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Snakebites are a serious problem in public health due to their high morbimortality. Most of snake venoms produce intense local tissue damage, which could lead to temporary or permanent disability in victims. The available specific treatment is the antivenom serum therapy, whose effectiveness is reduced against these effects. Thus, the search for complementary alternatives for snakebite treatment is relevant. There are several reports of the popular use of medicinal plants against snakebites worldwide. In recent years, many studies have been published giving pharmacological evidence of benefits of several vegetal species against local effects induced by a broad range of snake venoms, including inhibitory potential against hyaluronidase, phospholipase, proteolytic, hemorrhagic, myotoxic, and edematogenic activities. In this context, this review aimed to provide an updated overview of medicinal plants used popularly as antiophidic agents and discuss the main species with pharmacological studies supporting the uses, with emphasis on plants inhibiting local effects of snake envenomation. The present review provides an updated scenario and insights into future research aiming at validation of medicinal plants as antiophidic agents and strengthens the potentiality of ethnopharmacology as a tool for design of potent inhibitors and/or development of herbal medicines against venom toxins, especially local tissue damage.
Snakebites are a serious public health problem in many regions around the world, particularly in Africa, Asia, Latin America, and parts of Oceania . Conservative data indicate that, worldwide, there are between 1.2 and 5.5 million snakebites every year, leading to 25,000 to 125,000 deaths . Despite its significant impact on human health, this condition remains largely neglected by national and international health authorities, funding agencies, pharmaceutical companies, patients' organizations, and health advocacy groups . Thus, snake envenomation is included since 2009 in World Health Organization (WHO) list of Neglected Tropical Diseases (NTDs) . Envenoming and deaths resulting from snakebites are a particularly important public health problem in the rural tropics. Populations in these regions experience high morbidity and mortality because of poor access to health services, which are often suboptimal, as well as other NTDs, which are associated with poverty [3, 4].
Snakes with major clinical importance belong to the families Elapidae (African and Asian cobras, Asian kraits, African mambas, American coral snakes, Australian and New Guinean venomous snakes, and sea snakes) and Viperidae (Old World vipers, American rattlesnakes and pit vipers, and Asian pit vipers) . After production, snake venom is injected in the victim via tubular or channeled fangs . Biochemically, venoms are complex mixtures of pharmacologically active proteins and polypeptides, acting in concert to help in immobilizing the prey . The most common toxins in snake venoms are snake venom metalloproteinases (SVMPs), phospholipases A2 (PLA2s), snake venom serine proteinases (SVSPs), acetylcholinesterase (AChE), L-amino acid oxidases (LAAOs), nucleotidases, and snake venom hyaluronidases (SVHs) .
Biological properties of snake venom components are peculiar to each species, but in general, the main clinical effects of snake envenomation are immediate and prominent local tissue damage (including myonecrosis, dermonecrosis, hemorrhage, and edema), coagulation disorders (consumption coagulopathy and spontaneous systemic bleeding), cardiovascular alterations (hypotension, hypovolemic shock, and myocardial damage), renal alterations (which could evolve into acute kidney injure), neurotoxic action (descending paralysis, progressing from ptosis and external ophthalmoplegia to bulbar, respiratory muscle, and total flaccid paralysis), generalized rhabdomyolysis with myoglobinuria, and intravascular haemolysis [5, 8].
The only available specific treatment is the antivenom serum therapy, which consists of a pool of neutralizing immunoglobulins, or immunoglobulin fragments, purified from the plasma of animals hyperimmunized against snake venoms or specific toxins. Its effectiveness consists in its ability to provide to the patient antibodies with a high affinity to snake venom, aiming to eliminate the toxins responsible for toxicity of the envenoming, mitigating the progress of toxic effects induced by snake venom components . However, the antivenom has some limitations, such as poor ability to treat local effects, risk of immunological reactions, high cost, and difficult access in some regions [8–10]. If antivenom administration is initiated rapidly after envenomation, neutralization of systemic effects is usually achieved successfully; however, neutralization of local tissue damage is more difficult . Furthermore, the availability and accessibility of antivenoms is limited in many regions, such as Sub-Saharan Africa, Asia, and, to a lesser extent, Latin America, which could aggravate even more this picture . Thus, this inability to treat local effects, as well as the increased time between accident and treatment, is the main reason for the temporary or permanent disability observed in many victims, which can lead to serious social, economic, and health negative impacts, given that most victims live in rural areas .
In this context, the search for complementary therapies to treat snakebites is relevant and medicinal plants could be highlighted as a rich source of natural inhibitors and pharmacologically active compounds [6, 11–13]. There are several reports of the popular use of medicinal plants against snakebites around the world, especially in tropical and subtropical regions such as Asia, Africa, and South America [14, 15]. The rural and tribal people living in remote areas greatly depend on folk medicines for the treatment of bites from any venomous creatures . The use of medicinal plants against snakebites is a historical practice throughout the human history, and this knowledge has been transferred among the rural communities from generation after generation . Nowadays, these herbal antidotes used in folk traditional medicine gained much attention by toxinologists worldwide as a tool for design of potent inhibitors against snake venom toxins. The potential advantages of antiophidic plants are their possible low cost, easy access, stability at room temperature, and ability to neutralize a broad spectrum of toxins, including the local tissue damage [12, 15–17].
So, the objective of this review is to provide an updated overview of medicinal plants used popularly as antiophidic and discuss the main species with pharmacological studies supporting the uses, with emphasis on plants inhibiting local effects of snake envenomation, since this is a critical effect of snake venoms that could lead to relevant sequel to victims. A review of the main botanical families popularly used as antiophidic is presented, including the main species and forms of popular use of them. Then, studies supporting their popular use are discussed, as well as the advantages of this kind of approach for treatment of snake venom accident.
An extensive review of the literature was undertaken in different scientific sources, such as PubMed (https://www.ncbi.nlm.nih.gov/pubmed), Science Direct (http://www.sciencedirect.com/), Scopus (https://www.scopus.com/), Web of Science (http://www.webofknowledge.com/), “Literatura Latino-Americana e do Caribe em Ciências da Saúde” (LILACS) (http://lilacs.bvsalud.org/), Scientific Electronic Library Online (SciELO) (http://www.scielo.org/), Google Scholar (https://scholar.google.com.br/), Cochrane Library (http://www.cochranelibrary.com/), and Centre for Reviews and Dissemination (CRD) (http://www.crd.york.ac.uk/CRDWeb).
The study database included original articles published in peer-reviewed journals, as well as books, thesis, dissertations, patents, and other reports covering antiophidic plants (ethnopharmacological surveys, original articles, or reviews), dated until December 2016. For the online search, where applicable, the following search strategy was employed: (“plant” OR “plants” OR “plant extract” OR “vegetal” OR “vegetal species” OR “vegetal extract” OR “traditional medicine” OR “alternative medicine” OR “complementary therapy” OR “natural medicine” OR “ethnopharmacology” OR “ethnobotany” OR “herbal medicine” OR “herb” OR “herbs” OR “decoction” OR “tea” OR “infusion” OR “macerate”) AND (“snake venom” OR “snake” OR “snakes” OR “snakebite” OR “snakebites” OR “antivenom” OR “antivenoms” OR “anti-venom” OR “anti-venoms” OR “antivenin” OR “antivenins” OR “anti-venin” OR “anti-venins” OR “antiophidian” OR “antiophidic” OR “snake envenomation” OR “antitoxin” OR “antitoxins” OR “snake antidote” OR “snake antidotes” OR “snake venom neutralization” OR “snake venom inhibition” OR “snake toxins inhibition” OR “snake toxins neutralization” OR “viper” OR “viperidae” OR “crotalinae” OR “viperinae” OR “elapidae” OR “pit-viper” OR “bothrops” OR “jararaca” OR “crotalus” OR “micrurus” OR “lachesis” OR “cobra” OR “naja” OR “bitis” OR “vipera” OR “daboia” OR “trimeresus”).
All abstracts and/or full-text data were considered, without language restriction. Then, the publications covering ethnobotanical and/or pharmacological studies of antiophidic plants were selected and carefully analyzed. With the information gathered in these studies, the actual scenario of the use of plants against snake venom was pointed out. Main botanical families used, main countries where antiophidic plants are reported, and mode of use mostly employed in folk medicine were described. Regarding studies of pharmacological evidence, the snake species that were most studied, which plant species were tested and presented positive results, correlating with those species that also presented record of ethnopharmacological use, were also reported.
The accepted botanical name of each medicinal plant listed was confirmed in at least 2 botanical databases among the following ones: Flora do Brasil (http://www.floradobrasil.jbrj.gov.br/), Tropicos (http://www.tropicos.org/), The Plant List (http://www.theplantlist.org/), and NCBI Taxonomy Browser (https://www.ncbi.nlm.nih.gov/taxonomy). In some cases, where the same species was considered as different ones (different synonyms used) in different papers, the accepted name according to the botanical databases mentioned above was used in the present review, bringing the synonym used in the original work between parenthesis.
According to the literature search performed, a lot of ethnopharmacological studies showing medicinal plants claimed as antiophidic were found. A summary of these vegetal species can be observed in Table 1.
Along our survey were found 150 botanical families containing plants with reputation against snakebites, among which the most cited ones were the families Fabaceae, Asteraceae, Apocynaceae, Lamiaceae, Rubiaceae, Euphorbiaceae, Araceae, Malvaceae, and Acanthaceae (Figure 1(a)). In a cross-cultural comparison of medicinal floras used against snakebites, Molander et al.  identified five countries with a high number of antiophidic plants and representing different cultures, geography, and floristic zones: Brazil, Nicaragua, Nepal, China, and South Africa. From these countries, some “hot” families were identified, which were Apocynaceae, Lamiaceae, Rubiaceae, and Zingiberaceae , similar to the present review, except for the Zingiberaceae family which was not so reported in our survey.
Medicinal plants with reputation against snakebites are found all over the world, especially in tropical or subtropical regions of Asia, Americas, and Africa (Figure 2). This fact may be associated with richness of flora of these regions, as well as with relative need of complementary therapies to treat snakebites, considering geographical features that could limit the distribution and availability of the antivenoms in these areas.
As observed in Figure 3(a), leaves and roots are the parts of plants most used in folk medicine. Regarding the mode of use, the most frequent one is the topical application of the vegetal products directly on the place of the bite (Figure 3(b)). This is interesting especially in snake venoms that cause serious local tissue damage, such as Bothrops and Daboia species. Since these snakes produce intense local tissue damage, which has a very rapid onset, a topical treatment could be interesting for a rapid inhibitory action. On the other hand, interestingly, the use of some plant species is made by internal and external routes simultaneously, while for some other species the route of administration could be chosen among internal or external use. However, since in several cases this information is not clear, this differentiation was not considered in data tables. Regarding the mode of preparation, in general, paste and decoction were the most cited forms of use. However, for most of the plants enlisted, the information of mode of preparation was missing or confusing.
It is important to emphasize that these plant species, in addition to their use as antiophidic agents, present a series of another popular uses (data not shown) in popular medicine, mainly anti-inflammatory activity. For example, Jatropha gossypiifolia (Euphorbiaceae) has antiophidic, anti-inflammatory, analgesic, antipyretic, healing, and antihemorrhagic uses, among others .
Until date, according to our database, only a few numbers (less than 20%) of the species with reputation against snakebites were tested in preclinical assays with different snake venoms, which shows that there is still a great road for the study of antiophidic plants. From these tested plants which have popular use documented in our database, more than a half (almost 60%) showed positive results, which shows that in fact ethnobotany could be a good tool for bioprospecting of plants with antiophidic activity. In addition, the fact that among the tested vegetal species very significative results were obtained strongly suggests the potentiality of these natural products as a future source for development of snake venom inhibitors.
The plant families with most vegetal species showing positive results in antiophidic tests were Fabaceae, Euphorbiaceae, Apocynaceae, Lamiaceae, Asteraceae, Malvaceae, Melastomaceae, and Sapindaceae (Figure 1(b)). Crossing the data of popular use (Figure 1(a)) and of positive activity (Figure 1(b)), we can highlight these families as “hot” ones, that is, families that might be preferred or prioritized in studies searching for antiophidic plants.
Snakes from the genus Naja, Bothrops, and Bitis were the most evaluated ones in these antiophidic assays. However, although Naja and Bitis comprise a large fraction of the studies, virtually most of them are only in vitro studies, dealing with the in vitro enzymatic inhibition of classes of venom toxins relevant to local tissue damage, such as phospholipases A2 (PLA2s), hyaluronidases (SVHs), and proteases. More particularly, the great majority of these studies with Naja and Bitis snakes are part of the work undertaken by Molander et al. , aiming to investigate whether plants used in traditional medicine systems would be active against necrosis-inducing enzymes of snake venoms, having tested a total of 226 extracts from 94 plants from the countries of Mali, Democratic Republic of Congo, and South Africa against PLA2, SVHs, and proteases from Bitis arietans and Naja nigricollis (see Tables Tables22 and and4).4). Studies evaluating the inhibitory action of medicinal plants against these enzymes are very relevant, since they are involved in several pathological mechanisms produced by snake venoms; however, in vivo preclinical assays or, even better, clinical assays are essential for giving even stronger evidences of the effectivity of the use of medicinal plants against snakebites. In this scenario, the study of anti-Bothrops plants is more advanced, since quantitatively a higher number of in vivo scientific evidences are found in literature. Going the same way, studies with plants inhibiting local tissue damage of Daboia/Vipera, Lachesis, and Crotalus snakes could be also highlighted. However, studies of antiophidic medicinal plants in humans are very scarce: only one clinical study was found in literature, evaluating the inhibitory properties of a polyherbal formulation against local effects from Chinese cobra bite (see Section 4.9).
Hereafter, we describe the main plants with inhibitory potential against local tissue damage induced by snake venoms. It is important to emphasize that the focus of this review is plants against local tissue damage, mainly due to severity of these effects (which could cause permanent disabilities in victims) and the poor effectiveness of available antivenoms against them. So, studies with plants against systemic effects induced by snake were not considered; in addition some plants herein described possess inhibitory action upon systemic effects, although not stated here. For example, the vegetal species Jatropha gossypiifolia (Euphorbiaceae), a medicinal plant studied very much by our research group, had showed significative inhibitory action upon hemostatic disorders induced by B. jararaca snake venom . So, the antiophidic potential of this species (as well as some others) lies beyond the capacity of inhibit local tissue damage provoked by B. jararaca venom, although not described in this review.
In addition, it is important to analyze critically some works dealing with antiophidic activity of plant extracts, since some of them have limitations that could reduce, at least partially, the potentiality of these species. The major limitation is that various studies, especially the early ones, make the evaluation of the plants using a preincubation approach, which consists in the previous inactivation of venom by preincubating it with different proportions of the tested extracts. Although scientifically valid and even recommended by WHO for assessing antiophidic antivenoms , this preincubation approach makes a scenario unlikely to be possible in the field, where the medicine would be delivered after the snakebite. In fact, a recent study evaluated the inhibitory action of the medicinal plant Bellucia dichotoma (Melastomataceae) against Bothrops atrox snake venom using different protocols: preincubation, pretreatment, and posttreatment . The authors observed that while the extract was greatly active when preincubated, this inhibitory activity was drastically reduced or even lost when the extract was injected independently of venom, simulating traditional use. The authors observed that the extract has great amounts of tannins, which are compounds known to precipitate proteins. So, it was concluded that the “pseudo-inhibition” observed after preincubation may be due to the presence of these compounds, suggesting that the preincubation protocol overestimates inhibitory potential of medicinal plants, and for this reason, this kind of approach must be analyzed with caution for estimation of inhibitory potential of medicinal plants [13, 98]. In this sense, many recent studies have been done using protocols of pre- and/or posttreatment, to ensure the potentiality of antiophidic plants, and for most of them, positive results have been found [96, 98–102]. For this reason, studies using preincubation protocol are marked in the tables, for a critical analysis.
Also, it is interesting to note that several of the plants with inhibitory potential against snake venom local toxicities also present other relevant pharmacological activities. This is interesting since it is often discussed in the literature that several antiophidic plants did not neutralize snake venoms per se, but could have antiophidic use once they could relieve some of the symptoms of snake envenoming, especially the local effects. It is related that the presence of tranquilizing, antioxidant, immunostimulating, and/or anti-inflammatory activities in certain plants could be of great interest in the alleviation of snake envenoming symptoms [103, 104]. For example, some studies have shown that anti-inflammatory drugs could inhibit the edematogenic and other snake venom effects related to inflammation, such as necrosis and myotoxicity, induced by Bothrops venoms [105, 106]. In fact, many medicinal plants with antiophidic activity also possess significant anti-inflammatory activity in vivo [83, 96, 107–110]. Following the same reasoning, some plants with antioxidant activity also possess significant antiophidic effects [95, 96, 104, 111]. In fact, some authors suggest that molecules with antioxidant and/or anti-inflammatory effects could be interesting along with antivenom therapy, helping to reduce the occurrence of secondary/long term complication due to snakebites .
Bacterial infection secondary to snakebites is a common complication in envenomed victims [113, 114]. The main source of bacteria is the oral cavity of snakes, but the microbiota in the different layers of the victim's skin or even microorganisms from victim's clothes could also contribute [115, 116]. Abscess formation is a common complication found in patients bitten by Viperidae snakes, being a risk factor for amputation in these patients, and it may be associated with sepsis [113, 114, 117]. A large number of bacteria, including anaerobic species, aerobic gram-negative rods, and a small proportion of gram-positive cocci could be inoculated with snakebites and have been isolated from the abscesses of bitten patients [113, 114]. Microorganisms such as Staphylococcus, Pseudomonas, Salmonella, Escherichia, Providencia, Proteus, Enterococcus, and Bacillus were already identified in oral cavity of certain snakes . The use of antibiotics following snakebites is often recommended, usually therapeutically than prophylactically, mainly to avoid complications due to infections [114, 118]. In this context, medicinal plants presenting antimicrobial activities, especially against those microorganisms usually detected in snakebite victims' abscesses, could be interesting .
Medicinal plants having antimicrobial activities in association with some of the pharmacological properties discussed above (such as anti-inflammatory and antioxidant, e.g.) could be of great value to relieve especially local effects induced by snake venom. In another point of view, it is possible that several related plants in folk medicine as antiophidic agents do not act directly upon venom toxins but indirectly on its symptoms. Anyway, some studies have shown the potentiality of some vegetal species acting in two ways: directly, neutralizing venom toxins, or indirectly, by having some of the pharmacological activities mentioned above. For example, Jatropha gossypiifolia (Euphorbiaceae), a plant species studied very much in our research group, showed significant antiophidic properties, inhibiting biological and enzymatic activities from Bothrops venoms [96, 119], and presented anti-inflammatory, antioxidant, anticoagulant, and antimicrobial properties in preclinical assays . So, plants which possess these biological activities determined in previous studies might be preferred or prioritized in studies searching for antiophidic plants.
The mechanism by which medicinal plants neutralize the toxic venom constituents is still unknown, but many hypotheses have been proposed, such as protein precipitation, enzyme inactivation, proteolytic degradation, metal chelation, antioxidant action, and a combination of these mechanisms . In this context, some improvements in this understanding have been achieved in the last years, through the use of in silico methods (e.g., docking simulations) to analyze the interaction of compounds isolated from plants and certain classes of snake venom toxins such as PLA2 and SVMP [120–122].
The use of medicinal plants may present several advantages, such as low cost, being easily available, being stable at room temperature, and possibility of neutralization of a wide range of venom components . In addition, since medicinal plants are an extremely complex mixture, it is possible that there may be a synergistic action of different compounds in plant, acting in distinct targets, inhibiting a broad spectrum of venom toxins [12, 15]. According to literature, interestingly, there are some plants in which the crude extract is more active than the isolated constituents , which supports the hypothesis of the synergistic action of plant components.
A summary of active plants against Naja snakes local effects is presented in Table 2. Naja species are commonly called cobras. They typically occur in regions throughout Africa and Southern Asia. The outcomes of venom toxicity include nephro-, neuro-, and cardiotoxicity, respiratory and circulatory collapse, necrosis, hemorrhage, and edema . A great number of the plants showed in this review were tested against Naja species. However, it is important to mention that only a very small number of these plants were assessed in vivo, and so the scientific evidences of antiophidic activities of these species are based on enzymatic in vitro assays, especially against SVHs, a class of toxin particularly relevant in cobras. The study of Molander et al.  presented several medicinal plants identified as potent inhibitors of N. nigricollis SVHs, PLA2, and proteases, which could indicate a potential rich source of inhibitors of necrosis induced by these venom, which must be evaluated in vivo later . The same group, in a more recent study , investigated the skin permeation, ex vivo inhibition of venom induced tissue destruction, and wound healing potential of African plants used against snakebite, which included the most potent inhibitors identified in the previous work . A total of 30 plant species were tested against Naja nigricollis and Bitis arietans employing in vitro and ex vivo models . However, although plant extracts have showed potential in inhibiting snake venom enzymes, this study showed no effect against cell death and tissue damage.
A summary of active plants against Bothrops snakes local effects is presented in Table 3. More than 90% of the snakebites reported every year in Latin America are caused by Bothrops species . Envenomation by Bothrops snakes is characterized by a prominent and complex series of local pathological alterations, which appear rapidly after the bite in the anatomical site where venom is inoculated . In a number of Bothrops bite cases, lack of neutralization of local effects results in permanent sequelae, with significative tissue loss . So, the use of a therapeutic approach with high inhibitory potential and easy access and disponibility to victims, which could neutralize rapidly the onset of these local manifestations, is interesting. Most of the inhibitory studies with Bothrops snakes were performed in Brazil, which could be associated with richness of Brazilian flora as well as the epidemiological aspects of this country. The work performed by De Moura et al.  could be highlighted, where these authors performed an ethnopharmacological-guided screening of plants with reputation against snakebite in Santarém, Western Pará, Brazil. Twelve species were evaluated against Bothrops jararaca snake venom induced hemorrhage and some of them presented very significative results, showing, thus, the relevance of traditional knowledge in the survey of antiophidic plants .
A summary of active plants against Bitis snakes local effects is presented in Table 4. Snakes belonging to the genus Bitis are implicated in many accidents with humans in Africa. The envenomation by Bitis often results in severe local damage, hypotension, coagulopathy, thrombocytopenia, and spontaneous local bleeding and, in the absence of antivenom therapy, the accident can be fatal. Bitis arietans is one of the three species of snakes of medical importance in Africa and its venom is considered the most toxic venom of the viper group . Regarding the plants with inhibitory action upon Bitis snakes, only one in vivo study of antiophidic activity was found until date. Although many works have been showing the potential of medicinal plants against several snake venoms, only three works were identified evaluating the action of plants against Bitis, from which two are the same screening studies of plants against Naja snake venom discussed before (Section 4.2) [82, 123].
A summary of active plants against Daboia/Vipera snakes local effects is presented in Table 5. The Daboia genus is represented by a single species, named Daboia russelii, also popularly known as Russell's viper. This species is widespread in many parts of Asia and is responsible for large morbimortality due to snakebites in this continent [183, 184]. Russell's viper was formerly classified in Vipera genus and is therefore better known as Vipera russelii, since the new accepted nomenclature (Daboia russelii) is not yet universally followed . For this reason, to avoid confounding, we use the term Daboia/Vipera in some occasions.
In humans, Russell's viper bite causes severe local tissue damage; more frequently the necrosis results in an irreversible loss of tissue and requires amputation of the affected limb [182, 183, 185]. As observed with Bothrops snakes, several studies have showed the inhibitory potential of medicinal plants against local effects of Russell's viper venom, including several preclinical in vivo studies.
A summary of active plants against Lachesis snakes local effects is presented in Table 6. Lachesis muta is the longest venomous snake in the Americas and is distributed in the equatorial forests east of the Andes, ranging from eastern Ecuador, Colombia, Peru, northern Bolivia, and eastern and northern Venezuela, to Guyana, French Guyana, Surinam, and northern Brazil [100, 186]. L. muta snakebites are mainly characterized by systemic (generalized bleeding, coagulopathy, renal failure, and shock) and local effects (pain, hemorrhage, edema, and necrosis). In South America, Bothrops species has a higher incidence of accidents than L. muta, but, on the other hand, Lachesis bites led to more severe symptoms and have lethality indexes significantly higher than Bothrops [100, 186, 187]. Thus, the study of medicinal plants against these snakes, too, is of very much relevance. However, only a few studies were detected with plants against Lachesis snakes.
A summary of active plants against Crotalus snakes local effects is presented in Table 7. Snakes from Crotalus durissus complex, popularly known as rattlesnakes, are dispersed northward into North America and southward into South America. Species of the Crotalus durissus complex pose a serious medical problem in many parts of the America . Crotalic venom is considered highly toxic and more lethal in comparison with that of the genus Bothrops, having three main actions: neurotoxic, myotoxic, and coagulant [200, 201]. The crotalic accident is characterized by local and systemic manifestations, but while the local alterations are only discrete, the systemic manifestations are severe, leading to high chances of death . Probably due to this low local effect in envenomed victims, the inhibition of these effects by plants is, until now, little investigated, especially when compared to other species with characteristic severe local effects.
Besides the snakes discussed above, some other studies are found with plants inhibiting other snake species, such as those from Echis and Bungarus genus. For other snakes species such as Calloselasma rhodostoma, Philodryas olfersii, and Montivipera xanthina, only isolated studies with a single plant, in each one, were found. These plants are summarized in Table 8. Many reasons may be stated for this lack of studies, such as low level of local effects, incidence restricted to a small region of the world, and usual low efficacy of plant extracts due to possible extremely high toxicity. However, it is important to highlight that the lack of studies does not mean a lower medical relevance of these species. For example, the saw-scaled viper (Echis carinatus) and the common Indian krait (Bungarus caeruleus), along with spectacled cobra (Naja naja) and Russell's viper (Daboia russelii), are included among the referred “Big Four” venomous snakes of India, being responsible for the majority of morbid complications, characterized by persistent and progressive tissue necrosis even after treatment with antivenom [195, 202]. Therefore, future studies with plants aiming at the inhibition of the local effects induced by these snakes are encouraged.
Along our antiophidic plants database, only one clinical study was found in literature, evaluating the inhibitory properties of a polyherbal formulation, externally applied, against soft-tissue necrosis after Naja atra (Chinese cobra) bite . This polyherbal formulation, known in China as Jidesheng antivenom, is composed of the following ingredients: Ganchan (Succys Bufo), Dijincao (Herba Euphorbiae Humifusae), Chonglou (Rhizoma Paridis Chonglou), and Wugong (Scolopendra). This was a retrospective study performed with 126 patients with skin and soft-tissue necrosis due cobra bite, with the control group being treated externally with 40% glyceride magnesium sulfate (n = 52) and the treatment group performed by application of Jidesheng antivenom externally (n = 74). The authors observed statistically significant differences in maximum local necrotic area of skin and soft tissues, healing time, and skin-grafting rate between the control and treatment groups (P < 0.05), thus indicating that external application of Jidesheng antivenom may help to promote wound healing and reduce the skin-grafting rate in cases of skin and soft-tissue necrosis due to Chinese cobra bite . Considering the composition of the Jidesheng antivenom, the authors discuss that each ingredient in this product may exert antipyretic, antidotal, antiphlogistic, and analgesic effects, according to previous results with each ingredient isolated, which could contribute to the inhibitory effect observed by the formulation . The result obtained in this clinical study is very promising, since it shows that a plant-derived product showed significant results in humans, thus pointing to the potentiality of this kind of product in treatment of snake venom induced local effects. However, only one study is insufficient to ensure the potentiality of medicinal plants against snakebites, with performing more clinical studies, preferentially controlled and randomized ones, to bring more evidences of the viability of the approach for future safe and effective use in humans being necessary. So, more clinical studies, especially ones with those plants highlighted in this review and those presenting good preclinical in vivo evidences of antiophidic efficacy, are highly encouraged.
The popular use of vegetal species does not necessarily imply efficacy, but it gives a selected list of medicinal plants that can be primarily studied in pharmacologic assays for possible antiophidic effects, directing future studies in this area. In fact, a great number of these species that have been evaluated against local tissue damage induced by several snake species showed inhibitory potential against hyaluronidase, phospholipase, proteolytic, hemorrhagic, myotoxic, and edematogenic activities, among others. Therefore, considering the limitations of conventional antivenom serotherapy, especially its poor efficacy against local effects, the treatment with medicinal plants may provide a potential adjuvant alternative to treat snakebites, being used to complement the activity and effectiveness of available snake venom therapy. The main potential advantages of antiophidic plants are their low cost, easy access, stability at room temperature, and ability to neutralize a broad spectrum of toxins, including the local tissue damage.
Interestingly, some studies have showed that the crude extracts are more powerful than the individual herbal compounds, which could, at a certain extent, justify the development of herbal products containing these plants instead of medicines containing isolated compounds, which in turn could be more rapidly available in market, after proof of safety, effectiveness, and quality of these products. However, despite the existence of many plants with great potential, no natural antiophidic product is available in market, which points to question of the need for further studies. Only a few numbers of patents regarding herbal products against snakebites were found in literature. Some patents regarding the use of Chinese medicinal plants against snake and bug bites were found. In our research group, two patents were deposited concerning the processes of obtaining extracts, fraction, isolated compounds, and pharmaceutical compositions of some plants studied by our group applied in the treatment of accidents with venomous animals (BR 10 2013 034046 4 A2 and BR 10 2012 026958 9 A2). Thus, the number of patents with antiophidic herbal products is still relatively small. For this reason, we encourage pharmacologists and toxinologists around the world to intensify studies with antiophidic plants, especially prioritizing those with the greatest number of indications in traditional medicine and emphasizing clinical studies with the most active plants in preclinical studies, given that the low number of human studies is one of the major obstacles for the future application of herbal products with antiophidic potential. No less important, toxicological studies are also extremely necessary to ensure the safety of these products.
In conclusion, the data presented in this review provides an updated scenario for and insights into future research aiming at validation of medicinal plants as antiophidic agents and, based on scientific evidences, strengthens the potentiality of medicinal plants and ethnopharmacological knowledge as a tool for design of potent inhibitors and/or herbal medicines against venom toxins.
Matheus de Freitas Fernandes-Pedrosa is CNPq fellowship-honored researcher. Juliana Félix-Silva acknowledges CAPES for the Ph.D. scholarship.
The authors declare that there are no conflicts of interest regarding the publication of this paper.