Background

Traditional herbal medicine in Bangladesh has strong cultural and religious foundations. It manifests in different ways among indigenous groups in their ritual or ceremonial practices, spiritual practices, and self-healing practices. Indigenous communities have utilized this local knowledge for centuries to cure different diseases. Reportedly, more than 80% of the Bangladeshi use non-allopathic medicines (Ayurveda, Siddha, Unani, and homeopathy) for their healthcare, with herbs constituting a major ingredient of these alternative systems of medicine [1]. Bangladesh is a country that is considered rich in medicinal plant genetic resources, by virtue of its favorable agroclimatic conditions and seasonal diversity. With productive soils and a tropical climate, more than 5000 angiospermic plant species have been recorded in the country [2], of which about 250 have documented use in traditional medicine systems [3]. About 75% of the country’s total population lives in rural areas, and almost 80% is dependent on natural resources (e.g., medicinal plants) for their primary healthcare needs [4]. Rural/indigenous peoples are capable of identifying many species of plants yielding various products, including food, firewood, medicine, forage, and tools for daily needs. With such a high demand for herbal medicines, the medicinal plant sector has been cited as the most promising business sector in Bangladesh [5], with more than 500 companies producing herbal medicines, yet despite the biodiversity described above, more than 90% of the plants and products needed to meet domestic demands are imported from other countries, such as India, Nepal, and Pakistan.

Many indigenous Bangladeshi live in deep forest zones. They include those people living within the three Chittagong Hill Tract districts (CHTs) of south-eastern Bangladesh, within which there are 12 indigenous communities [6]. The smallest of these communities is the Pangkhua, who dwell in the remote Pangkhua paras, an isolated part of the Bilaichari Upazilla of the Rangamati CHT. In the wet season, the only way to reach Pangkhua paras is by motorboat, taking 6 h, while in the dry season it takes more than 8 h on foot. Like other remote communities, the Pangkhua have their own distinct traditional healthcare system and practices. In fact, the nearest conventional medicine facility is in Belaichari Upazilla sadar, the only Government health facility nearby (about 15 km), with basic health facilities. Services there are provided by two medical practitioners alongside three paramedics. The Pangkhua people thus have inadequate access to modern treatments, and in any case, allopathic medicine is largely unaffordable to them. Traditional medicinal knowledge, on the other hand, is orally transmitted from one generation to the next. Typically, every elderly man and woman of the community can prepare herbal formulations for the treatment of common ailments, such as fever, cough, cold, dysentery, diarrhea, and gastritis. Typically, they visit professional healers only when they suffer from more serious symptoms or conditions, such as jaundice, cholera, malaria, or cancers. The headmen (karbari) of each village also act as professional healers. In fact, many Pangkhua believe that they lose their community spirit if they receive allopathic care. Local government has had to enforce modern treatment in instances of contagious disease.

Several studies on ethnomedicinal plants of Bangladesh have been conducted in the past, and comprehensive works have already been published [7,8,9,10,11,12,13]. However, few studies focus on the Rangamati district [10, 14, 15] with almost nothing on the Pangkhua indigenous community. With this in mind, the Pangkhua indigenous community was selected for the present study, as their ethnomedicinal practices have not been thoroughly investigated to date. It was important to ascertain who among them represent the custodians of such knowledge and to document their uses of medicinal plants. To the best of our knowledge, this is the pioneer quantitative documentation of medicinal plants in the studied area.

Methods

Study area

The present study was carried out in the Pangkhua areas of the Belaichhari Upazila within the Rangamati District (Fig. 1). This district is part of the Chittagong division and Chittagong Hill Tracts. Belaichhari thana (now an upazila) was established in 1976. It consists of 3 Union parishads, 9 mouzas and 30 villages. The Belaichhari Upazila is situated approximately between 20° 50′ and 22° 35′ N latitude and between 90° 38′ and 92° 17′ E longitude. The Rainkhiang is the main river of the upazilla. The district lies in the south-east of Bangladesh and has a tropical monsoon climate. There are three main seasons: the dry season (November to March), which is sunny and dry; the pre-monsoon (April to May), which is very hot and sunny with occasional showers; and the rainy season (June to October), which is warm, cloudy, and wet. Temperatures of the Belaichhari Upazila are moderate, with a mean monthly average temperature in Rangamati of 25.8 °C and annual monthly average temperatures ranging from 13.4 to 34.6 °C. The mean annual rainfall is 2865.4 mm, with mean monthly maxima and minima of 679 mm (July) and 7.4 mm (January), respectively [16].

Fig. 1
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A map of the studied area [16]

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Methods of study

The success of ethnobotanical documentation depends on the cooperative relationship between the researcher and local informant. Knowledgeable informants are very important for the study of ethnobotany [17, 18]. Various techniques are recommended for ethnobotanical studies, including (i) direct or participant observation, (ii) checklist interview, (iii) group interview, (iv) field interview, (v) plant interview, and (vi) market survey [19, 20]. All of these techniques were followed in this study except the use of checklist interviews. The interview is a dynamic process involving spoken interactions between two or more people. In general, open-ended and semi-structured techniques were followed. Initial contacts are very important to understand an area and its people. Initial contacts were made with headmen, teachers, and students within the area to select informants. Upon identification of informants, if necessary, interpreters were also appointed. Ethnobotanical information regarding the usage of medicinal plants available in the local area for treating various ailments and diseases was collected through direct interviewing of traditional healers and other informants possessing traditional knowledge about medicinal plants. During the interviews, information was noted using data documentation sheets; in addition, audio recording was performed with a digital voice recorder. Contact in the field was conducted over a total of 43 days, in different seasons, with interviews conducted in the Chittagonian language, accompanied by a local student (Bathue Pankhua) and with consultancy with a local doctor (Dr. Mizanur Rahman).

Quantitative analysis

To analyze the data, we adopted the following quantitative ethnobotanical techniques:

Factor of informant consensus (FIC)

The level of homogeneity between information provided by different informants was calculated using the factor of informant consensus (FIC) [21, 22]. It is calculated as FIC = Nur – Nt/(Nur – 1), where Nur is the number of use reports from informants for a particular plant-use category and Nt is the number of taxa or species associated with that plant-use category across all informants. FIC values range between 0 and 1, with FIC = 1 indicating the highest level of informant consensus. A high value (close to 1) indicates that relatively few taxa (or, more usually, species) are used by a large proportion of informants, while a low value indicates that informants differ on the taxa to be used in treatment within a category of illness. Therefore, if informants use few taxa, then a high degree of consensus is reached and medicinal tradition is thus viewed as well-defined [23].

Jaccard index (JI)

We also wished to calculate similarities between our studies with prior ethnobotanical studies carried out in other parts of Bangladesh, as well as those from neighboring countries. This may be expressed using the Jaccard index (JI), which uses the following formula [24, 25]:

JI = C × 100/A + B − C, where, A is the recorded number of species of the current study area a, B is the documented number of species of another study area b, and C is the number of species common to both areas a and b.

Results

Enumeration of taxa

The ethnobotanical survey was carried out three times during summer and winter seasons from January 2016 to September 2017. All plant materials were collected and identified through expert consultation, by comparison with herbarium specimens and through use of literature references. Following preservation, plant materials were numbered and deposited as voucher specimens in the Chittagong University Herbarium. Descriptions and current nomenclature were compared with the recent “Dictionary of Plant Names of Bangladesh-Vascular Plants” [2] and with www.theplantlist.org. The ethnomedicinal value of each plant was cataloged as follows: botanical name (with voucher number in brackets), Bangla name, Pangkhua name, family, habit, plant part(s) used, disease(s)/illness treated, usage information, and prior documentation in the allied literature (Table 1).

Table 1 List of the ethnomedicinal plant species used by the Pangkhua community of the Rangamati district, Bangladesh
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Demography of informants

A total of 218 people, including traditional healers and other community members, mostly the elderly men and women, with ages ranging from 27 to 86 years were interviewed, and of them, the majority (65.6%) belonged to the age group of 51–70. We considered as informants those reporting one or more ethnomedicinal uses of a species (see Additional file 1 as an example). Demographics by gender, age, education, and occupation of participants are summarized in Table 2. Detailed clarification of informants is presented in an additional file (see Additional file 2).

Table 2 Demographics of the Informants
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Ethnomedicinal plants and part(s)

The present investigation details 117 species of ethnomedicinal plants distributed across 104 genera and belonging to 54 families (Table 1). The highest numbers of ethnomedicinal plants recorded were from the Fabaceae (12 species). The second largest used families represented were the Asteraceae and Zingiberaceae (10 species each), followed by the Lamiaceae (5), Caesalpiniaceae (4), and Amaranthaceae, Apiaceae, Cucurbitaceae, and Poaceae having 3 species each. The remainder of families was represented by two or one species. However, most of these families are documented to contain active constituents and feature in different traditional systems of medicine. Of all recorded species, herbs (55 species) were found to account for the greatest number, followed by trees (35 species), shrubs (13 species), climbers (10 species), and under-shrubs (4 species). Different parts of ethnomedicinal plants are used in herbal formulations by local traditional healers for the treatment of different ailments. Among such plant parts, leaves (34.07%) were found to be the most frequently used for the preparation of herbal drugs, followed by other parts (Fig. 2).

Fig. 2
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Plant parts used for the preparation of herbal medicines

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Considering the mode of preparation of traditional medicines by the Pangkhua community, the range of methods reported for various species included decoctions, juices, extracts, pastes, powders, infusions, oils, and the use of fresh plant parts. Among these, the most common formulations were decoctions (25.93%) and fresh plant parts (23.46%), followed by juices (16.05%), pastes (14.81%), extracts (13.58%), oils (3.70%), and infusions and powders (1.23% each). Decoctions are often the most commonly encountered preparation method reported [26,27,28,29,30]. In some cases, processing involved drying of the plant material followed by grinding into a fine powder. Water was most commonly used if a solvent was required, with cow’s milk or honey sometimes used as a matrix or as an adjuvant to increase viscosity. Within the study community, plant medicines were usually administrated orally. Bathing in a decoction or rubbing and massaging using the plant parts were also encountered.

Conservation status

The conservation status of all recorded plant species was checked using the International Union for Conservation of Nature (IUCN) Red List of Threatened Species [31]. A total of 12 species, namely Acorus calamus, Amorphophallus paeoniifolius, Ammania multiflora, Azolla pinnata, Breonia chinensis, Centella asiatica, Cyperus rotundus, Commelina diffusa, Hygrophila difformis, Lasia spinosa, Mimosa pudica, and Ottelia alismoides were recorded as “of Least Concern,” while only one species (Saraca asoca) was recorded as “vulnerable,” with all other species not included on the list.

Quantitative analysis

The present study records the use of ethnomedicines to treat 11 categories of ailments. Of these, the most common uses were for digestive system disorders (49 species), followed by respiratory complaints (39 species) (Table 3). To ascertain the level of agreement among the informants of the Pangkhua community regarding the use of plants to treat certain disease categories, FIC values were determined. The FIC values are presented in Table 3. It is clear that the FIC values showed variation, varying from 0.50 to 0.66. In the treatment of digestive system disorders, the highest FIC value (0.66) was encountered, with 141 use-reports for 49 plant species. This was followed by plants used to treat respiratory system disorders (FIC = 0.64) and so on (Table 3). In contrast, the least agreement (FIC = 0.50) between informants regarding therapeutic uses was observed for plants used to treat urinary disorders. The calculated JI indices (Table 4) ranged from 1.65 to 33.00. The highest degree of similarity was found with a study conducted in Bangladesh, while the lowest degree of similarity was found with a study conducted in Pakistan.

Table 3 Categories of ailments and Informant Consensus Factors (FIC)
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Table 4 Jaccard similarity index (JI) for local and neighboring countries
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New ethnomedicinal plant species and uses

Our comparative analysis revealed that out of 117 ethnomedicinal plant species documented, 37 species had either no similar or any use (Table 1). Therefore, these species were compared with the research databases of SCOPUS, PubMed, Biomed Central, and Google Scholar, and the results showed that use of 12 of these species has heretofore been unreported in Bangladesh (Table 5), while 6 species have never been screened pharmacologically.

Table 5 List of new ethnomedicinal plant species and species as yet unscreened for pharmacological activity
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Discussion

Overall, this study revealed the traditional use of 117 plant species, distributed among 104 genera and belonging to 54 families to treat 11 categories of ailments, recorded from 218 traditional healers and elderly men and women. The highest number of species belonged to the Fabaceae; this dominancy may be due to the worldwide distribution of species from this family [32, 33] and, furthermore, that the Fabaceae constitute the second largest family in the flora of Bangladesh [2]. Similar results have been reported by other ethnobotanists [10, 27, 34] while [7] reported the Asteraceae as the largest family and the Fabaceae the third largest family in their study conducted in Bangladesh.

Herbs are naturally abundant in the study areas, which were mostly hilly and covered by a forest canopy, creating favorable conditions for their growth. Similar results were observed with other studies conducted in different regions of Bangladesh [3, 27, 34,35,36].

The preference for the use of leaves in the preparation of herbal medicines by the healers is likely due to the year-round availability of leaves, and the fact that they are easier to collect, store, process, and handle. Similar observations have been reported in allied studies in Bangladesh and other countries [28, 35, 37, 38]. Healers usually however prefer to use fresh plant materials instead of dry and stored ones for herbal preparations.

In the study area, digestive system disorders are common, largely due to a deficiency of pure water, especially in the dry season, coupled with a lack of awareness of its importance among those living in hilly and remote areas. Similarly, respiratory system disorders were second in occurrence, due to prevalence of smoking and chewing of leaves of Nicotiana tabacum with those of Piper betel. Analogously to our results, digestive system disorders were found to be the major ailment category in many other ethnomedicinal studies conducted in Bangladesh [7, 8, 14, 39, 40]. High FIC values also indicate that such species are worth investigating for bioactive compounds.

As discussed earlier, some medicinal plant species used by the healers of the studied community are also used by the healers of different communities in different parts of Bangladesh as well as in neighboring countries and beyond.

A total of 19 ethnomedicinal plant species which are commonly used by the indigenous communities of Bangladesh were selected and their known uses compared with our results (Table 6), to ascertain whether the Pangkhua community has any novel uses of these species. Alongside, we evaluated the phytochemical literature on these species. From our review, 11 species, namely Acorus calamus, Aegle marmelos, Arecha catechu, Calotropis procera, Centella asiatica, Curcuma longa, Justicia adhatoda, Phyllanthes emblica, Saraca asoca, Terminalia chebula, and Zingiber officinale have distinct uses within the Pangkhua community. For example, Centella asiatica is used analogously by the Marma community in Bandarban [35], the Rakhaing community in Cox’s Bazar [34], the Tripura community in Chittagong [3]; the local people in the Panchagarh [36], Garo, Hazong, and Bangalee communities in Durgapur [8]; the local people of 11 districts in Bangladesh [27]; and the ethnic people of western Nepal [41]. This species was also used differently in traditional medicine by traditional healers of Bangladesh and other countries [37, 42,43,44,45]. Interestingly, its use in one ailment, asthma, has been documented for the first time in this study. Similarly, the use of Acorus calamus as an anthelmintic has not been reported before, and the use of fruit of Aegle marmelos to treat asthma is recorded herein for the first time, while its leaves were used in combination with other plants [46]. Other unreported uses of established ethnomedicinal species include Arecha catechu as a carminative, Calotropis procera to treat asthma and snake bite, Curcuma longa as a laxative and to treat fever, Justicia adhatoda and Phyllanthes emblica to reduce high blood pressure, Saraca asoca to treat diarrhea and leucorrhea, Terminalia chebula to reduce pain during menstruation and to treat bronchitis, and Zingiber officinale as a laxative and to treat dyspepsia and tuberculosis.

Table 6 Comparative ethnobotanical uses of selected species among the Pangkhua and wider Bangladesh
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To illustrate homogeneity of use or otherwise, the JI was used to compare our study with 43 previous investigations. In total, the JI was calculated for 28 regions of Bangladesh with the Cox’s Bazar district emerging as the most similar to our study area with JI = 33.00, followed by the Panchagarh, Chittagong, and Bandarban districts (JI = 22.83, 19.44, and 18.80 respectively), while the lowest JI (2.77) was found with the study conducted by Rahman [47]. The high JI may reflect that the study area is located in the same geological zone, with similar socioeconomic and cultural characteristics. On the other hand, among three neighboring countries (India, Pakistan, and Nepal), the highest similarity was found with the adjacent state of Tipura, India (JI = 11.74) while the lowest (JI = 1.65) was from Pakistan.

Limitations of the current study

Ethnobotanical documentation constitutes field-based research. Nevertheless, the field is not always a safe environment. A majority of the indigenous communities we studied live in forest areas, and there have been security risks due to rebel movement in these areas. It is risky to carry valuable field equipment like cameras, recorders, etc. Route access was limited to foot traffic. Language barriers were encountered, as most participants did not speak the national Bangla language requiring the use of interpreters. Seasonal variation is an important factor in the collection of voucher specimens, as in the rainy season it is difficult to both access and dry the specimens, while in the dry season the aerial parts of many plants have withered, coupled with the clearing of forest areas for cultivation during that period.

Indigenous peoples are sometimes unwilling to share their knowledge of medicinal plants with others, specifically the Bangali (Bangladeshi). They maintain the secrecy of medicinal plant use because there is a belief among them that the medicines lose their efficacy if too many people know of them, and additionally, they may be conscious about economic losses [48]. There may also be resistance to allowing themselves to become the subject of study by outsiders [48]. Therefore, potential informants must be encouraged using several techniques. Firstly, emphasis must be given to help them understand that shared information will be preserved for the benefit of their children and future generations. As their children are less frequently adopting the role of healers, without documentation, much knowledge of medicinal plants may disappear forever.

Research highlights

  1. 1.

    The present study revealed that the Pangkhua community depends on a variety of ethnomedicinal plants to treat various diseases.

  2. 2.

    Local herbalists are predominantly aging men and women, and the Pangkhua younger generation lacks interest in following the traditional role of the healer.

  3. 3.

    While in many cases, the plants utilized by the Pangkhua are documented in allied literature, their preparation, mode of use, and clinical indication often differ from that of other indigenous communities.

  4. 4.

    The information compiled herein constitutes a rich knowledge source for taxonomists, phytochemists, environmentalists, pharmacists, and allied professionals.

Conclusions

It can be concluded that the Pangkhua indigenous community of the Rangamati district of Bangladesh possess rich ethnomedicinal knowledge, as they use many medicinal plant species in their healthcare system. The novelty of this study is that 12 ethnomedicinal plant species have been recorded with new uses, and 6 of these species have never been screened pharmacologically. The traditional plants utilized have in some cases been validated scientifically by isolation of active ingredients, thus showing that traditional remedies are an important and effective part of indigenous healthcare systems in the district. Our findings will be helpful to ethnobotanists and phytochemists for conducting research into the isolation of active principles from these species. The preservation of these plant species is the gateway toward developing efficacious remedies for treating disease. Enhancing the sustainable use and conservation of indigenous knowledge of useful medicinal plants may benefit and improve the living standards of poor people. Hence, it is necessary to document the indigenous knowledge of useful plants and their therapeutic uses before they are lost forever.