Introduction

Since antiquity, plants have offered an invaluable source of preventive and curative therapies for many ailments affecting humans and farm animals (Kareru et al. 2007). Currently, the exploitation of plant-based products and their commercialization is rising globally (Sahoo and Manchikanti 2013; Ros et al. 2018; WHO 2013a). Previous reports indicate that over 80% of the population in developing countries and 40% in Western world utilize herbal products to treat various diseases (Lu and Lu 2014; Tugume and Nyakoojo 2019). In Kenya, over 90% of its inhabitants have, at least once, used herbal therapies to treat and manage health conditions and to promote healthy living (Gakuya et al. 2020). Different communities throughout the world have specialized traditional medicine practice and demonstrate profound knowledge on the application of medicinal plants in preventing, managing, and treating various diseases (Yuan et al. 2016). Medicinal plants are arguably accessible, affordable, and associated with fewer side effects, hence preferred by over 80% of the world population (WHO 2013b). As a result, in some countries like China and India, traditional medicine has been incorporated into the conventional healthcare system as a fully functional entity, owing to their proven therapeutic efficacy (WHO 2013b; Mordeniz 2019; Abbott 2014; Sen and Chakraborty 2017). In fact, research has shown that most patients, especially in the rural settings of the African continent, combine traditional medicines with conventional medicines to accelerate recovery and avert undesirable effects (Mwaka et al. 2020). Moreover, the commercialization of herbal products in Kenya, especially in the Western Region, including Kisii, is profound due to their perceived potency and safety (Chebii et al. 2020).

Due to the promising potential of medicinal plants in offering therapeutic solutions, research has been heightened to elucidate their efficacies in the claimed diseases (Yuan et al. 2016). Currently, there are enormous reports on the efficacy of various plant extracts on various disease conditions. However, there are scanty recapitulations of these reports, presently available, which forestall literature collation to advance research.

The people of the Abagusii community are endowed with a wealth of traditions and cultures, which spun many ages (Ondicho 2015a; Omare 1999). Herbal medicine practice is the major and essential component of the Abagusii culture, which has fostered their health and animals (Mwalimu 2019). However, ethnomedical documentation and empirical validation of medicinal plants used by the Abagusii people (Omare 1999; Nyarang and Bonareri 2021) has dragged due to the locals' fear of losing traditional knowledge. Additionally, due to lifestyle change that has contributed to the loss of biodiversity, rural–urban migration, and the death of the elderly, who are usually the custodians of the heritage (Gakuya et al. 2020), urgent interventions to preserve the ethnomedical knowledge for heritage, and the advancement of science are warranted.

Despite the available ethnomedical studies on various medicinal plants used by the Abagusii people to meet their healthcare needs (Charles and Bonareri 2020), these studies are far between, thereby necessitating the need for detailed recapitulation to promote and enhance ethnomedical research. The current study compiled and analyzed available scientific literature on plants used in traditional medicine practices of the Abagusii community in Western Kenya. The findings contribute to the conservation of indigenous knowledge of medicinal plants used in traditional medicine by the Abagusii community.

Materials and methods

Study area

The area covered for the current study was Gusiiland, inhabited by the native Abagusii people. The Gusiiland covers a total land area of 2230 Km2 of South Western Kenya, about 50 Kilometers East of Lake Victoria. The region is composed of Kisii and Nyamira Counties with a population of 1,872,436 as per the 2019 Kenya population and housing census, respectively (2019). Gusiiland is demarcated by the coordinates 0° 30' and 1° 00' S and 34° 30' and 35° 00' E. The area is characterised by Gusii highlands at elevations of 1190 m in the north-western corner of the territory and 2130 m in the central highlands. It borders Narok County to the South and Homabay and Migori Counties to the West, Bomet County to the East, and Kericho County to the North East. The mean maximum temperatures range from 28.4 °C at the lowest elevation to 22.8 °C at the highest elevation, while the mean minimum temperatures are 16.4 °C and 9.8 °C, respectively. Rain falls throughout the year with an annual average of 150–200 ml. There are two peak seasons of rainfall: the major rainy season (March to May) and the minor rainy season (September to November). In the nineteenth century, much of present-day Gusiiland was covered by moist montane forests; however, due to the increasing population, all major forests have been cleared with scant indigenous vegetation remaining, and no large mammals can be found.

In the past, Gusiiland was heavily forested with old indigenous broadleaf rainforest trees and other flora and was part of the old Congo Basin forests. The only remnant of this old forest in Kenya is the Kakamega Forest, the westernmost tip of the Equatorial rainforest. Indigenous plant diversity in Gusiiland is constantly on a declining trend and is aggravated by the need for agricultural land and settlement area due to the increasing human population. On the other hand, the reduced need for traditional herbal medicine, which has now been surpassed by modern medicine in hospitals and medical care, is also why the Abagusii are not keen on indigenous plants Mutie et al. (2023). As a result, several traditional medical practitioners in the two counties operate without recognition, with only about twenty registered in Kisii county and none in Nyamira county (County Government of Nyamira 2018; Kisii County Government 2018).

Data collection and analysis

Data extraction and management

Data was extracted from one textbook (Sindiga et al. 1995), articles published in peer-reviewed journals (Matsuzono 1993; Wainaina et al. 2019; Omwenga et al. 2015; Furnas 1985; Muregi et al. 2003; Bjorå et al. 2015), and published and unpublished Masters and Ph.D. theses (Omare 1999; Mwalimu 2019, Gisesa 2004; Wainaina 2019; Ondicho 2015b; Mogwasi 2016; Bosire 2014; Amboga 2009). Online electronic databases used for literature search were Google Scholar, Web of Science, Scopus, PubMed, Science direct, and flora databases of different countries, accessed between April 2020 and March 2021. Specific search terms used were "medicinal plants for Abagusii people", "traditional medicines for Abagusii", and "Indigenous medicine, Abagusii, Kenya". After identifying potential literature sources, an inventory of medicinal plants used by the Abagusii people was created using Excel Spreadsheet (Microsoft 365).

The consolidated inventory included scientific plant names with their families in parentheses, common name, reference, origin of the plant, growth habit, county of original research, original use reports, plant parts used, preparation methods, route of administration, and medicinal use category. The classification of various use reports into various categories of health disorders was based on the International Classification of Primary Care—2nd Edition (ICPC-2 and International classification of diseases (ICD-11) 2015; ICD-11 for Mortality and Morbidity Statistics 2020).

Inclusion and exclusion criteria

Only primary data obtained from original research was used to develop the inventory. Plants whose use reports were referred by their vernacular names or genus names, and their identities were authenticated after collecting and depositing voucher specimens at the East Africa herbaria, hosted at the National Museums of Kenya, were included in this study.

Plants whose use reports were referred by their vernacular names or genus names or even were assigned scientific names without proper authentication were excluded from this study. Also, plants with incomplete, unauthenticated, and incorrect information were not included in this study.

Scientific names of plants

Scientific names of medicinal plants from primary sources were corroborated and updated to accepted names as stated in The Plant List (2013). After that, the medicinal plants were assigned their respective families according to the Angiosperm Phylogeny Website (2020).

Use reports and medicinal use categories

Raw data was classified by year of publication, medicinal plant species and family, origin, habit, plant parts used, mode of preparation and administration, use report, and medicinal use category. Use report in this study referred to the local uses of medicinal plant species as reported in the primary reference.

Homogeneity

The index of Informant Consensus Factor (ICF) was calculated using the Eq. (1) described by Trotter and Logan (1986):

$${\text{ICF}} = \frac{{{\text{Nur}} - {\text{Nt}}}}{{{\text{Nur}} - 1}}$$
(1)

where Nur is the number of use reports in a particular category, and Nt is the number of species used as medicine in a particular use category (Figs. 1, 2).

Fig. 1
figure 1

Map of Gusiiland showing Kisii and Nyamira Counties

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Fig. 2
figure 2

A flow chart summarizing the literature screening procedure

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The index of Informant Consensus Factor (ICF) values closest to one indicate homogeneity in the use of medicinal plants among the informants, who in this case were the Abagusii people from different localities that use the same medicinal plant species for the same ailment. On the other hand, the index of Informant Consensus Factor (ICF) values close to zero suggests little or no exchange or sharing of knowledge among the informants (Trotter and Logan 1986).

Results and discussion

Diversity of medicinal plants

In the current study, two hundred and fifteen (215) plant species were used by the Abagusii people of Western Kenya. The plants belonged to sixty-nine families of which Asteraceae (Sindiga et al. 1995) had the most number of species, followed by Fabaceae (Nyarang and Bonareri 2021), Lamiaceae (Mwaka et al. 2020), Solanaceae (Sen and Chakraborty 2017), Malvaceae (Yuan et al. 2016), Euphorbiaceae (Gakuya et al. 2020), Poaceae (Gakuya et al. 2020), Papilionaceae (Lu and Lu 2014), Moraceae and Phyllantaceae (Lu and Lu 2014), Asparagaceae and Amaranthaceae (WHO 2013a), Apocynaceae, Rutaceae, Myrtaceae, Verbenaceae and Meliaceae (Ros et al. 2018), Anacardiaceae, Cactaceae, Ebenaceae, Oxalidaceae, Salicaceae, Amaryllidaceae, Arecaceae, Polygonaceae, Proteaceae, Apiaceae, Urticaceae, Rhamnaceae, Celastraceae, Crassulaceae, Bignoniaceae (Sahoo and Manchikanti 2013) and 35 families were represented by one medicinal plant species (Table 1).

Table 1 Plant families and the number of medicinal plants used by Abagusii people of Western Kenya
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The plant diversity results recorded in this study corroborate Zhou et al. (2017), who described Asteraceae, Fabaceae, Lamiaceae, Malvaceae, Euphorbiaceae, and Poaceae as the largest families of the vascular flora of Kenya. Therefore, there was a higher likelihood of many plant species of medicinal significance belonging to the larger families. However, other families with more than five medicinal plant species, including Solanaceae, Cucurbitaceae, Papilionaceae, Moraceae, and Phyllanthaceae, have also been described as families with medicinal plant species that are used by other communities in Kenya (Kigen et al. 2019; Mbuni et al. 2020; Nankaya et al. 2020).

The recorded plant species in the current study were mostly herbs (80), followed by shrubs or trees (39), shrubs or trees (29), herb or shrubs (9), woody annuals (2), fern and sedge (1), respectively (Fig. 3). Nineteen (19) plant species were not identified with any listed growth form (Fig. 3). Of the 215 plant species, 68% were indigenous, 24% exotic, and 8% were not classified.

Fig. 3
figure 3

Growth form of plant species used by the Abagusii community

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Medicinal plant use reports

Of the two hundred and fifteen medicinal plant species recorded in the current study, twenty of them had ten or more use reports (Fig. 4 and Table 2). Urtica massaica had the highest number of use reports (Furnas 1985), followed by Senna didymobotrya (Omwenga et al. 2015), Carissa edulis and Aloe secundiflora (Nyarang and Bonareri 2021), Erythrina abyssinica and Solanum incanum (Omare 1999), Toddalia asiatica and Bidens pilosa (Ondicho 2015a), Citrus aurantium (Chebii et al. 2020), Warburgia ugandensis and Croton macrostachyus (Pandey et al. 2013), Persea americana, Solanum aculeastrum and Capsicum frutescens (2021), Coffea arabica (Abbott 2014), Eucalyptus globulus, Ocimum lamiifolium, Daucus carota, Carica papaya and Rotheca myricoides (Mordeniz,Mordeniz 2019), respectively. The high number of medicinal plant use reports in our present study has also been demonstrated by other researchers in different communities (Mbuni et al. 2020; Nankaya et al. 2020; Njoroge 2012; Njoroge et al. 2010).

Fig. 4
figure 4

Twenty medicinal plant species which had ≥ 10 use reports

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Table 2 A recap of the ethnomedicinal usage of medicinal plants by the Abagusii people of Western Kenya
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Parts of medicinal plants used by the Abagusii people of Western Kenya

Twenty-one (21) different plant parts were used to prepare medicines by the Abagusii people of Western Kenya. The parts were broadly classified as unorganized (4.6%) and organized (95.4%). The former constituted plant parts with no cellular structures included juice, latex, seed oil, and gum. On the other hand, drugs that consisted of cellular organization, in the form of anatomical features of plants, were leaves, roots, whole plants, stem bark, peels, stems, seeds, flowers, root barks, shoots, woods, bulbs, root tubers, rhizomes, twigs and maize cobs (Fig. 5).

Fig. 5
figure 5

Parts of medicinal plants used by the Abagusii people of Western Kenya

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The leaves were the most commonly cited plant parts used by the Abagusii people to treat different categories of diseases, yielding 224 use reports (Fig. 5). Similarly, other researchers have demonstrated that leaves are preferred in traditional medicine because of their role in photosynthesis and the related pathways, which are responsible for the synthesis of secondary metabolites like alkaloids and phenolics, with pharmacologic significance (Mbuni et al. 2020; Fortini et al. 2016; Kipkore et al. 2014; Hassan-Abdallah et al. 2013; Nanyingi et al. 2008). Additionally, the high frequency of leaves in plants and availability in large quantities compared to other plant parts contributes to their high preference.

The second highly used plant parts by the Abagusii people were the root, with one hundred and twenty-two (122) use reports, which was about half the leaves' use frequency (Fig. 5). Some studies elsewhere have indicated high use of roots (Nankaya et al. 2020). Nevertheless, there is a great similarity of chemical composition between the leaves and roots, therefore, appealing to use leaves as alternatives renewable plant parts such as leaves, young stems, and fruits in place of bark and underground parts like root, rhizome for sustainable use of medicinal plants (Jena et al. 2017).

Preparation and administration of medicinal plants used by the Abagusii people

The current study revealed different methods to prepare traditional medicines by the Abagusii people (Table 2). The methods identified were: decoction, infusion, concoction, pounding, roasting, steaming, chewing, and poultices. The decoction was the most common preparation method with 218 use reports, followed by infusion (98) and concoction (82). Our findings are consistent with those of other researchers, who have shown that decoctions, concoctions, and infusions dominate other methods in different cultures because they are easy to prepare (Tugume and Nyakoojo 2019; Jaradat and Zaid 2019). The addition of honey, sugar, salt, or milk to the decoctions, infusions, and concoctions was noted and can be attributed to taste improvement, as demonstrated among the Maasai people of Kenya (Nankaya et al. 2020). Macerates, juices, pastes, and eating without preparation were the least common preparation methods used by the Abagusii people.

The routes of administration of traditional medicines by the Abagusii community that were predominant included oral, topical, inhalation, poultice, and dropping. The oral route was the most common with 514 use reports, followed by topical (152). Inhalation and drops were the least common methods (< 10 use reports). Various studies have demonstrated the supremacy of oral and topical routes of administration of herbal medicines (Wambugu et al. 2011; Tabuti et al. 2012; Boadu and Asase 2017). In addition, these routes may be preferred because of the associated advantages that include simplicity, convenience, safety, economic, and non-requirement for special sterility, knowledge, and special supplies (Talevi and Quiroga 2018; Raj and Raveendran 2019).

Disease categories managed by the medicinal plants of the Abagusii people

Eight hundred and seventh (870) use reports of medicinal plants by the Abagusii people of Western Kenya were documented in this study. The use reports were classified into twenty (20) health disorder categories according to the international classification of Primary care classification system 2nd edition (ICDP-2) and the International classification of diseases version 11 (ICD-11) (Nyarang and Bonareri 2021; Charles and Bonareri 2020). The study revealed that the Abagusii people mostly utilize medicinal plants to treat gastric/intestinal disorders, positing the highest number of use reports (228 use reports). The second most common medicinal plant use was for managing skin infections and disorders (131 use reports). Other health disorders managed by the medicinal plants of the Abagusii people with more than fifty (50) use reports were: musculoskeletal disorders and swelling/oedema (97 use reports), urinogenital infections (92 use reports), and respiratory diseases (76 use reports) (Table 2).

Informant consensus factor (ICF) (homogeneity of use)

Nervous disorders were recorded with the highest ICF value of 0.59. followed by infections and disorders of the skin, intestines, and urinogenital with records of ICF values of 0.55, 0.54, and 0.5, respectively. Zero ICF values were recorded for anthrax, cardiovascular disorders, and evil eyes (Table 3).

Table 3 Disease category and the informant consensus factor (ICF) (homogeneity of use) of medicinal plants by the Abagusii people
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The revelation that intestinal disorders were most commonly treated by the Abagusii people ethnomedicinally is consistent with other previous findings in different Kenyan cultural communities, as demonstrated by Kigen et al. (2019) and Mbuni et al. (2020). In addition, intestinal disorders have also been reported to be predominantly treated by people of various ethnic groups of other African countries (Tugume and Nyakoojo 2019) and other continents (Fortini et al. 2016; Zank and Hanazaki 2017; Rakotoarivelo et al. 2015).

The high ICF values indicated that there was sharing of plant species information used to treat nervous disorders, skin infections and disorders, intestinal disorders, and urinogenital infections and disorders by the Abgusii people of various parts of Nyamira and Kisii Counties. The high ICF values indicate the efficacy of the used plant species (Singh et al. 2020; Faruque et al. 2018). However, the low levels of information shared for oral and dental problems, respiratory disorders, Animal bites, Circulatory system disorders, Musculoskeletal disorders, and swelling/oedema, malaria, ear, nose, and throat (ENT) problems, fevers, metabolic disorders, birth-related disorders, and eye problems were possibly due to the lack knowledge/information sharing across geographical locations or within groups (Nankaya et al. 2020).

Besides, our study revealed not homogeneity of use of medicinal plants against anthrax, cancer, cardiovascular disorders, and evil eyes. Our findings indicated that there is limited information sharing about plant species used to treat these conditions among the Abagusii people of Nyamira and Kisii County. However, other communities have demonstrated high levels of information sharing over the plant species used to treat anthrax, cancer, cardiovascular system disorders, and evil eyes (JimaJima Jima and Megersa 2018; Simegniew Birhan et al. 2017; Chekole et al. 2015; Musa et al. 2011; Wubetu et al. 2017; Issa et al. 2018; Woldemariam 2016).

Conclusions

Our study reveals the integral role of medicinal plants in promoting health among the Abagusii community of Kenya, with 870 documented use reports in managing various diseases. Based on the study findings, we concluded that the Abagusii people mostly utilize medicinal plants to treat gastric/intestinal disorders, followed by skin infections and disorders, musculoskeletal disorders and swelling/oedema, urinogenital infections, and respiratory diseases. Moreover, there is substantial ethnomedicinal knowledge sharing among the Abagusii people on various disease categories, as revealed by the ICF values in this study. However, limited homogeneity of medicinal plant use against anthrax, cancer, cardiovascular disorders, and evil eyes among the Abagusii people was noted and attributed to lack of information or knowledge sharing within and across different geographic locations of Kisii and Nyamira Counties. Generally, our study lays a framework for empirical studies towards the validation of medical plants of the Abagusii people in the management of various ailments and the possible development of alternative, efficacious, affordable, accessible, and safe therapeutic remedies.