INTRODUCTION: Herbal medicine has been a cornerstone of healthcare for centuries, with an estimated 80% of the world’s population relying on it. In Africa, herbal medicine is the backbone of rural healthcare, serving 80% to 90% of the population. Despite its widespread use, the safety of herbal medicine raises a significant concern considering the lack of regulation and testing, particularly in Africa. Microbial contamination is a primary safety risk threatening consumer health. In this systematic review, we aimed to synthesise evidence on microbial contamination in herbal medicines across Africa, provide a clear understanding of the problem, and inform effective public health interventions regarding microbial contamination of herbal medicines in Africa.
METHOD: The systematic review was conducted in accordance with the PRISMA guidelines. A literature search was conducted across PubMed, Web of Science, Science Direct, Scopus, and Google Scholar using appropriate search terms. Eligible studies were selected based on predetermined criteria, and data were extracted and analysed.
RESULTS: The review included fifty eligible studies in Africa, with a combined sample size of 1996, of which 1791 showed microbial contamination. Bacterial contaminants were reported in 98% of studies, with Escherichia coli (62%) being the most reported bacteria, followed by Staphylococcus aureus (57%), and Bacillus spp. (55%). Fungal contaminants were reported in 70% of studies, with Aspergillus spp. (40%) being the most reported, followed by Penicillium spp. (27%) and Candida spp. (26%). Parasitic contaminants were reported in 2% of the studies reviewed. A total of 70 bacterial species, 37 fungal species, and 6 parasite species were identified in this review.
CONCLUSION: Herbal medicines in Africa pose significant health threats to consumers due to the high prevalence of diverse microbial contaminants and clinically significant pathogens. This emphasises the need for stricter regulations and quality control measures in the production, sale and use of herbal medicines.
Introduction
Herbal medicine has been an integral resource for health in communities globally for centuries. It is estimated that 80% of the world’s population use herbal medicines.1-4 In Africa, herbal medicine is the backbone of rural healthcare, providing essential support to a significant number of the population (an estimated 80%-90%).5 The patronage and use of herbal medicines have increased due to their availability, accessibility, and affordability.6-8 They provide a practical alternative to healthcare services in the rural communities of developing nations.9
Considering the expanding market for herbal remedies across African countries, it is important to address all safety concerns associated with their use.10 Several herbal products used in Africa remain untested and unregulated,1,4,11,12 posing significant health risks to consumers. According to a survey conducted by the World Health Organisation (WHO), only 43% of African member states currently have regulations in place for herbal medicines.4 The lack of effective regulation and monitoring make consumers vulnerable to diseases.13 Typically, the safety risks associated with herbal medicines include contamination by microbiological agents (such as bacteria and fungi), and chemical agents (such as metals, pesticides, residual solvents, and mycotoxins).14 Microbial agents are however, the most implicated contaminants in herbal medicines.11,12,15 The presence of pathogenic microbial contaminants in herbal medicines has generated increased apprehension, as they can lead to the development of serious infections.16
Across Africa, research on the microbial safety of herbal medicines is only largely conducted within individual countries. Thus, the fragmented nature of the relevant research hinders the development of continent–wide, comprehensive herbal safety guidelines and public health policies. It is therefore important to curate evidence that reflects the extent and diversity of microbial contamination in herbal medicines throughout the African region. To the best of our knowledge, no published review specifically collates and synthesises the evidence on microbial contamination in herbal medicines across Africa. While a previous review by Opuni et al.15 examined various contaminants in herbal medicines across low– and middle–income countries, it did not address parasitic contaminants, limiting our understanding of the full spectrum of microbial risks associated with herbal medicines. Similarly, the review by Walusansa et al.12 only examined bacterial contaminants in herbal medicines from East Africa, overlooking other microbial contaminants. Our systematic review aimed to provide a holistic and up–to–date analysis of the microbial contaminations associated with herbal medicines in Africa, integrating findings that have emerged since the publication of previous reviews. This study assessed original research articles that explored the presence and diversity of microbial contaminants in herbal medicines across African countries, spanning from 2000 to 2024. By examining this body of research, we sought to identify emerging trends and challenges concerning microbial contamination in herbal medicines in the 21st century. This knowledge is necessary to guide the development of relevant public health interventions and offer direction for future research on herbal medicine safety in Africa.
Method
Search strategy
This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta–Analyses (PRISMA) guidelines.17 Literature search was conducted between June 7th and 15th, 2024, across PubMed, Web of Science, Scopus, Science Direct, and Google Scholar, to identify articles related to the microbial contamination of herbal medicines in Africa, from year 2000 to 2024. The primary search strategy incorporated both Medical Subject Headings (MeSH terms) and keywords such as ‘Herbal Medicine’[Mesh] OR herbs OR ‘Plant medicine’ OR ‘Plants, Medicinal’[Mesh] OR ‘Plant Preparations’[Mesh] AND ‘Microbiology’[Mesh] OR microbes OR ‘Bacteria’[Mesh] OR ‘Fungi’[Mesh] OR ‘Viruses’[Mesh] OR ‘Colony Count, Microbial’[Mesh] AND ‘Africa’[Mesh]. The citations and references of the identified articles were carefully reviewed to include all relevant studies. The full electronic search strategy for all the databases used is shown in Supplemental Table 1.
Eligibility criteria
The review exclusively examined studies conducted from 2000 to 2024, which presented evidence of microbial contamination in herbal medicines across Africa. These studies were required to identify the specific microbial contaminants isolated from herbal medicines and determine the prevalence and/or load of these microbes. To ensure the reliability of the findings, only peer-reviewed articles that employed standardised laboratory methods for assessing microbial contamination and were published in English were included. Studies that investigated contaminants and adulterants other than microbes were excluded. Additionally, studies conducted outside Africa, review articles, and studies that did not specify microbial contaminants or provide sufficient information on contamination levels in herbal medicines were excluded from the review.
Study selection
The selection of studies for this review involved a three–phase screening process to retrieve articles of interest. In the initial phase, duplicates were identified and manually eliminated using the systematic review tool ‘Rayyan QCRI’.18 Two researchers then independently screened the remaining articles by reading the titles, abstracts, and keywords to identify relevant studies. Finally, the full texts of the remaining articles after the second screening phase were thoroughly reviewed to determine which studies met the inclusion criteria and were ultimately included in the review. The PRISMA flow diagram below (Figure 1) illustrates the article selection process.
Quality assessment
To evaluate the quality of the included articles, we employed the modified Oxman and Guyatt score,19 an analytical tool adapted from previous systematic reviews on herbal medicines.15,20,21 This tool assessed the study methodology, country of origin, and specific microbial contaminants reported, with 1 point allocated per dimension for a maximum score of 10. Two authors independently assessed and scored the articles, and discrepancies were resolved through consensus among all 3 authors. Articles with total scores ranging from 8 to 10 were considered to be of good quality, those scoring from 5 to 7 as fair, and those scoring from 0 to 4 as poor quality. The scoring system for quality appraisal and the assessment of included studies are presented in Supplemental Tables 2 and 3, respectively.
Data extraction and analysis
Two independent researchers extracted data from 50 articles that met the inclusion criteria. Each researcher entered the extracted data into spreadsheets, documenting various attributes, including author(s) name, year of publication, country, sample size, number of contaminated samples, identification methods, type of microbial contaminant, microbial loads, and specific microbial contaminants isolated. The geographical distribution of the included articles was visualised using a map, while bar charts and tables were used to visualise the distribution of study characteristics and findings. Bar graphs illustrating microbial contaminants and their occurrence levels in the included studies were created using Microsoft Excel. The data for these graphs were sourced from the individual studies included in this review.
Results
From an initial pool of 8005 search results, 50 research articles were selected for inclusion after the three–phase screening process. The quality of the selected articles ranged from fair to good based on the quality assessment parameters used.
Characteristics of the eligible studies
Table 1 shows a summary of the characteristics of the 50 studies included in this review. The highest number of eligible studies came from Nigeria, 25 (50%), followed by Ghana with 9 (18%), Kenya with 4 (8%), South Africa with 3 (6%), and Tanzania with 2 (4%). Côte d’Ivoire, Malawi, Cameroon, Sierra Leone, Benin, Uganda, and Lesotho, contributed 1 study each (2%). The distribution of these studies across different regions is shown in Figure 2.
Table 1.
Characteristics of the included studies in this review.
Of the 50 studies reviewed, 49 (98%) reported on bacterial contaminants, 35 (70%) reported on fungal contaminants, and only 1 (2%) study reported on parasitic contaminants in herbal medicines. Some studies examined multiple types of contaminants, resulting in a combined total that exceeds the total number of individual studies. Conventional culture and identification methods, encompassing gram staining, biochemical reactions, and physiological techniques, were employed in 96% of the studies to identify bacterial, fungal, and parasitic contaminants in herbal medicines. Molecular techniques for isolation and identification were used in only 4% of the studies. The prevalence of microbial contamination in herbal medicines varied widely, ranging from 14% to 100%.
Collectively, the included studies examined 1996 herbal medicine samples, with 1791 of the samples harbouring microbial contamination. This equates to an overall contamination prevalence of 90% in herbal medicines across Africa. Sixty-two percent (62%) of the reviewed studies reported a 100% prevalence of microbial contamination. The majority of studies included in this review, 39 (78%), were published from 2014 to 2024, while 11 (22%) were published between 2000 and 2013.
Bacterial contaminants of herbal preparations in Africa
A significant number of studies (98%) reported diverse bacterial contaminants in herbal medicines.8,10,22–68 Across these studies, 70 bacteria from 37 different genera were isolated. Escherichia coli emerged as the most frequently identified bacteria, reported in 62% of the studies. Other commonly reported bacterial contaminants include Staphylococcus aureus (60%), Bacillus spp. (54%), Pseudomonas spp. (46%), Salmonella spp. (44%), Klebsiella spp. (44%), Enterobacter spp. (38%), Proteus spp. (22%), Serratia spp. (16%), Citrobacter spp. (16%), Enterococcus spp. (12%), Streptococcus spp. (10%), Pantoea spp. (10%), Shigella spp. (8%), Acinetobacter spp. (8%), Providencia spp. (6%), Rahnella spp. (4%), Chryseomonas spp. (4%), and Moraxella spp. (4%). Each of the following bacterial contaminants was reported in 2% of the included studies: Edwardsiella spp., Cedecea spp., Flavimonas spp., Stenotrophomonas spp., Ewingella spp., Bordetella spp., Pasteurella spp., Aeromonas spp., Arizona spp., Kocuria spp., Rhizobium spp., Leclecia spp., Sphingomonas spp., Raoultella spp., Paenibacillus spp., Corynebacterium spp., Micrococcus spp., and Yersinia spp. Figure 3 illustrates the percentage distribution of the common bacterial isolates identified in this review.
From the studies included in this review, reports on the bacterial loads of the various herbal medicines analysed, revealed varying levels of contamination across different countries. The bacterial loads documented in this review generally ranged from 0 cfu/mL to 3.54 × 1012 cfu/mL. The highest bacterial load recorded (3.54 × 1012 cfu/mL) was reported in Nigeria by Igbeneghu and Lamikanra.36 The samples in this study were sourced from unregulated herbal medicines on the market. Similarly, another study by Nwankwo and Olime,28 which investigated microbial contamination in registered herbal preparations on the Nigerian market, reported bacterial loads ranging from 3.10 × 102 cfu/mL to 2.56 × 103 cfu/mL in liquid formulations, and 9.0 × 101 cfu/g to 1.5 cfu/g × 102 cfu/g in powdered herbal preparations.
In a study conducted in Nigeria by Tatfeng et al.,33 it was noted that ‘schnapps’ and palm wine–based preparations were mostly contaminated with Bacillus spp. (aerobic spore bearers), while water-based preparations had several bacterial isolates, including Staphylococcus spp., Pseudomonas aeruginosa, Escherichia coli 0157, Proteus mirabilis, Enterococcus faecalis, Serratia marcescens, Staphylococcus aureus, and Bacillus spp.
Also in the studies outlined, Brooks and Takim25 reported a Total Viable Bacterial Count (TVBC) of 2.2 × 104 cfu/g to 5.6 × 104 cfu/g for solid dosage forms and 3.8 × 104 cfu/mL to 6.8 × 104 cfu/mL for liquid forms of herbal medicines sold in Calabar, Nigeria. Nwankwo and Olime28 reported a Total Heterotrophic Bacterial Count (THBC) of 3.1 × 10² cfu/mL to 2.65 × 10³ cfu/mL for liquid preparations and 1.1 × 10² cfu/g to 1.5 × 10 ²cfu/g for powdered preparations. Walusansa et al31 in a study conducted in Uganda reported a mean viable load of 126.407 × 104 cfu/mL or g across 140 samples. Kaume et al.35 reported bacterial loads ranging from 3.03 × 104 cfu/mL to 4.22 × 10⁵ cfu/mL in some herbal medicines in Kenya.
Omoruyi et al.54 in Nigeria found microbial counts ranging from 2.8 × 104 cfu/mL to 3.1 × 104 cfu/mL for regulated products and 3.8 × 104 cfu/mL to 12.6 × 10³ cfu/mL for unregulated products. A study conducted by Onyambu et al.61 on regulated and unregulated herbal medicines in Kenya reported a bacterial load count of 1.50 × 1010 cfu/mL in unregulated herbal medicines and counts below 100 cfu/mL in registered herbal products. Adounkpe et al.57 reported bacterial loads ranging from 9.15 × 107 cfu/mL to 3.65 × 109 cfu/mL in herbal medicines from Benin. Osei-Adjei et al.45 reported bacterial loads ranging from 1.0 × 102 cfu/mL to 1.0 × 109 cfu/mL in herbal medicines from Ghana. Kira et al.62 reported a mean bacterial load of 1.64 × 108 cfu/mL in herbal medicines from Tanzania.
Fungal contaminants of herbal medicines in Africa
Fungal contaminants were reported in 35 (70%) studies.10,24-29,32,33,35-43,45-47,50,52,53,56-61,63,65-67,69 Forty (40) fungal species from 24 different genera were identified (Table 1). Aspergillus spp. was the most commonly reported fungal species, appearing in 40% of the studies. This was followed by Penicillium spp. (28%), Candida spp. (24%), Mucor spp. (20%), Rhizopus spp. (20%), Fusarium spp. (8%), Cladosporium spp. (6%), Saccharomyces spp. (6%), Trichosporon spp. (4%), Scedosporium spp. (4%), and Geotrichum spp. (4%). Other fungal contaminants reported include Phaeoacremonium spp., Curvularia spp., Cryptococcus spp., Trichoderma spp., Alternaria spp., Mycelia spp., Rhodotorula spp., Sporobolomyces spp., Phialophora spp., Torula spp., Trichophyton spp., Microsporium spp., and Syncephilastrum spp., each reported in 2% of the reviewed studies.
Several studies included in this review also documented a wide range of fungal loads in herbal medicines. The total fungal loads reported across the studies ranged from 0 cfu/mL to 3.54 × 1012 cfu/mL. Some of the highest fungal loads reported in this review were: 3.54 × 1012 cfu/mL recorded in Nigeria,36 1.60 × 109 cfu/mL from Sierra Leone,37 6.0 × 108 cfu/mL from Lesotho,27 1.3 × 108 cfu/mL from Nigeria,43 4.4 × 107 cfu/g recorded in Cote d’Ivoire,58 4.7 × 107 cfu/mL33 and 1.5 × 107 cfu/mL both recorded in Nigeria. Akande et al.47 reported total fungal counts of 1.0 × 105 cfu/mL to 8.0 × 105 cfu/mL in herbal medicines from Nigeria. The fungal counts as presented by the studies included are shown in Table 1. Figure 4 shows the percentage frequency of the most reported fungal isolates.
Parasitic contaminants of herbal medicines in Africa
Only 1 study (2%) reported parasite contamination in herbal medicines in the reviewed studies. The study by Onyemelukwe et al.66 reported a 53% occurrence of parasites in 80 herbal medicine samples from Nigeria. Ascaris lumbricoides, was the most prevalent parasite in that study, detected in 53.7% of the samples, followed by hookworm ova (19.5%), and Toxocara canis (12.2%). The least prevalent parasites were Entamoeba coli, Giardia infestinalis, and Entamoeba histolytica/dispar each found in 4.9% of the samples.
Discussion
The increasing use of herbal medicines and other crude concoctions in Africa raises concerns about their safety to consumers, particularly relating to their microbial quality. This study analysed fifty studies that investigated the prevalence and loads of microbial contaminants in herbal medicines across Africa; Nigeria, Ghana, Kenya, South Africa, Tanzania, Cote d’Ivoire, Malawi, Cameroon, Sierra Leone, Benin, Lesotho, and Uganda. Most (78%) of these studies were conducted in the recent decade (2014-2024). This trend is supported by other findings,12 indicating a significant increase in research addressing the microbial contamination of herbal medicines in Africa.
This review reported on bacterial, fungal, and parasitic contaminants in herbal medicines across the African region. The majority of included studies (98%) reported on bacterial contaminants. Escherichia coli was the most reported bacterial pathogen in herbal medicines across the African region. This finding is consistent with the report from the study conducted by Walusansa et al, which identified Escherichia coli as the most prevalent bacterial contaminant in herbal medicines.12 Findings from another study conducted by Opuni et al.,15 also reported Escherichia coli as the most reported bacterial contaminant found in herbal medicines across low–and middle– income countries. The presence of this pathogen in herbal medicines suggests possible faecal contamination, raising concerns about the potential for direct or indirect exposure to human or animal waste during preparation.27,31 According to the World Health Organisation, (WHO),70 the presence of E. coli not only indicates faecal contamination but also raises concerns about the potential presence of more virulent strains, such as shiga toxin-producing E. coli. These strains are implicated in life–threatening diseases such as haemolytic uraemic syndrome, particularly in vulnerable populations like young children, the elderly, and HIV/AIDS patients.15,70 Notably, some studies included in this review26,35 investigated herbal medicines marketed to HIV/AIDS patients. These studies revealed alarming levels of bacterial contamination exceeding acceptable limits. As reported in these studies, liquid formulations recorded bacterial counts as high as 1.19 × 109 cfu/mL,26 while solid dosage forms recorded 7.1 × 108 cfu/g,35 exceeding the acceptable limits of 105 cfu/mL for liquid samples and 107 cfu/mL for solid samples.71 This poses a significant health threat to an already immunocompromised population.
This review also identified Staphylococcus aureus, Bacillus spp., Pseudomonas spp., and Salmonella spp. as commonly reported bacterial pathogens (60%, 54%, 46%, and 44% of included studies respectively) from herbal medicines in Africa. These findings are consistent with findings from low–and middle–income countries in other regions. Studies conducted by Opuni et al.15 and De Souza Lima et al.11 identified Salmonella spp., Bacillus spp., Pseudomonas aeruginosa and Staphylococcus spp. as common bacterial pathogens in herbal medicines. These organisms which are also indications of faecal contamination, reveal poor hygiene conditions in the preparation and storage of these herbal medicines, thus making them unsafe for consumption.15,72 In this review, Govender et al.26 identified diarrhoeal toxins produced by Bacillus cereus in herbal medicines from South Africa. Additionally, other studies26,73,74 highlight the potential health risks posed by toxins when consumed. The potential for severe infectious diseases among the African population due to contaminated herbal remedies is a serious concern, given the presence of numerous medically important pathogens. Staphylococcus aureus, for example, which was reported in 60% of studies included in this review causes staphylococcal gastroenteritis, scalded–skin syndrome, toxic shock syndrome, endocarditis, lung infection, folliculitis, among other diseases.75-77 These diseases are life–threatening in older people and immunocompromised adults.76
According to the World Health Organisation (WHO), ‘Salmonella and Shigella species must not be present in herbal medicines intended for internal use, at any stage’.71 Contrary to this guideline, Salmonella spp., and Shigella spp. were reported in 44% and 8% of the studies respectively. These organisms have the potential to cause large disease outbreaks due to their low infectious dose.78 They are responsible for a significant disease burden worldwide, causing diarrhoea and a spectrum of associated symptoms, from mild to life–threatening.79 The CDC estimates that Salmonella spp. causes approximately 1.4 million infections, 26, 500 hospitalisations, and over 400 deaths annually in the United States.80 This poses a significant threat to public health in Africa, where many people rely on herbal remedies and may lack access to adequate medical care. Similar to our findings in Africa, gram–negative bacteria such as Escherichia coli, Klebsiella spp., Pseudomonas spp., Shigella spp., and Salmonella spp. in addition to several species of Staphylococcus have been reported as major contaminants in herbal medicines from other continents, particularly Asia.81-83
Another popular finding across multiple studies included in this review is the presence of fungal isolates from the genera Aspergillus, Penicillium, Candida, Mucor, Rhizopus, Fusarium, Cladosporium, and Scedosporium in herbal medicines across the African region. These fungal species have been identified in herbal medicines across various regions globally as evidenced in studies conducted by De Souza et al., Kneifel et al., Lee & Yoon, Opuni et al., and Zheng et al.15,72,84-86 The study by Kneifel et al.84 revealed that fungal isolates in herbal medicines can degrade active ingredients reducing their effectiveness, and potentially produce mycotoxins. These toxins are mainly produced by fungi from the genera Aspergillus, Penicillium and Fusarium.87 Exposure to these toxins can have devastating effects on human health, potentially leading to liver cancers, weakened immunity, altered protein metabolism, seizures, and respiratory problems among other health complications.88-90
Herbal medicines to a large extent are mostly contaminated with bacterial and fungal elements.15 However, one study included in this review reported the contamination of herbal products from Nigeria with parasite forms such as helminths and protozoans. The study by Onyemelukwe et al.,66 reported the presence of helminth eggs and protozoan cysts in herbal preparations at a staggering 53% occurrence. The parasites found in these herbal preparations included Ascaris lumbricoides, hookworm, Toxocara canis, Entamoeba coli, Entamoeba histolytica/dispar and Giardia intestinalis.66 Data from other regions such as Asia supports the occurrence of parasitic contaminants in herbal medicines. A study conducted by Posadzki et al.,21 found parasitic contaminants similar to those identified in the study from Nigeria in herbal medicines.
The problem of microbial contamination of herbal products in Africa is exacerbated by widespread environmental pollution and unsanitary conditions66,91 which is common in Africa. Several studies included in this review8,10,22,24-27,34,44,56 attributed the high prevalence of microbial contamination in herbal medicines to a combination of factors, including lack of regulation, and pollution throughout the production chain, from harvesting raw materials, to handling, processing, storage, and transportation. According to Onyemelukwe et al., the trees and plants from which medicinal preparations are made could have microorganisms adhered to their stems, barks, leaves, flowers, fruits, and roots eventually leading to contamination of the product.66 Other factors contributing to the high prevalence of microbial contamination in herbal medicines as reported in the reviewed studies include the use of untreated water supply, poor quality of packaging materials, use of contaminated containers, working from polluted faecal environments, and poor personal hygiene behaviours during handling.29,31-33,59
A survey conducted by the World Health Organisation (WHO) in 2019, indicated that 43% of African member states regulate herbal medicines, compared to 26% in 2005.4 However, despite the progress in regulatory efforts, this study found a significant 90% overall prevalence of microbial contamination in herbal medicines, highlighting the need for stricter regulations in the African region. The prevalence of microbial contamination in herbal medicines is a public health concern in Africa. To address this challenge, it is important that existing regulations are enforced and novel regulations adopted in countries where they are lacking. Also, producers of herbal medicines should ensure strict quality control measures and Good Manufacturing Practices (GMP) are followed throughout the production and distribution processes to minimise the proliferation of microorganisms in these products. Failure to address this issue could lead to widespread health problems in Africa. Research on the microbiological safety of herbal medicines in Africa must expand beyond fungal and bacterial contaminants to include parasites for a comprehensive understanding of the unique challenges associated with these remedies.
Limitations of the Study
While this systematic review provides valuable insights, it is subject to some key limitations. Firstly, the literature search was restricted to peer-reviewed studies published in English language, potentially excluding grey literature and other relevant studies not published in English. Secondly, the studies captured in this review were mainly from the western, eastern, and southern parts of Africa, limiting its generalisability.
Conclusion
This systematic review provided a comprehensive overview of the microbial contaminants reported in herbal medicines across Africa, revealing a disturbingly wide range of bacterial, fungal, and parasitic species with varying degrees of contamination. The presence of pathogens such as Escherichia coli, Staphylococcus aureus, Bacillus spp., Pseudomonas spp., Salmonella spp., Klebsiella spp., Shigella spp., Aspergillus spp., Penicillium spp., Candida spp., Mucor spp. and Entamoeba histolytica among others, poses a significant risk to consumer safety. The findings of this review underscore the urgent need for stricter regulations and quality control measures to ensure the safety of herbal medicine products in Africa, ultimately protecting the health and well-being of consumers.
Author Contributions.
Conceptualisation, ESD; methodology, WKA, SD, and ESD; validation, SD, and ESD; formal analysis, WKA, and SD; resources, ESD; data curation, WKA; writing—original draft preparation, WKA, SD and ESD; writing—review and editing, WKA, SD, and ESD; visualisation, WKA, and SD; supervision, ESD.
Supplemental Material.
Supplemental material for this article is available online.
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