which do not provide any information about the effective antiparasitic compounds and their mechanism of action. Therefore, pure comounds isolated from plants should be given focus for in vitro, in vivo evaluation, toxicology, and pharmacology studies in future research, this will provide base-line information for developing new ecofriendly and cost effective drugs with lesser side effects.