Plants used in traditional medicine in the Comoros archipelago. A review

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Résumé

Les plantes utilisées en médecine traditionnelle dans l’archipel des Comores (synthèse bibliographique)

Introduction. Dans l’archipel des Comores, comme dans de nombreuses régions d’Afrique, le premier réflexe adopté par les populations quand il s’agit de se soigner est la médecine traditionnelle. Ce travail illustre la diversité des remèdes à base de plantes que l’on retrouve dans cette région du monde. C’est à l’aide de travaux similaires à celui-ci que des espèces potentiellement intéressantes pour le monde pharmaceutique et cosmétique peuvent être identifiées. De plus, ce type d’étude contribue à la préservation d’un savoir ancestral en voie de disparition.

Littérature. Les informations mentionnées dans ce travail sont issues de bases de données construites par des ethnobotanistes ainsi que d’articles scientifiques validés par les pairs. Une partie des données proviennent aussi de travaux réalisés par des locaux en collaboration avec des organismes reconnus.

Conclusions. La littérature scientifique concernant la pharmacopée traditionnelle de l’archipel des Comores cite 207 espèces différentes. Parmi ces espèces, 9 sont endémiques de l’archipel. La totalité des espèces a été comparée aux autres iles de l’océan Indien ainsi qu’aux régions avoisinantes du point de vue des usages respectifs. À l’issue de ce travail, il s’avère que seulement 3 % de ces espèces sont utilisées de manières similaires dans ces différentes régions.

Abstract

Introduction. In the Comoros archipelago, as in many places in Africa, traditional medicine is the first reflex people have when it comes to finding a cure. This work illustrates the diversity of remedies found in this group of islands. The plant species potentially effective from a pharmaceutical point of view can be targeted through the comparison of different databases. The present study also illustrates the importance of preventing the loss of traditional knowledge based on hundreds of years of observations.

Literature. The information in this paper originates from databases built by ethnobotanists as well as peer reviewed scientific articles. In addition, some information also come from work done by locals working with recognized organisms.

Conclusions. The scientific literature cites 207 different species that are used for traditional practices in the Comoros archipelago, among which 9 are endemic. These species were compared to the pharmacopoeias of other islands and surroundings from the Indian Ocean in terms of similarities and differences between targeted ailments. Only 3% of the cited species present similarities in use among the islands of the Indian Ocean and surroundings.

Tabla de contenidos

1. Introduction

1As long as humankind can remember, plants have been part of Human’s development. In addition to the management of the carbon equilibrium, they can be a source of food, medicine, cosmetics, fabric, energy, as well as of construction materials (Cartier, 1994; Bouloc, 2006; Hoffman et al., 2007; Soidrou et al., 2013; Rakotoniaina et al., 2018). The earliest records of plant use for medicine, among other habits and beliefs, were found in ancient Egypt and are estimated to date from 2500 BCE (Kelly, 2009). Even though the way plants are used has been changing, their use is still a common practice nowadays. In the 19^th century, as science and medicine progressed, traditional uses of plants in medicine provided an ever-growing source of inspiration for the development of new drugs and treatments (Farnsworth, 1966; Heitzman et al., 2005; Katiyar et al., 2012), starting with the isolation of morphine from Papaver somniferum L. by Friedrich Sertüner in 1806 (Brownstein, 1993).

2These discoveries were the early stages of what would become the birth of ethnopharmacology in 1967 (Heinrich, 2015). This field of study is defined as “the interdisciplinary scientific exploration of biologically active agents traditionally employed or observed by man, putting in relation plants, fungi, animals, microorganisms, minerals, the way people use them and the biological and pharmacological effects of those ingredients. It is a discipline tightly linked to ethnobotany and phytochemistry” (Holmstedt, 1991). The ethnobotanical part of this discipline led to the gathering of huge amounts of data. In order to simplify the access to that immense source of knowledge, ethnobotanists have been creating many very complete databases such as: NAPRALERT (United States of America); PHARMEL (Belgium); PRELUDE (Belgium); PROTA (The Netherlands); Kew MPNS (Great Britain); MNHN (France). These databases contain information that can prove very interesting for laboratory scientists in order to target the species on which detailed analyses should be implemented (Farnsworth, 1994).

3Ethnobotany has proven to be effective for the discovery of important medicine: e.g. in Madagascar, Catharanthus roseus (L.) G.Don has been used traditionally to treat many pathologies, such as pancreatic pain, colitis, chest pain, heartburn and gastritis (Razafindraibe et al., 2013; Randriamiharisoa et al., 2015). Phytochemical studies were conducted on this species and led to the discovery of many alkaloids such as vindoline I, vindolidine II, vindolicine III, vindolinine IV as well as vinblastine and vincristine. Among these alkaloids some are used nowadays as anti-cancer drugs in modern medicine (Tiong et al., 2013; Dugé de Bernonville et al., 2015). Other plants, originating from different places have been subject to similar investigations and led to the discovery of interesting new compounds. As an example, in the Reunion Island, Centella asiatica (L.) Urb. is used traditionally to treat many benign and malignant ailments (e.g. aspergillosis, scabies, fungal infections). After a phytochemical study was undertaken, compounds such as asiaticoside, brahmoside, brahminoside or kaempferol were identified (Hashim et al., 2011). Thus, C. asiatica was proven to be useful against serious immune disorder diseases. Even though not all compounds and mechanisms have been investigated and studied in depth, this species is currently used as an ingredient in patented phytomedicines (Rangel, 2009).

4The two species mentioned above come from places known for their wide biodiversity, which are called biodiversity hotspots. The Indian Ocean is home to many biodiversity hotspots as 25% of the world’s biodiversity can be found there, as well as in subsaharan Africa (Gurib Fakim, 2011).

5Among the large number of different species found in the western part of the Indian Ocean, many are endemic. Mauritius, Réunion, Rodrigues, Seychelles, the Comoros archipelago and Madagascar together are home to 11 endemic plant families, including at least 310 endemic genera, leading to a total of around 10,000 endemic species (Rasoanaivo, 2011).

6While several studies on Madagascar, Reunion Island, Mauritius and Rodrigues review current knowledge in ethnobotany in these areas, it is surprising to note that despite the incredible potential of the Comoros archipelago, no study has been devoted to them.

7People from the Comoros archipelago live in rural areas. Poverty and difficult access to a modern health care system have led them to develop their own health system based on natural products. The cultural background of these people, being a mix of African Bantu and Arab-Muslim, gave to this region of the world a diversified and rich knowledge when it comes to traditional medicine. The conservation of this knowledge is based on an oral transmission from one generation to the next (Kaou et al., 2008; Soidrou et al., 2013).

8We thus focused on this part of the world in this work for the previously mentioned reasons, that are:

9– it is known as a biodiversity hotspot and the birthplace for many different species (Tatayah, 2011);

10– due to its location, it has had the cultural influences of many ethnicities (Kaou et al., 2008; Soidrou et al., 2013);

11– lastly, only very little data compilation has been done on the botanicals used in traditional medicine in this part of the world.

12Mainly peer reviewed documents were taken into account and most of the ethnobotanical information can be linked to one or several vouchers stored in herbaria. As some work was done by locals, in collaboration with a recognized botanic conservatory (CBNM – Conservatoire botanique national de Mascarin), these data were also taken into account.

13In the end, this work has led to the creation of an exhaustive list of plant species and their traditional uses, based on all available and significant literature. The perspective of development, risks and limitations linked to the use of plants from the Comoros archipelago in traditional medicine are also discussed.

14To find the information mentioned in this article, systematic bibliographic research was conducted in the PRELUDE database as well as in Google Scholar. As the first database already targets ethnobotanical information the keyword used was Comores. The keywords used to do the research in Google Scholar were the following: Comoros archipelago, Moheli, Mwali, Mayotte, Maore, Anjouan, Ndzuwani, Grande Comore, Nagzidja linked to ethnobotany, traditional practices, traditional medicine, cosmetics. This process led to the identification of 100 bibliographic references.

2. Context and study area

15In a context where exports equal a tenth of the imports (13,8 M $ versus 129,6 M $) in 2007 (UNCTAD, 2011), it is crucial to find new ways to finance the region. One way to reach that goal is to seek high added values in available goods. In this case, the rich and diverse flora of the archipelago has been targeted. As mentioned by El Hilaly et al. (2003) and Mhame (2004), folk medicine can be an asset helping the financial status of a region. Through the discovery of valuable plant species for the pharmaceutical and cosmetic business, the import and export balance could be influenced towards a healthier situation. It is vital for the exploitation of these goods to be carried out in a sustainable manner or else this type of work cannot guarantee solid change in the long term (Abdurazag et al., 2003).

2.1. Geography and climate

16The Comoros archipelago lies in the northern region of the Mozambique Channel. It is composed of four main islands, respectively Grande Comore (950 km²), Anjouan (378 km²), Mayotte (370 km²) and Mohéli (216 km²). In addition, around 60 islets are found in the surrounding seas, especially south of Mohéli and in the Mahoran lagoon. The archipelago is the result of the separation of the Malagasy and African plates, between the Miocene and the late Pleistocene, which led to the creation of this volcanic pack of islands (Nougier et al., 1986). Due to their volcanic origin, these islands present a hilly landscape with summits reaching 2,361 m (Mount Karthala) for Grande Comore, 1,595 m (Mount Ntringui) for Anjouan, 790 m (Mzé Koukoulé) for Moheli and 660 m (Mount Benara) for Mayotte (Quod et al., 2000). The climate is tropical, with a hot and rainy season from December to April, during which the monsoon prevails and is characterized by an average temperature reaching 27 °C during the day. The dry season starts in May and ends in November, with an average temperature reaching 23 °C during the day. The pluviometry of the different islands is strongly influenced by their relief as clouds tend to form and stay in places with high altitudes. On mount Karthala, rainfall can reach up to 5,000 mm per year, whereas the average yearly pluviometry of Moroni (located west of Anjouan Island, near the sea) only reaches 2,700 mm per year. The average pluviometry of Moheli, Grande Comore and Mayotte reach respectively 2,100 mm, 2,300 mm and 1,250 mm per year (OMM, 2018). Differences in pluviometry can also vary within the islands: e.g. in Mayotte, the south of the island measures rainfall below 1,300 mm per year and the north of the island regularly records rainfall reaching more than 2,000 mm per year (Boullet, 2016). All these characteristics within the archipelago and within the different islands are some of the reasons for the wide biodiversity (Rasoanaivo, 2011).

2.2. Flora

17The flora of the Comoros archipelago has not been studied in depth, in opposition to other islands in the Indian Ocean (Morat & Lowry, 1997; Pascal et al., 2001; Pascal, 2002). Floristic studies of the Comoros archipelago began in the first part of the 20^th century, when Voeltskow published “Flora und Fauna der Komoren. Reise in Ostafrika in den Jahren 1902–1905” (Vos, 2004) in which he identified 935 vascular plant species, including 416 endemic species. His work was completed by a more recent project aiming to identify the flora of the Comoros archipelago. The main studies made in Comoros were carried out by Moinjoin in 1981, Adjanohoun in 1982 and the PLARM (study of the characteristics and composition of aromatique and medicinal plants) project (Adjanohoun et al., 1982; Gurib Fakim & Guého, 1999). More recently, studies of Kaou et al. and Soidrou et al. were added to this list (Kaou et al., 2008; Soidrou et al., 2013). The latest botanical studies for the Comoros archipelago estimate the number of species as being over 2,000 (indigenous and introduced) (Adjanohoun et al., 1982; Pascal, 2002; Barthelat & Boullet, 2005; Barthelat &Viscardi, 2012).

3. Methods

18Targeting significant literature to provide valuable data for those who will use it afterwards is of paramount importance. Likewise, we must highlight that within the identified posology mentioned in the literature, some remedies are more strongly linked to rites than to biological material, which calls for a cautious review on how the species are used. The ideal criteria in the ethnobotanical literature are, according to Trotter & Logan (1986), as follows: the database should be significant, the scope should be comparable and complete, the plant specimens must be properly identified, vouchers need to be stored for further verification and, if possible, activity tests in the field should have been carried out so as to prove the potency of the concerned remedy. Considering how little ethnobotanical work has been done in the Comoros archipelago, only a few articles meet the criteria mentioned above. Therefore, the selected data originate from peer reviewed articles or from work done by recognized organizations such as the Conservatoire botanique national de Mascarin (CBNM) in collaboration with local inhabitants.

19In fine, the data collected consist of the family name, the scientific name, the endemism of the plant as well as the targeted ailment and the part of the plant used when available (Table 1). The collected data were then studied so as to identify any type of consensus on the way the species were used in the Comoros and in the surrounding islands.

20Based on the work carried out on the collected information, it was observed that in the Comoros archipelago, as well as in the other islands of the Indian Ocean, the number of uses per species varies widely. As this phenomenon was observed worldwide by many ethnobotanists, several data reduction tools have been developed to enhance the significance of the work. First to develop these tools were Trotter & Logan in 1986, then Prance et al. (1987) mentioned the concept of quantitative ethnobotany. Following on from this, many researchers have used different approaches towards developing the significance of the data they have collected. Most indicators require information such as the number of informants, number of ailments per species, number of mentions for each species or treatment; this type of information is found when doing in field observations. As this work is a review, only the body system impacted and the number of health issues targeted per species were available, leading to the selection of the indicators as follows.

21Considering these elements, the RII (Relative Importance Index) was established for each species using the work of Bennett & Prance (2000). This indicator reflects the versatility of a species based on the normalized number of pharmacological properties and the normalized number of body (BS) systems it affects, using the following formula:

22where BS_T = total number of body system as mentioned in the literature (12) (Bennett & Prance, 2000), BS_S = body system specifically targeted by the concerned species, HI_S = number of health issues claimed to be managed by a specific species, HI_M = maximal number of health issues claimed to be managed by a specific species within this data set.

23In 2003, as explained by Tardío & Pardo de Santayana (2008), Pardo de Santayana developed a new indicator also called Relative Importance Index (RI). Similar to the RII mentioned previously, it is based on the relative number of use-categories (RNU); however, it does not take the body systems into account, rather it integrates the relative frequency of citation (RFC). In this present work, it is not possible to determine the RFC as there is no survey linked to the data in table 1 (Tardío & Pardo de Santayana, 2008). On its own, the RNU can be considered as an indicator of the type of ailment that is more of a concern for the inhabitants of the archipelago. All the ailments mentioned in table 1 were sorted into 86 uses (data not shown); then the following formula was applied to the data set:

24where NU_s = number of mentions of a specific use, NU_max = maximum number of specific uses mentioned within the whole data set.

4. Data collection

25All the collected data have been gathered in table 1. A total of 207 different species from 80 different families have been mentioned in the literature, when it comes to the traditional use of plants of the Comoros archipelago. Among these families, the most frequently found are Fabaceae (9%), Asteraceae (7%), Euphorbiaceae (5%), and Malvaceae (5%) (Table 1).

26When applying the two different indicators on the data in table 1, the following information stand out. Leptadenia madagascariensis Decne. and Ocimum canum Sims. both affect 7 body systems, however, L. madagascariensis is claimed to treat 7 health issues, giving a RII of 41.67 where O. canum is claimed to treat 19 different health issues, leading to a RII of 63.10. Ergo the latter is more versatile and could be studied in many different contexts. For our data set, the highest value, the average and the lowest values are respectively: 100 for Bidens pilosa L., 18.9 for the average and 1.79 for Canaga odorata (Lam.) Hook.F. & Thomson, Struchium sparganophorum (L.) Kuntze, Commelina africana L., Acacia farnesiana (L.) Willd., Chrysopogon zizanioides (L.) Roberty and Cestrum nocturnum L. The common link between the 6 very low RII species resides in the fact that they are only used for cosmetics and therefore do not impact body systems. Based on this indicator, the plants to be targeted in further studies could be: B. pilosa, Kalanchoe pinnata (Lam.) Pers., Adansonia digitata L., Cassia occidentalis L. and all locally available Aloe species. These plants have in common the fact that they are used traditionally for many different ailments targeting many body systems, thereby enhancing the chances of having some significant biological activity.

27When it comes to the RNU, it seems that the main concern of the inhabitants of the Comoros archipelago are ailments linked with the following symptoms: diarrhoea (RNU = 100), eczema and skin conditions (RNU = 100), abdominal pain (RNU = 86.84) and rheumatism (RNU = 81.58). This information is interesting for further research in ethnopharmacology when focusing on diseases linked to these symptoms.

5. Tendencies and cultural consensus

28Based on table 1, information on the consensus was also collated, leading to the following noteworthy points:

29– 56 species with one or more specific uses are known to be used only in the Comoros archipelago. Among these the one most mentioned is: Pandanus mayotteensis H.St.John. The dried roots of this species are ground together with the roots of Woodia fruticose (L.) Kurz, Desmodium ramosissimum G.Don., Triclisia capitate Baillon and Monanthotaxis glaucocarpa (Baill.) Verdc. The powder obtained is used as a decoction and drunk two times a day for seven days. This preparation is used to treat impotence (Pascal et al., 2001; Mchangama & Salaün, 2012; Saive et al., 2018);

30– Only 6 species are used for the same purpose in several islands of the Indian Ocean, thus showing an agreement on use between traditional healers. Among these, the ones most mentioned are: Averrhoa bilimbi L. and Acalypha lyallii Baker. In the Comoros archipelago, the fruit of A. bilimbi are used to treat itching; they are crushed with water and ashes to make a paste that is applied to the affected area (Daruty, 1886; Mchangama & Salaün, 2012; Abe & Ohtani, 2013). Acalypha lyallii is used as a leaf decoction to massage the body to treat rheumatism (Adjanohoun et al., 1982; Gurib Fakim & Brendler, 2004; Seebaluck et al., 2015).

31– 145 species are used widely in the Indian Ocean and surroundings, including the Comoros archipelago islands, for many different ailments. Among these the one most mentioned is: Cardiospermum halicacabum L. Leaf decoctions are used to treat eczema, anasarca, ear diseases and wounds. Roots are used to treat dizziness, eye diseases, rheumatism and stiffness. Stems are used to manage fever in India. In the islands of the Indian Ocean, stems and roots are used as laxative and emetic as well as to treat cysts, bladder catharsis and gonorrhoea. The leaves are used to treat boils, rheumatisms, eczema and impetigo (Sussman, 1980; Adjanohoun et al., 1982; Gurib Fakim & Brendler, 2004; Jain & Srivastava, 2005; Mchangama & Salaün, 2012; Abe & Ohtani., 2013; Sreekeesoon & Mahomoodally, 2014).

32Throughout the classification in table 1, these 145 species are found the most frequently.

33Even though C. halicacabum is the most often mentioned (7 sources), it does not have the widest variety of uses (RII = 54.17), ergo there is a certain consensus for the specific uses of this plant.

34On the other hand, Plectranthus amboinicus (Lour.) Spreng. has fewer sources (6) but has many different uses (RII = 65.48), therefore pointing towards a higher versatility; e.g. a total of 21 traditional uses for this plant have been mentioned in the literature. In the Comoros archipelago, it is used as a cure for ailments ranging from a simple cough to colitis and flatulence, as well as rheumatism, malaria and furuncles. In Madagascar, leaves are added in meals as a food complement and are also used in decoctions or in fumigation to treat colds (Nicolas, 2012). In the Reunion Island, leaves of P. amboinicus are used in baths, infusions and for massage, to treat the symptoms caused by the chikungunya virus (fever, sore muscles and articulations) (Minker, 2007).

35When it comes to similarities between different regions, the species Moringa oleifera Lam. (RII = 61.31) is used in the Comoros archipelago to treat asthma, hiccups and spasms among other uses (Table 1) (Adjanohoun et al., 1982; Gurib Fakim & Brendler, 2004; Mchangama & Salaün, 2012; Saive et al., 2018). In Mauritius and Seychelles, this species is linked to the treatment of throat related infections. For the anti-spasmodic effect of M. oleifera, people from Madagascar, Mauritius, Seychelles, as well as peoples from the Comoros archipelago tend to agree. As for the other uses mentioned in the Comoros archipelago, other places have their own use versatility: only inhabitants of Rodrigues and Seychelles use this species to treat high blood pressure, coughs and as an abortifacient, whereas in Madagascar, Mauritius and Rodrigues it is used to treat helminthiasis; in Mauritius, this plant is also used to treat nervous disorder, ear infections and fever and in Seychelles, this species is consumed as a refreshing drink (Pernet, 1957; Adjanohoun, 1983; Gurib Fakim & Brendler, 2004).

36In some cases, some plants are used in other islands of the Indian Ocean and surroundings and are part of the flora of the Comoros archipelago, but are not known to be part of the Comorian pharmacopoeia. Therefore, these species are not mentioned in table 1. E.g.: Cabucala erythrocarpa (Vatke) Markgr. is endemic to Madagascar and the Comoros archipelago. In Madagascar, a bark decoction is drunk to treat viral hepatitis, malaria, stomach pain and diarrhoea. The bark is also used as a bitter ingredient in some alcoholic beverages, which are considered as strongly aphrodisiac. Leaves are used to treat skin infections and are consumed as decoctions to treat hypertension (Schmelzer & Gurib Fakim, 2008). In the Reunion Island, an infusion of Hypercum lanceolatum Lam. yellow flowers is considered as refreshing, meaning that it helps manage fever as well as heartburn caused by the ingestion of hot and spicy food. This infusion is also considered helpful against urinary tract inflammation and will often be used as a depurative and to regulate menstruations (Lavergne, 1989). These plants are found in the Comoros archipelago; however, no use reports are mentioned there.

37As expected, plants with fewer mentioned medicinal uses are more likely to be used the same way in different places, in comparison to plants with multiple medicinal uses. Acalypha lyallii is only used for rheumatism in the Indian Ocean, whereas Bidens pilosa L. is known for many uses in the Comoros archipelago but has even more uses in the surrounding islands. Out of the 207 different species mentioned in the present work, only 27% are used exclusively in the Comoros archipelago for a variety of purposes, 3% are used in the Comoros archipelago and in several islands and surroundings of the Indian Ocean for the same purposes and 70% are used in the Comoros archipelago and in other islands and surroundings of the Indian Ocean for comparable or different purposes. For the latter category, its size can be explained by the fact that many non-endemic species have been transported through the years and have diverse origins. Therefore, they might be used very differently in the different areas of the world where they are found.

38When looking at different ethnobotanical studies carried out in the Maurice archipelago, Mauritius Island and Rodrigues Island are studied separately. On the other hand, most of the studies that were performed in the Comoros archipelago did not separate the different islands; in the present work however, separating the islands is important as some species are endemic to only one or two islands of the archipelago: e.g. Syzygium humblotii Labat & Schatz and Eugenia choungiensis Byng & N.Snow have been reported to be endemic to Mayotte. Syzygium tringiense Byng & N.Snow is known to be endemic to the island of Anjouan (Byng et al., 2016) and Gyrostipula comorensis J.-F. Leroy was only endemic to Grande Comore and Mohéli but its endemicity has been widened to include Anjouan (Mouly, 2009). As these species are not found in all the Comorian islands, the archipelago should therefore not be studied as a whole but rather be investigated in its separate parts. When a clear separation between the geographic regions is made, the collected data have a higher impact when integrated into ethnobotanical indicators such as the informant agreement ratio.

39Identification of species of interest for modern medicine is not the only reason to study the relationship the local people have with the flora that surrounds them. Other factors such as biodiversity management and protection (Nazarea, 1999), as well as the preservation of ancestral knowledge are two important objectives for ethnobotany studies, especially as traditional knowledge is mostly shared through oral tradition. Due to the growing access to modern medicine, the interest for traditional medicine tends to disappear; therefore, the number of traditional healers and their knowledge plummet. Ethnobotanists store plant specimens in herbaria, gather information on the uses of species and thereby preserve ancestral knowledge from extinction (Kaido et al., 1997).

40The fact that plants have been used for centuries as medicines does not mean they are harmless. Their presumed innocuousness is based on hundreds of years of empirical observations (Fennell et al., 2004). Just as for modern medicines, there can be some deleterious effects. Due to the way traditional knowledge is transmitted, adverse effects are not always fully understood and serious poisoning due to traditional medicines is not uncommon. In South Africa, the estimated mortality due to traditional medicines ranges between 10,000 and 20,000 cases per year. This huge variability is due to a lack of precise data. Indeed, many cases of poisoning which are not recorded could possibly be linked to traditional medicine (Thomson, 2000; Popat et al., 2001).

41As mentioned previously, some species are traditionally used for numerous diseases, which means that these species are under a strong anthropic pressure due to their biological value (Adjanohoun, 1983; Gurib Fakim, 1990; Lartigau Roussin, 2002; Gurib Fakim & Brendler, 2004; Mchangama & Salaün, 2012). If there is no management of these plants, there are chances that they will become extinct (Rasoanaivo, 2011).

42When focusing on the case of the Comoros archipelago, it is important to remember that it came to exist through volcanic activity and therefore it has a very hilly landscape (Nougier et al., 1986), which is not ideal for crop production, even though the soil is rich and fertile (Clement et al., 2016). The discovery of medicinal values in plants in such places is an important opportunity to promote the development of these regions through the sustainable exploitation of interesting species. The high added value compensates for the lack of infrastructure for more traditional crops. Needless to say, the knowledge gathered from the traditional healers should be returned in one way or another to the population from which it originated (McManis, 2003; Rasoanaivo, 2011).

6. Conclusions

43This review is an attempt to gather and examine numerous species used in the traditional pharmacopoeia of the Comoros Islands. Since the 19^th century, ethnobotany and ethnopharmacology have been an important part of the drug and cosmetic industry. Based on that affirmation, the study of biodiversity hotspots, especially the ones found on islands seems essential as they present a high concentration of different species in a defined area. In addition, the ethnobotanical work done in these places on Earth will allow us to maintain the knowledge developed by the people who have been working with these species for centuries and who have had the time to test the effectiveness of the remedies empirically. In the present work, 207 species were identified as part of the traditional pharmacopoeia of the Comoros archipelago. Some species are already known worldwide for their properties. However, many are still to be studied in order to validate their biological activities.

44The specific uses for the different species mentioned herein can be compared to other databases and then using the previously mentioned tools, species that are more likely to be really effective and interesting for medicinal and cosmetic purposes can be identified and integrated into more hands-on studies. These differences and similarities will probably be of interest when this database is compared to data from around the world, giving more clues as to which species are likely to produce new cures.

Acknowledgements

45The authors are grateful to the University of Liège (Belgium) for allowing us to gain access to all the information required for an exhaustive study of the subject.

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