Botanical Products in the Treatment and Control of Schistosomiasis: Recent Studies and Distribution of Active Plant Resources According to Affected Regions

Schistosomiasis, a parasitic disease caused by trematodes of the genus Schistosoma, is the second most prevalent parasitic disease in the world. It affects around 200 million people. Clinical treatment, prophylaxis, and prevention are performed in countries susceptible to schistosomiasis. In the pharmacological treatment for an acute form of schistosomiasis, the use of antiparasitics, mainly praziquantel, is more common. As an alternative way, prevention methods such as reducing the population of intermediate hosts (mollusks) with molluscicides are important in the control of this disease by interrupting the biological cycle of this etiological parasite. Despite the importance of pharmacological agents and molluscicides, they have side effects and environmental toxicity. In addition, they can lead to the development of resistance enhancing of parasites, and lead to the search for new and effective drugs, including resources of vegetal origin, which in turn, are abundant in the affected countries. Thus, the purpose of this review is to summarize recent studies on botanical products with potential for the control of schistosomiasis, including anti-Schistosoma and molluscicide activities. In addition, species and plant derivatives according to their origin or geographical importance indicating a possible utility of local resources for countries most affected by the disease are presented.


Introduction
Schistosomiasis is also commonly known as bilharziasis or snail fever. It is a parasitic disease caused by trematodes of the genus Schistosoma [1], which in turn are transmitted to humans through their intermediate hosts such as planorbids belonging to genus Biomphalaria (S. mansoni), Oncomelania (S. japonicum), and Bulinus (S. haematobium) [2,3]. According to the World Health Organization (WHO), schistosomiasis is the second most prevalent parasitic disease in the world after malaria. It is associated with socioeconomic problems and water supply. It directly affects around 290 million people, with more than 700 million of them in risk areas. It is distributed in 78 countries, with major prevalence in South America, Sub-Saharan Africa, and Asia [4,5]. The worldwide burden associated with schistosomiasis has been projected to be at 2.6 million disability adjusted life years [6].
In the case of S. mansoni and S. japonicum infections, the acute form of this disease causes symptoms such as fever, myalgia, fatigue, malaise, dry cough, bloody mucus, diarrhea, diffuse abdominal pain, hepatosplenomegaly, eosinophilia, and the release of viable Schistosoma eggs in faeces, whereas dysuria, painful hematuria, urinary obstruction, vaginal discharge, or pain/bleeding after intercourse and the release of viable eggs in urine occurs in infections caused by S. haematobium. The chronic stage is caused by an egg deposition by individuals of the genus Schistosoma and reactions of the host's immune system. The fundamental characteristic of this form in S. mansoni and S. japonicum infections is the development of portal hypertension, leading to splenomegaly, hepatosplenic, and hepatointestinal forms. Varicose veins of the esophagus, spleen enlargement, hepatic cirrhosis, and urinary, intestinal, hepatic, and ectopic forms of the disease are other symptoms that may appear at this stage of the disease. In the infections caused by S. haematobium, the chronic stage is initiated by lodged eggs in the urogenital system, causing granulomatous host response and subsequent tissue inflammation [18].
Another form of the disease is neuroeschistosomiasis, a more frequent and disabling ectopic form compromising the central nervous system. Its diagnosis is based on epidemiological, clinical, and laboratory data. In some countries such as Brazil, this form of the disease has shown a considerable increase in the last two decades [19].
Clinical treatment, prophylaxis, and prevention are usually performed in countries susceptible to schistosomiasis. In the pharmacological treatment of acute schistosomiasis, the use of oxamniquine and praziquantel, often associated, is common. Prevention methods such as reducing the population of intermediate hosts (mollusks) are also important in the control of the disease since they can interrupt the biological cycle of the etiological parasite [20,21].

The Use of Drugs in the Treatment of Schistosomiasis
Pharmacotherapy is the most effective method for reducing the number of infection cases of schistosomiasis. In the last four decades, access to more effective drugs has reduced the prevalence and morbidity of this disease in various countries [4,27]. In the therapeutic treatment for an acute form of schistosomiasis, the use of prednisone, a corticosteroid, or its association with oxamniquine or praziquantel, is recommended. For the treatment of chronic disease without advanced lesions, praziquantel and oxamniquine are mostly indicated. In patients with involvement of the spinal cord (schistosomal myelopathy), the use of schistosomicides and steroids has been shown to be effective in most cases. At this rate, corticosteroids should be maintained for several months after clinical improvement and they should be withdrawn slowly. Such combination is also the therapy of choice for individuals with an advanced hepatosplenic form presenting portal or pulmonary hypertension who may develop hepatitis or pneumonitis due to the embolism of dead worms after treatment [28].
Mechanisms of action of the two main antiparasitics (praziquantel and oxamniquine) are very different. Praziquantel probably interferes with the muscular activity of the parasite, causing paralysis and preventing its binding with the host tissue. It also causes electrolyte imbalance and leads to destruction of the schistosome [29]. Oxamniquine acts by its anticholinergic effect. It can irreversibly inhibit enzymes that synthesize nucleic acids [30,31].
Despite the historical use of these schistosomicides, side effects such as metallic taste in the mouth, abdominal pain, diarrhea, asthenia, headache, dizziness, decreased therapeutic efficacy, and resistance have been reported [28]. Furthermore, the use of corticosteroids can culminate in the development of several side effects of this pharmacological class, such as Cushing's syndrome, metabolic dysregulation, and immunosuppression [32]. Other drugs used for this purpose also have many side effects with broad parasitic resistance and/or poor efficacy and some of them have been discontinued, such as metrifonate, an organophosphate with good efficacy against S. haematobium, but it caused abdominal pain, nausea, vomiting, diarrhea, headache, and vertigo [33]. On the other hand, artemisinine derivatives useful in the treatment of schistosomiasis and other parasitic diseases such as fasciolosis and triphostomy have been gradually neglected due to the increasing resistance in individuals co-infected with the malaria parasite [34], although these drugs can act synergistically with the heme group, generating free radicals that are toxic to the schistosomulae and are highly active against juvenile worms, whereas praziquantel is active against adult forms [35].

Biological Control of Intermediate Hosts as an Alternative Way
The control of the malacological population is an alternative way to prevent parasitic diseases that have mollusks in the life cycle of the parasite. The practice of combating natural breeding sites of intermediate hosts through the use of molluscicides has been one of alternative ways to decrease the incidence of some diseases such as schistosomiasis. However, most of these molluscicides have disadvantages such as damage to the ecosystem. Thus, there has been increasing interest in searching for new molluscicides from natural products that are less harmful to the environment [36].
Clinical treatment, prophylaxis, and prevention are performed in countries susceptible to schistosomiasis. In many cases, prevention methods such as reducing Schistosoma intermediate hosts (Biomphalaria and Oncomelania, for example) are important in controlling the disease, when combined with other strategies, such as the improvement of water sanitation, education, and access to clean water. However, vectors have developed resistance to chemical substances that are commonly used to inhibit the development and propagation of snails, leading to the search for new drugs and substances to be used in snail control [20].
Metallic salts such as copper sulphate were the first molluscicidal agents used. However, over time, they caused serious ecological imbalance since they limited the growth of algae that served as food for fish [3]. With the development of new molluscicides such as nicotinanilide, organotin, dibromo-nitrobenzene, sodium pentachlorophenate, tritylmorpholine, acetamide, and niclosamide, the environmental imbalance problem has become less drastic [37]. However, resistance in mollusks, residual toxicity, and low selectivity of these agents continue to be important reasons for the search and development of more selective, safe, and effective molluscicides [36].
Niclosamide is a drug of choice for controlling mollusks involved in the schistosomiasis cycle. It is also used as anthelmintic in humans [38]. This substance acts by inhibiting the anaerobic phosphorylation of adenosine diphosphate (ADP) by mollusk mitochondria, thus blocking the process of obtaining energy dependent on the fixation of CO 2 [39]. In addition to the problems mentioned above, niclosamide is costly and easily degraded under sunlight [40].
In recent years, the aim to control and/or prevent schistosomiasis has been listed as a top priority in the agenda of the government, pharmaceutical companies, and international agencies triggered by initiatives derived from the World Health Agency resolutions [41]. The WHO program targets deworming of at least 75% of school children. However, only 28% of this figure was met in 2015 [42] mainly due to the scarce access to praziquantel and prophylactic chemotherapy. In addition, being the sole drug marketed for schistosomiasis therapy, there are some evidence points that praziquantel might be inefficacious against distinct developing strains of the parasite [43][44][45]. Other instances have demonstrated that intensive use of praziquantel can result in declined cure rates, higher resistance, and ultimately treatment failure [46]. The rising cost of praziquantel has also contributed to less successful prophylactic programs since most prevention programs depend on the availability of charitable funding organizations. In addition, in many countries, the control of schistosomiasis is not integral of the national budget, meaning that resources are limited to overcome the morbidity and mortality associated with the disease [41,42,47]. Given the limitation in available resources and the fact that there is no available vaccine for this disease, there is a dire need to develop alternative medicines that are both cost-effective and treatment-effective for this disease [48,49]. In this advent, natural products present an interesting opportunity toward the development of novel pharmacological agents to triumph over this disease.

Recent Studies on Natural Resources of Affected Regions as an Alternative in the Treatment and Control of Schistosomiasis
Within the context mentioned above, the search for new and effective drugs with fewer side effects is necessary. Hence, the active agents of vegetal origin that are abundant in number and diversity in many affected countries could be a promising alternative. According to Ali (2011), several plant species have been used in different regions of the world for the treatment of parasitic diseases, including Allium sativum, Chenopodium ambrosoides, Curcubita pepo, Olea europaea, Mentha crispa, Citrus reticulata, and phytopharmaceuticals such as berberine and lapachol [33,58]. However, an increasing number of studies have demonstrated the importance of endemic plant species and their derivatives with a specific action against the genus Schistosoma or against one of their intermediate hosts. Such endemic plant species and its derivatives can serve as prophylactic and/or curative agents in countries where they are found. The objective of this review is to summarize recent studies (since 2010) on endemic species, especially species from the most affected countries. In the following sections, we describe the most promising plant derivatives according to their native region.

Brazil
Brazil is one of the most affected countries by schistosomiasis, presenting 12,009 new cases in 2016 and about 1.5 million people are at risk of contracting this disease. Schistosoma mansoni, the main etiologic agent of this disease in Brazil, can be transmitted to the population through three species of mollusks: Biomphalaria glabrata, B. straminea, and B. tenagophila [59]. This country has a vast area of equatorial, tropical, and subtropical climate, so that it is suitable for the development of different strains of snails, as well as the parasite's cycle. On the other hand, the country also has the greatest biodiversity on the planet, which potentially has a large number of plant resources to be researched in the combat against schistosomiasis [60].
In 2010, Parreira et al. [61] described the activity of essential oil (10, 50, and 100 µg/mL) from leaves of Baccharis dracunculifolia, a species used by folk medicine, against Schistosoma mansoni adult worms. After the treatment, worms showed a significant decrease in their motor activities. In addition, most pairs of coupled adult worms were separated into individual males and females. Furthermore, adult worms of S. mansoni showed integumentary alterations after treatment with the essential oil. Their study also demonstrated that the essential oil had no toxicity against VERO cells. Furthermore, it showed that nerolidol and spathulenol were major substances in the essential oil [61]. In the same year, Magalhães et al. [62]. showed that the action of fluoroglucinol derivatives isolated from species from genus Dryopteris is known to have a global distribution against S. mansoni. Aspidin at 25 to 100 µM, flavaspidic acid at 50 and 100 µM, methylene-bisaspidinol at 100 µM, and desaspidin at 25 to 100 µM were the most active derivatives, causing death of adult worms. These fluoroglucionols at 100 µM also inhibited egg development. Furthermore, the authors suggested that schistosomicidal effects of phloroglucinols derivatives might be related to the inhibition of oxidative phosphorylation pathway in S. mansoni [62].
Ageratum conyzoides L. (Asteraceae), an annual aromatic weed from Southeastern Brazil, can produce leaf essential oil rich in Precocene I and (E)-caryophyllene [63]. The synergic effect of main compounds against adult worms of S. mansoni has been reported. Plectranthus neochilus, another Brazilian southeastern plant, has also been evaluated for its activity against S. mansoni. The essential oil of its leaves at a concentration of 100 ppm caused 100% mortality in a period of 24 h. Furthermore, it caused separation of coupled pairs, decrease of motor activity, and tegumental alterations. The main substances of its essential oil were β-caryophyllene, α-thujene, and α-pinene [63]. In the same year, another group of researchers also demonstrated the antiparasitic activity of cramoll-1,4-lectine isolated from seeds of Cratylia mollis, an endemic species from the Brazilian northeast [64]. Treatment with this substance at 50 mg/kg for 40 days or at 7 mg/kg for seven days reduced egg excretion (79% or 80%), adult worm recovery (71% or 79%), and liver granulomas (40% or 73.5%) caused by S. mansoni in infected mice [64].
In vitro activity of piplartine, an amide isolated from inflorescences of Piper tuberculatum found in Brazil, has been reported [65]. Piplartine at a concentration of 15.81 µM reduced the motor activity of worms of S. mansoni and caused their death within 24 h. In addition, this substance induced morphological changes on the tegument of adult worms. A quantitative analysis revealed an extensive tegumental destruction, represented by the number of damaged tubercles. In addition, this amide was not toxic to the VERO cells when it was used at concentrations up to three times higher than the one showing schistosomicidal effects (31.51 µM) [65]. Another study demonstrated that imidazole alkaloid epiisopiloturine had an anti-S. mansoni activity. This substance was found in leaves of Pilocarpus microphyllus, a native plant species of Amazonia and Brazilian savannah. This alkaloid has been shown to be active against parasites of different developmental stages, including adults, schistosomulas, and eggs. Epiisopiloturine at a concentration of 300 µg/mL caused the death of all schistosomula within 120 h. Extensive tegumental alterations and death were observed when adult schistosomes were exposed to 150 µg/mL of epiisopiloturine. At the highest sub-lethal dose (100 µg/mL), a 100% reduction in egg laying of paired adult worms was observed. Furthermore, it exhibited no cytotoxicity to mammalian cells [66].
Oliveira et al. [67] have investigated the activity of Baccharis trimera against S. mansoni. This species is commonly used as a phytotherapic agent in Brazilian traditional medicine due to its several pharmacologic activities. They found a significant decline in the motility of worms with a mortality rate of 100% at 30 h after exposition to the leaf essential oil at a concentration of 130 µg/mL. Male worms were more susceptible, producing a dose-response effect within a shorter exposition period than female worms. The essential oil of B. trimera also induced a peeling on the tegument surface as well as destruction of tubercles and spines, resulting in smooth areas on the body surface. The essential oil also caused tegument destruction in female worms. In addition, it caused destruction of oral and acetabular suckers. Moreover, the cytocidal effect was only observed at the highest concentration (250 µg/mL), indicating a low cytotoxicity of this essential oil [67].
In 2014, Brazilian researchers evaluated the molluscicidal activity of Schinopsis brasiliensis against Biomphalaria glabrata. Extracts in chloroform and ethyl acetate from the stem bark caused mortality of B. glabrata, with IC 90 values of 68 and 73 µg/mL, respectively [59]. In 2018, Faria et al. investigated effects of plants on the control of B. glabrata. Manilkara subsericea, an endemic plant to the Brazilian sandbanks of Rio de Janeiro State, has a wide range of biological activities. At a concentration of 250 ppm, the M. subsericea leaf crude extract and ethyl acetate fraction induced 80 ± 4.13% and 86.66 ± 4.59% mortalities of adult snails after 96 h of exposure. LD 50 values were 118.7 ± 1.62 and 23.41 ± 1.15 ppm, respectively. Substances isolated from M. subsericea were also found to be active in controlling B. glabrata. Treatment with quercetin, myricetin, and ursolic acid at a concentration of 100 ppm for 96 h induced mortalities of 100%, 80%, and 53.33%, respectively [69]. One year later, another study reported the activity of a Brazilian sandbank species against B. glabrata [70]. Nanoemulsified essential oil from leaves of Xylopia ochrantha, an Annonaceae plant, showed activity against three species of Biomphalaria: B. glabrata, B. tenagophila, and B. straminea. The similar action was observed on mollusks of different ages and its oviposition. Treatment with this essential oil at a concentration of 78 ppm for 24 h caused 100% mortality of all adult species [70]. Table 2 shows recent studies with Brazilian plants with antischistosoma or molluscicide activity.

Africa
In Africa, schistosomiasis is endemic in rural and coastal regions. Its geographical conditions are suitable for infestation of this parasite due to multiple creeks, creeklets, lakes, ponds, rivers, and almost stagnant water sources [73]. Being a neglected tropical disease, schistosomiasis strikes mostly poor, marginalized communities where people are in contact with natural water sources, posing an enormous societal, healthcare, and economic burden, mostly in Sub-Saharan Africa (SSA) [74,75]. Furthermore, this disease has a profound adverse effect on maternity, the development of children, and agricultural output. Schistosomiasis remains one of the prime factors contributing to poverty among 500 million SSA residents. It has been estimated that around 120 million individuals were present with schistosomiasis-related symptoms in Sub-Saharan Africa. This constitutes around 85% of the Sub-Saharan Africa population which represents 13% of the world's populace [76]. In addition, 20 million individuals from the same region undergo an incredible torment due to chronic manifestations of this disease which ranks the second right after hookworm infection in SSA. Among African countries with the highest prevalence of schistosomiasis, Nigeria has the highest number (29 million) of cases, followed by the United Republic of Tanzania, Ghana, and the democratic republic of Congo with 19 and 15 million cases each [17,34]. Nonetheless, it is believed that most cases of the disease are not reported. The true incidence of this disease might be 400-600 million cases worldwide [34].  [77][78][79][80][81].

Plants Traditionally Used against Schistosomiasis
Among the plant species traditionally employed against Schistosoma mansoni, only a few have been subjected to scientific screening (Table 3). Principally, in vitro assays have been performed on these plant species and the most potent result was retrieved for the root extract of Abrus precatorius L. subsp. africanus Verdc. which nonetheless deserved to be evaluated further in vivo and in randomized clinical trials.

Recent Scientific Studies on Molluscicide Activity
In 2010, Adetunji and Salawu [84] reported that Terminalia catappa and Carica papaya possessed molluscicide activity against Bulinus globosus and Biomphalaria pfeifferi, respectively, the main intermediate hosts of S. haematobium and S. mansoni. Despite the fact that both vegetal species are distributed in several tropical regions of the world, they also have good adaptation in Nigeria and affect mollusks and parasites of great importance in this country. In that study, an ethanolic extract of T. catappa leaf showed LC 50 values of 864.1 and 2716.3 ppm against B. pfeifferi and B. globosus, respectively. However, an ethanolic extract of C. papaya leaf showed LC 50 values of 1095.7 and 619.1 ppm against B. pfeifferi and B. globosus, respectively [84]. One year later, Nigerian scientists described activities of several extracts of leaves and fruits of Blighia unijugata, an endemic plant in tropical Africa, against B. glabrata. LC 50 values of these extracts were found to be 7.60 and 13.00 µg/mL. Among various extracts, the ethyl acetate extract of its fruit was the most active one [57]. Part of these authors also evaluated the activity of Zanha goluogensis against the same mollusk. It was found that the ethanolic extract of its stem showed an LC 50 value of 60 ppm [82].
In 2013, Angaye [85] investigated the activity of different extracts of Jatropha curcas against mollusks Bulinus globosus, Bulinus rholfsi, and Biomphalaria pfeifferi. It was found that the methanolic extract of its leaves was active against all species, showing LC 50 values of 0.3 and 30 ppm against Bulinus and B. pfeifferi, respectively, while the crude ethanolic extract showed less activity (LC 50 > 500 ppm) [85,86]. Previously, other authors have also reported activities of root and seed methanolic extracts against different species of Bulinus [87,88]. Two years later, Angaye et al. [89] reported activities of two typical mangrove species, Avicennia germinans and Rizhophora mangle, from Niger Delta. For the first species, the methanolic leaf extract was active against Biomphalaria pfeifferi, Bulinus globosus, and Bulinus rholfsi, showing LC 50 values of 175, 89.21 and 123.74 ppm, respectively, whereas the methanolic leaf extract of R. mangle showed LC 50 values of 87.50 and 108.22 ppm for the two first mollusks and B. rholfsi, respectively [89]. Aqueous and ethanolic extracts of Vernonia amygdalina have been found to be toxic to adult Biomphalaria pfeifferi, showing an LC 50 value of 338.8 ppm and an LC 90 value of 614.8 ppm [90]. Interestingly, previous studies with this species have reported its antischistosomal activity [91][92][93] and its use in etnomedicine as a folkloric treatment against haematuria resulting from Schistosoma haematobium infection and stomach complaints due to infestation by Schistosoma mansoni [51]. Studies of African plant species and their molluscicidal activity are summarized in Table 4.  [98]. Moreover, in Egypt, another study on asparagaline A, a triterpenoid isolated from roots of an African plant Asparagus stipularis Forss., demonstrated its activity against S. mansoni [99]. The administration of asparagalin A resulted in a retardation of worm growth and locomotion on the first day. It also showed a significant activity of egg-laying suppression at a 200 µg/mL concentration [99]. In this same year, Ramalhete et al. [100] reported that triterpenes isolated from the methanol extract of aerial parts of Momordica balsamina L. had a potent in vitro schistosomicidal potential (100% within 24 h). Balsaminol F and karavilagenin C showed LC 50 values of 14.7 ± 1.5 and 28.9 ± 1.8 µM, respectively, after 24 h of incubation. Both compounds at 10-50 µM induced significant reductions in the motor activity of worms and significantly decreased egg production. Furthermore, at 10-100 µM, they were able to separate adult worm pairs into males and females after 24 h [100]. In 2015, a study investigated the effect of an aqueous extract (1.25-40 mg/mL) of aerial parts of Sida pilosa Retz. (Malvaceae) and derived n-hexane, dichloromethane, ethyl acetate, and n-butanol fractions (0.25-8 mg/mL) against Schistosoma mansoni. Among these extracts, the n-butanol fraction was the most active one, showing an LC 50 value of 1.25 mg/mL [45]. Two years later, Tekwu et al. [101] demonstrated that the stem bark and root extracts of Rauwolfia vomitoria at concentrations of 250-1000 µg/mL were active against Schistosoma mansoni worms after 120 h of incubation. In addition, the cytotoxicity (MTT) assay conducted on HepG2 and Chang liver cells demonstrated that these extracts were safe. They inhibited the proliferation of these cell lines with an IC 50 > 20 µg/mL [101].

Prevalence of Schistosomiasis in Asia
In Asia, the major foci of human schistosomiasis infection caused by parasite Schistosoma japonicum are in China, the Philippines, and small pockets of Indonesia. To a lesser extent, along the Mekong river on borders of Cambodia and Laos People's Democratic Republic, the infection is caused by S. mekongi [102,103]. S. japonicum and S. mekongi are traditionally considered as zoonotic [104]. Specific intermediate snail hosts for S. japonicum and S. mekongi are Oncomelania hupensis and Neotricula aperta, respectively [102,105]. Approximately 600 million people are estimated to be at risk of infection in China. Approximately 0.3 million people are currently infected. In the Philippines, 6.7 million people live in endemic areas. Of these, 1.8 million people are considered to be directly exposed to infection through water contact activities [106,107]. Approximately 140,000 people are estimated to be at risk for S. mekongi infection (80,000 people in Cambodia and 60,000 in Lao PDR) [102,108].
In China, schistosomiasis japonica remains a major public health concern. It is listed as one of the top priorities in communicable disease control defined by the central government [109,110]. The major endemic foci are concentrated on marshland and lake regions of Southern China which cover a vast area of five provinces (Jiangsu, Anhui, Hubei, Jiangxi, and Hunan). Cases within the area account for 86% of the total number of people infected in China [103,111]. It should be noted that the schistosomiasis transmission period in China lasts only for five months annually over two distinct transmission periods, whereas in the Philippines it is throughout the year [103,112]. For S. mekongi, its transmission period is March to April, coinciding with the dry season when the water level is low and host snail populations reach their maximum [102]. In Japan, eradication of the disease was achieved through transmission control by environmental management (i.e., land reclamation to enhance agricultural production and cementing ditches used for rice irrigation) and social economic development [103,113]. China has used extensive, long-term, repeated praziquantel chemotherapy to control the morbidity and reduce the prevalence and intensity of the S. japonicum infection in the country over three decades. Although S. japonicum has not shown resistance to praziquantel yet [114], the emergence of drug resistance has been experimentally induced in the laboratory, proving that S. japonicum may develop resistance to praziquantel under drug selection pressure [115,116]. As the potential development of praziquantel resistance may pose a great threat to the elimination of S. japonica in China and other Asian countries, there is an urgent need to find antischistosomal agents, especially from natural sources that are generally safer.

Plants Traditionally Used against Schistosomiasis in Asian Countries
Ginkgo biloba is a Chinese-specific rare relict species with relatively high economic and medicinal values. Its sarcotesta is usually discarded. However, it contains high levels of ginkgolic acids. Ginkgolic acids are long-chain phenolic compounds that are derivatives of sumac acid. Reports have shown that ginkgolic acids possess biological activities, including anti-tumor, neuroprotective, anxiolytic, and antibacterial activities [117][118][119][120]. Considering such biological virtues of G. biloba, Li et al. [121] have evaluated the petroleum ether fraction of the ethanolic extract of fallen leaf of G. biloba (PFGB) against O. hupensis. This extract showed a high toxicity (100% mortality) after treatment at a concentration of 100 mg/L for 72 h. Out of five fractions of the ethanolic extract of G. biloba tested for molluscicidal activity, the petroleum ether fraction (PFGB) exerted the most pronounced activity in a time-and dosedependent manner. Experimental results demonstrated that the LC 50 value of PFGB was decreased from 72.38 mg/L at 24 h to 9.22 mg/L at 72 h. Upon assessment, it was found that the glycogen and total protein contents of the snail's tissues decreased at a rate parallel to molluscicidal activity. Therefore, abnormal energy metabolism might be a factor contributing to its molluscicidal activity [121].
Buddleja lindleyana is a medicinal plant distributed in Eastern China, including Jiangsu, Anhui, Jiangxi, and Hubei provinces. This plant has been traditionally used for treating various ailments such as rheumatism, cough, and blood stasis [122]. Chemical investigations of B. lindleyana have revealed compounds such as phenethyl alcohol glycosides, phenylpropanoid phenolic glycosides, sesquiterpenes, diterpenes, triterpenes, flavonoids, and other constituents. One study has revealed that the N-butanol fraction of B. lindleyana (NFBL) is potent against the snail O. hupensis (LC 50 = 39.1 ppm, LC 90 = 59.3 ppm) [120]. Han et al. [123] have further reported that active components of B. lindleyana can induce remarkable decreases of activities of five key enzymes: Succinate dehydrogenase (SDH), lactate dehydrogenase (LDH), cytochrome oxidase (CC0), cholinesterase (CHE), and nitric oxide synthase (NOS). These decreased activities of enzymes affected the supply of neurotransmitters (CHE and NOS) and energy supply (CCO, LDH, SDH), leading to a physiological function disorder or loss which ultimately resulted in the death of snails [123]. A further study has revealed that acacetin-7-rutinoside is the molluscicidal component of NFBL. In addition, acacetin-7-rutinoside at a concentration of 100 mg/L was found to be non-toxic to zebrafish, implying that this active compound might have applications as a potent molluscicide against O. hupensis [124].
Pulsatilla chinensis (Bunge) Regel (PRS) is a botanical with a long history in medical use in China. It displays "blood-cooling" and detoxification activities. Roots of Pulsatilla chinensis (Bunge) Regel have been widely used for treating various ailments such as intestinal amebiasis, malaria, trichomoniasis, bacterial infections, and malignant tumor [125] , and lactate dehydrogenase (LDH) activities in cephalopodium and liver of snails were assessed. It was found that there were significant alterations in CHE, ALP, and ALT activities in the cephalopodium and the liver of snails after exposure to 40% and 80% LC 50 of PRS or NIC for 24 h. Furthermore, the zebra fish lethality test was performed to assess its toxicity to non-target aquatic species. Results showed that PRS which contained 15 compounds was less toxic to zebra fish than the control (niclosamide) [126]. In 2018, Kang et al. investigated whether hederacochiside C (HSC) isolated from roots of P. chinensis possessed antischistosomal effects and anti-inflammatory activities in S. japonicum-infected mice. Mice infected by schistosomula or adult worms by an intravenous injection were treated with different concentrations of HSC twice a day for five consecutive days. Experimental results demonstrated that the total worm burden, female worm burden, and egg burden in livers of mice treated with 400 mg/kg HSC were fewer than those in non-treated ones. Following the HSC treatment, murine immune responses were assessed by enzyme-linked immunosorbent assays (ELISA). Results showed that 200 mg/kg of HSC was sufficient to reduce the expression of IgG, tumor necrosis factor (TNF)-α, interleukin (IL)-4, and IL-17 in comparison with those in the infected group, exhibiting remarkable immunomodulatory effects [127]. More recently, Kang et al. reported antischistosomal properties of hederacolchiside A1 (HSA) isolated from Pulsatilla chinensis against S. japonicum and S. mansoni. Its antischistosomal activity was higher than praziquantel and artesunate against one-day-old juvenile schistosome. In vivo assays confirmed that the HSA-mediated antischistosomal activity was partly due to morphological changes in the tegument system when juvenile schistosomes were exposed to HAS [127]. Extensive tegumental disruption such as sloughing and erosion was observed. The tegument of schistosome is a protective sheath that plays a crucial role in host-parasite interactions as well as in defense, uptake of nutrients, osmoregulation, and excretion and hence morphological changes in the tegument is critical for the survival of schistosome [56,128].
Reports have highlighted the molluscicidal activity of the leaf extract from C. camphora against O. hupensis [129,130]. Yang et al. [131] have investigated molluscicidal and larvicidal activities of leaf extracts of C. camphora growing in China against O. hupensis and S. japonicium. Gas chromatography coupled to mass spectrometry (GC-MS) was performed to identify bioactive components from the leaf extract of C. camphora. Results showed that Linalool-rich C. camphora leaf extracts had high molluscicidal effects against O. hupensis (LC 50 = 0.25 mg/L) and cercaricidal activity against S. japonicium (LC 50 = 0.07 mg/L) [131].
Wan et al. [133] evaluated cercaricidal activities of Allium sativum (garlic) oil against S. japonicum larvae both in vitro and in vivo. Their findings revealed that exposure to garlic emulsions at concentrations of 10 −6 (v/v) or higher for 30 min induced a 100% mortality of S. japonicum cercariae. The toxicity of the garlic oil against S. japonicum was determined by pre-treating mice with garlic emulsion on the shaved abdomen, followed by an S. japonicum cercariae challenge. The in vivo assay revealed that the pre-exposure treatment with ≥ 10 −4 (v/v) garlic emulsions in mice caused a 100% inhibition of S. japonicum infection in mice, while the pre-treatment with 10 −5 and 10 −6 (v/v) emulsions achieved 20-40% inhibition of S. japonicum infection and 35.2% to 63.6% worm burden reduction, respectively [133]. Similarly, curcumin, a major polyphenol isolated from rhizomes of Curcuma longa L., a dietary spice widely used in Asian cuisine and in folk medicines worldwide, has been reported to be active in vitro against every life stage of S. japonicum [134]. Furthermore, curcumin was observed to exert an optimal activity against the adult stage without differential sensitivity between male and female worms. After 72 h of incubation with 5 mg/mL of curcumin, a decrease in the motor activity of these worms without tegumental alterations was observed by Ke et al. [134].
Macleaya cordata (Willd) R. Br. is a plant with high content of alkaloids. The molluscicidal effect of alkaloid components against snail Oncomelania hupensis was determined by Ke et al. [135]. Alkaloid AN2 was found to be the most toxic one against snail O. hupensis, showing 48 h LC 50 and LC 90 values of 6.35 and 121.23 mg/L, respectively. Responses of some critical enzymes to AN2, including activities of alanine aminotransferase (ALT), alkaline phosphatase (ALP), malic dehydrogenase (MDH), aspartate transaminase (AST), and succinate dehydrogenase (SDH), in cephalopodium and liver were also detected. Results showed that AN2 significantly inhibited activities of MDH, SDH, and esterase isozyme. AN2 also significantly stimulated activities of ALT, ALP, and AST to increase at a low concentration (25 mg/L), while IT significantly inhibited activities of these enzymes at a high concentration (100 mg/L). These results indicate that AN2 not only can inhibit protein synthesis and respiratory chain oxidative phosphorylation, but also can cause hepatocellular injury and reduce the detoxification ability of the liver [132]. Based on the aforementioned facts and information summarized in Table 6, many Chinese herbal medicines are potential molluscicides. They can be used to control intermediate hosts of schistosome and are safer to use than their synthetic counterparts.

Conclusions
Schistosomiasis is a worldwide disease and needs the integration of several measures to promote its control, which includes clinical treatment with antiparasitic agents, as well as the use of molluscicides to promote the biological control. For these purposes, the use of plant-based actives available in the affected countries is one of the viable, ecologically friendly and cheaper alternatives, since most of them are considered developing countries. Thus, this review demonstrates that several researches with natural resources present a broad diversity of species/substances that have action in different phases of the schistosomiasis cycle and can conduct further studies with active formulations and optimization of the best candidates to be used in the combat against this disease in each country.