Natural products (NPs) are chemical substances in the form of primary and secondary metabolites which are produced by living organisms such as plants, animals, marine organisms, bacteria, and fungi ^[1][2][1,2]. They are mostly referred to as small molecules or secondary metabolites, representing a unique scaffold of compounds that are distributed in a broad variety of chemical classes, such as alkaloids, cardiac glycosides, flavonoids, phenolics, saponins, sterols, and terpenoids [3]. Essential oil compounds form part of these classes as volatile and non-volatile aromatic substances, which comprised mostly of terpenoids and some phenylpropanoid derivatives ^[4][5][4,5].

Chemically, terpenoids form the largest group of essential oils, such as monoterpenes (C10) and sesquiterpenes (C15) and a few diterpenes and phenylpropanoids [6]. Many of them have functional groups such as aldehyde, ether, ester, alcohol, carboxylic acid, phenol, amines, and amides [6].

Essential oils are naturally distributed in many higher plants and are most abundant in aromatic plant families, including Lamiaceae (Peppermint family), Myrtaceae (Eucalyptus family), Rutaceae (Citrus family), and Zingiberaceae (Ginger family) [7]. They have been reported in the seeds, fruits, peels, flowers, buds, leaves, young stems, barks, resins, woods, bulbs, roots, and rhizomes of many plants [8] and are extracted by methods of hydro-distillation, steam distillation, hydro-diffusion, solvent extraction, maceration, cold-press extraction, supercritical fluid (CO₂) extraction, sub-critical liquid extraction, microwave-assisted extraction, and enfleurage ^[4][9][4,9].

The use of EOs predates modern history. They are one of the most important NPs that have ever been utilized by many cultures, for many centuries, around the world for domestic (cosmetics, perfumery, food, and beverages) and medicinal purposes [10]. Essential oils are popularly known to help relieve the airways during the cold (winter) or flu season, and evidence has emerged of their medicinal importance in the amelioration of respiratory tract infectious diseases, such as the common cold, pneumonia, and influenza [11]. The flu (influenza) is a disease that often occurs during the winter or cold season, and so can be referred to as seasonal influenza [12].

Influenza is an infectious zoonotic respiratory disease caused by the influenza A, B, C, and D viruses, where A and B are the common virus type that causes seasonal flu ^[13][14][13,14]. Influenza is characterized by the sudden onset of fever, dry cough, sore throat, runny nose, headache, and joint pains [12]. It is a major public health problem, accounting for about 3–5 million cases worldwide, with about a 10% annual death rate [12]. It is responsible for high hospitalization and deaths among high-risk individuals, such as the elderly and those with co-morbidities [15].

Vaccination remains a primary strategy to prevent and control influenza, due to the waning immunity that is associated with it [16]. So far, an effort made to reduce the burden of influenza through the administration of vaccines has yielded promising results, with more expectations for success. Vaccine selectivity is still a major issue because the currently available influenza vaccines are virus-type- and sub-type-specific [17]. In addition, many people are still in doubt about the harmlessness of vaccines due to cultural beliefs, myths, and conspiracy theories, thus causing vaccine hesitancy [18]. Therefore, there is a need for deliberate and continuous vaccine education and advocacy to help bring influenza to a halt.

Currently, four FDA-approved antiviral drugs of three different classes are recommended for use against the recently circulating influenza viruses, Baloxavir marboxil, Oseltamivir, Peramivir, and Zanamivir [19]. Despite the availability of these drugs, influenza is still on the rampage, taking its toll on human health and well-being. In fact, the problem is further exacerbated because the available drugs still lack strong antiviral activity against all the influenza virus strains; in addition, there is always an emergence of new resistant strains and the resurgence of old virulent strains that were not adequately reduced in past influenza outbreaks [20].

2. Influenza (Flu)

Influenza is a contagious viral disease that affects the upper and lower respiratory tract ^[21][92]. Influenza viruses can be found in humans and some animals such as Aves and cattle, and are generally categorized as type A, B, C, and D influenza viruses ^[22][23][93,94]. The common types are the influenza A and B viruses, which affect humans and are mostly characterized as the seasonal flu ^[21][92]. Influenza A viruses are largely implicated in the flu pandemic and are a common cause of zoonotic infections, often characterized by virulent infections in humans ^[14][24][14,95]. The influenza C viruses are predominantly responsible for mild illness in animals and are rarely implicated in human epidemics ^[14][24][14,95]. Influenza D viruses mostly affect animals, with rare cases of human-to-human transmission ^[22][93]. Symptoms associated with influenza virus infections include a fever, sore throat, runny nose, cough, fatigue, and headache, owing to disease of the upper respiratory tract, while the lower respiratory tract may present with severe or acute pneumonia ^[25][96]. The influenza disease caused by the type A influenza virus of zoonotic origin, is a major public health concern, as it is responsible for both the common seasonal influenza epidemic (seasonal flu) and the sporadic and unpredictable (10–50 years of occurrence) global influenza pandemic outbreaks ^[26][97]. Seasonal influenza outbreaks typically occur during the winter season in temperate regions (Europe, Southern Africa), due to favourable conditions of low humidity and low temperatures ^[26][27][97,98]. However, in tropical countries, it is characterized by a complex pattern of occurrences due to an interplay of climatic factors such as temperature levels, hours of sunshine, and the level of rainfall ^[27][98]. On the other hand, pandemic influenza is characterized by a fast spread of the influenza A virus from the virus origin to the rest of the world in several waves over a short period, as witnessed in the first influenza pandemic of 1918 by the influenza A H1N1 virus strain, and subsequent pandemics of 1957, 1968, and 2009, caused by the Influenza A H2N2, H3N2, and H1N1 virus strains, respectively ^[26][28][97,99]. According to the World Health Organization, up to 1 billion influenza virus infection cases are reported annually, with about 4 million of the cases leading to severe illness, and around 400,000 reported deaths ^[24][95]. The most vulnerable groups are often infants between the ages of 0–9 months and adults 65-years-old and above ^[29][100]. Vaccination remains an effective means to reduce the burden of influenza ^[30][101]. The National Advisory Committee on Immunization (NACI) recommended the use of two classes of influenza vaccines, the Inactivated Influenza Vaccines (IIVs) and the Live Attenuated Influenza Vaccines (LAIVs) ^[31][102]. Just prior to the COVID-19 pandemic, it was reported that vaccination prevented an estimated 3.7 million cases of influenza, 105,000 influenza-related hospitalizations, and 6300 influenza-associated deaths worldwide ^[32][103]. However, more success in flu vaccination is still desired. Some key issues to be addressed include the complexity involved in predicting the pattern of seasonal influenza, reduced vaccine efficacy based on repeated annual immunization, an antigenic mismatch between the developed vaccines and the circulating virus strains, an age difference of the different cohorts involved in vaccination, and the issue of variability in the virulence level of different seasonal strains of the virus ^[33][34][35][104,105,106]. Even though vaccination is the most effective means of reducing the burden of influenza, antiviral drugs can be very useful in delaying the spread of new pandemic viruses, and they have also been found useful for the treatment of critically ill influenza patients ^[36][107]. There have been significant strides in the development of influenza antiviral drugs (IADs), and there are currently three classes of FDA-approved IADs: M2 proton channel antagonists, neuraminidase inhibitors, and polymerase acidic endonuclease inhibitors ^[37][108]. The drugs Amantadine and Rimantadine, are M2 proton channel antagonists, which used to be effective for the treatment of influenza A virus infection but have lost their efficacies over the years due to the emergence of more virulent strains of the type A virus, such as the 2009 H1N1 influenza A virus [19]. The FDA-approved neuraminidase inhibitors such as Oseltamivir, Peramivir, and Zanamivir are more efficacious and less toxic for the management of influenza than the M2 proton channel antagonists ^[38][109]. However, these drugs are associated with adverse effects, such as skin rash, diarrhea, anaphylactic reaction, headache, nausea, vomiting, cough, and gastritis ^[37][108]. Baloxavir is a cap-dependent, polymerase acidic endonuclease inhibitor which is similar in potency to neuraminidase inhibitors, except for the fact that it is newer and offers a different mechanism of action from the earlier developed neuraminidase inhibitory drugs ^[37][39][108,110]. Based on the rapid and unlimited variabilities of influenza viruses and the emerging resistance of new influenza virus strains to the currently used drugs, there is a dire need to discover more lead anti-influenza agents with a novel mechanism of action and develop (synthesize and optimize) more effective analogs from the already existing ones ^[38][109]. Natural products, including EOs, continue to offer an inexhaustible reservoir of bioactive compounds as lead therapeutic agents for the management of diseases. Some EOs and their compounds have been reported to demonstrate remarkable biological activities against a wide range of viruses, including influenza viruses ^[40][111]. It is against this backdrop that the anti-influenza potentials of EOs and their compounds were discussed, vis a viz the mechanism of action and structure-activity relationships of lead antiviral compounds, to source newer anti-influenza agents.

3. Anti-Influenza Properties of Plant-Derived Essential Oils and Their Compounds

Evidence has emerged on the anti-influenza potentials of many aromatic plants that are used for the treatment of flu and flu symptoms (cold, cough, sore throat, bronchitis, and pneumonia) by various ethnomedicines ^[41][112]. For instance, the EOs of Cynanchum stauntonii roots demonstrated an in vitro activity against Influenza A/NWS/33 (H1N1) virus at an IC₅₀ value of 64 µg/mL and selectivity index of 8, with the main EOs used comprising (E,E)-2,4-decadienal, 3-ethyl-4-methypentanol, 5-pentyl-3H-furan-2-one, (E,Z)-2,4-decadienal, 2(3H)-furanone,dihydro-5-pentyl, and caryophyllene oxide ^[42][113]. Further investigation revealed considerable inhibitory effects on influenza-induced deaths with 40, 70, and 100% survival rates when administered 50, 150, and 300 mg/kg doses of the EO, respectively, in male albino mice ^[42][113]. The leaf EOs of Melaleuca alternifolia (tea tree oil) contain terpinen-4-ol, terpinolene, and α-terpineol, which showed considerable in vitro activity against the influenza A virus in MDCK cells by interference with acidification of intra-lysosomal compartment ^[43][114]. Mosla dianthera is an aromatic herb used in the TCM to treat colds, coughs, nasal congestion, bronchitis, fever, and headache ^[44][115]. The EOs derived from the aerial part of M. dianthera exhibited significant in vivo inhibitory activity against the influenza A virus at 90–360 mg/kg body weight in mice, with elemicin, thymol, β-caryophyllene, iso-elemicin, asarone, and α-caryophyllene implicated as the major active ingredients ^[41][112].

4. Mechanisms of Action and Structure-Activity Relationships of Some Lead Anti-Influenza Essential Oil Compounds

Basically, the molecular mechanisms of action of lead anti-influenza agents can be summed up under two major categories: those agents that target influenza virus proteins or genes and those that target the various components within the hosts for replication and propagation ^[36][107]. These mechanisms can be used to further categorize anti-influenza agents (virus inhibitors). First, entry and attachment (fusion) inhibitors, which are commonly used as an adjuvant in the preparation of anti-influenza vaccines ^[47][48][132,133]. The aerial EO of Melaleuca alternifolia, known as tea tree oil (TTO), has been shown in an in silico simulation study to interfere with the entry and fusion activities of the influenza virus ^[45][116]. The anti-influenza activity has been attributed to its hydroxylated monoterpenes, terpinen-4-ol, and α-terpineol ^[43][114]. Other known groups are hemagglutinin inhibitors ^[49][134], M2 ion channel protein inhibitors ^[50][135], endosomal and lysosomal inhibitors, also implicated in the TTO ^[51][125], protease inhibitors ^[52][136], RNA polymerase inhibitors ^[53][137], pathway inhibitors ^[54][138], neuraminidase inhibitors ^[55][139], non-structural protein inhibitors ^[36][107], caspase inhibitors ^[56][140], glycoprotein/glycosylation inhibitors ^[57][141], phospholipase inhibitors ^[58][142], release inhibitors ^[59][60][143,144], and autophagy ^[61][145]. Natural antiviral agents including EO compounds can act as inhibitors during the influenza virus activity stages of binding, penetration, uncoating, genome replication, assembly, and release of the virus; thus, they may offer considerable protection and efficacy as anti-influenza agents ^[62][146]. Three major compounds, curdione, curcumol, and germacrone, were implicated in the antiviral EO components of the TCM Zedoary oil ^[63][147]. The compounds impaired influenza A (H1N1) virus replication in vitro and in vivo, with germacrone exhibiting the highest anti-H1N1 effect ^[63][147]. Germacrone was shown to activate the transcription of interferon genes and protect the peripheral cells from influenza virus infections ^[63][147]. It also showed a marked decrease in the expression of antiviral proteins, RIG-I, IFNs, OAS, IRF3/7, MX, and EIF2AK2/PKR, viral replication, and viral load, with increased TAP1 expression, inhibited TAK1 phosphorylation, and consequently inhibited the NF-κB signaling and intrinsic antiviral responses ^[63][64][147,148]. The biological properties of germacrone have been linked to its ketone and non-conjugated double bonds ^[65][149]. Some anti-influenza active bisabolane-type sesquiterpenoids from turmeric oil (Curcuma longa) have also been reported ^[66][150]. Generally, turmeric oil is used in ethnomedicine for the treatment of flu-related and/or airway inflammatory diseases, such as bronchitis, pneumonia, and influenza ^[67][151]. The compounds significantly acted as pathway inhibitors against the influenza A/PR/8/34 (H1N1) virus replication in MDCK and A549 cells in vitro ^[66][150]. The compounds act by inhibiting the expression of pro-inflammatory cytokines (IL-6, IL-8, IP-10, and TNF-α), and regulating the activity of the NF-κB/MAPK and RIG-1/STAT-1/2 signaling pathways ^[66][150]. The presence of ketone, α, β-unsaturation, and presence of an electron-withdrawing group (OH, OCH₃, NH₂, SH, and halogens) have been reported to influence the bioactivity of this group of compounds. Eucalyptol (1,8-cineole), a monoterpenoid principally from Eucalyptus plants, is another lead anti-influenza agent to discuss ^[68][153]. Eucalyptus oil is used in traditional medicines as a remedy for respiratory ailments ^[69][154]. Eucalyptol is a major EO component of the oil, and it has been shown to exert considerable protection against influenza viral infection in vivo ^[70][155]. The oil efficiently decreased the levels of cytokines, IL-4, IL-5, IL-10, and MCP-1 in nasal lavage fluids, as well as the levels of IL-1β, IL-6, and necrosis factors TNF-α and IFN-γ in the lung tissues of mice infected with the influenza A virus ^[68][153]. It also reduced the expression of the inflammatory response, NF-kB, p65, intercellular adhesion molecule (ICAM)-1, and vascular cell adhesion molecule (VCAM)-1 in lung tissues ^[69][154]. The findings thus suggest that eucalyptol is capable of augmenting protection against influenza virus infection in mice by inhibiting pulmonary inflammatory responses in the tissues ^[69][154]. In another study, eucalyptol (12.5 mg/kg) demonstrated lower viral titers, less pulmonary edema, less weight loss, less inflammation, a lower mortality rate, and a longer survival time when it was co-administered with 0.2 µg of haemagglutinin influenza vaccine, compared to when the vaccine was administered alone ^[71][156]. The mechanism of action of eucalyptol has been reported to be an increase in the antiviral activity of IRF3 as well as the IκBα- and JNK-dependent inhibitory effect of IRF3 on the NF-κB p65 and NF-κB proinflammatory signaling pathways ^[71][72][156,157]. The presence of an epoxy functional group and the unique inter-atomic distance between the R1-C-O-C-R2 of eucalyptol have been linked to its remarkable biological effects ^[73][158]. In a recent study, isocaryophyllene acetamides (ICAs) and some S-containing derivatives of caryophyllene oxide (caryophyllane thiosesquiterpenoid, CTS) were shown to inhibit the replication of rimantadine-resistant influenza virus A/California/07/09 (H1N1) pdm09 and influenza virus A/Puerto Rico/8/34 (H1N1) strains, respectively ^[74][75][159,160]. Due to the natural bicyclic framework of ICAs (they are known to show marked anti-influenza activity by blocking the M2 protein of the influenza virus and by inhibiting the cleavage of hemagglutinin ^[75][160]. This led to an aggregation of the virus and lysosomal vacuole membranes and virus inactivation ^[75][160]. Gyrdymova and others demonstrated the influence of the S-containing functional group on anti-influenza activity, showing that bisulfide-containing CTS compounds possess high virus-inhibitory activities and suggesting S-containing derivatives of caryophyllene oxide as promising substrates for the design of newer anti-influenza and/or antiviral agents ^[76][161]. Some caryophyllene derivatives (Ginsamides, GAs) were reported to demonstrate dose-dependent virus inhibition and subtype-specific virus-inhibiting activity (IC₅₀ = 0.15 µM) against the influenza virus H1N1 and H1N1pdm09 strains in a pool of influenza virus A/Puerto Rico/8/34 (H1N1) strains in MDCK in vitro cell cultures ^[77][162]. Ginsamides showed considerable in vivo protective ability against the virus at 150 mg/kg/day and inhibited the fusogenic activity that is typical of influenza A/Puerto Rico/8/34 (H1N1) viruses ^[78][163]. According to the report, GAs can act as lead inhibitors against the viral infection of normal cells and may offer the host an opportunity to maintain a complete immune response ^[77][79][162,164]. Structurally, the bicyclic backbone and the amide functional group of GAs are known to confer a high level of antiviral and anti-influenza activities ^[36][76][78][107,161,163]. Eugenol and citronellol are major EO compounds of Cinnamomum zeylanicum and Pelargonium graveolense, respectively ^[80][81][165,166]. The combined EOs demonstrated in vitro antiviral activity (MIC = 100 3.1 µL/mL) against the influenza A (H1N1) virus ^[82][167]. It acted by targeting the virus surface before and during the adsorption event in the viral lifecycle, thus making it a natural neuraminidase inhibitor ^[82][167]. Structurally, eugenol and citronellol contain phenolic hydroxyl and primary alcohol functional groups, respectively, which confer some biological properties, such as antioxidant, anti-inflammatory, and antiviral activities, amongst others ^[83][84][168,169]. A novel camphor-based anti-influenza agent, camphecene, has been reported to cause a significant decrease in the number of influenza virions fusing their envelopes with endosomal membranes ^[85][170]. This nitrogen-containing camphor derivative has been reported to possess unique chemical properties that bind it effectively to the active sites of hemagglutinin (HA), acting as an HA inhibitor, and thus causing a decrease in viral pathogenicity ^[85][170]. Based on the pharmacokinetic study, camphecene demonstrated a remarkable decrease in virus titer in the lungs and mortality at 7.5 mg/kg, following a 6 h dose regime in vivo ^[86][171]. It also demonstrated an additive effect with Tamiflu, a synthetic anti-influenza drug, which suggests it is an anti-influenza drug candidate ^[85][86][170,171]. Several analogs of this compound have been synthesized, and the structure-activity relationship analysis suggests that camphecene analogs should bear an oxygen atom with a short linker (C2–C4), either as a hydroxyl or ketone group, or as part of a heterocycle, for optimal anti-influenza activity ^[87][172].