A healthy person tends to have a good immune system that combats potential infectious diseases caused by insects, fungi, bacteria or viruses. The immune system is the natural defence mechanism against diseases whereby it can produce an unlimited variety of cells and molecules to inhibit the manifestation of a variety of infections and undesirable substances (Sharma et al. 2017). It plays a crucial role in guarding against most external disease-promoting factors and malicious cells using helper and suppressor cells and soluble products (Mirabeau & Samson 2012). The immune system may be compromised and may result in either sickness or even death in some cases as experienced in the recent outbreak of the coronavirus pandemic (Cucinotta & Vanelli 2020). However, to survive infections, medical experts advised the consumption of food items or medications capable of boosting the immunity of a compromised immune system to maintain the high level of immunity required for survival (Aman & Masood 2020).
Medicinal plants are natural resources used by many local communities since time immemorial for the treatment of infectious diseases and as natural immune boosters (Street & Prinsloo 2013). The international communities now understand and appreciate the important role of medicinal plants in healthcare systems amongst many other benefits (Geldenhuys & Mitchell 2006; Street & Prinsloo 2013). The pharmacologically active compounds in medicinal plants and horticultural produce responsible for boosting or modulating the immune system, especially in immunocompromised body systems, are now of great interest, especially during coronavirus disease outbreak. Furthermore, many vegetables such as eggplants, potatoes, mushrooms and tomatoes provide alternative forms of quality nutrition because of their richness in natural substances and enzymatic systems that protect against oxidation and free radical attack during metabolism (Huang et al. 2010; Scalbert et al. 2005). They are also important sources of polyphenols that protect cells against oxidative damage and invariably provide immunity against infectious diseases associated with oxidative stress inflicted by free radicals (Huang et al. 2010; Meccariello & D’Angelo 2021). Thus, consumption of these vegetables will potentially enhance human immune systems against diseases such as coronavirus disease 2019 (COVID-19). Apart from the contribution of fruit and vegetables in stimulating the immune system of humans, several medicinal plants are used in the African traditional medicine system to boost the immune system and treat chest pain, cough, high fever and shortness of breath – these are some of the major symptoms associated with coronavirus and other respiratory diseases. In addition to their efficacy in treating these symptoms and helping to develop immunity, they have been used to treat other respiratory infections, such as measles, tuberculosis (TB) and influenza amongst others in Africa and other parts of the world.
Several viral infectious diseases have plagued the world and led to high mortality and morbidity rates. Viral infections such as measles, smallpox, chickenpox, hepatitis, TB and polio amongst others have left apparent scars on mankind, especially amongst Africans (Greenwood 2014; Tulchinsky & Varavikova 2014). Before the discovery of the measles vaccine in 1963, there was measles epidemic every 2–3 years and almost everyone had the infection during childhood, with 90% of individuals infected by the age of 10 years (Hendriks & Blume 2013; WHO 2009). In urban areas, measles epidemic would occur every 1–2 years affecting children between the ages of 1.5 and 2.5 years. Outbreaks were less frequent in the rural areas and infections were commonly recorded amongst children between the ages of 2.5 and 5.0 years (Cutts et al. 1991). Whilst traditional healers often come up with treatments of natural origin to combat these diseases during another wave of the pandemic, it is believed that the infection naturally provides lifelong immunity (Goodson et al. 2011) meaning that once infected with measles during childhood, it is most improbable to be reinfected during adulthood.
Although the measles vaccine was administered throughout Africa and measles incidence was drastically reduced (Centres for Disease Control and Prevention [CDCP] 2009), there are still measles outbreaks, with case fatality rates amongst young children as high as 5% – 10% during outbreaks (WHO 2009). This resulted in an estimated 28 000 measles-related deaths each year (CDCP 2009). By implication, measles remains a major public health problem in Africa to date (Goodson et al. 2011). The developing countries of Africa and Asia have the highest incidence of measles infections in the world (Onoja & Ajagbe 2020). Endemic areas are largely confined to the tropics where the transmission rises after rainy seasons (Onoja & Ajagbe 2020). To curb the infection and transmission of measles, several medicinal plants are now being used traditionally because of their therapeutic components that could modulate complex immune systems to prevent infections rather than just treating immune-related diseases (Jantan, Ahmad & Bukhari 2015). Many plants contain immunomodulatory compounds, such as lactones, alkaloids, flavonoids, polysaccharides, diterpenoids and glycosides, that make them useful as medicinal plants (Jantan et al. 2015). Table 1 displays a summary of some active ingredients responsible for the immune-boosting potentials of horticultural crops whilst Table 2 shows some diseases and infections that are currently being treated with the use of medicinal plants.
Similarly, onion is a perennial herbaceous plant, which is rich in vitamin C, selenium, sulphur compounds and zinc. It is usually used as spices in the daily preparation of most cuisines around the world and contains protein, flavonoids and fibre (Lanzotti 2006). Onions have high antioxidant properties with antiviral functions (Griffiths et al. 2002) and can be found in almost every region of the world, including Africa, Europe and North America (Suleria et al. 2015). The bulb makes an important vegetable that can be consumed raw, cooked or processed into different products but is mostly used as food preparation and culinary agent (Corzo-Martínez & Villamiel 2012). The bioactive compounds responsible for the characterisation of onion as an immune-boosting vegetable include organosulphur compounds (diallyl sulphide and diallyl sulfoxide), peptides, proteins and flavonoids (Bystrická et al. 2013; Suleria et al. 2015).
Carotenoids that are also referred to as tetraterpenes are fat-soluble pigments and they include provitamin A carotenoids such as β-carotene and β-cryptoxanthin and non-provitamin A carotenoids such as lutein and lycopene (Raiola et al. 2014). These compounds protect plants against photodamage (Rao & Rao 2007). More than 600 carotenoids have been identified in nature, of which around 40 are naturally present in food included in the human diet (Gerster 1997). The health benefits of carotenoids are related to their antioxidant ability, immune system stimulation and antitumor activity (Ciccone et al. 2013; González-Vallinas et al. 2013; Maiani et al. 2009). Digested purple carrot extract has the potency of decreasing oxidative DNA damage by up to 20.7% to protect colon cells against reactive oxygen species (ROS) stress (Olejnik et al. 2016). Lycopene can increase the expression of differentiation-related proteins, such as cell surface antigen (CD14), oxygen burst oxidase and chemotactic peptide receptors (Sharoni et al. 2004). Carrot possesses a diverse range of phytochemicals such as phenolic compounds, polyacetylenes and ascorbic acid serving as immune-boosting agents against microbial infections (Ahmad et al. 2019).
Scientific reports have indicated that carotenoid intake from tomatoes is the most significant nutritional contribution of the fruit, and hence the highest weight in the index of antioxidant nutritional value is attributed to carotenoids content (Frusciante et al. 2007). Tomatoes contain 8 μg – 40 μg per gram fresh weight of the compound lycopene and around 80% of the total dietary intake of this carotenoid (Kong & Ismail 2011; Renju, Kurup & Saritha Kumari 2014). Research has shown that field-grown tomatoes contain higher levels of lycopene, with a range of 5.2 mg/100 g to 23.6 mg/100 g fresh weight (FW) compared with greenhouse-grown tomatoes (0.1 mg/100 g FW and 10.8 mg/100 g FW) (Sahlin, Savage & Lister 2004). In fresh tomatoes, lycopene is normally found in trans-conformation, whilst heat, light, acids, oxygen and digestion can cause conversion into the more bioactive cis-form (Boileau, Boileau & Erdman 2002). Lycopene is the major compound in tomato fruit responsible for its strong antioxidative role, which is associated with its ability to act as free radical scavengers from reactive oxygen species produced by partial reduction of oxygen (Friedman 2013). Tomato’s protective capacity is usually attributed to lycopene (Raiola et al. 2014). However, tomato products contain other compounds such as vitamins A, B and E (Raiola et al. 2014).
There is increasing evidence to indicate a relationship between the consumption of antioxidant-rich foods and the incidence of obstructive airway disease (Reyes-Munguía et al. 2016). The consumption of fresh fruit has been related to healthy pulmonary function both cross-sectionally (Butland, Fehily & Elwood 2000; Cook et al. 1997; Strachan et al. 1991; Tabak et al. 1999) and longitudinally (Reyes-Munguía et al. 2016). Consumption of fruit has also been linked to reduced prevalence of respiratory symptoms, particularly airway obstruction, such as wheeze (Butland, Strachan & Anderson 1999; Forastiere 2000). Studies have shown a positive relationship between plasma nutrient levels and respiratory health as a link between pulmonary function and plasma vitamin C, (Hu & Cassano 2000; Ness et al. 1996), vitamin A (Chuwers et al. 1997; Morabia et al. 1990), vitamin E (Hu & Cassano 2000), and β-carotene (Chuwers et al. 1997; Hu & Cassano 2000). Furthermore, lower incidence of bronchitis and wheeze has been attributed to high plasma levels of vitamin C (Schwartz & Weiss 1990).
Several medicinal plants have been used to treat measles in Africa. For instance, finely powdered particles prepared from the whole plant of Allium cepa L. and Allium sativum L. are taken orally with cow’s milk to treat measles amongst the Keffi people of Nigeria. The leaves of Citrus aurantifolia and Hibiscus cannabinus L. are independently made into powder and taken orally with cow milk for a few days to treat measles (Mustapha, Owuna & Uthman 2013). In Peru, leaves and aerial parts of Momordica charantia L. are utilised for measles treatment (As et al. 2009). The succulent ripe fruit of Nauclea latifolia S. is used in the treatment of measles and as a prophylactic against measles epidemics by the Ibo people of Nigeria. It is prepared by roasting ripe fruit in a pot over a hot firewood flame till the whole fruit is well-cooked (Okwu & Uchenna 2009).
Frequently used medicinal plants for the treatment of measles in the Ijebu area of Ogun State in southwest Nigeria include Elytraria marginata Vahl, Peperomia pellucida L. Humb., Bonpl. and Kunth, Vernonia amygdalina Del., M. charantia L., Newbouldia laevis (P. Beauv.) Seem. ex Bureau, and Ocimum gratissimum L. (Sonibare, Moody & Adesanya 2009). Whilst Cucurbitaceae species are the most frequently mentioned (Sonibare et al. 2009), Citrullus colocynthis L. Schrad. seeds, Momordica augustisepala L. bark and M. charantia L. have also been mentioned in the treatment of measles (Sonibare et al. 2009). Lagenaria breviflorus, referred to as ‘Itagiri’ in Yoruba, is a potent antiviral plant. The fruit of ‘Itagiri’ is commonly used to ward off viral diseases such as chickenpox and measles in Nigeria by placing it under the bed.
Amongst the several medicinal plants used for the treatment of viral infections in Africa, M. charantia is often used globally in various pharmaceutical products to treat viral diseases and their symptoms. The plant is traditionally used against chickenpox and measles in Togo and is topically applied to treat fever and measles in the Ejisu-Juaben Municipality of Ghana (Appiah et al. 2018). Kedrostis foetidissima (Jacq.) Cogn. is mixed with silverfish, boiled and given to children to drink to treat measles in communities around Mabira central forest reserve in Uganda (Tugume et al. 2016). In Southwest Nigeria, the leaves of M. charantia are boiled in water and the decocted material is used to bathe every day and night until the measles is cured (Fatoba et al. 2019). However, in Cameroon water macerate and palm wine macerate are used to treat measles and chickenpox, respectively (Ngono Ngane et al. 2011). The bark of Rauvolfia caffra Sond. (Quinine tree) is applied topically to treat measles in South Africa (Hutchings 1996; McGaw, Jäger & Van Staden 2000). The aromatic herb Lippia javanica leaf infusions are commonly used as a tea for the treatment of influenza and measles in Africa (Mitra 2012). Momordica charantia L. has been reported to contain many potent antiviral activities (Pongthanapisith et al. 2013) because its proteins strongly inhibit several viruses such as hepatitis B virus, dengue virus, herpes simplex virus and human immunodeficiency virus (Bourinbaiar & Lee-Huang 1996; Jiratchariyakul et al. 2001; Tang et al. 2012; Waiyaput et al. 2012). Momordica charantia L. possessed effective antiviral activity to a broad range of influenza A subtypes including H1N1, H3N2 and H5N1 (Pongthanapisith et al. 2013). In Uganda, leaves of Momordica foetida Schum. are reported to be used by traditional medicine practitioners for the treatment of TB and it is amongst the most frequently mentioned plant for the treatment of the disease (Tabuti, Kukunda & Waako 2010). Therefore, the plant may be considered a legendary medicinal plant. Other plants that have been found helpful in the treatment of TB include Eucalyptus spp. (leaves), Ocimum suave (leaves), Persea americana Mill. (leaves), Acacia hockii De Wild. (Stem bark), Zanthoxylum chalybeum Engl. (Root) (Tabuti et al. 2010) and Warburgia salutaris (G. Bertol.) Chiov. (Root and bark).
Warburgia species are used to treat cold, cough, sore throat, fever and respiratory ailments (Maroyi 2014), throat infections (Jansen & Mendes 1984) and chest complaints (Gertner 1938). In Kenya, W. salutaris bark is chewed and juice swallowed or bark is mixed with animal fat and rubbed on the chest to treat chest pain (Kiringe 2006; Wamalwa 2005). In Kenya and Tanzania, the bark is chewed and juice is swallowed for the treatment of cough (Kiringe 2006; Masinde 2010). In South Africa, W. salutaris is sold in tablet form to treat bronchitis, chest infections and ulcers (Botha, Witkowski & Shackleton 2004). As a traditional remedy for cough, powdered bark of W. salutaris and leaves of Cannabis sativa L. (Cannabaceae) are smoked as a herbal remedy for cough (Hutchings 1996). Warburgia salutaris is used as an expectorant for colds, sinus clearing, pneumonia, headache and cough (Van Wyk & Gericke 2000; Watt & Breyer-Brandwijk 1962).
African potato is a legendary African endemic medicinal plant with immunostimulatory activities, and it is one of the most popular medicinal plants in the South African traditional medicine system. This is because of its extensive use by the South African people for human immunodeficiency virus (HIV) and Acquired immunodeficiency syndrome (AIDS) symptoms and opportunist infections. Traditionally, it is used for the treatment of TB, asthma and several other conditions (Hutchings 1996; Makunga 2010; Mills et al. 2005; Van wyk & Gericke 2000) and as an immunostimulatory agent (Drewes et al. 2008). The corms of Hypoxis hemerocallidea are being used for immune-related illnesses, which include common cold, flu, cancer and HIV and AIDS (Mills et al. 2005). The extracts of the corm of African Potato are used to make decoctions, which are taken as tonics against TB, testicular tumours, other cancers and HIV and AIDS (Drewes et al. 2008). Within the genus, two species, H. hemerocallidea and Hypoxis colchicifolia, are predominantly known and utilised as African traditional remedies and in preparation of herbal tinctures and teas (Mills et al. 2005).
L-canavanine (2-amino-4-guanidinooxybutyric acid) is the most important nonprotein amino acid found in the leaves of Sutherlandia. L-canavanine is a structural analogue of L-arginine and a natural insecticide. As a result of its close similarity to arginine, it can interfere with arginine metabolism and be incorporated into proteins and thus lead to the formation of dysfunctional proteins (Mitri et al. 2009). Canavanine has been reported to be a potent anticancer agent (Swaffar et al. 1994; Tsirigotis et al. 2001) and has antiviral activity against influenza and retroviruses (Green 1992).
This herbal treatment is well sought after in European countries, the Commonwealth of the Independent States, the Baltic States and Mexico (Kolodziej 2007; Kolodziej & Kiderlen 2007). EPs® 7630 is the most widely investigated extract of the plant that revealed its potent antiviral activity, which inhibits the replication of respiratory viruses and the enzymes haemagglutinin and neuraminidase (Kolodziej 2011). The high local and international demands for P. sidoides herbal mixtures have proven that traditionally utilised herbal medicines could provide the basis for the development of a modern and innovative phytopharmaceutical that meets vital requirements such as quality, safety and efficacy for an evidence-based therapy (Kolodziej 2011). The immunomodulatory activity of P. sidoides is assumed to be a result of the combination of phenolic compounds and the numerous coumarins (Brendler & Van Wyk 2008). The main constituents of EPs 7630 include coumarins (e.g. umckalin) and flavanols (polyphenols) (Kolodziej 2007). According to Kayser, Kolodziej and Kiderlen (2001), the high content of gallic acid and its methyl ester in P. sidoides and its active extracts have been identified to be responsible for the immunomodulatory property of this herbal medicine (Kayser et al. 2001).
There is a possibility that curcumin could inhibit oxidative damages caused by chronic stress in vital organs, including the brain, liver and kidney (Boroumand et al. 2018). It has been established that curcumin plays this vital role by maintaining the superoxide dismutase and glutathione peroxidase activity in addition to reversing the stress-induced inhibition of catalase (Boroumand et al. 2018). These activities of curcumin could ultimately reduce lipid peroxidation and, hence, ameliorate the negative effect of chronic stress on tissues (Samarghandian, Farkhondeh & Samini 2017). Curcumin is traditionally known for its anti-inflammatory effects and has been reported to be a potent immunomodulatory agent that can modulate the activation of T cells, B cells, macrophages, neutrophils, natural killer cells and dendritic cells (Allam 2009; Jagetia & Aggarwal 2007). Supplementation in rabbit diet with curcumin (2, 4 and 6 g/kg) significantly increased the serum levels of immunoglobulin G and M, thus signifying the ability of curcumin to improve immune functions (Alagawany, Ashour & Reda 2016).
Studies on mice spleen immunised with sheep red blood cells established various immunostimulatory activities of curcumin, which include an increase in total white blood cell count, circulating antibody titter and plaque-forming cells (Antony, Kuttan & Kuttan 1999). Curcumin raised bone marrow cellularity, α-esterase positive cells and phagocytic activity of macrophages (Antony et al. 1999). Churchill et al. (2000) reported that curcumin treatment stimulates the proliferation of B cells in the mucosa of the intestine of C57BL/6J-Min/+ (Min/+) mice, thus indicating its immunostimulatory activity. Curcumin affects several autoimmune diseases because of its ability to modulate immune cells and immune cell cytokines (Jagetia & Aggarwal 2007). Curcumin is expected to be helpful in the therapy of autoimmune disorders as inflammation plays a crucial role in most autoimmune diseases (Jagetia & Aggarwal 2007).
Several reports have highlighted that curcumin may have therapeutic potential against AIDS (Jagetia & Aggarwal 2007). Curcumin’s mode of action in the suppression of replication of HIV is through inhibition of HIV long terminal repeat (Barthelemy et al. 1998) and HIV protease (Sui et al. 1993), inhibition of HIV-1 integrase (Mazumder et al. 1995; Vajragupta et al. 2005), inhibition of p300/CREB binding protein-specific acetyltransferase and the repression of acetylation of histone/nonhistone proteins and histone acetyltransferase-dependent chromatin transcription (Balasubramanyam et al. 2004). Curcumin treatment before and after the onset of sepsis could reduce tissue injury and mortality and decrease TNF-α expression in septic rats (Siddiqui et al. 2006). Curcumin could reverse the post-transplant lymphoproliferative disorder commonly associated with the use of cyclosporine in the process of organ transplant (Ranjan et al. 1998).
Tuberculosis is a highly contagious and dangerous lung disease for which medicinal plants have been used in its treatment in Africa. Although it is not a viral infection, it has left terrible scars of anguish and pain in the lives of the African people, with very high incidences in the Southern African Development Community countries because of HIV and AIDS infections leading to high mortality rates. South Africa has the seventh highest incidence of TB in the world and the second highest in Africa and the fifth highest burden of drug-resistant TB cases globally (National Department of Health 2011). A survey by Lawal, Grierson and Afolayan (2014) revealed 30 plants belonging to 21 families that are commonly used by traditional healers in the Eastern Cape region of South Africa for the treatment of TB. Clausena anisata, Haemanthus albiflos and Artemisia afra were the leading set of investigated plants (Lawal et al. 2014). In other parts of South Africa, several medicinal plants are being used for the treatment of TB and its symptoms. Traditional healers in Amathole District in the Eastern Cape province use A. afra to treat flu and TB, and it is also utilised by the Zulu people for the same purpose amongst others (Buwa & Afolayan 2009; Makunga 2010; Semenya & Maroyi 2013). Amongst the Zulu people, infusion of Cannabis sativa is inhaled in the process of the traditional treatment of TB (Hutchings 1996). Madikizela et al. (2014) documented Asparagus africanus and Ficus sur, which showed positive results against M. tuberculosis. Clausena anisata is used for the treatment of measles and bronchial problems in Nigeria. Its leaves are used for the treatment of respiratory ailments (Ajibesin et al. 2007; Hutchings 1996). Similarly, there is the widespread use of C. anisata for the treatment of TB in Nkonkobe Municipality (Lawal et al. 2014).
Nature has bestowed a variety of health-benefiting food and medicinal plants on humanity. These natural resources could help maintain good health and boost the body immune system. In the past, viral or bacterial infections have affected humanity severely. However, cures for these diseases or infections and their symptoms were discovered from plant origin. Hence, the plant kingdom is a hub for natural therapeutic compounds. Whilst some of the active therapeutic compounds available in the plant have been fully developed into usable products, ongoing research and future research will lead to the discovery of more therapeutic compounds. However, consumption of the horticultural fruit mentioned in this review could help maintain a good immune system to protect mankind against infectious diseases, such as coronavirus. Nonetheless, in-depth research into the antivirus potential of the medicinal plants reported in this review, especially those with antiviral and antibacterial activities against respiratory infections, could offer a pathway to the discovery of medicinal plants combined with active ingredient(s) that can combat COVID-19 and its symptoms. This review has highlighted a variety of medicinal plants traditionally used for fever, cough, chest pain, flu, sore throat and respiratory infections. These are some of the COVID-19 symptoms. Hence, it will be worthwhile to investigate the antiviral activities of these plants or a combination of these against the disease. This could lead to innovative discoveries towards the formulation of a plant-based cure for COVID-19, which is currently ravaging the world.
The authors would like to acknowledge Mangosuthu University of Technology for postdoctoral funding.
The authors have declared that no competing interest exists.
With the submission of this manuscript, the authors would like to undertake that this work is originally put together by the authors and no part thereof has been submitted or published elsewhere. All authors agreed with the contents of the manuscript and its submission to the journal. All authors have contributed significantly to the work. No part of the research has been published in any form elsewhere.
This article followed all ethical standards for research without direct contact with human or animal subjects.
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
Data sharing is not applicable to this article as no new data were created or analysed in this study.
The views and opinions expressed in the article are those of the authors and do not necessarily reflect the official policy or position of any affiliated agency of the authors.
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