Plant Gums as Vaccine Delivery Agents for Major Poultry and Small Ruminant Vaccine-Preventable Diseases

Abstract

Plant gums have found applications in various industries, including food, cosmetics, and pharmaceuticals. They offer unique properties and act as adjuvants, and when employed as a mucoadhesive vaccine delivery system, have immense potential of enhancing the immune response to animal diseases. Novel studies have in recent times, shown growing interest in their use as vaccine delivery agents for poultry and small ruminant diseases and these studies have empirically demonstrated that combining certain plant gums with vaccines for mucosal immunization results in earlier and sustained immune response. Incorporation of vaccine antigens into plant gum formulations protects the vaccine antigen from enzymatic degradation on mucosal surfaces and allows for prolonged vaccine residence at the administration site, leading to improved antigen uptake by the antigen presenting cells, resulting in enhanced host mucosal and systemic immune responses. However, challenges such as standardized extraction methods and gum composition variability need to be addressed. Overall, plant gums have significant potential as vaccine delivery agents and may contribute to the development of effective and affordable vaccines for mucosal immunization against major poultry and small ruminant viral diseases.

1. Introduction

The need to exhaustively harness the potential of mucosal vaccines for poultry and ruminants, administered using mucoadhesive delivery systems, arises from recognition of the limitations and challenges associated with the invasive methods of vaccination. The traditional injection-based vaccines, while effective in conferring immunity, present several drawbacks. The invasive nature of injections can cause stress and discomfort to animals, leading to reduced overall welfare. Additionally, these methods often require skilled personnel for administration, making mass vaccination campaigns in large livestock populations logistically challenging and expensive. Mucosal vaccines, on the other hand, offer a non-invasive, user-friendly alternative with the potential for broader vaccine coverage.

Plant gums are potentially ideal carriers for mucosal delivery of drugs and vaccines [1, 2, 3]. Their natural adhesive and emulsifying properties make them conducive for creating stable formulations that can be administered orally, intranasally, or through other mucosal routes. By incorporating vaccines into plant gum-based formulations, the need for injections is circumvented, eliminating the stress associated with invasive procedures. This not only enhances animal welfare but also facilitates the scaling up of vaccination efforts, particularly in extensive farming systems where there may be a need to vaccinate large numbers of animals. Moreover, mucosal vaccines have the advantage of stimulating both local and systemic immune responses [4, 5, 6]. This dual action enhances the overall effectiveness of the vaccination, providing a robust defense against pathogen invasion at mucosal surfaces as well as systemically throughout the body. Mucosally administered vaccines have shown promise in conferring protection against a variety of diseases affecting poultry and ruminants [1, 7, 8, 9]. Plant gums, known for their biocompatibility and stabilizing properties, emerge as promising carriers for mucosal vaccine delivery, ensuring efficient antigen protection and sustained immune responses [3].

Plant gums are complex polysaccharides produced by various plants. These substances are often exuded by the plant when it is wounded or stressed and serves the function of a protective barrier against invading pathogens and insects, facilitation of wound healing, as well as acting as a sealant to prevent fluid loss in plants [10, 11]. Plant gums have diverse applications in industries due to their unique properties. They are often chosen for their ability to provide viscosity, stability and other desirable characteristics in diverse products. The viscous and emulsifying qualities of numerous plant gums have led to their widespread application in the food, cosmetics, textile, and biomedical/pharmaceutical industries [12]. Plant gums have emerged as promising candidates for vaccine delivery agents in livestock, may revolutionize the approach to immunization in animal husbandry [13]. These natural polysaccharides possess unique properties that make them ideal for enhancing the efficacy of vaccines. Utilizing plant gums in vaccine formulations can offer several advantages, including biocompatibility, biodegradability, and low toxicity. Additionally, the use of plant gums in vaccine delivery systems offers advantages such as reduced production costs, increased safety, and simplified administration methods, and widespread availability of these plant-derived materials make them attractive options, especially in resource-limited agricultural settings. When utilized for mucoadhesive vaccine delivery, plant gums protects the vaccine antigens from enzymatic degradation at mucosal surfaces and dependent on it mucoahesiveness, promotes vaccine residence time at the site of application, facilitating controlled antigen release, and results in a sustained immune response [1, 2]. Through biotechnological approaches, researchers have harnessed the adhesive characteristics of plant gums, like Irvingia gabonensis plant gum [2, 7], Cedrella odorata and Khaya senegalensis [8, 9], to administer vaccines for avian and small ruminant viral diseases. The application of plant gums as vaccine carriers holds great promise, presenting an innovative and potentially transformative approach to enhancing the efficacy, accessibility, and sustainability of vaccination strategies in the field of veterinary medicine.

2. Vaccine delivery agents

Vaccination may be considered the most effective and economical infectious disease control strategy. The success of vaccination leverages on the fact that vaccines, which primarily contain live, attenuated, or a sub-unit of a particular pathogen, can train the immune system to respond to future infection by the pathogen or its antigenic relatives, resulting in the establishment of immune memory cells without themselves inducing symptoms of the disease [14]. Adjuvants are delivery agents or immunostimulatory compounds added to vaccine formulations to improve the effectiveness of the vaccine. For mucosal vaccinations, adjuvants primarily improve the vaccine effectiveness by enhancing their bioavailability in mucosal sites where innate defense mechanisms like pH, enzymatic activities, mucous blanket, and ciliary action may limit or inhibit the interaction of vaccine antigens with antigen presenting cells or result in antigen degradation [6]. Microparticles, emulsions, ISCOMS, and liposomes are common delivery adjuvants, and they basically function to encapsulate antigens to improve its transportation across mucosal surfaces to antigen-presenting cells (APCs) like macrophages and dendritic cells while protecting it from natural defenses.

2.1 Plant gums as adjuvants

Plant gums have emerged as promising candidates for vaccine adjuvants, playing a crucial role in enhancing the effectiveness of vaccines administered through mucosal sites. Plant gums as delivery vehicles, work to promote the slow release of antigens, hence prolonging their targeted interaction of vaccine antigens with antigen-presenting cells at the mucosal sites.

From a broad perspective, adjuvants stimulate the immune system in five distinct ways. This includes, the depot effect, the targeting effect or antigen dissemination, the immunological activation/modulation effect, the antigen presentation effect, and the CTL induction effect [15]. Plant gums as adjuvants, may mostly elicit its immune response via the depot effect, whereby a “bank” is established in which antigens can be slowly released, presented, and circulated to immunocompetent cells, thereby maintaining stimulation of immunocompetent cells and resulting in elevated antibody titres in vaccinated humans or animals (see Figure 1). Using plant gums as adjuvants also increases the resistance time of the dosage formulation at the site of absorption by interacting with the mucus layer on the mucosal epithelial surfaces and mucin molecules, compensating for the dilution of vaccine formulations in mucosal fluids and their loss in mucosal turnover [6].

Figure 1.

Graphical representation of the depot effect of immune response stimulation following a mucosal delivery of vaccine using a mucoadhesive delivery system.

Many plant gums have been tested for their adjuvant potential in vaccination with successful outcomes (see Table 1). Xanthan, a natural polymer gum has been used in bioadhesive formulations for intranasal influenza virus immunizations [3]. In another study by Schuch et al. [21], an improved immune response was observed against ovalbumin antigen, following the use of xanthan as adjuvant. In the study, higher serum antibody and INF-ɣ levels were observed in the xanthan-adjuvanted rats, compared to the control. Another group of researchers evaluated the effect of xanthan on humoral and cellular immune responses to DNA vaccines in mice [22], demonstrating a significant increase in antibodies and IL-17 in the xanthan – adjuvanted group.

Plant		Citation
Botanical name	Common names
Irvingia gabonensis	Bush mango, Wild mango, African Mango	[2, 9, 16]
Abelmoschus esculentus	Okra, Ladies finger, Bhindi, Gumbo	[2]
Khaya senegalensis	Senegal mahogany, African mahogany, Senegal Khaya, Bisselon	[1, 7, 8]
Cedrella odorata	Spanish cedar, Cuban cedar	[1, 7, 8, 17]
Boswellia carteri	Frankincence, Olibanum	[18]
Boswellia frereana	Frankincence, Olibanum	[18]
Commiphora myrrha	Myrrh, African Myrrh, Somali myrrh	[18]
Cyamopsis tetragonoloba	Guar, Cluster bean	[19]
Chrysanthemum	Mums, Chrysanths	[20]

Table 1.

Plant-based mucoadhesive polymers with potential for mucosal vaccine delivery.

Current research studies on the potential of plant gums for mucoadhesive vaccine delivery in veterinary medicine are actively exploring the unique properties of plant-derived polymers for enhancing mucosal vaccination strategies. Scientists have investigated various plant gum extracts, such as Khaya senegalensis, Cedrela odorata, [7, 8], Abelmoschus esculentus and Irvingia gabonensis [2, 9] gums, to develop formulations that can adhere to mucosal surfaces and improve the delivery of vaccines to animals. These studies aim to harness the mucoadhesive properties of plant gums to enhance the stability and bioavailability of vaccines, ensuring prolonged contact with mucosal membranes for improved antigen uptake. Emikpe and co-workers [1] investigated ex-vivo, the mucoadhesive properties and immunopotentiating effects of Khaya senegalensis and Cedrela odorata gums; individually and combined, using tracheal and duodenal tissues of cattle, chicken, pig, sheep and goats. They were able to demonstrate that these gums, singly or in combination, had extended adhesion times on the animal tissues, and also stabilized incorporated vaccine antigens ex vivo, a property desirable for an ideal mucoadhesive vaccine delivery agent. This paved the way for subsequent effectiveness and efficacious studies on these gums with promising results. Oyebanji et al. [7] evaluated the mucoadhesive strengths of Cedrela and Khaya gums ex vivo. Subsequently, they used the gums as mucoadhesive vaccine delivery agents for Newcastle disease virus (NDV) vaccination in chickens and recorded higher protective immunity compared to the controls. A similar outcome was reported by Adeniran et al. in 2019 [8] for infectious bursal disease (IBD) in chickens. In another study, Ezeasor and co-workers evaluated the mucoadhesive and immunopotentiating effects of gum extracts from Abelmoschus esculentus and Irvingia gabonensis ex-vivo, using cattle and goat nasal, tracheal and duodenal mucosa [2]. In the study, Irvingia gabonensis gum extracts showed better mucoahesiveness and were used in a subsequent study, for intranasal administration of pest des petits ruminants vaccine in goats. The antibody titres in the adjuvanted group were significantly higher than the non-adjuvanted groups. Also, it was observed in that study that the PPR virus specific antibodies at 28 days post-vaccination was comparable to the subcutaneously vaccinated groups and did not differ significantly [9]. A similar study by Mumin and co-workers [18] further corroborated the potentials of plant gums for mucoadhesive vaccine delivery. They evaluated the mucoadhesive properties as well as the immunopotentiating effects of Boswellia carteri gum on intranasal PPR vaccination in sheep and goats. Vaccine-to-gum ratios of 1:1 and 1:2 were assessed in this study and it was discovered that vaccine-to-gum ratio of 1:1 was better, showing higher PPR virus-specific antibody titres comparable to animals vaccinated via the convention route (subcutaneous route) up to 56 days post-vaccination. The biocompatibility and low toxicity of plant gums make them promising candidates for veterinary applications. By understanding the intricate interplay between plant gums and mucosal surfaces, researchers can develop innovative and effective vaccine delivery systems that can contribute to the advancement of veterinary medicine, ultimately promoting the health and well-being of diverse animal populations.

2.1.1 Ex-vivo evaluation methods and principles of mucoadhesive properties of gums

The bioadhesive strength of numerous polymer compounds has been evaluated in vivo and ex vivo using a wide range of techniques and procedures. Shear stress is frequently measured using adhesion experiments, which measure the force required to separate two polymer-coated glass slides that are separated from one another by a layer of mucus. In ex vivo tensile stress measurements, Wilhelmy plates and electromagnetic force transducers are frequently employed. This idea was used by Mikos and Peppas [23] to develop the flow chamber approach. The rotating cylinder method is also commonly utilized in the ex vivo evaluation of mucoadhesive polymers [1]. The mucin-polymer bioadhesive strength has also been measured using viscometric methods [24] and rheological methods [19, 25] to assess the in vivo properties of mucoadhesive polymers. Emikpe et al. [1] modified the rotation cylinder method using a tablet dissolve machine to assess the interaction between mucosal tissue surfaces and the gum polymer. In all these methods, the dosage forms vary and are not inter-applicable, hindering comparison across studies. Hence, a standardized method is not yet available. However, Amoros-Galicia and co-workers [26] recently proposed a standardized method based on the parameters of peak force and work of adhesion as a single systematic in vitro method for measuring bioadhesion or mucoadhesion that is applicable to various pharmacological dosage forms.

2.1.2 Measurement of the mucin-polymer bio adhesion strength

Mucin-polymer bio-adhesion measurements are vital to development of adjuvants for mucoadhesive vaccine delivery, representing a critical aspect that influences the effectiveness of mucosal vaccines. Mucins, the glycoprotein components of mucus, form a protective barrier on mucosal surfaces, acting as the first line of defense against pathogens. Understanding the interaction between mucoadhesive formulations and mucins is paramount, as it directly impacts the adhesion, retention, and subsequent release of antigens. Accurate measurements of mucin-polymer bio-adhesion provide crucial insights into the formulation’s ability to adhere to mucosal tissues, ensuring prolonged contact for optimal antigen delivery. This is particularly significant in mucosal vaccination, where the mucosal immune system’s response is essential for robust protection against infections. The effectiveness of the mucin polymer bio glue has been evaluated in vitro, using a flexible tablet dissolving device (Copley dissolution equipment). Prabhu et al. [19] made this adjustment to the standard method of measuring shear stress. This device was used to determine the mucin-polymer adhesive strength by measuring the shear strain of the gum on living tissue and by tracking the peak adhesion time in a wet environment (temperature, pH, and physiological buffer) [17]. The peak adhesion time is the amount of time it takes for an animal tissue to separate from a mucoadhesive material crushed into a tablet under physiological settings that mimic the interaction in vivo. Fast and reliable adhesion to the mucosa surface was observed for phytogenic gum gels utilized in the investigation. This may be due to the fact that the gum polymer and phytogenic mucoadhesive have been shown to have complementary actions on the tissues.

Further evidence of the phytogenic mucoadhesive’s better viscoelastic capabilities in a wet environment came from the persistent binding of mucin to the mucosal tissue with little change and disintegration at the mucosal surface of adhesion. As a result, interactions (such hydrogen bonds and mild van der Waals forces) between intermediate contacts (glycoproteins) led to the creation of a stable hydrated gel. There is a breakdown in adherence at the mucus layer compared to the mucoadhesive-mucus interface plane of adhesion, demonstrating that the mucoadhesive from these phytogenic sources is strong enough to establish continuous and enduring interaction with the mucus layer of mucosal surfaces. According to Parthasarathy et al. [27], this method is highly suited for an in vivo test since it allows for continuous antigenic presentation, stimulation, and response. Hydrodynamic circumstances, pH, and vaccination components all caused some interference. Due to the mucoadhesive’s excellent penetration of vaccine antigens, retention of mucoadhesive properties, and enhancement of gum polymer-vaccine mix’s haemagglutination properties in veterinary subjects, the mucoadhesive exhibits tremendous promise as a vaccination vehicle. The development of the best adjuvant for inducing an immunopotentiating response to vaccinations in vivo should focus on this latter function. The lectins, which are assumed to be ubiquitous in plants and their derivatives, may also account for the mucoadhesives’ observed haemagglutination activity. According to Ingale and Hivrale [28], these widely distributed sugar-binding proteins also possess cytotoxic, mitogenic, antiviral, and antifungal activities.

In general, precise bio-adhesion measurements help researchers tailor vaccine formulations, incorporating polymers that enhance mucoadhesion and prolong the residence time of vaccines at mucosal surfaces. This extended contact time facilitates better antigen uptake and presentation to immune cells, ultimately leading to a more potent and sustained immune response. A thorough understanding of mucin-polymer interactions aids in the development of vaccines that are not only efficacious but also safe and well-tolerated. By fine-tuning the bio-adhesive properties of vaccine formulations, researchers can mitigate potential side effects and improve the overall safety profile of mucosal vaccines. In essence, mucin-polymer bio-adhesion measurements serve as a crucial tool for optimizing the design and formulation of mucosal vaccines, contributing to advancements in vaccine technology that hold great promise for preventing a wide array of infectious diseases in both human and veterinary medicine.

2.2 Assessment of immune response in chickens vaccinated with Newcastle disease vaccine using plant gums

Emikpe et al. [13] evaluated the immunological response of challenged chickens given the Newcastle disease vaccine combined with Cedrela odorata and Khaya senegalensis gums. The gum-vaccine groups a more robust and earlier serum and mucosal antibody titres, especially in the oral group. In addition, starting in week 2 following the challenge, the gum-only groups displayed a comparable peak titer response. Evidence from an ex-vivo experiment [1] lends credence to the idea that phytogenic mucoadhesives possess immuno-potentiating capabilities and can stimulate immunologic memory processes. The immunological response may result from a complex interaction between the gum mixture and antigen-producing cells like the M-cells of the Follicle Associated Epithelium (FAE). The research discovered that the combination improved mucosal response in the gum-vaccine-ocular group before the oral groups. Booster vaccination clearly separated the gum-vaccine groups from the vaccine-alone groups. In both gum-vaccine groups, the post-infection titer was higher than in the vaccine-alone groups, which may be related to the immunologic memory process. A recent study by Liu et al. [20] has attributed the development of immunologic memory to the ability of plant gums to extend antigen contact time, improve antigen uptake by the APCs, enhance mucosal immune responses, and influence the activation of T cells, which are central to the development of immunologic memory. By creating a favorable microenvironment at mucosal surfaces, these polymers can promote the activation and differentiation of T cells, leading to the establishment of memory T cell populations.

2.2.1 Hematological profiles of chicken vaccinated with Newcastle disease vaccine using phytogenic gums as delivery agents

Despite the enormous potential of plant gums for use as ideal mucoadhesive delivery agents, the possibility of exerting an adverse effect on recipients is considered. Some researchers have evaluated Khaya senegalensis and Cedrela odorata gums using the Haemagglutination (HA) technique [1, 7]. Their findings showed that the gums possess strong haemagglutination properties (log225) either individually or in combined ratios, presenting a risk of causing haemagglutination in hosts under field conditions. This HA property from these gums is hypothesized to stem from the presence of immunogenic large carbohydrates that make up the macromolecular structure of these gums such as Rhamnose, Lectins, Arabinose [28]. A checkerboard dilution conducted to determine a concentration with minimal HA property while retaining the mucoadhesive and desired immunopotentiating property was conducted and a 1:8 dilution with HA property of Log22 was proposed safe in an in-vivo study. Oyebanji et al. [7] used combined Cedrela odorata and Khaya senegalensis gum at ratio of 1:8 as the delivery vehicle for Newcastle disease vaccination in chicken, and studied some selected hematological parameters of Newcastle disease-vaccinated chicken. The gums used as the vehicle for the delivery of Newcastle disease immunizations were found to be safe, since no discernible disruption in the hematological indices was detected. Hematological parameters were within the safe range and similar to the control group, demonstrating that 1:8 dilutions of Cedrela odorata and Khaya senegalensis did not produce any hematological derangement in chicken, in-vivo. The authors recommended that in order to avert possible occurrence of Cedrela/Khaya adjuvant-induced haemagglutination in chickens, vaccination of chickens should therefore be carried out using the gum mixture at the recommended dilution ratio of 1:8.

2.2.2 Development and evaluation of oral phytogenic micro-beaded vaccine delivery of Newcastle disease vaccine in chicken

Due to the common practice of backyard poultry production in Africa, there is a pressing need for conveniently accessible, low-cost poultry vaccines that can be used to vaccinate these birds. The development and evaluation of micro-beaded oral vaccination against Newcastle disease (ND) in backyard chickens is timely because, with the production of large-doses vaccines, there is a need to provide simple and adaptable technology that could be used for smaller flocks and backyard poultry. Controlling ND in backyard chickens has been shown to be possible through vaccination with a locally made, live, thermally stable vaccine that may be blended with poultry feed [29]. Successful local chicken production can be improved by the creation of a micro-beaded vaccination that is inexpensive and easy to administer. In a study by Ola et al. [30], micro-bead, which contains a vaccine, was produced by the ionotropic gelation method, using aluminum sulphate as cross-linker with Sodium alginate and Boswellia carterii gum extract (Figure 2). The use of plant gums as vaccine delivery agents is reliant on its ability to increases vaccine residence time at the site of application and improve immune response. The ability of plant gums to increase vaccine residence time when used as mucosal vaccine delivery agent is greatly influenced by the mucoadhesive properties of the gums. Ex vivo evaluation of the mucoadhesive properties of Boswellia carterri gum showed a sustained mucosal peak adhesion time, reflecting its superior mucoadhesiveness and suggesting great potentials as delivery agents for mucosal delivery of vaccines [18, 30]. In the study by Ola and co-workers, the beaded formulation gave a delayed but more robust and prolonged immune response compared to the study control. The merits of this study may be attributed to the fact that micro-beads allows for precise and controlled delivery of the vaccine, ensuring accurate dosing for each bird. This method is particularly advantageous in backyard settings where individual bird handling may be more feasible than in large-scale commercial operations. More so, micro-beaded vaccines can potentially overcome challenges of vaccine degradation, as they offer protection against internal and external environmental factors that may compromise vaccine stability. Additionally, the oral route minimizes the stress associated with traditional injection methods, greatly mitigating the vaccination stress on the birds as well, hence, making it more suitable for backyard poultry owners. However, certain drawbacks associated with the use of micro-beads to achieve disease free backyard poultry should be considered. These may include the need for careful and meticulous administration to guarantee that each bird receives an effective dose, as uneven uptake could compromise vaccine efficacy. Furthermore, the cost and availability of micro-bead technology may pose challenges for widespread adoption. Despite these considerations, the benefits of micro-beaded oral vaccination in terms of precise dosing, reduced stress, and potential stability advantages make it a promising approach for Newcastle Disease control in backyard chicken populations.

Figure 2.

Micro-beads containing Newcastle disease vaccine, produced by the ionotropic gelation method, using aluminum sulphate as cross-linker with sodium alginate and Boswellia carterii gum extract.

2.3 Assessment of immune responses in chickens vaccinated with infectious bursal disease vaccine using plant gums as delivery agents

The best strategy for controlling IBD and its efficiency rate in field settings depends on the field pressure, level, hygienic management, diversity in maternally produced IBD antibodies, as well as the IBD vaccination types to be utilized. Adeniran et al. [8] used phytogenic adjuvants delivery method to immunize chickens against the infectious bursal disease virus (IBDV), and they were able to demonstrate that both oral and ocular vaccinated birds, using phytogenic adjuvants had robust, prolonged and protective humoral and mucosal immune responses. Specific antibody titres in the serum and mucosal washings were significantly higher in the gum-vaccine group after only a few days of administration. These observations corroborates earlier reports that phytogenic mucoadhesive adjuvants have immunopotentiating properties and may induce immunologic memory processes, as was shown in previous ex vivo studies. The employment of phytogenic carriers in the vaccination process, however, has been seen to have the potential to further immunopotentiate the observed immune response. In the aforementioned study, mucilage from two plants, Cedrela odorata and Khaya senegalensis was used. Apart from ability to increase the vaccine residence time, the superior immune response facilitated by this gum may be attributed to the phytochemical constituents of these gums. Cedrela gum consists of oligosaccharides while Khaya gum consists of arabinose, galactose, and rhamnose which possess adjuvant action mediated through glucan and mannan receptors on macrophages. Activation of these receptors leads to enhanced phagocytic activity facilitating antigenic uptake, processing and presentation, cytokine secretion, leukotrienes release, and prostaglandin production [31].

2.3.1 Clinico-pathological evaluation of infectious bursal disease vaccine using gums in challenged broilers

Since chicken production suffers continuous setbacks as a result of inconsistency in vaccination outcomes against IBD, there is a need to pragmatically consider the incorporation of plant gum adjuvants in poultry production to remedy this challenge. Effectiveness studies have been conducted to ascertain and describe the effects of selected plant gum in the induction of protective immune responses against IBD in chickens. In one of such studies, Adetunji and co-workers [8] evaluated the protective effects of oral administration of IBD vaccines, adjuvanted with equal proportions of Cedrela odorata and Khaya senegalensis in broiler chickens. In broilers that had received a vaccination against IBD using a combination of the IBD vaccine and plant gums (Khaya senegalensis and Cedrela odorata), the clinicopathologic effects of IBD virus infection post vaccination was assessed. The vaccinated and gum-fed broilers chickens had better survival and the clinical disease was well ameliorated compared to the control group. In that study, the enhanced response observed in the vaccine-gum combination corroborates the earlier reported potentiating properties of the selected gums, whose primary mechanism of action is postulated to be via increase in the residence time of the vaccine antigen on the mucosa, progressive release of the vaccination antigen and/or activation of memory cells [7, 8]. The observation may also be associated with the phytochemical constituents of the plant gums as discussed in the earlier section. Nagakawa [31] reported that Khaya gum consists of arabinose, galactose, and rhamnose, and these possess adjuvant action mediated through glucan and mannan receptors on macrophages. Activation of these receptors leads to enhanced phagocytic activity facilitating antigenic uptake, processing and presentation, cytokine secretion, leukotrienes release, and prostaglandin production. In another report, Emikpe et al. [1] proposed using a combination of Cedrela odorata and Khaya senegalensis gums at a ratio of 1:1 to maximize the immune response to IBDV. Supporting the method’s efficacy and the amount of protection it offers against Gumboro disease in birds, it was evident that the vaccine gave effective protection [32]. Similar tissue tropism for Newcastle disease was shown in a previous study, using the same mucilage [33]. Adjuvants from macromolecules, synthetic materials, or phytogenic sources have been recommended for use in the delivery of vaccines in a number of studies [34, 35, 36]. These mucilages should be employed in the delivery of vaccinations to better address the prevalence and widespread dispersal of IBDV in chicken, as they are cheap and easily accessible.

To combat PPR, a stomatitis pneumo-enteritis complex that primarily affects sheep and goats in the tropics, researchers also assessed the use of plant gums for vaccine delivery in small ruminants.

2.4 Immunogenicity and vaccines applications using phytogenic delivery systems in ruminants

The utilization of various plant gums for vaccine delivery in ruminants is of paramount relevance in veterinary medicine, offering a versatile and effective means to enhance immunization outcomes. Different plant gums have been shown by numerous researchers, to present unique potentials in mucosal vaccine delivery. The mucoadhesive properties of these plant gums play a pivotal role in promoting the adhesion of vaccines to mucosal surfaces, such as in the respiratory and gastrointestinal tracts, which are significant sites for immune induction in ruminants. This enhanced adhesion ensures prolonged exposure of antigens to induction sites of the mucosal associated lymphoid tissues (MALT), fostering robust and sustained immune responses. The relevance of using different plant gums lies not only in their ability to improve the stability and bioavailability of vaccines but also in their potential to modulate immune responses [31]. Plant gums may possess immunomodulatory properties that influence the activation of immune cells, such as macrophages and T cells, leading to a more tailored and effective immune response [31]. By selecting and optimizing plant gums based on their specific properties, veterinary researchers can tailor vaccine formulations to address the unique physiological characteristics of ruminants. The ability of Irvingia gabonensis gum, Abelmoschus esculentus gum, and Boswellia carteri gum to enhance the immunogenicity of vaccines and restrict the growth of harmful microorganisms, as well as their use as mucosal delivery systems, were discussed in relation to the intranasal administration of the peste des petits ruminants (PPR) vaccine in sheep and goats. Emikpe & Odeniyi [13] explored the possible use of plant-based gums/polymers for vaccine delivery and highlighted the necessity for creating efficient adjuvants and delivery systems to increase the immunogenicity of vaccinations and defend against hazardous pathogens. Phytogenic polymers and gums have great promise as veterinary medicine therapeutic agents, however, it vital to consider how different extraction procedures may affect the gums’ mucoadhesive qualities. The extraction methods employed in obtaining plant gums can significantly impact their mucoadhesive properties, influencing their suitability for vaccine delivery [2]. Diverse extraction techniques, such as solvent extraction or aqueous extraction, may yield gums with varying molecular structures. The purity, molecular weight, and structural integrity of extracted plant gums may directly affect their mucoadhesiveness. Gentle extraction methods that preserve the native structure of the gums are essential, as harsh processes can lead to structural alterations and diminish mucoadhesive performance. Therefore, meticulous selection of extraction methods is pivotal to ensuring the plant gums maintain their inherent mucoadhesive properties for effective use in vaccine delivery formulations.

2.5 Evaluation of the extraction method of Abelmoschus esculentus and Irvingia gabonensis gums mucoadhesive strength

Variations in extraction processes, such as solvent type, temperature, and duration, impact the composition and structure of extracted plant gums. These factors may alter the molecular weight and configuration of gum molecules, subsequently affecting their mucoadhesive capabilities. Hence, it is necessary to study and define optimal extraction methods so as to preserve the natural characteristics of plant gums and ensure that their mucoadhesive potential remains intact. In a recent study, gum from Abelmoschus esculentus and Irvingia gabonensis was extracted using various procedures, and its mucoadhesivity and antigen release in vaccine-gum formulations was evaluated. Gums from the plants Abelmoschus esculentus (AE) and Irvingia gabonensis (IG) were extracted with acetone or sodium chloride (NaCl) and then oven-dried or freeze-dried, respectively, to determine how the extraction method affected mucoadhesion and the gums’ ability to release vaccine antigen in vaccine-gum formulations [2]. Due to their capacity to improve antigen absorption and lengthen the residence period of the immunizing antigen, mucoadhesive polymers have been discovered to provide considerable promise for mucoadhesive vaccine administration. In the ex vivo study, the peak adhesion times (PAT) were measured for both Abelmoschus esculentus and Irvingia gabonensis gum on bovine and caprine nasal, tracheal and duodenal mucosa. The PAT of NaCl-extracted and freeze-dried Irvingia gabonensis gum extract was substantially greater than that of acetone-precipitated and oven-dried Abelmoschus esculentus gum extract on all the test mucosa. The higher mucoadhesivity observed for the I. gabonensis gum extract was attributed to the NaCl that was utilized during the extraction protocol, as it may have altered the surface charge of the polymer gum extract. According to Park et al. [37] and Tiwari et al. [38], the charge of a polymer is very important for effective mucoadhesion. These authors also reported that non-ionic polymers undergo a weaker adhesion than ionic polymers. It has also been suggested that cationic polymers may likely have notable mucoadhesive strength [39], hence the possibly cationic extraction of I. gabonensis may have reacted with the anionic mucin coat of the mucous membranes [40], resulting in strong ionic bonding between the phytogenic extract and the test mucous membranes. The study highlights the need to optimize extraction methods, as it is essential for preservation of the integrity of plant gums, and ensuring consistent and desirable mucoadhesive properties. Hence, understanding the relationship between plant gum extraction techniques and its mucoadhesivity is vital for developing effective mucoadhesive vaccine delivery systems that harness the full potential of plant gums in enhancing mucosal immune responses.

2.5.1 Effect of extraction methods on antigen release from vaccine-gum formulation

To better understand the relationship between plant gums and incorporated vaccines, an experiment was conducted by Ezeasor and co-workers [2] using the same plant gum extracts and extraction methods as described earlier. A modified agar gel diffusion (AGID) method was used to evaluate the antigenicity/antigen release of I. gabonensis and A. esculentus gums in combination with reconstituted peste des petit ruminants vaccine over the course of 72 hours in a humid chamber at 25°C by carefully observing the presence and type of precipitation lines as well as the time it took for them to form. The PPR vaccine-I. gabonensis gum mixes at a ratio of 2:1 and 1:1 caused a robust reaction with high-intensity precipitation lines after 24–48 hours, however, the vaccine-gum mixture at a ratio of 1:2 only mildly produced a good response. When the PPR vaccine to A. esculentus gum ratio was 2:1 after 48 hours, a significant reaction was seen with high intensity precipitation lines, while the vaccine-to-gum ratios of 1:2 and 1:1 were negative. This demonstrates that the PPR vaccine’s antigenicity was unaffected by its addition to the gum extracts. Additionally, the study showed that I. gabonensis gum extract loaded with PPR vaccine antigen may release vaccine antigens faster than A. esculentus polymer gum extract suggesting an immunomodulatory effect. The authors suggested that NaCl used in the I. gabonensis gum extract protocol may have played a role, because an earlier study on synthetic mucoadhesive by Yan and Huang [41] showed that the addition of NaCl in the protocol enhanced immune response by interfering with the electrostatic interaction between the liposome carrier and the protein antigen resulting in enhanced antigen release from the carrier. Thus, when developing a mucoadhesive delivery system for the mucosal distribution of medications or immunizations, it is crucial to carefully consider the extraction methods.

2.6 Hematological and humoral immune responses associated with intranasal Peste des petits ruminants virus vaccination of goats using Irvingia gabonensis gum as delivery agent

The goal of this study was to examine if goats could be given an intranasal PPR vaccination using the attenuated Nigeria 75/1 PPR vaccine and Irvingia gabonensis (IG) gum extract’s to harness its immune-modulating properties. When employed as a mucoadhesive intranasal vaccine delivery medium, I. gabonensis seed gum extract exerts an immunomodulatory effect on the systemic immune response following PPR intranasal vaccination. By day 7 post-vaccination, both intranasal vaccinations with and without plant polymer gum as the mucoadhesive carrier produced a significant systemic PPR-specific IgG response as determined by H-based PPR blocking ELISA, with the reaction reaching its peak by day 21. This promotes the concept that intranasal administration of PPRV vaccine is the most effective method of mucosal immunization against the virus [42, 43]. It also supports the assertions made by a number of authors [4, 5] that intranasal vaccination produces significant mucosal immune responses in addition to high systemic immunological responses. The experimental goats showed no clinical symptoms of muco-haemorrhagic nasal discharges, coughing, or persistent sneezing during or after delivery, suggesting the safety of the intranasal PPR vaccine-gum and its lack of irritation on mucosal surfaces. An ex vivo investigation that focused on the potential value of I. gabonensis gum extract for the delivery of mucoadhesive vaccines employed the PPR vaccine [2, 9]. Irvingia gabonensis gum extracts after PPR vaccine-gum synthesis showed outstanding mucoadhesivity on goat nasal mucosa and efficient vaccination antigen release. In the study [9], Irvingia gabonensis’ potent nasal mucoadhesivity may have enabled the antigen-charged gum to adhere to the nasal mucosa for a longer period of time, giving the specialized antigen-presenting cells more time to bind the antigen and produce high-affinity antibodies. Hematological testing revealed that all trial groups had erythrocytic indices well within the normal range. Therefore, it may be concluded that the erythrocytic profile was not affected by any of the PPR immunization protocols which further corroborates that the gum extract is not toxic. Goats that received intranasal PPR immunization had normal erythrocytic profiles, as shown by the studies of Ezeasor et al. [42] and El-yuguda et al. [44]. In contrast, the differential counts of blood leukocytes showed clear differences even if the results were not statistically significant. All of the vaccinated groups showed an increase in relative lymphocyte counts beginning on day 14 post-immunization. A greater immune response to the vaccination may be indicated by a higher lymphocyte count in the total leukocyte count. Because it combines information on leukocytic differentials outside of usual reference ranges into one variable, the neutrophil to lymphocyte ratio (NLR) is a frequently utilized biomarker of systemic inflammation [45, 46]. Absolute lymphocyte counts beyond the lower normal limits are suggested as a biomarker by Mavinkurve-Groothuis et al. [47] for predicting the immune response to influenza vaccination. The ratio between neutrophil and lymphocyte counts declined from day 14 in all the immunized groups until day 28, despite the fact that both counts in this study were within the normal reference ranges. Not neutropenia, but a proportionate increase in lymphocytes is to blame for the decrease. Since peripheral blood NLR is a biomarker that predicts a favorable immunological response to immunization, it may 1 day be utilized to rule out vaccination failures in a clinical context. An intranasal immunization against PPR virus in goats may stimulate a systemic immune response. The PPRV-specific systemic immune response is further improved by intranasal PPR vaccination using plant gum as mucoadhesive adjuvant. Since the most synthetic mucoadhesive polymers, including chitosan, are expensive, plant gums offer a cheaper means with promising outcomes for mucosal administration of vaccines.

2.6.1 Evaluation of mucoadhesive property and the adjuvant effect of Boswellia carterri gum on intranasal PPR vaccination in small ruminant

To ascertain the mucoadhesive and immunomodulatory effects of phytogenic gums from the Boswellia frereana, Boswellia carterri, and Commiphora myrrha, intranasal vaccination of goats and sheep against Peste des Petits Ruminants (PPR) was tested in both ex-vivo and in-vivo models. Because the intranasal method produced comparable antibody titres to subcutaneous injection following vaccination with the Boswellia carterri PPR vaccine combination, it may be used as a non-invasive alternate delivery technique for PPR immunization of small ruminants.

Gums from B. frereana, B. carterri and C. myrrha were examined for their mucoadhesive properties on goat tissue, and results showed that they were either short-lived or ineffective. The duration of an attachment is often determined by the amount of surface mucus present, therefore this finding could be explained likewise [13]. Mummin et al. [18] found that the parenteral and intranasal injection of the PPR vaccine is protective because all inoculated animals with B. carterri gum vaccine combinations were clinically healthy throughout the study. The study found that the strongest immune response was achieved with intranasal delivery of B. carterri gum and PPR immunization, with an inhibitory proportion almost similar to that of conventional subcutaneous therapy. The results of this study show that similar antibody titres were produced by all gum-vaccine combinations in sheep and goats, suggesting that this methodology can be safely applied to both species. Less of an immunological response was seen with a 2:1 mixture than with a 1:1 mixture of gum and vaccine. Because the optimal immunological response is achieved through a balanced polymer and vaccine combination, increasing gum polymer beyond 50% may be unsuccessful [13]. In conclusion, intranasal injection of the Boswellia carterri PPR vaccine combination represents a less invasive, more secure, and immunogenic alternative to the intrusive subcutaneous delivery route commonly employed for PPR immunization.

2.6.2 Influence of PPR vaccine application methods on clinicopathological and immunohistochemical immune responses in goats

The choice between nasal spray and nasal drop methods of administering intranasal vaccines is clinically important, as it has been shown to influence vaccine efficacy. Immune responses in goats following intranasal vaccination can be affected by how the vaccine is administered, but this is not well understood. Live attenuated PPR vaccine was administered intranasally to goats using a nasal dropper or nasal spray. The purpose of this research was to compare the efficacy of two PPR intranasal vaccine application methods in stimulating immunological responses in goats. The study showed that in protection of the lungs from PPR-induced pulmonary pathology, the intranasal spray technique has a better chance of success than the intranasal drop technique, even though the latter may produce an earlier peak in the PPR-specific antibody titres. The unique anatomy and location of the induction sites of the NALT may have influenced these observed outcomes. Nasal spray vaccinations, as reported by Ramvikas et al. [48], mostly deposit antigen in the anterior nasal cavity. It has also been pointed out that where in the nasal cavity the vaccination is deposited has an effect on how much antigen is absorbed by the formulation, with more antigen being absorbed when the vaccine is placed in the posterior end of the nasal cavity rather than the rostrum [48]. Small ruminants have the NALT, and accompanying lymphoid tissues in the back of their nasal cavities, just beyond the eustachian tube [49]. Conversely, nasal drops may offer a more controlled and precise delivery, allowing for individualized dosing, which may be particularly beneficial in smaller herds or specific case scenarios. The NALT is the primary inductive site for nasal vaccines, and the liquefied vaccine administered using a dropper may have allowed posterior vaccine antigen deposition and coating of the nasal mucosa and the NALT upon administration, increasing the availability of vaccine antigen to the specialized follicle associated epithelial (FAE) cells and antigen presenting cells (APC). Davis [4] states that soluble antigens can enter the body through the nose, where they can interact with dendritic cells, macrophages and lymphocytes before being drained to the superficial lymph node and spleen to set off a more systemic immune response. This study shows that a strong systemic immune response can be induced by intranasal administration of the PPR vaccine using the nasal dropper, but that a local immune response in the lower respiratory tract is better induced following intranasal PPR vaccination using nasal spray. Therefore, the nasal spray method of PPR immunization may have greater promise for pulmonary protection against the pneumonic form of the disease than the dropper approach of intranasal administration. However, unlike nasal drops, nasal spray administration may require specialized equipment, potentially posing challenges in resource-limited settings. Nasal drops administration is simpler, and more practicable in diverse husbandry conditions. Balancing these practical aspects is crucial for successful intranasal PPR vaccine programs in ruminants, where the choice between nasal spray and nasal drop methods should align with herd size, and infrastructure availability, ensuring effective immunization strategies and disease prevention.

2.6.3 The influence of intranasal phytogenic mucoadhesive delivery of peste des petits ruminants (PPR) vaccine on PPR outbreak outcomes in goats

In the local settings, goat farmers frequently present their livestock in public markets, which then become a hub facilitating the transmission of contagious disease because sick and healthy animals are occasionally tethered together in the same stalls as they wait for clients [50]. PPR, also known as peste des petits ruminants, is a highly contagious disease of small ruminants in Africa, Southeast Asia, Middle East and Eastern Europe. Outbreaks are common in these areas and the results of the study are discussed in relation to how PPR vaccination influenced the clinicopathological PPR findings during an epidemic [16]. The results of the study advocate that intranasal immunization, with or without plant gum, may be more successful than subcutaneous vaccination in preventing the spread of PPR infection during an epidemic. In the study, following the observation of clinical signs suggestive of PPR, all the animals were vaccinated accordingly. Group A received the PPR vaccine intranasally together with phytogenic mucoadhesive gum; Group B received the vaccine without the gum; Group C received the vaccine subcutaneously; and Group D received no vaccination. The majority of the experimental animals demonstrated prompt onset of pertinent clinical signs following the vaccination. The clinical observations were scored and this peaked 4 days after vaccination in Groups C (subcutaneous vaccination) and D (Not vaccinated), while they did so 6 days later in Groups A (IN + gum) and B (IN Only) with higher survival. This illustrates that the route of administration of a vaccination may influence the disease outcomes in the face of a PPR outbreak. Because PPR infection occurs naturally through the nasal passages, it is probable that the clinical presentation of the disease was different in goats from Groups A and B because they were able to mount an immune response earlier than the other experimental groups. Mucosa has a much higher dendritic cell density than subcutaneous tissues, as shown by a number of studies in the field of immunology [51]. For examples, see [52, 53]. When comparing vaccination by the mucosal route to immunization via the subcutaneous approach, this may explain why the former produces a more rapid immune response. This study’s findings on erythrocyte and leukocyte populations are consistent with goat PPR infection. All groups had erythrocytosis and leukopenia on day 7 post-viral. As was also observed in our experiment, physiological polycythemia in PPR frequently comes from fluid loss brought on by a high body temperature and severe diarrhea. PPR is linked to significant leukopenia in goats [54, 55, 56]. All of the unvaccinated animals died prior to the hematological assessment on day 7 pv. Due to this event, drawing conclusions from the data was challenging. Regardless of the delivery method or route, the vaccinated animals displayed mild leukopenia as opposed to the significant leukopenia typical with PPR in goats that was anticipated. Clinical lesions can enlarge and become more severe due to the lymphocytolytic effects of the PPR-associated leukopenia virus, which can last for weeks in lymphoid organs and peripheral blood circulation [54, 56]. The unvaccinated groups eventually developed mild leukopenia (7 dpv), but they recovered by 14 dpv. The findings of this study could imply that increasing antibody titres may have reduced viral load and its associated lymphocytolytic effects in the lymphoid organs because all vaccinated animals seroconverted by day 7 of vaccination, peaking on days 14, and 21 for subcutaneously vaccinated and intranasally vaccinated groups, respectively.

Clinical assessments and pathomorphological information strongly suggest that the different vaccination strategies utilized in this experiment were able to elicit a systemic immunoglobulin G (IgG) response in the face of an epidemic, but it is possible that this outcome may have been influenced by infection. Hence there is need for more research using DIVA vaccines. Also, in the study, the pulmonary pathologies were evaluated using the consolidated lung lesion scores (CLLS) system, and the unvaccinated group had the highest prevalence of lung consolidation, followed by the subcutaneously and intranasally vaccinated groups. This observed pattern is possibly linked to the choice of vaccination route. Similar patterns were seen in the pulmonary histomorphology, with fewer lesions in the intranasally immunized animals than in the subcutaneously immunized animals and the control group. Bronchus-associated lymphoid tissue (BALT) has developed in the pulmonary tissues of Group B. Following intranasal PPR vaccination, a local mucosal immune response in the lower respiratory tract has been demonstrated to be highly protective against PPR-induced caprine pneumonia [43, 54], which may have been akin to the observation in this study. PPR mortality is mostly caused by metabolic acidosis from severe diarrhea and pulmonary acidosis from pneumonic impairment [57, 58]. In this analysis, higher mortality rates were associated with both the clinical score and the consolidated lung lesion score. The intranasal vaccination shielded the goats from PPR lung involvement, which could account for the decreased mortality rate and lower frequency of clinical disease. This study implies that PPR vaccination in the face of an epidemic may have an impact on the course of the disease based on the percentage mortality, clinical scores, clinicopathological, and serological findings. Additionally, the tendency suggests that intranasal PPR vaccination, whether it includes or excludes phytogenic mucoadhesive gum, may have a more profound impact on health outcomes. To distinguish immune responses from likely immunological responses brought on by spontaneous infection, more research is required to better understand immune responses.

3. Conclusion

Implementing the use of plant gums as vaccine delivery agents in real-world scenarios requires careful consideration and adherence to specific recommendations to optimize vaccine efficacy and safety. Firstly, thorough preclinical studies are essential to understand the mucoadhesive properties, stability and immunomodulatory effects of the chosen plant gums. Researchers should assess the compatibility of different plant gums with the vaccine formulation and evaluate their impact on antigen stability. The selection of an appropriate plant gum should also consider its sourcing, ensuring sustainability and ethical practices.

In the formulation stage, researchers should focus on achieving a balance between mucoadhesion and controlled-release properties. The concentration of plant gums must be carefully optimized to enhance adhesion to mucosal surfaces while avoiding excessive viscosity that could hinder administration. Compatibility with vaccine antigens, stability throughout storage, and ease of administration should guide the formulation process.

In field applications, the method of administration is crucial. Whether through oral, nasal or other mucosal routes, the chosen plant gum should facilitate targeted delivery to the specific mucosal site associated with the desired immune response. Practical considerations, such as ease of administration and the suitability for large-scale vaccination campaigns, play a significant role in selecting the delivery route.

Additionally, continuous monitoring and refinement of plant gum-based vaccine formulations are necessary. Real-world conditions may introduce variables such as temperature fluctuations and variations in animal physiology. However, regular assessments of vaccine stability and immunogenicity under these conditions will ensure sustained efficacy.

Consideration of regulatory guidelines is paramount when introducing plant gum-based vaccines. Complying with regulatory standards ensures the safety and effectiveness of the vaccine. Collaboration with regulatory bodies can aid in obtaining necessary approvals and facilitating the integration of plant gum-based vaccines into routine vaccination programs.

Furthermore, communication and education are crucial components of successful implementation. Veterinarians, farmers, and other stakeholders should be informed about the benefits and proper use of plant gum-based vaccination methods. Clear guidelines on storage, handling and administration will help maximize the impact of these vaccines in real-world settings.

In conclusion, the successful implementation of plant gums as vaccine delivery agents necessitates a comprehensive and systematic approach. From preclinical studies to formulation optimization, administration methods, regulatory compliance and stakeholder education, each step plays a crucial role in ensuring the real-world effectiveness of plant-gum-based vaccines in promoting animal health and advancing veterinary immunization strategies.

Acknowledgments

We appreciate the efforts of the Research team which comprises Biologists, Veterinarians and Nurses who actively aid the thinking process. They include Ruth Fiadjoe, Belinda Dogbe, Derrick Adu Asare, Abigael Emikpe, Theophilus Jarikre and Obadiah Opoku Bamfoh who searched, typed and edited the chapter.