The research study was conducted in Jos North Local Government Area (LGA) of Plateau state (Figure 1), with a population density of 439 217 constituting 220 856 men and 216 361 women (National Population Commission 2006). Jos North LGA has a land area of 291 km² inhabited by various ethnic groups, such as Irigwe, Berom, Ngas, Anaguta, Rukuba and Mwaghavul, as well as different tribes, including Fulani, Hausa, Igbo and Yoruba.

This study focused on traditional healers who use medicinal plants for the treatment of various human ailments in Jos North LGA. This was conducted through interviews and discussions with 100 traditional healers who were registered with the traditional healers’ association, Jos North LGA. The choice of the sample size and the purposive selection were based on a reconnaissance survey and interview, which were conducted before the study.

The interviews and discussions were conducted in the local languages of the traditional healers (Hausa, Yoruba and Igbo) using a well-structured questionnaire. The questionnaires consisting of both open-ended and closed-ended questions were administered to the respondents. Interview or discussion themes targeted the names of the common diseases or disorders, local names of the medicinal plants, life form, the plant parts used, method of preparation and administration of the herbal medicine. Where necessary, the aetiology and symptomatology of some diseases were described to the traditional healers in order to enable them to recollect the exact medicinal plants that they usually use to manage the diseases. The authenticity of claims was verified by cross-checking the acquired data in different locations either by showing the medicinal plants or telling the local names to other informants (Afolayan et al. 2014). The medicinal use of a particular plant was validated only when the answers of two or more traditional healers coincided with the same use of a similar part of the plant, irrespective of the method of preparation. Data were also obtained from secondary sources, such as relevant literature, books, journals, conference proceedings, newspapers, magazines and online sources. The results of the study are tabulated to include scientific or botanical name, plant family, local name, plant part used, and method of preparation and application.

The traditional healers assisted in the collection of the cited medicinal plants to ensure that the correct plants were collected. The fresh specimens of the collected medicinal plants were pressed and later transported to the Forestry Research Institute of Nigeria (FRIN), Ibadan, where they were identified by botanists using floristic works of Nigeria. Voucher specimens were deposited at FRIN herbarium, Ibadan.

Descriptive statistics, such as means and percentages, were used for the demographical information of the respondents, whilst two analytical tools (informant consensus factor [ICF] and fidelity level [FL]) were used to analyse the ethnomedicinal data that were obtained during the study.

The user’s variability of herbal plants was determined using the informant or respondent consensus factor (F_IC) with the method of Heinrich et al. (2001) and Canales et al. (2005). The consensus value ranged between 0 and 1. According to Heinrich et al. (2009), high value connotes high usage of one or few plant species to treat a specific ailment, whilst a low value indicates disagreement between informants over the use of any particular plant. Informant consensus factor was obtained from Eqn 1:

where N_ur refers to the number of individual plant use reports for a certain illness category, whilst N_t connotes the total number of herbal species utilised by all respondents for this illness category.

The potency of a particular medicinal plant to treat or heal an ailment in a cultural group was quantified using the FL. Fidelity level is derived from the comparative healing power of the herbal plant species concerning the proportion of respondents’ agreement on the usage of a particular medicinal plant for treatment against a specific ailment category (Teklehaymanot 2009). However, reported diseases were classified into major ailment categories before the determination of the FL (Heinrich et al. 2001). Fidelity level was obtained from Eqn 2:

where ‘N_p’ refers to the number of respondents who claimed using a medicinal plant to treat a given disease or some major ailments, whilst ‘N  ’ connotes the number of respondents who use the plants as a medicine to treat any specific disease or ailment.

Ethical clearance and approval for the research study were obtained from the Forestry Research Institute of Nigeria’s ethics committee.

The demographic information of the traditional healers who participated in the study is presented in Table 1. Close to half of the respondents (47%) were in the age range of 51-60 years and 30% were with the age bracket of 61-70 years. Generally, the majority of the respondents were older than 51 years. Amongst the respondents, 62% were men, whilst 38% were women. Most of the traditional healers practised Islamic religion (60%), whilst 35% of them professed Christianity. However, 78% of the traditional healers had formal education. Majority of the traditional healers have been engaged in this practice for last 7–10 years (31%), whilst 47% and 27% of them have their own private jobs and personal businesses, respectively.

As observed in this study, the traditional healers were within the age brackets of 51–60 years, that is, generally above 51 years. In a similar consonance, the study also reported that elders (> 60 years) were the ones who possess significant traditional knowledge and were practising traditional medicine. As similarly reported (Cheikhyoussef et al. 2011; Mahwasane, Middleton & Boaduo 2013), elders above 60 years are usually endowed with significant traditional knowledge and practice of traditional medicine. This consequently validates the fact that a wide gap exists between the elderly and the younger generation on the knowledge of traditional healing (Ohemu et al. 2014). This finding also suggests that the legacy of traditional knowledge in the study area is endangered, especially if the knowledge is not being passed by the elders to the younger generation. However, Shapi, Matengu and Mu Ashekele (2009) posited that with regard to traditional knowledge, a considerable period is required for the indigenous people to acquire the inherent knowledge of traditional healing.

The findings of Arowosegbe, Olanipekun and Kayode (2015), however, contradict with those of this present investigation. They reported that the age bracket of 61 years and above had the least number of traditional medicine practitioners in Ekiti State, Nigeria. This could be ascribed to the fact that locality can be a determinant factor in the integration of new healers in traditional medicine practices. In consonance with the report of findings of Regassa (2013) and Arowosegbe et al. (2015), men are more involved in traditional medicine practices than women. This negates the assertion of Cheikhyoussef et al. (2011), who reported more female (70%) traditional healers than male (30%).

The medicinal plants cited in this investigation are presented in alphabetical order, including their botanical and local names, plant parts used, methods of preparation and administration, as shown in Table 2. Thirty-nine plant species, distributed in 33 families and 39 genera, were cited by the respondents as being used in the treatment of one or more ailments. The Fabaceae was the most representative family with three species, followed by the Euphorbiaceae and Myrtaceae with two species each.

Leaves (29%) and tree bark (23%) were the most commonly used plant parts for treatment. Other plant parts include seeds (13%), fruit (13%), roots (11%), flowers (9%) and pulp (2%) (Figure 2). Out of the 39 plant species cited for the treatment of diverse ailments by the traditional healers, 48% were trees, 31% were herbs and 21% included shrubs (Figure 3). The herbal remedies (Figure 4) were mostly administered orally (53%) and topically (26%) (Figure 5), whilst the most common method of preparation was decoction (42%), followed by maceration (25%), pounding (16%), crushing (9%) and chewing (8%).

Amongst the plant parts used against various ailments in the study area, leaves and barks are reported to be the most frequently used (Figure 6), contrary to the findings of Steenkamp (2003) and Shrivastava (2013) who both reported a high frequency of roots plant parts. The observed inference of high frequency of leaves can be attributed to the ease of collection and processing of abundant quantities (Rehmana et al. 2017; Tugume et al. 2016). This occurrence can also be ascribed to existence of biologically active compounds against diseases, compared with other plant parts (Faruque et al. 2018; Passulacqua et al. 2007). The leaf area of any plant constitutes the major photosynthetic area and the most metabolically active part – this enhances the production of secondary metabolites (through various biochemical pathways), which are responsible for its healing potential (Ghorbani 2005).

Majority of the herbal remedies in the study area were sourced from trees, contradicting the reports of Tumoro and Maryo (2016) and Faruque et al. (2018) that herbs were the most common life forms. From a conservationist perspective, the exploitation of leaves from trees is more sustainable as opposed to herbs because trees are more resilient, probably because of their large sizes. Notwithstanding, the harvesting of leaves should be regulated - harvesting in large quantities is destructive as it restrains the physiological processes in flowers, seeds and fruits, and this subsequently affects the regeneration potential of the plant (Tugume et al. 2016).

The administration of the herbal medicine mostly by oral means, as well as through topical, nasal and optical routes, corroborates previous findings (Kankara et al. 2015; Keo et al. 2018; Tugume et al. 2016; Tumoro & Maryo 2016; Umair, Altaf & Abbasi 2017). A high frequency of oral administration as reported was believed to be associated with the easy conveyance of the herbal remedies, which are mixed with some solvents or additives (Tugume et al. 2016).

As revealed in the study findings, different methods of herbal drug preparation were utilised by the traditional healers, which ranged (in ascending order) from decoction, maceration, pounding, crushing to chewing. Decoction was found to be the widely adopted method as corroborated by similar findings (Afolayan et al. 2014; Keo et al. 2018; Tugume et al. 2016; Umair et al. 2017). Conversely, Tumoro and Maryo (2016) and Habtamu, Mulatu and Tsdeke (2014) reported crushing, whilst Faruque et al. (2018) indicated paste as the most popular mode of herbal medicine preparation. Nonetheless, decoction is generally utilised in preparing herbal remedies because of the ease of preparation, in which the medicinal plant is boiled in a solvent, such as water, soup and tea (Rokaya, Münzbergová & Timsina 2010). Hot extraction by boiling was found to have more efficacy compared with cold extraction as it helps to preserve the efficacy of the herbal decoction over a long time (Hirt & M’pia 2008).

The diseases that were mentioned by the traditional healers and their ICFs, as shown in Table 3, were classified into seven categories: blood-related ailments, skin diseases, skeletal, gastrointestinal, STDs or genital, respiratory and others, and their ICFs ranged from 0.33 to 1. The highest consensus factor (1.00) was detected for chickenpox or measles or leprosy (four use reports), haemorrhoids (two use reports), and sickle cell ailments (two use reports). The lowest ICFs were revealed for the following categories of ailments, such as paralysis or stroke (ICF = 0.60; six use reports), ulcer (ICF = 0.50; 13 use reports) and cancer (ICF = 0.33; four use reports). Malaria or fever had the highest number of use reports (180), followed by diarrhea or cholera (71) and rheumatism or arthritis (70). However, the lowest use reports were observed for haemorrhoids (2), sickle cell (2), chickenpox or measles or leprosy (4) and cancer (4). Malaria or fever (20), diarrhea or cholera (17), dysentery or purging (15), and rheumatism or osteoarthritis or arthritis (15) had the highest number of medicinal plant species (Table 3).

The level of agreement by the respondents in using a particular medicinal plant for treating several ailment categories (Rehmana et al. 2017), as well as a means of establishing the most widely used plant species for the treatment of various diseases (Chekole 2017), is measured by ICF. Higher ICF (0.67 and 1) indicates ailments with a high frequency of prevalence in the study location. According to Kankara et al. (2015), this high ICF might be attributed to the efficacy of the plants in providing a remedy to the reported diseases. It can also be ascribed to communication and sharing of information on the potency of the medicinal plants, prolonged usage and feedback on the effectiveness of the medicinal plants. Albeit, the highest ICF observed for chickenpox or measles or leprosy, haemorrhoids and sickle cell ailment categories (Table 3) with fewer use reports implies that information on medicinal treatments as regards these ailment categories are kept secret amongst the traditional healers (Chekole 2017). In contrast, the low ICF values reported for paralysis or stroke (0.60), ulcer (0.50) and cancer (0.33) revealed a gap in communication and knowledge usually common with herbal healers divulging information for fear of copyrights (Chekole 2017).

As revealed in the study findings, the identified medicinal plants used for the treatment of different categories of ailments has FL ranging from 44.4% to 100%. Cymbopogon citratus, Dioscorea dumetorum, Grewia venusta, Piliostigma thonningii and Waltheria americana revealed a 100% FL (Table 2). Cola acuminate (50%), Garcinia kola (50%), Olea europaea (50%) and Carica papaya (44.4%) indicated the lowest FLs (Table 2).

Medicinal plants showing high FLs indicate higher pharmacological potentials because of the presence of potent phytochemicals, which requires further laboratory screening for the discovery of novel curing possibilities (Hassan-Abdallah et al. 2013). The percentage of FL (Table 2), as revealed in this study, ranged from 44.4% to 100%. This aligns with the range of values (45% – 100%) affirmed by Rehmana et al. (2017). They also acknowledged high FLs amongst plant species such as Cymbopogon citratus, Dioscorea dumetorum, Grewia venusta, Piliostigma thonningii and Waltheria americana.

Cymbopogon citratus (Poaceae), commonly known as lemon grass, was cited by respondents as being used in the treatment of convulsion, epilepsy and spasms. Many plants that contain essential oils (EOs) have been reported to exhibit anticonvulsant activity and might benefit people with epilepsy (Bahr et al. 2019). For example, clove, lemon grass and lavender contain constituents, such as citral, carvone, linalool and eugenol, which are phyto-constituents with potential antiepileptic activity. Whilst further research studies are needed to confirm their modes of action, it seems that the anticonvulsant properties of EOs are mainly because of their ability to regulate the GABAergic system of neurotransmission and their capacity to change ionic currents via ion channels (Bahr et al. 2019). A decoction of the leaves of G. venusta and W. americana was reported to be used for the treatment of diarrhea and dysentery by the traditional healers in the study area. The Grewia genus (Tiliaceae) encompasses about 150 species of small shrubs and trees, distributed in tropical and subtropical regions of the world, and is the sole genus that yields edible fruits in the family (Zia-Ul-Haq et al. 2013). The genus is reputed to cure stomach upsets, intestinal and skin infections, and seem to have mild antibiotic properties (Zia-Ul-Haq et al. 2013). Waltheria americana is considered as one of the 10 most important medicinal plants in Hawaii used as an aspirin-like drug for the management of inflammatory conditions, and the plant has also been reported for the treatment of diarrhea by traditional healers in other parts of Nigeria (Zongo et al. 2013). The high FL, as revealed in this study, indicates a wide acceptance and preferred usage of such herbal plants in treating specific ailments (Bibi et al. 2014; Islam et al. 2014; Zhang et al. 2015).