Anti-Trypanosomal Activity of Nigerian Plants and Their Constituents

African trypanosomiasis is a vector-borne parasitic disease causing serious risks to the lives of about 60 million people and 48 million cattle globally. Nigerian medicinal plants are known to contain a large variety of chemical structures and some of the plant extracts have been screened for antitrypanosomal activity, in the search for potential new drugs against the illness. We surveyed the literatures on plants and plant-derived products with antitrypanosomal activity from Nigerian flora published from 1990 to 2014. About 90 plants were identified, with 54 compounds as potential active agents and presented by plant families in alphabetical order. This review indicates that the Nigerian flora may be suitable as a starting point in searching for new and more efficient trypanocidal molecules.


Introduction
African trypanosomiasis is a parasitic disease caused by a protozoan of the genus Trypanosoma. Trypanosoma vivax (T. vivax), Trypanosoma congolense (T. congolense) and to a lesser extent Trypanosoma brucei brucei (T. b. brucei) are the main species responsible for African animal trypanosomosis (AAT) called nagana in West Africa while T. b. rhodesiense and T. b. gambiense cause sleeping sickness (human African trypanosomiasis, HAT). Surra and Dourine are caused by the other trypanosome species T. evansi and T. equiperdum respectively. The disease is transmitted by a bite of the vector-tsetse fly (Glossina species) [1].
In Nigeria, trypanosomiasis seems to be re-emerging as an important livestock disease, assuming major clinical importance in small ruminants and extending to previously designated tsetse-free zones [2,3]. Apart from the old Gboko endemic focus remaining active, there have been reports of the disease outbreak in many other communities in Nigeria [4,5].The prevalence rate in different breed of animals in Nigeria for the past few years have been studied and ranged from 8.4% to 15.53% [6,7].
In Africa, the annual loss in livestock production and mixed agriculture alone due to the disease is valued at 5 billion US dollars. In 1995, WHO Expert Committee estimated that 60 million people were at risk with an estimated 300,000 new cases per year in Africa, with fewer than 30,000 cases diagnosed and treated. In 2004, the number of new reported cases fell to 17,616 and WHO considered in that due to increased control, estimated cumulative rate to be between 50,000 and 70,000 cases. In 2009, the number of new cases reported dropped below 10,000 (9878) for the first time in 50 years and the estimated number of actual cases is currently 30,000. This trend has been maintained in 2012, with 7216 cases reported [8][9][10].
The current chemotherapy of HAT relies on only six drugs (suramin, pentamidine, melarsoprol, eflorinithine, arsobal and mel B), five of which were developed more than 30 years ago. Others such as homidium, isometamidium and diminazene aceturate are used in animal infections. Each of these drugs has one or more of these challenges: expensive, highly toxic, need parenteral administration and parasites increasing resistance. However, tireless effort being made by WHO, private partners and local governments to eliminate HAT is yielding significant success. The Drugs for Neglected Diseases initiative (DNDi) is developing fexinidazole to a new oral drug for HAT with a good chance of success. It has entered Phase II/III clinical study in patients with late-stage sleeping sickness. It is hoped that fexinidazole would solve the problems and limitations of current chemotherapeutic options [11][12][13].
Several reviews on medicinal plants used in treatment of trypanosomiasis have been published [14][15][16][17][18][19][20][21]. It is estimated that 66%-85% of the World's population depends directly on plants as medicine and search for drugs derived from plants has accelerated in recent years [22][23][24][25][26][27][28][29][30]. Nigeria, located in West Africa on the Gulf of Guinea, has a rich biodiversity. There are many reports documenting the potentials of medicinal plants in Nigeria against several diseases except trypanosomiasis [31,32]. This paper documents works on Nigerian medicinal plants and derived products as source of trypanocidal agents which could be further investigated for the development of better drug molecules for the disease. We present the plants which are 84 by their plant families (40) in alphabetical order. trypanocidal towards T. b. brucei at 200 mg/kg [53]. Further investigation of the ethyl acetate extract of T. procumbens led to the isolation of four flavonoids; 3-hydroxyflavone (5), quercetin (6), 7,8-dihydroxyflavone (7) and catechin (8). Compounds 6 and 7 were described to exhibit trypanocidal activity in vitro and in vivo as pure compounds without affecting normal human cell [54][55][56]. In vitro studies showed T. b. brucei was immobilized by 0.4 mg/mL of Artemisia maritime (Asteraceae) chloroform and petroleum ether extracts using short assay duration of less than one hour [57]. Boswellia dalzielii (Burseraceae), a tree of the Savannah forest of Nigeria, is used for the treatment of wound, diarrhoea, syphilis and to induce vomiting [58,59]

Capparaceae, Celastraceae, Clusiaceae, Combretaceae, Cucurbitaceae, Ebenaceae and Euphorbiaceae
The plants in the families listed above are summarized in Table 2 and potential active isolates are represented in Figure 2. Leaves of Crateva adansonii (Capparaceae) are used to treat ear infections while its root is employed to treat syphilis, jaundice and yellow fevers [62]. The ethyl acetate and hexane crude extract of the plant demonstrated moderate in vitro activity (MIC 12.5 μg/mL) against T. b. brucei [63]. Two phytoconstituents (oleanolic acid (9) and 4-epi-hederagenin (10)), which were not tested for activity, had also been isolated from C. adansonii and could account for the activity of the plant. The seed extract of Bucholzia coriacea (Capparaceae) is locally used in treatment of feverish conditions in Eastern Nigeria [64]. T. b. brucei was cleared in infected mice after administering 1000 mg/kg of aqueous and methanol seed extracts of B. coriacea i.p. for five consecutive days [65,66]. MIC of 0.625 μg/mL has been reported for both compounds 11 and 12 isolated from the hexane-ethyl acetate fraction of Maytenus laevis (Celastraceae) root [44,67]. The highest in vivo trypanocidal activity of Garcinia kola (Clusiaceae) seeds was observed in the alkaloid fraction which brought about 92.25% reduction in parasitaemia at 100 mg/kg in T. b. brucei infected rats. The antitrypanosomal property of the alkaloids from G. kola has been suggested to be due to DNA intercalation in combination with protein biosynthesis inhibition [68].
In North-Eastern Nigeria, leaves and fruits of Strychnos spinosa (Loganiaceae) are consumed by lactating Fulani women to stimulate breast milk production [96]. Dichloromethane leaf extract of the plant yielded: saringosterol (16), 24-hydroperoxy-24-vinylcholesterol (17), akagerine (18) and 10-hydroxyakagerine (19). Out of the four compounds, only 16 and 17 exhibited significant antitrypanosomal in vitro activity with IC50 values of 7.8 ± 1.2 and 3.2 ± 1.2 µM respectively [97]. The essential oil from the plant's leaves was active on T. b. brucei with IC50 13.5 µg/mL. Hoet et al., went further to show that nerolidol (20) and linalool (21), components of this oil, had a higher potency on the trypanosomes with IC50 values of 7.6 and 16.3 µM [98]. Ten and 4 mg/mL methanol extracts of Tamianthus globiferus (Loranthaceae) and Lawsonia inermis (Lythraceae) leaves respectively drastically ceased motility of T. congolense within 5 min [39,99]. A dose of 200 mg/kg of ethyl acetate extract of Punica granatum (Lythraceae) leaf reduced anaemia and promoted weight gain in mice infected with T. b. brucei [100]. The stem barks of Bombax buonopozense (Malvaceae) and Heterotis rotundifolia (Melastomataceae) methanol extracts (200 and 300 mg/kg body weight, respectively) cleared T. b. brucei within seven days of treatment duration, while 67% of T. b. brucei infected mice survived for over 43 days when treated with 400 mg/kg methanol seed extract of Adansonia digitata (Malvaceae) [70,101,102]. Dissotis rotundifolia (Melastomataceae) is a medicinal plant widely used in Nupe (Nigeria) ethno-medicine to treat trypanosomiasis [103,104]. Oral and i.p. administration of 800 mg/kg of ethanol leaf extract of the plant to rat infected with T. b. brucei significantly reduced parasitemia by 66.7% and 78.4%, respectively. The parasite was killed within 45 s of in vitro exposure to the same concentration of the extract. Hyptis spicigera (Lamiaceae) is locally known as "Bunsuru fadama" in Hausa language, Northern Nigeria. When the plant is crushed and applied to the head, it relieves headache [59].  Table 4 and Figure 4 respectively. Various extracts of leaf, bark, stem and seed of Khaya senegalensis (Meliaceae) were reported to treat several human diseases [109]. Rats infected with T. b. brucei showed a significant decrease in blood parasite burden within six days when treated with aqueous stem bark extract of K. senegalensis at 60-100 mg·kg −1 bw i.p. [110]. Umar et al. reported that the stem bark of K. sengalensis possessed the highest in vitro activity among the six extracts tested as it eliminated the parasites within 5 min of incubation time at 1 mg/mL [111]. The following triterpenoids: gedunin (22); methyl-angolensate (23); methyl-6-hydroxyangolensate (24), isolated from K. senegalensis stem bark, though not tested, could be responsible for its trypanocidal activity [112]. aqueous stem bark extract of Securidaca longepedunculata (Polygalaceae) and methanol stem bark extract of Pseudocedrella kotschi (Meliaceae), methanol stem bark extract of Ficus sycomorus (Moraceae), chloroform stem bark extract of Canarium schweinfurthii (Poaceae) and chloroform stem bark extract of Syzygium guineense (Myrtaceae) (4 mg/mL) ceased T. b. brucei motility in vitro within the incubation time of less than one hour [39,113]. Adansonia digitata to treat cancer SD [101] Sterculia setigera used as a thickener and emulsifier R [39] Melastomataceae Heterotis rotundifolia malaria, rheumatism, diarrhoea WP [102] Dissotis rotundifolia trypanosomiasis treatment L [104] R = root, RB = root bark, SB = stem bark, L = leaves, S = stem, WP = whole plant, Re = rhizome, B = bark, SD = seed.

Conclusions
This review, the first of its kind on antitrypanosomal medicinal plants from the Nigerian flora, represents an overview of the potentials of these plants in combating the disease. It is intended to serve as the scientific baseline information for the use of documented plants as well as a starting point for future studies for the discovery of better trypanocidal molecule(s). Most of the plants were evaluated as crude extracts. Only compounds 1, 3, 4, 9, 11-12, 14-17, 20-21, 37-38, 47-54 were actually tested for antitrypanosomal properties. Mode of action of the compounds has almost never been thoroughly studied; only possible mechanisms have been suggested. This calls for more detailed investigations in this direction. In general, even though not all plants reviewed here are native or unique to Nigeria or West Africa, this survey suggests that Nigerian flora is a potential suitable starting point to discovering new and better trypanocidal drug molecules.

Author Contributions
NJN, AI, MUA and CJM conceived the ideas, FNK provided access to CBIC database and contributed in data analysis, NJN and AI wrote the first draft of the paper and all authors agreed on the final version before submission.

Conflicts of Interest
The authors declare no conflict of interest.