Anthelmintic Agents from African Medicinal Plants: Review and Prospects

Soil-transmitted helminthiasis affects more than 1.5 billion people globally and largely remains a sanitary problem in Africa. These infections place a huge economic burden on poor countries and affect livestock production, causing substantial economic losses and poor animal health. The emergence of anthelmintic resistance, especially in livestock, and the potential for its widespread in humans create a need for the development of alternative therapies. Medicinal plants play a significant role in the management of parasitic diseases in humans and livestock, especially in Africa. This report reviews anthelmintic studies that have been conducted on medicinal plants growing in Africa and published within the past two decades. A search was made in various electronic databases, and only full articles in English were included in the review. Reports show that aqueous and hydroalcoholic extracts and polar fractions obtained from these crude extracts form the predominant (80%) form of the extracts studied. Medicinal plants, extracts, and compounds with different chemical groups have been studied for their anthelmintic potential. Polyphenols and terpenoids are the most reported groups. More than 64% of the studies employed in vitro assays against parasitic and nonparasitic nematode models. Egg hatch inhibition, larval migration inhibition, and paralysis are the common parameters assessed in vitro. About 72% of in vivo models involved small ruminants, 15% rodents, and 5% chicken. Egg and worm burden are the main factors assessed in vivo. There were no reports on interventions in humans cited within the period under consideration. Also, few reports have investigated the potential of combining plant extracts with common anthelmintic drugs. This review reveals the huge potential of African medicinal plants as sources of anthelmintic agents and the dire need for in-depth clinical studies of extracts, fractions, and compounds from African plants as anthelmintic agents in livestock, companion animals, and humans.


Introduction
Parasitic worms afect more than one-quarter of the world's population, with soil-transmitted helminthiases (STH) accounting for about 1.5 billion infections [1,2]. STH is one of the neglected tropical diseases (NTDs) that afects mainly people living in regions of high poverty, without adequate sanitation, and in close contact with infectious vectors, domestic animals, and livestock [2,3]. Tey occur globally in the tropics and subtropics, including the Americas, Asia, and sub-Saharan Africa. Tese areas are more impacted because of the low levels of development [2,4].
Helminth infection is largely a sanitary problem and is associated with the human-animal food chain. Parasite eggs present in human faeces contaminate the soil where they embryonate and are taken back into the intestinal tract through poorly treated drinking water and foods [5]. Tis creates a vicious cycle of recurrent infections that is often difcult to break or interrupt [3,6].
Although helminthiases have a low fatality rate, they have a huge impact on human health and livestock production. Te severity of symptoms in humans depends on the worm burden and whether monospecifc or mixed infections are involved [2,7]. Whilst children constitute the most vulnerable group to worm infestation, pregnant women also sufer impaired immunity and a lower quality of life [5,[8][9][10].
Based on the location of the adult parasite in the body, helminthiases may clinically present as intestinal (whipworms, intestinal roundworms, and hookworms), or tissue (trematodes, hydatid tapeworms, and tissue roundworms) parasites [5,11]. Te most common and widespread intestinal nematodes in humans include Ascaris lumbricoides, Ancylostoma duodenale, Necator americanus, Trichuris trichiura, and Strongyloides stercoralis, which have been classifed as soil-transmitted helminthiases [3,12]. Mild symptoms include abdominal pain, nausea, diarrhoea, and loss of appetite, and in children, severe cases may lead to anaemia, eosinophilia, stunted growth, malnutrition, pneumonia, and poor physical and cognitive development [3,13]. High-intensity infections could result in intestinal obstruction requiring surgery and death in cases of Strongyloides stercoralis [14].
Unlike for some viral and bacterial diseases, there are currently no vaccines developed for human intestinal parasites [5,15,16]. In livestock, however, the frst vaccine (Barbervax   ® ) against H. contortus, which is derived from an intestinal surface antigen of the nematode, has proven to be a sustainable control measure in small ruminants [17]. Control measures mainly include periodic deworming, health education, and improvements in environmental sanitation. Seasonal chemotherapy with synthetic anthelmintics remains the primary measure to eliminate or reduce infecting helminths. Health education helps to prevent reinfection, while improved sanitary conditions reduce egg transfer to soil [14].
Morbidity due to helminthiases has been greatly reduced by the annual or biannual mass drug administration (MDA) in vulnerable populations. Te two benzimidazole drugs, mebendazole and albendazole, are the core agents recommended by the World Health Organization (WHO) for MDA in children of school age. Both drugs are efective, cheap, easy to administer, and have been used in large populations for several years with minor side efects [14]. Other classes of anthelmintic drugs available include macrocyclic lactones, imidazothiazoles, tetrahydropyrimidines, and amino-acetonitrile derivatives. Other drugs, including levamisole, pyrantel pamoate, niclosamide, ivermectin, and piperazine, have contributed immensely to tackling livestock and human parasites [18].
Te increase in cost, availability, continual reinfection, emergence of drug-resistant parasites, adverse events associated with population-wide drug use [9,17,19], and lack of coverage for other infectious agents like Strongyloides have become major drawbacks to the success of anthelmintic chemotherapy [20]. Tese threats have spurred the quest to discover and develop new, innovative, sustainable, efective, safe, alternative, and complementary treatment options, mostly from natural products [21][22][23][24].
In Africa, about 80% of the population largely depend on traditional remedies for their primary healthcare needs [25][26][27]. Compared to orthodox medicines, these remedies are relatively accessible and cheaper, perceived to be safe and efective, and form part of folkloric practices [22,24,28]. Plants form the larger part of these traditional remedies and have historically been used in treating internal parasites and other diseases in humans and livestock [15,29,30]. Tey constitute a viable source of chemically diverse molecules with broad-spectrum activity and can be a ready means to combat parasite resistance. From January 1981 to September 2019, 71 new approved drugs were entirely derived from natural products, 14 as natural botanicals, and 356 as semisynthetic derivatives of natural molecules [31]. However, there is currently no anthelmintic drug product approved that has been developed from plant sources [18].
Even though the chemical constituents and mechanisms by which medicinal plants elicit the observed activities are less known [28,32], technological advancement has reignited research using in vitro and in vivo assays to evaluate ethnopharmacological claims and, where possible, identify such chemical entities and their mechanisms [22,28].
Tis review is unique in the sense that it gathers information on anthelmintic extracts, fractions, and compounds from African medicinal plants. It seeks to reveal the potential of African medicinal plants as sources of new anthelmintic molecules and alternative therapies against helminthiases.
Whereas the African continent has a huge natural resource pool that is widely used by local people, especially indigenous people, for the management of many disease conditions, the continent remains one of the hardest hits by intestinal parasites [2]. Tere is increasing research into natural products, especially medicinal plants, as sources of new antiparasitic agents. Despite eforts to gather the library of these plant products, be they extracts, fractions, or purifed compounds [7,23,33], those available from African medicinal plants are scattered and limited to certain geographical regions. Tis review, therefore, sought to expand this pool of information and to create a clear picture of the situation as far as studies of anthelmintic agents from African medicinal plants are concerned. Here, we elaborate on the various studies that have been conducted on medicinal plants native to Africa and espoused on the very promising plant families and species. 2 Evidence-Based Complementary and Alternative Medicine

Anthelmintic Resistance in Humans and Livestock
Parasite susceptibility to the existing anthelmintic drugs continues to rapidly decline, leading to the emergence of drug-resistant parasites. Several studies have reported the development and spread of resistance to all major classes of anthelmintics [34][35][36], especially in livestock and, to a lesser extent, in companion animals and humans [9,37]. Te main contributing factors to drug resistance include selective pressure induced by high treatment frequencies, single-drug regimens, preventive mass treatments, inadequate dosing, indiscriminate use, and overreliance on synthetic drugs to control helminthiases [18,34,38].
High-frequency preventive chemotherapy in humans and livestock, as a result of the high disease burden and limited number of anthelmintics, causes a reduction in worm refugia-enhancing mutations and resistance development [5,21,39]. Prolonged use of single drugs, for example, the use of ivermectin for Onchocerciasis control in West Africa and praziquantel against Schistosomiasis in Egypt, has been associated with widespread resistance [34].
Te development and spread of drug-resistant traits at the molecular level have been well investigated in the model organism C. elegans [40] and the barber's pole worm (H. contortus) [41]. Mutations in genes coding for drug receptor sites or the expression of genes involved in drug efux, detoxifcation, or amphidial drug uptake have for instance been reported as possible causes of drug resistance [42,43]. Resistance to the benzimidazoles in trichostrongylid nematodes in ruminants has been ascribed to mutations in the isotype 1 β-tubulin gene (E198A, E198L, F167Y, and F200Y) [44][45][46].
Nematodes, generally upon hatching, undergo multiple larval developmental stages into adult worms [5], and this multistage cycle poses a challenge to drugs that target just a few stages. Broad-spectrum activity against egg hatching, larval metamorphosis, and adult worms is therefore an ideal requirement for anthelmintic agents [7].
Te use of plant extracts may signifcantly delay and reduce the spread of resistance among parasite populations [47,48]. Tese multicomponent systems with natural products from very diferent classes could interact with multiple developmental stages, help reduce natural selection pressures, and delay resistance development, which are typically found in such multitarget systems [49][50][51]. Selective treatment of individuals, multidrug therapy, and environmental parasite control strategies slow down the emergence of selective resistance alleles [38].
Pyrethrum, nicotine, and rotenone are some drug products of plant origin that have been used as antiparasitic agents in veterinary practice [60].
Te discovery and development of anthelmintic agents from natural sources and the isolation and characterization of bioactive constituents have therefore become the end goal of research in this less-funded area [7,60]. Whereas several eforts have been made towards the isolation and characterization of antiparasitic compounds from plant sources in the past two decades, little is seen in other organisms such as bacteria and fungi [7]. Te disadvantage of the isolation approach, however, is the unavoidable loss of so-called pharmacological synergy or toxicological antagonism associated with multicomponent extracts and fractions [7].
Issues of availability of bioactive minor compounds from the plant material in sufcient amounts, stability, formulation, delivery, compatibility, and many years of development have also kept some promising plant molecules out of the market. Because of their bulky nature, semisynthetic measures to modify the chemical structures and properties of plant molecules have proven difcult and expensive [60].
Generally, medicinal plants have not competed favourably with orthodox medicines as anthelmintics. Te expensive human clinical efcacy and safety trials and bureaucratic licensing procedures, accompanied by a limited drug market, disincentivize the pharmaceutical industry from plant-based anthelmintic product development. Te yield and nature of phytoconstituents are also variedly infuenced by environmental factors like climate, altitude, soil type, rainfall, and herbivore predation. Tis erratic and unpredictable outcome afects the establishment of consistent quality control measures and hence reduces the interest of pharmaceutical investors [7].
Proprietary issues of ownership, royalties, access, government charges, and patency for plant-based drugs further defate the hopes of drug-producing companies, which have high expectations for investment returns [60]. Te presence of "pan assay interference compounds," often referred to as "PAINS," is militating against the advancement of preliminary bioassays on plant extracts. PAINS is an unorganised group of promiscuous molecules that occur as unspecifc hits in several enzyme assays and in vitro screenings. Tese subversive compounds have often been considered for optimisation steps but end up consuming a lot of resources of investigators [61]. Te presence of these molecules in high-throughput screens, on the other hand, should be recognised as a cautious group requiring further assessment rather than an outright rejection as irrelevant [62].
Contrary to the popular notion, not all natural compounds are innocuous [7]. Often produced as defence mechanisms or in response to external stress, some natural products from plants are potentially toxic to humans and animals and can be deleterious to physiological functions [63,64]. An investigation by Ali et al. [65], for instance, revealed that even though the crude saponins from aerial parts of Achillea wilhelmsii and Teucrium stocksianum have signifcant anthelmintic activities, they were cytotoxic in the brine shrimp assay. Tis, therefore, implies that any anthelmintic hit from plant extracts must be verifed for its safety in mammalian cells [66] and, if possible, in living organisms.
With the emergence and rapid spread of multidrug resistance to existing synthetic anthelmintics, the prospects for drug development from natural sources remain high [67]. To attract investors, herbal anthelmintics must be developed to a stage where rigorous and reproducible quality control can be assured. Standardized extracts have a huge market potential due to the current drive for organic food supplements [60]. Multicomponent plant extracts may potentiate efcacy, counter toxicity, enhance bioavailability, or improve the stability of each other in formulations [68][69][70][71].
A more interesting addition to the herbal industry is the advancement in the genetic engineering of specifc metabolic pathways [72]. Genetic modifcation and tissue cultures can increase the yield of target molecules and improve the turnover rate [59]. Tis also counters the risk of plant depletion through wild harvesting and reduces the impact of an unfavourable climate on raw materials [72].

Brief Comparison of In Vitro and In Vivo Anthelmintic Assays
Preliminary screening of natural products for pharmacological activities requires the use of validated methods to guarantee reproducible outcomes [73]. Since helminth infections are complex and often involve mixed parasites with varied lifecycles, models for testing for antiparasitic activities also widely difer between species. Tese investigations are grouped into the in vitro and in vivo techniques [74]. Most primary investigations of plant materials for anthelmintic activity employ in vitro bioassays [22,[74][75][76], which rapidly screen large numbers of samples, are simple in design, easy to perform, cheap to implement, require minimal ethical considerations, require a small quantity of samples, and quickly churn out reproducible results [49,75,77,78]. Tese assays target various stages of the parasite lifecycle, including egg laying or hatching, larval development, migration, motility, motor paralysis, and lethality [5,79]. Te current gold standard for assessing the susceptibility of adult and larval worms to drugs is in vitro worm motility assays using read out by microscopy [53]. Some in vitro assays use nonparasitic worm models, whereas others involve the isolation of eggs or larvae from experimentally or naturally infected animal hosts and the growth of larvae in vitro, during which periods the test substances can be applied and activity evaluated [80].
Te results of basic in vitro assays of test samples ("hits") are confrmed by the higher test models, which are specialized in vitro and in vivo studies to defne possible "lead" status [78]. It is however often difcult to reproducibly extrapolate results from in vitro investigations to in vivo activity owing to pharmacokinetics issues [81]. Te growth and maintenance of parasitic nematodes for long periods outside the host is often a laborious, expensive, and slow process that hinders efective in vitro studies [82]. Te versatility, availability, ease of culture maintenance, and high proliferation rate make C. elegans, a free-living, nonparasitic nematode, a suitable model for many nematocidal and mechanistic studies [40,76,82].
In vivo assays involve the use of whole animals and are models that remain close to the patient, which is the fnal target for drug development [73]. Studies involve the in vivo investigation of anthelmintic potential using animals infected with the relevant parasites. Te outcomes of in vivo studies are infuenced by the mode of administration, nature, and dose of the test substance, host organism, and parasite species involved. Faecal worm, egg count, worm shedding, and host immune response are usually the parameters evaluated. Egg counts evaluate the efect of treatment on adult parasite fecundity, whereas parasite load depicts the efect on larvae or adult worms. Te density-dependent fecundity efect, however, limits the signifcance of the FECR assay [49,53] and cannot be used for Strongyloides spp., whose eggs are not passed in stool but larvae.
Although in vivo studies provide superior and reliable outcomes for pharmacological screening due to the natural, biological, pharmacological, pharmacokinetic, and toxicological environments, they are expensive, slow for large-scale investigations, labour-intensive, and often bedevilled with ethical and animal welfare issues [73,75].

General Overview of Anthelmintic Evaluation of African Medicinal Plants
From this review, it is evident that African medicinal plants have great potential as sources of anthelmintic agents. An ideal anthelmintic agent should have broad spectrum activity, afecting almost all stages of the lifecycle of the nematodes and sufcient safety. Whereas some studies that considered more than one parasite stage reported such broad spectrum activities, a few others reported disparities in efcacy against various forms of the parasites [83][84][85]. Te majority (78%) of the studies or reports reviewed employed in vitro assays in evaluating the anthelmintic activities of medicinal plants. In vitro models mainly focused on the ability of drug candidates to inhibit egg hatching, larval migration, motility, larval development or exsheathment, and survival [86][87][88][89][90][91][92][93][94].
H. contortus and related gastrointestinal nematodes (GIN) of small ruminants are the most widely investigated organisms in in vivo models, whereas sheep and goats are the major animals in which such clinical investigations have been reported [86,87,111,112]. Tere are few reports involving trials in pigs, chickens, goldfsh, snails, rats, and mice [113][114][115][116][117]. Te ability of test substances to reduce faecal egg count (FEC), a typical measure of efects on fecundity, and postmortem intestinal worm burden are the parameters measured in in vivo assays. A few other studies evaluated the physiological impact of test substances on haematological indices of host animals in addition to the antiparasitic investigations [95,111,116,[118][119][120]. Tere was no report cited that investigated the clinical efcacy of extracts or isolated compounds in human subjects, neither was any activity testing reported on commercially available herbal anthelmintic products from these medicinal plants. Tere is, therefore, is a need to clinically evaluate some of these plant products and establish quality parameters for their development into standardised remedies for helminthiases.
Te anthelmintic activities reported vary widely depending on the plant species, type of extract, strain of nematode, and its parasitic stage of development. A similar observation was reported in a review of anthelmintic agents used in goats [74]. Most in vivo studies, however, produced lower efcacies compared to their in vitro counterparts regarding the same plant samples [74]. Te efects of pharmacokinetic processes such as absorption and metabolism and the biological variations of host animals could be accountable for these observations [75]. Also, many of the studies report activity lower than that observed for the standard anthelmintic drugs often used as positive controls [121][122][123][124]. An in vitro study of Carica papaya extracts against the Indian earthworm P. posthuma, however, was reported to show better paralytic (p < 0.0001) and wormicidal (p < 0.0001) activity than albendazole [6].
Almost 80% of the studies evaluated aqueous or hydroalcoholic extracts evaluated aqueous or hydroalcoholic extracts of various plant materials including root barks, stem barks, fowers, seeds, and whole plants, oils, and latex or exudates (Table 1). A few organic extracts, fractions, and crude powdered plant materials (mostly as feed) have also been studied [65,84,112,169,172,197]. Tis trend is expected since many studies seek to replicate traditional applications of the study materials.
Te pharmacological potential of medicinal plants is attributed to their specifc natural product composition, which can be infuenced by various factors, including changes in environmental conditions [203]. To assess the pharmacological activities of individual constituents, they must frst be isolated and characterized. Te elucidated chemical structures provide grounds for quality control, structural modifcation, syntheses, elucidation of biosynthesis pathways, and quantitative structure-activity relations (QSAR) studies [7]. In this review, several bioactive anthelmintic compounds belonging to diferent biosynthetical classes have been isolated from medicinal plants native to Africa. Tough there have been eforts to gather these antiparasitic phytochemical libraries [7,23,33], there exists no such profle for African medicinal plants. Te majority of the studies reporting anthelmintic activities of medicinal plants focused on extracts or fractions with Evidence-Based Complementary and Alternative Medicine Ethanolic extract Leaves of Amaranthus spinosus L.
West Africa In vivo FEC assay against Ascaris suum, Hyostrongylus rubidus and Trichuris trichiura in pigs More than 80% reduction of FEC on day 7 posttreatment with 0.5 g/kg [125] (4)

Ethanolic extract
In vitro activity against GIN from goats 60% mortality at 100 mg/mL [126] (5) Aqueous and methanolic extracts Whole plant of Chenopodium album L.  Evidence-Based Complementary and Alternative Medicine 7 West Africa In vitro assay against egg hatching of C. elegans strains Both extracts, respectively, yield 53.8% and 69.1% egg hatch inhibition at 2 mg/ml [106] 8 Evidence-Based Complementary and Alternative Medicine In vitro activity against C. elegans (wild type) larvae Ethanolic extract of the bark was the most active with LC 50 of 380 μg/ml [144] Methanolic and chloromethylenic extracts Leaves, roots, and bark of Anogeissus leocarpus (DC.) Guill. & Perr.
In vitro activity against Rhabditis pseudoelongata All extracts possess anthelmintic action with EC 50 between 2.5 μg/ml and 10 μg/ml contortus [147] Evidence-Based Complementary and Alternative Medicine 9  [88] Evidence-Based Complementary and Alternative Medicine Aqueous extracts Leaves, stem, and root barks of Azadirachta indica A. Juss.
In vitro EHIA and larval inhibition against strongylid nematodes of small ruminants Dose-dependent activity with over 90% inhibition of survival and about 50% inhibition of egg hatch at 100 mg/ml [167] 14 Evidence-Based Complementary and Alternative Medicine  [149] li- Aqueous extract Stem bark of Nauclea latifolia Sm.
In vivo efcacy study against strongyle ovine nematodes FEC was signifcantly reduced (93.8%) in infected animals treated with 1600 mg/kg body weight for 5 consecutive days [178] Rutaceae

PAC fractions
In vitro action against C. elegans larvae All PAC fractions exhibited a minimum of 70% inhibition against the larvae [200]

Ethanolic extracts
In vitro activity against GIN from goats 80% mortality at 100 mg/ml [126] 22 Evidence-Based Complementary and Alternative Medicine mited bioactive constituents [8]. Like the extracts, most of their activities have only been evaluated at the in vitro stage, with not much clinical reporting in animals or humans. Some specifc compounds isolated from plants have been reported to exhibit anthelminthic activity (Table 2 and Figure 1). Phenolic compounds such as tannins and favonoids constitute a large class of natural molecules with potential anthelmintic or antiparasitic activities [74,79,244]. Because of their bulky structure and ability to bind several macromolecules, phenolic compounds have been reported to possess a broad range of biological activities. Oligomeric and polymeric proanthocyanidins, hydrolysable tannins, and favonoids, for example, have been more intensively studied than any other class of natural anthelmintic compounds [79,245]. A study conducted by Engström et al. [244] isolated and studied the in vitro activity of 33 hydrolysable tannins and gallic acid against egg hatching and larval motility of H. contortus. Tese compounds, isolated from various plants in Finland, showed varying anthelmintic activities [244]. Other studies isolated diferent types of proanthocyanidins from P. pinnata root bark and C. mucronatum leaves and reported varied activities against C. elegans and some animal intestinal parasites [150,200]. Recent reviews on polyphenolics with anthelmintic potential have been published by Spiegler et al. [79] and Mukherjee et al. [246]. Alkaloids, coumarins, triterpenes, terpenoids, lignoids, prenylated derivatives, isothiocyanates formed after fermentation from glucosinolates, and saponins have also been widely isolated and studied [74]. Several fatty acids and aromatic compounds have also been reported to possess anthelmintic activities [247,248]. Pineda-Alegría et al. [249] recently reported that long-chain fatty acids, including β-sitosterol, palmitic, pentadecanoic, stearic, and linoleic acids have nematocidal activity.
With the ever-increasing emergence of drug-resistant parasites and polyparasitism in animal and human helminthiases, there is a great need to explore the potential of developing some of the studied plants and their compounds into commercial drug products. Te African herbal drug market should, therefore, explore the possibility of developing polyherbal formulations especially for use in livestock and companion animals, and as chemopreventive food supplements for humans.

Anthelmintic Activities of African Medicinal Plants
Medicinal plants belonging to diferent genera and families have been reported to have anthelmintic activities. However, some families have been frequently reported than others.

Fabaceae.
Tis is the plant family with the highest reported number of plants with anthelmintic activities. Acacia nilotica (L.) Del. (Fabaceae), for instance, is a popular remedy for helminthiasis in Kenya [250]. Bachaya et al. [97] reported signifcant in vitro and in vivo activity of methanolic extracts of its fruits against the eggs (LC 50 � 512.86 μg/ mL) and larvae (LC 50 � 194.98 μg/mL) of H. contortus and related ovine gastrointestinal nematodes. It also induced 78.5% reduction in FEC on day 13 post 3.0 g/kg treatment in sheep [97]. Similar efects were reported of its hydroalcoholic extracts against the larvae of C. elegans (wild type strain) and Onchocerca ochengi microflariae with LC 50 of 350 ± 1.1 μg/mL and 10.8 ± 0.3 μg/mL, respectively [110]. Te aqueous extract of the root bark of Albizia anthelmintica Brongn, a plant traditionally used as an anthelmintic [251,252], revealed potent in vitro ovicidal (ED 50 of 144.2 μg/mL) and larvicidal (ED 50 of 65.2 μg/mL) activities against strongyle nematodes of sheep [86]. Gathuma et al. [157] also reported 89.8% in vivo efcacy of similar extracts of the plant in FECR assays against mixed GIN infections in sheep.

Combretaceae. One important species in the family
Combretaceae is Anogeissus leiocarpus (DC.) Guill. and Perr. (common name: Axlewood tree). It is widely used in African traditional practices and by livestock farmers for managing various parasitic disease conditions [89,143,145,178]. Aqueous extracts of A. leiocarpus leaves caused 39.5% reduction in faecal egg count and 33% reduction in faecal worm burden in sheep treated with 400 mg/kg extract [143]. It also exhibited in vitro ovicidal (ED 50 � 409.5 μg/mL) and larvicidal (100% excludability inhibition at 1.2 mg/mL) actions against H. contortus [89], whereas its acetone extract inhibited egg hatch (LC 50 � 360 μg/ml) and larval development (LC 50 � 509 μg/ml) [92]. Ndjonka et al. [144] reported that an ethanolic extract of A. leiocarpus bark was more active than an aqueous extract with LC 50 of 380 μg/ml, signifcantly retarding the development of larvae into adult worms [144]. Another study reported signifcant in vitro activity of methanol and DCM extracts of leaves, roots, and bark against larvae of Rhabditis pseudoelongata with EC 50 between 2.5 and 10 μg/ml [145].
Combretum mucronatum Schumach and Tonn, traditionally used for various ailments including helminthiases in Africa [149,253], has also been reported to exhibit anthelmintic activities against various test models. Ethanolic extract of C. mucronatum leaves exhibited in vitro nematocidal efects with 10 μg/mL minimum lethal concentration against T. muris and induced 85.3% reduction of worm burden in mice [148]. Hydroalcoholic extracts also inhibited C. elegans larvae with LC 50 of 1.67 mg/mL. A partition of this extract revealed that the ethyl acetate portion possessed stronger anthelmintic activity than the remaining aqueous fraction. Te respective activity can be related to the presence of oligomeric proanthocyanidins Evidence-Based Complementary and Alternative Medicine with diferent structures [150]. Subsequent ultrastructural studies showed that the tannin-rich extract caused visible effects on the cuticle without overt efects on the intestines/gut of the worms [151].
6.3. Cucurbitaceae. Some plants belonging to this taxonomic family have also been investigated for anthelmintic efects. Ethanolic extracts of Momordica charantia leaves collected from diferent ecological zones in Togo exhibited varying degrees of inhibition against C. elegans larvae with LC 50 values between 473 and 997 μg/ml [155]. Similar extracts of its fruits also caused 100% mortality of GIN larvae at 100 mg/mL in vitro and 78% FECR on day 9 posttreatment of goats with 100 mg/kg [126]. Aqueous and ethanolic extracts of seeds of Citrullus lanatus, Cucurbita pepo, and Telfairia occidentalis all exhibited signifcant mortality and paralysis in vitro against the earthworm Lumbricus terrestris at 50 mg/mL [154].
6.5. Meliaceae. Azadirachta indica A. Juss. (neem) is a widely known plant in African traditional medicine and contributes immensely to the management of livestock diseases and pests [254][255][256]. Almost every part of this plant has been reported to have anthelmintic activity. Polar extracts of neem seeds exhibited signifcant ovicidal and larvicidal action in vitro against H. contortus, with the ethyl acetate fraction causing 83% wormicidal efects at 50 mg/ml 1 h postexposure [162]. Aqueous and methanolic extracts of the seeds induced 29.3% and 40.2% reduction in EPG in sheep naturally infected with H. contortus and Trichostrongyus spp. on day 15 posttreatment with 3 g/kg [163]. Neem leaves also exhibited in vitro anthelmintic efects against the earthworm Pheretima posthuma, the tapeworm Raillietina spiralis, and the roundworm Ascaridia galli [164]. In vivo studies of feed in sheep reported signifcant inhibition of bovine nematodes, causing 98% reduction in FEC on day 14 posttreatment [165]. Te leaf extracts also inhibited microflariae of Setaria cervi in vitro [166] and caused a signifcant reduction in FEC and TWC against H. polygyrus in mice [84]. Sujon et al. [126] reported a 100% in vitro mortality at 100 mg/mL against GIN with 81% reduction in EPG on day 9 posttreatment of goats [126]. Leaves, stems, and root barks extracts inhibit strongyle nematodes, causing over 90% mortality of larvae at 100 mg/ mL [167].
Another medicinally relevant species from the family Meliaceae is Khaya senegalensis (Mahogany). Ethanolic extract of K. senegalensis bark induced in vitro LC 50 of 0.51 mg/mL and 88.82% FECR at 500 mg/kg in sheep against strongyle nematodes [168]. Similar extracts of the bark and leaves had LC 50 of 470 μg/mL and 1.0 mg/mL, respectively, against C. elegans larvae [144]. Methanol-dichloromethane extract of the whole plant of Lansium domesticum also inhibited adult C. elegans, signifcantly reducing their survival to 59% [107]. x paradisiaca exhibiting the highest activity against the three worms [173]. M. x paradisiaca also caused signifcant FECR when fed to lambs infected with H. contortus [172] and in vitro ovicidal efects (LC 50 � 2.13 μg/mL) against the same parasite [181]. Other preparations of various parts of Musa spp. demonstrated ovicidal activities against T. colubriformis in sheep [169], in vitro ovicidal and larvicidal efects against H. contortus, and reduction of FEC in sheep infected with H. contortus [170,171]. 6.7. Rubiaceae. Morinda lucida and Nauclea latifolia are two plants from this family for which anthelmintic potential has been widely reported. Methanol and DCM extracts of leaves and roots of M. lucida inhibited larvae of R. pseudoelongata with EC 50 of 2.5 μg/mL [145], whereas hydroethanolic extracts of the stem bark induced dose-dependent paralysis (18.17 ± 0.03 min) and death (24.34 ± 0.21 min) at 50 mg/mL against P. posthuma [176]. Ethanolic extract of M. lucida leaves demonstrated concentration-dependent ovicidal action against T. colubriformis [177]. Te aqueous and ethanolic extracts of N. latifolia leaves induced ovicidal activities (LC 50 of 0.704 and 0.650 mg/ml, respectively) against ovine GIN and reduced faecal egg count when administered to naturally parasitised sheep [95]. Onyeyili et al. [178] reported signifcant reduction (93.8%) in FEC when sheep, infected with nematodes were treated with 1600 mg/kg body weight of aqueous extract of N. latifolia stem bark for 5 consecutive days [178]. Ethanolic extract of Canthium mannii stem bark induced 90% inhibition of egg hatching against Ancylostoma caninum at 1 mg/mL after 48 h incubation [96].       . Although the only plant in this family whose anthelmintic activities have been reported, Carica papaya (pawpaw) is one African medicinal plant whose anthelmintic potential has been widely investigated. Investigations on various extracts and parts of this plant have all reported some level of anthelmintic activity. Latex exudate from unripe fruits of C. papaya signifcantly reduced FEC (77.7%) of Ascaridia galli and Cappilaria spp. in poultry after one week of treatment [186]. After one week of posttreatment with 100 mg/mL, aqueous extracts of papaya seeds caused 100% reduction in FEC of GIN in goats [187] and sheep [188]. Te seed extract also induced 100% reduction in FEC two weeks posttreatment in chicks [116] and goats [111]. In vitro studies reported that C. papaya seed extracts caused signifcant paralysis and death of adult P. posthuma [142] and inhibited egg hatch, larvae, and adult worms of T. colibriformis [177]. An in vitro comparative study of the leaves, stem bark, and seeds extracts of C. papaya reported that the seed extracts were the most active against adult P. posthuma [6]. Aqueous extract of papaya seeds was again reported to have more active LD 50 of 49.94 and 49.32 mg/ml against H. contortus egg hatch and larval development, respectively [94]. Te anthelmintic activity of pawpaw can be related to the isothiocyanates, which are formed from the genuine glucosinolates.

Garcinia kola Heckel (Clusiaceae).
Commonly referred to as "bitter kola," Garcinia kola is used to treat gastrointestinal helminthiases [190] and has been shown to possess this activity in pharmacological screenings. Hydroethanolic extract of G. kola seeds induced 76.5% irreversible paralysis of H. bakeri larvae at 50 mg/mL [91] whilst its aqueous extract exhibited 53.3% larvicidal efects against strongylid nematodes of goats at same concentration [189]. At 50 mg/mL, the stem bark extract induced dose-dependent paralysis and death of the adult P. posthuma at 39.29 ± 0.12 and 54.29 ± 0.01 min, respectively, for 50 mg/mL [176]. Both the seed and stem bark extracts were ovicidal (98.9% and 100%, respectively) at 100 mg/mL against strongylid nematodes [190]. 6.8.4. Paullinia pinnata L. (Sapindaceae). Paullinia pinnata is used in sub-Saharan Africa as an anthelmintic agent, especially for treating ancylostomiases [149,258]. Te anthelmintic properties of its root bark and leaves have been explored, revealing a huge potential as a source of nematocidal molecules, mainly oligomeric proanthocyanidins in the bark. Okpekon et al. [145] reported that extracts of both leaves and root bark of P. pinnata have in vitro inhibitory efects on R. pseudoelongata with EC 50 of 2.5 μg/ml each [145]. Te hydroethanolic extract of the root bark also reduced the survival of C. elegans larvae to 85.2% at 1 mg/mL [149]. Further in vitro investigations of this extract against some animal parasites and C. elegans revealed that the extract had  Table 2 against helminthiases.

Conclusion
Te World Health Organization's 2030 targets for STH can only be achieved with renewed investments in new and efective drugs. African medicinal plants will serve as a useful source of remedies for integrated parasite control, along with other measures such as education and the provision of sanitation facilities to at-risk populations. Te foregoing data validate the claims that African medicinal plants have huge potential for the discovery and development of new, innovative, and alternative anthelmintic agents. Te majority of reports and studies evaluated extracts of plants, with a few isolated compounds also characterized. Even though in vivo animal studies abound, 78% of the studies reported in vitro activities against parasitic nematodes. Tere are no reports available on clinical investigations of extracts or purifed compounds in humans, nor have any commercial products been reported or evaluated for their efectiveness. Terefore, clinical evaluation of these plant products and mechanistic studies, especially on isolated compounds, will advance the goal of identifying and developing drug candidates from plant sources. [256].

Data Availability
No data were used to support this study.

Conflicts of Interest
Te authors declare that they have no conficts of interest.