Plant as a Source of Natural Antiviral Agents

INTRODUCTION

Virus "a piece of bad news wrapped in a protein coat" has been defined by Peter Medawar (Oldstone, 1993). It appears as the perfect definition after considering the list of top ten causes of death in low, middle and high income countries. Lower respiratory infections, diarrhoeal diseases and HIV/AIDS are the common death causes among low and middle income countries (WHO, 2011b). All of these three health disorders are directly or indirectly caused by viruses. Except lower respiratory infections none of the above mentioned factors are prevalent among high income countries. It clearly indicates that how severely these viral diseases are affecting the people health in low and middle income/developing countries.

Our planet contains nearly 10³¹ viruses and their ubiquity also invaded the marine environment, where in every 200 liter of water nearly 5000 viral genotypes are present (Breitbart and Rohwer, 2005; Suttle, 2005). Moreover viruses are moving between the environments and they are present almost everywhere e.g. deep sea, polar ice, alkaline, hot and saline waters and more than 2000 m deep in terrestrial environment. There are almost 20 families of viruses that actually infect humans (Harvey et al., 2006) and some of them also cause diseases in animals (Mahzounieh et al., 2006). The diseases they cause in human include chickenpox, influenza, skin rash, hepatitis, bronchiolitis, acquired immunodeficiency syndrome, liver infection and many others. Virus particles enter in the living system and if they overwhelm the body’s immune system then it is almost impossible to stop their spread in body. They direct the host metabolic pathway for the sake of their repeated replication; this makes their treatment difficult. But fortunately, it is now well known that viruses are unique in their mode of replication, which can be easily targeted (Selisko et al., 2007; Syed et al., 2010). They use specific enzymes to infect and replicate, whose inhibition could arrest their metabolism. For example, the proteolytic enzyme promotes virus maturation by separating the viral polyprotein precursor, whose inhibition will stop its maturation (Wapling et al., 2007). So the virus metabolism or replication can be stopped by specific inhibitors.

Today many synthetic antiviral drugs e.g. moroxydine, ganciclovir, valganciclovir, valaciclovir etc. are used, which inhibit the virus replication via different mechanisms (Biron, 2006; Czeizel et al., 2006). But difficulty in drug treatment arises due to their low efficiencies, cytotoxicity and development of viral resistance against them. Another antiviral treatment; vaccination, can be applied but they are still under development, as they often provide incomplete protection against virus and their reliability needs more research (Pervez, 2000b; Subbarao and Joseph, 2007). Thus the treatment through antiviral synthetic drugs and vaccines need more scientific investigation. Nature provides another, more reliable source of antiviral agents; viz. plants phytochemicals; almost 40% of currently available drugs are direct or indirect derivatives of plants. A number of ethnobotanical studies aiming to identify potential therapeutic plants for more effective control of health issues demonstrate the importance of plant species in health care system (Shinwari and Khan, 2000; Heneidy and Bidak, 2004; Appidi et al., 2008; Ky et al., 2009; Ansari and Inamdar, 2010; Makambila-Koubemba et al., 2011). Plants are rich source of phytochemicals like alkaloids, anthocyanins, carotenoids, flavonoids, isoflavones, lignans, monoterpenes, organosulfides, phenolic acids, saponins and many more (Al-Yahya, 2005; Hassan et al., 2006; Anitha and Ranjitha Kumari, 2006; Akomo et al., 2009; Rahman et al., 2009; Amabeoku and Kinyua, 2010; Ndjonka et al., 2010). These phytochemicals have been proved to be responsible for their antimicrobial (Sampathkumar et al., 2008; Krishnan et al., 2010), antihypertensive (Amalia et al., 2008), anti-diabetic (Qureshi et al., 2009), antioxidant (Momtaz and Abdollahi, 2010), hepatoprotective (Mahalakshmi et al., 2010; Ansari et al., 2011), cardioprotective (Ojha et al., 2008; Fard et al., 2008) and other therapeutic activities. Thus this study is aimed to analyze the previously reported antiviral plants and identify potential mode of action and compounds that are responsible for their antiviral activity. Better understanding of natural antiviral agent’s mode of action and identification of responsible compounds will be helpful to provide a new insight for the development of new antiviral drugs for more effective viral control.

Basic viral structure and its mode of action: Viruses are organic objects, which are metabolically inactive outside the host body but become active on their entry into the host cell (Dupre and O'Malley, 2009). These are mainly composed of proteins and nucleic acid; the proteins majorly contribute to their specific shape and form a coat called capsid (Andersson, 2010). Thus viruses are of various shapes e.g. simple, helical, icosahedral or complex and some viruses are surrounded by a lipid bilayer, derived from host membrane, which is called as envelope (Geng et al., 2007; Raja et al., 2003). Some capsid proteins are also associated with virus nucleic acid and called as nucleocapsid, while nucleic acid proteins, are the direct part of the nucleic acid, known as nucleoproteins. The nucleic acid of virus is either made up of DNA or RNA, is the basic source of information required for the regulation of its metabolic activities. Theses DNA and RNA can be further divided into two types depending upon the number of strands i.e. single stranded or double stranded DNA/RNA (Firth et al., 2010; Pichlmair et al., 2006). The single stranded RNA viruses can be further distinguished depending upon the sense of strand as some RNA viruses have positive-sense RNA (+VE ssRNA) and some viruses have negative-sense RNA (-VE ssRNA) (Gorbalenya et al., 2006). The shape of nucleic acid (DNA/RNA) is also an important source of differentiation, because all the viruses did not contain same-shape nucleic acid (Gao and Hu, 2007). It can be either in circular, linear or coiled form.

Virus (either DNA or RNA) life cycle can be divided into some predefined stages; adhesion, adsorption (entry), replication, maturation and release, which involve some enzymes and proteins. For example, the process of virus entry is carried out by cell surface proteins; HCV entry involves claudin-1, occludin, tetraspanin as main receptors proteins (Burlone and Budkowska, 2009). Its entry is also mediated by some other lipoproteins and an enzyme; lipoprotein lipase. On the other hand the influenza virus infection is mediated by protease enzyme, which activates the viral surface protein haemagglutinin (Zambon, 2001). The protease enzyme is also important in the expression of viral proteins; it splits the proteins into groups depending upon their structural and nonstructural functions (Appel et al., 2006). But the RNA viruses need two additional enzymes for their survival; reverse transcriptase and integrase, former transcribes the viral RNA into DNA at the time of replication (Briones et al., 2010; Sluis-Cremer and Tachedjian, 2008). While the second enzyme is used to incorporate the viral DNA into host genome, furthermore it is also needed for proper uncoating of virus core proteins. Thus virus is needy of enzymatic and non-enzymatic proteins, which can be targeted to stop their replication and infection.

Antiviral plants: In this review a total of 105 plant species have been identified that were reported for their potential antiviral activities (Fig. 1). Maximum number of plants were reported for their activity against herpes viruses, indicating that herpes viruses were the highly studied viruses with respect to antiviral plants. After herpes virus, HIV, influenza and hepatitis were among other viruses that were addressed in most of the studies in order to discover the plant with antiviral properties. In next sections a brief description with respect to the available antiviral plants against some important viruses has been provided.

Plants with antiviral activity against herpes virus: An enveloped double stranded DNA virus with linear genome; it belongs to the family Herpesviridae, which is recently reclassified to separate the mammal’s virus from other non-mammalian viruses (Davison et al., 2009). It is also known as human herpesvirus and Varicella-zoster virus.


Fig. 1:	Mostly studied viruses found during current review

It is highly infectious and prevalent disease especially in developing countries with almost 50 % prevalence in adults, disease symptoms are often unnoticeable (WHO, 2001). It is a sexually transmitted disease and highly prevalent in females as they often get infected with it before the age of fertility (Ziyaeyan et al., 2007). It also causes febrile rash and graft-versus-host disease in humans, who got hematopoietic stem cell transplantation (Pichereau et al., 2011). It is also the casual agent of genital herpes disease; vaccines against it are currently under developmental stages (Soleimanjahi et al., 2007).

Total 57 plants were identified with antiviral activity against herpes virus (Table 1). Binns et al. (2002) reported that h-hexane extracts of Echinacea purpurea showed in vitro antiviral activity against herpes virus in another study polysaccharide and cichoric acid were found among the active phytochemicals of various plant parts extracts (Vimalanathan et al., 2005). Another compound of Phyllanthus urinaria from its acetone extract (hippomanin A) showed its activity against herpes simplex virus type 2 (Yang et al., 2007); in another study Cheng et al. (2011) identifies a compound (excoecarianin) from this herb with same activity. Anthraquinones has been identified as potential compound responsible for antiviral properties of Rhamnus frangula, Rhamnus purshianus and Rheum officinale (Sydiskia et al., 1991). This organic compound has also been reported in many studies to be responsible for positive health attributes of many medicinal plants (Kumar et al., 2007; Hussein et al., 2010; Karou et al., 2011; Mbaya and Ibrahim, 2011; Sonibare et al., 2011). Kurokawa et al. (1999) reported the antiviral activity of Rhus javanica against herpes virus and found moronic acid as potential active agent from its herbal extract. Plants mainly showed activities like restriction of entry host into cell (Weber et al., 1992; Zandi et al., 2007), reduced viral replication (Duarte et al., 2001; Chiang et al., 2003; Alche et al., 2003) and partial destruction of viral envelope (Sydiskia et al., 1991). Active compound present in most of the plants were anthraquinones, terpenes, quercetin, lectins and phenolics (Amoros et al., 1987; Sydiskia et al., 1991; Chiang et al., 2003; Ooi et al., 2004; Kan et al., 2009). Allium sativum is a medicinal plant with a lot of health benefits (Ishtiaq et al., 2007; Sukandar et al., 2010; Abdelaziz and Kandeel, 2011; Weber et al., 1992) found its extract effective against this virus. The extract of Aloe vera another important medicinal plant (Alqasoumi et al., 2008; Semalty et al., 2010) has been reported to be effective against this virus by inhibiting the viral entry and replication into the host cell(Zandi et al., 2007).

Plants with antiviral activity against HIV: HIV is highly infectious enveloped virus of family Retroviridae; it has liner ssRNA positive sense genome and cause high morbidity. During year 2004, HIV infection was one of the leading factors responsible for almost 2.65 million deaths in low income countries (Patton et al., 2009). It is also prevalent in high income countries, as in United States almost 55400 new HIV cases were observed each year from 2003-06 (Hall et al., 2008). It increases the chances of bacteria and other viruses infections, which poorly effects the health; transmission of HIV from mother to child could lead to the death (Corbett et al., 2003; Ahasan et al., 2004; Ilboudo et al., 2007; Abongo et al., 2008). Its chemo-treatments are scarce, because of their potential side effects on human body, non-significant efficiencies and increased divergence in HIV genome (Fokunang et al., 2006; Jaffary et al., 2007; Kagone et al., 2011). But a nutrient rich food can reduce the HIV caused decreased weight and help in improving the patient’s health (Oguntibeju et al., 2007).

In present study 26 plant species were found effective against HIV. Terpenoids, lectins, alkaloids and flavonoids were common among the active compound of these plants (Table 2). Roja and Heble (1995) reported the positive activity of an alkaloid (castanospermine) isolated from seeds of Castanospermum austral against this virus.

Table 1:	Detail description of studies focusing on antiviral plant activities against herpes virus

n/a: Not available

Table 2:	Detail description of studies focusing on antiviral plant activities against HIV

n/a = Not available

Wang and Ng (2001) reported that gossypol and alkaloids of Corydalis yanhusuo might be responsible for it inhibitory activity on HIV virus. Ethanolic extract of Monotes africanus, which is a rich source of various flavonoids has been reported for its antiviral activity against HIV (Meragelman et al., 2001; Reutrakul et al., 2007). Most of the plants mainly affect the Reverse Trancriptase (RT) activity or penetration of virus particles into host cell. Calanolides which exhibits potential RT inhibitory activity have been found in the hexane extract of Calophyllum brasiliense leaves (Cesar et al., 2011). The hexane extract of this plant was found to have inhibitory effect on RT enzyme of HIV. Lectins derived from Phaseolus vulgaris are reported for their potential inhibitory effect on HIV RT activity (Ye et al., 2001; Fang et al., 2010). Another seed protein isolated from Vigna unguiculata possesses the same effect on RT activity in addition to inhibitory effect on glycohydrolases α- and β-glucosidases (Ye et al., 2000; Ye and Ng, 2001). Oh et al. (2011) reported that aqueous extract of Prunella vulgaris interferes with the viron post binding events. Extract of Rhizophora mucronata and Rhizophora apiculata (a rich source of polysaccharides) blocks the viral binding to the cell surface (Premanathan et al., 1999a; Premanathan et al., 1999b). In another study Lee-Huang et al. (1995) studied the effect of a protein isolated from Gelonium multiflorum and found that it reduces the virus replication by inhibiting the integration of viral DNA into host genome. Panax ginseng, which is also effective against cardiovascular diseases (Jun et al., 2007) has been reported to be effective against HIV by inhibiting the RT activity (Ng and Wang, 2001). Ricinus communis an important medicinal plant (Onwuliri and Anekwe, 2001) showed inhibitory effect on RT and N-glycohydrolases of HIV (Wang and Ng, 2001). Another plant Terminalia chebula, which has been proven to have many beneficial medicinal properties (Gupta et al., 2008a, b; Shinde et al., 2009; Anam et al., 2009) posses significant anti-HIV activity (Lee et al., 2011). Its methanolic extract showed antiviral activity against HIV when tested on virus infected baby hamster kidney cells.

Plants with antiviral activity against influenza virus: It is a single stranded RNA virus with negative sense linear fragmented genome enclosed in a capsid, it belongs to family Orthomyxoviridae and infects both mammals and birds. It has seasonal epidemiology and mainly spread through air when the environment is dry and cold (Lowen et al., 2007). The hospitalization and death rate associated with influenza vary with the age and type of virus, as influenza virus A-caused infection rate is higher than type-B (Thompson et al., 2004). Influenza virus, especially of avian origin is highly virulent disease causing agent in humans and birds; it has long history, put huge burden on human health since 1580 (Farooq et al., 2006; Lazzari and Stohr, 2004). It is responsible for deaths of millions of people; its pandemic nature is due to its variable strains which develop via the reassortment of genetic information. Thus the development of vaccine against it is difficult both in humans and birds.

Sixteen plants were identified which showed antiviral activity against influenza virus. Anthocyanin and polyphenols were among the commonly found active phytochemicals in these plants (Table 3). Camellia sinensis is an important herbal plant having significant antioxidant, photochemoprotective, hyperglycemic, antibacterial and many other beneficial heath benefits (Adiloglu and Adiloglu, 2006; Bakar et al., 2006; Shokrzadeh et al., 2006; Hassani et al., 2008; Mohamed and Metwally, 2009; Amutha et al., 2010; Chakraborty and Chakraborti, 2010; Kaur and Saraf, 2011; Obaid et al., 2011). Catechin derivatives obtained from the tea of this plant showed significant inhibitory effect on influenza strains (Song et al., 2005).

Table 3:	Detail description of studies focusing on antiviral plant activities against influenza virus

n/a = Not available

Ehrhardt et al. (2007) found that Cistus incanus possess the antiviral activity by modulating the viral surface in order to inhibit its entry into Madin-Darby Canine Kidney (MDCK) cells. Inhibitory effect against influenza virus has also been reported in human patients (Kalus et al., 2009). Another herb Echinacea purpurea also restrict the viral entry by inhibiting the binding of viral receptors to the host cells surface (Pleschka et al., 2009). Geranium sanguineum, a rich source of polyphenols showed antiviral activity by effecting the expression of viral proteins on cell surface (Serkedjieva, 1996; Sokmen et al., 2005). A lectin isolated from bulbs of Narcissus tazetta showed significant antiviral activity against various strains of influenza virus (Ooi et al., 2010). In another study Ooi et al., 2004) reported the anti-influenza activity of a lectin isolated from saline extract of Pandanus amaryllifolius leaves. Scutellaria baicalensis an important medicinal plant (Yeh et al., 2010) possess a compound (isoscutellarein-8-methylether) which is thought to be responsible for it antiviral activity against influenza virus (Nagai et al., 1995).

Plants with antiviral activity against Hepatitis C Virus (HCV): Hepatitis C is an enveloped virus with +VE single stranded linear RNA, which belongs to the family Flaviviridae. It cause mild-chronic liver disease infecting more than three million people each year, which results in almost 350 000 deaths (WHO, 2011a). It has worldwide spread with 4.8% infection rate in Pakistan. It is a global problem nearly affecting 130 million people; moreover, alone it is responsible for about 27% of cirrhosis and almost 25% of world’s hepatocellular carcinoma (Alter, 2007). Its high spread is attributed to poor moral and health conditions, use of drugs, alcohol and contaminated syringes (Roshandel et al., 2007; Zakizad et al., 2009). It is detectable through serum proteins and blood analysis; its chemotherapeutic treatment is difficult but can be treated through herbal products (Ansari et al., 2011; Joseph and Raj, 2011; Pervez, 2000a; Moundipa et al., 2007; Tabassum et al., 2000).

In current study six plants have been identified with proved antiviral activity against HCV. Mainly plant showed inhibitory effect on HCV protease. Hussein et al. (2000) reported the inhibitory effect of Trachyspermum ammi and Embelia schimper methanol extract on HCV protease. Solanum nigrum which also exhibits hepatoprotective activity (Subash et al., 2011) has also been reported to have inhibitory effect on HCV (Javed et al., 2011). Cannabis a commonly used drug (Richardson, 2010) is a phytochemical of Cannabis sativa a medicinal plant with many beneficial health effects (Arshad and Khan, 2000; Qureshi et al., 2001; Tehranipour and Ebrahimpour, 2009). Acacia nilotica has remained the focus of many studies for its multipurpose applications (Shirazi et al., 2001; Banerjee et al., 2004; Ghosh et al., 2004; Elkhalifa et al., 2005; Emtehani and Tabari, 2007) its acetonic and methanolic extracts have sown anti-HCV effect on liver cells (Rehman et al., 2011). Sylvestre et al. (2006) reported the anti-HCV activity of cannabis in human patients. Mainly these plants showed their activity against HCV by targeting its protease (Table 4).

Plants with antiviral activity against respiratory syncytial virus: It belongs to the family Paramyxoviridae and is single stranded RNA virus, which is enclosed in an envelope. It is a highly prevalent virus throughout the world with large variation in its genome (Trento et al., 2010). It cause bronchiolitis and other respiratory problems and is a major cause of lower respiratory tract infections; it mostly infects the children under age of six months and also responsible for asthma problems (Mohapatra and Boyapalle, 2008). It causes an infection in 3-7% of healthy adults and 4-7% in the non-healthy adults, who already suffers from lung and heart diseases (Falsey et al., 2005).

Table 4:	Detail description of studies focusing on antiviral plant activities against hepatitis, respiratory syncytial virus and vesicular stomatitis virus

n/a: Not available

Moreover it cause burden on population equal to that of influenza A virus, as the patients needs more health facilities and care. Ma et al. (2001) found that ethanol extract of Selaginella sinensis possess significant activity against respiratory syncytial virus. They further elaborated that this activity of plant could be due to the presence of a biflavonoid (amentoflavone) in its plant extract. A lectin isolated from Narcissus tazetta also showed antiviral activity against this virus (Ooi et al., 2010). Li et al. (2005) reported the Schefflera heptaphylla antiviral activity against respiratory syncytial virus and concluded that this activity was due to the inhibition of fusion of viral particles to the host cell.

Plant with broad spectrum antiviral activity: Based on the data presented in Table 1-5 plant with broad spectrum antiviral activity (plants reported for their antiviral activity against two or more than two viruses) were identified. Total 24 plants were found that were resistant to two or more than two viruses (Fig. 2). Two plant species showed antiviral activity against four viruses.


Fig. 2:	Plant with broad spectrum antiviral activity

Table 5:	Detail description of studies addressing rarely studied viruses

n/a = Not available

Allium sativum showed resistance against Herpes virus, Parainfluenza virus-3, Rhinovirus and Vesicular stomatitis virus. Daucus maritimus has been reported for its activity against Dengue virus, Hepatitis C virus, HIV and West Nile virus. Helichrysum aureonitens, Pterocaulon sphacelatum and Quillaja saponaria have been reported for their antiviral activity against three viruses each. As shown in figure 2 Helichrysum aureonitens is effective against Coxsackievirus, Herpes virus and Reovirus. Pterocaulon sphacelatum showed antiviral activity against Herpes virus, Picornavirus and Polio virus. Quillaja saponaria has been reported for its activity against Herpes virus, HIV and Reovirus. Almost all plants with broad spectrum activity were effective against herpes viruses. Moreover five plants which were effective against Herpes virus, also showed antiviral activity against Influenza virus. In the same way three plant species showed resistance against both Herpes virus and Vesicular stomatitis virus.

CONCLUSION

Majority of the studied viruses belongs to the Flaviviridae, Herpesviridae and Picornaviridae family. A large number of plants are available in nature which could act as a source of lead antiviral compounds. Mainly these plants target the enzymes that are involved in replication and integration of virus into host cell. In case of DNA viruses restricted entry of viral particles into host cell or inhibition of viral replication into the host cells were most frequent mode of actions. Destruction of viral envelop was also one of the identified mode of action against DNA viruses. RT plays a significant role in replication of RNA viruses and most of the plants restrict the activity of RT enzyme of virus. In order to design effective drugs against viruses their enzymes that are involved in the key metabolic activities (integration, replication) should be focused. A lot of plant population with possible potential is still uncovered as few plants have been studied in detail in order to identify the active phytochemicals against these viruses. More detailed studies in future will help not only to identify the potential antiviral compounds but also in better understanding of their mode of action for more effective control of these lethal viruses.