Discussion
Traditional use of herbal remedies to treat animals by livestock owners is still prevalent in Yemen due to poverty, the availability of medicinal and aromatic plants, and better accessibility and low cost of herbal drugs, especially when compared to both inaccessible and expensive conventional drugs. However, few studies have reported on the ethnoveterinary use of some plants in Yemen [2], [3], [4]. Our work aimed to go a step further by searching for scientific confirmation of the ethnoveterinary uses of the 14 Yemeni plants documented in these studies by reviewing their bioactivities and toxicological properties. The results of researching scientific literature have revealed various in vivo and in vitro experimental studies demonstrating different pharmacological activities that may support some of the claimed healing activities of 11 herbal remedies ([Table 1]).
The use of Aloe latex as a laxative by Yemeni livestock owners and elsewhere [5] is strongly supported by scientific evidence on its laxative properties, based on the well-established cathartic properties of anthraquinone glycosides such as aloin and aloe-emodin found in aloe latex [88]. The laxative activity of the Aloe vera latex is due to a common metabolite, aloe-emodin-9-anthrone, as well as aloe-emodin and other metabolites in the colon, where they produce their cathartic effects by multiple mechanisms leading to an increase in intestinal peristalsis, water, electrolyte content, and mucous secretion [6]. The laxative activity of fenugreek seeds and barley grains, on the other hand, is based on the well-known fecal bulking effect of dietary fibers, especially insoluble fibers contained in fenugreek seeds [78] and barley grains [79]. The use of Aloe latex to treat worms could be justified by the in vitro anthelmintic activity of its constituents (aloin and aloe-emodin, [Table 1]).
Worldwide, disease caused by gastrointestinal strongyles (GIS) is considered one of the most critical health constraints affecting productivity in small ruminants. GIS may limit sheep production by causing retarded growth, weight loss, reduced food consumption, lower milk production, impaired fertility, and even mortality in heavy parasite burden [8]. Aloin and aloe-emodin effectively targeted the larval developmental stages of gastrointestinal strongyle larvae. When used at the same concentration, aloe-emodin especially showed the same efficacy as the reference drug thiabendazole (0.1%, [Table 1]).
A recent publication [112] confirmed via its in vitro and in vivo results the anthelmintic action of A. vera extract in sheep. The antimicrobial, anti-inflammatory, and antioxidant activities play an essential role in wound healing by reducing the microbial load, decreasing prolonged and chronic inflammation, and protecting vital tissue of the wound bed from free radicalsʼ destructive effect, respectively [89]. Consequently, these activities produced by Aloe species latex and its anthraquinone constituents (aloe-emodin and aloin) and by B. sacra underbark and its content of oleo-gum resin [90] (with its constituents, α- pinene [42], [91] and boswellic acids [90], [92]) alongside the wound healing activity of aloe-emodin and
analgesic effect of B. sacra oleo-gum resin and boswellic acids, could substantiate their use for wound healing activity ([Table 1]).
The mechanisms of action of these pharmacological activities were reported for some constituents. The antimicrobial activity of anthraquinones was ascribed to several mechanisms such as the inhibition of the microorganisms enzymes (such as penicillinase) by rhein, emodin, and aloe-emodin [93]; the inhibition of nucleic acids synthesis in B. subtilis by anthraquinones extracted from different Aloe species [94]; the inhibitory effect of aloe-emodin on the initial adhesion and proliferation stages of Staphylococcus aureus biofilm development [14]; and the partial disruption of virus envelopes by aloe-emodin [16]. Moreover, aloe-emodin showed a concentration-dependent reduction of virus-induced cytopathic effect and inhibition of the replication of influenza A in MDCK cells. A 50% inhibitory concentration value of aloe-emodin on virus yield
was less than 0.05 µg/ml [95]. The antibacterial mechanism of action of acetyl-11-keto-β-boswellic acid (AKBA) was attributed to its ability to disrupt the permeability barrier of microbial membrane structures [44]. α- Pinene was also found to induce toxic effects on the membrane structure (membrane expansion, increased membrane fluidity) and functions (inhibition of a membrane-embedded enzyme) of yeast and bacteria [56].
The anti-inflammatory mechanisms of action of Aloe latex, B. sacra, and A. bracteolate were reported for some of their constituents ([Table 1]). AKBA, as the most important inhibitor of 5-lipoxygenase, binds to 5-lipoxygenase in a calcium-dependent and reversible manner and acts as a nonredox type, noncompetitive inhibitor [51]. In addition, studies have revealed that boswellic acids can also inhibit microsomal prostaglandin E2 synthase (mPGES)-1 in cell-free, cellular, and in vivo studies [96]. AKBA was also found to exert its anti-inflammatory effect by inhibiting NF-κB. The inhibitory effects of boswellic acids on NO production and the expression of pro-inflammatory cytokines and mediators via inhibition of phosphorylation of MAPKs, JNK, and p38 have been observed [97]. In addition to the anti-inflammatory effects
of boswellic acids (beta-boswellic acid, 3-acteyl-beta-boswellic acid, 11-keto-beta-boswellic acid, and 3-acetyl-11-keto-beta-boswellic acid) through systemic application by inhibiting 5-lipoxygenase, boswellic acids were effective against inflammatory disorders through topical application by using different models of acute and chronic inflammation (i.e., arachidonic acid and croton oil-induced mouse ear edema, carrageenan-induced rats paw edema, and adjuvant-induced developing arthritis in rats). The effect observed through this route is in accordance with the study conducted with the systemic route, thus establishing that boswellic acids, when used through topical application, are as effective as through the systemic route [98].
The analgesic effect of AKBA could be linked to its highly specific inhibition of 5-lipoxygenase, the key enzyme for the biosynthesis of leukotrienes. It has been shown that leukotrienes play a role in acute pain. Leukotriene LTB4 and 5-hydroxyeicosatetraenoic acid are potent chemotactic factors for the polymorphs, which lower the firing threshold of pain fibers and therefore stimulate the nociceptors directly. In addition, the metabolic products of the lipoxygenase are reported to activate capsaicin receptors, which were suggested to play a role in thermal and chemical pain. Moreover, 5-lipoxygenase products act as a second messenger, downstream to protein kinase A and protein kinase C, and modulate their action in primary afferent nociceptors to mediate their sensitization to the mechanical and chemical stimuli [53].
The antioxidant activity of several extracts and chemical constituents of the studied plants ([Table 1]) was assessed by several mechanistic methods based on either a single electron transfer reaction or a hydrogen atom transfer reaction from an antioxidant or oxidant to a free radical. The change of optical absorbance of either antioxidant or oxidant is measured for the quantitation for antioxidant capability [99].
Mastitis is a disease with a severe economic impact, causing production losses in the dairy industry. S. aureus is one of the leading causes of mastitis, resulting in an annual economic loss of $2 billion globally. S. aureus is also responsible for the emergence of resistant organisms and antibiotic residues in dairy products [55]. Acetyl-11-α-keto-β-boswellic acid-mediated silver nanoparticles (BANS, 0.12 mg · kg−1, intramammary and intraperitoneal) were reported to exert superior antibacterial (36%), anti-inflammatory (95%), and antioxidant effects compared with the antibiotic cefepime (1 mg · kg−1, intraperitoneal) in S. aureus-induced murine mastitis. BANS treatment significantly (p < 0.05) reduced bacterial load, C-reactive protein, superoxide dismutase, catalase activities, and neutrophil infiltration in affected mammary glands [55]. The antibacterial,
anti-inflammatory, and antioxidant effects of acetyl-11-α-keto-β-boswellic acid could lend some support for the local use of oleo-gum resin of B. sacra, which may be able to target pathogens such as S. aureus present at the teat opening and thus help to prevent bacteria that cause mastitis from entering the teat canal and colonizing the mammary tissue, as well as reduce the inflammation of the udder surface.
S. aureus is among the most common causes of ocular infections, including blepharitis, dacryocystitis, conjunctivitis, keratitis, and endophthalmitis. Humans are not the only reservoir for this organism because the organism can be isolated from companion animals, livestock, and wild animals [100]. In the infection process, myeloperoxidase, NADPH oxidase, and NO synthase generate reactive species [101]. Therefore, the antibacterial activity of both Rumex nervosus and salt against S. aureus, together with the antioxidant activity of R. nervosus to reduce the oxidative stress triggered by this bacterial infection, could justify their combined use to treat pimples in goatsʼ eyes.
Bloating is a disorder of ruminants caused by gas retention in the stomach. It results most commonly from excessive foaming of the contents of the rumen, which is the primary cause of bloating in animals feeding on legumes or lush young grass, and of feedlot bloating. Nonfoamy or free gas bloating is less common than foamy bloating and results from a variety of causes. It is predominantly a disorder of cattle but may also occur in sheep and other domestic ruminants. The total economic cost of bloating comes in many forms: losses by death and culling of bloating prone animals; losses of production from animals that were bloated and survived and the disruption of regular farm work and management programs; losses due to the use of less productive but safer pastures; and the cost of preventive measures and treatment [102]. Bloating can develop by the action of ruminal microflora, such as bacteria, fungi, and protozoa, that break down cellulose and
protein by microbial fermentation. As such, antibiotics can control bloating based on the principle of reduced microbial activity. Penicillin was the first antibiotic used to control legume bloating, but its use was soon discontinued because of the drugʼs rapid development of microbial resistance. Recently, the ionophore antibiotics monensin (rumensin) and lasalocid have been used for bloating protection [103]. Consequently, the antimicrobial activity, together with the anti-inflammatory, antioxidant and antiulcer activities of A. bracteolate (with its constituent aristolochic acid) and the antiulcer effect of S. bicolor grains ([Table 1]), could support their combined use to reduce livestock bloating by decreasing microbial fermentation, and inflammation (e.g., inflammation of the peritoneum, a common cause of vagal nerve damage, in the case of free gas bloating [104]) as
well as the stress facing the animals during bloating.
In cattle, the source of abdominal pain can originate from visceral stretch receptors within the mesentery, organ capsules, or ligaments, muscular spasms or inflammation, and ischemia. Pain can also arise from parietal sources, including the parietal peritoneum, abdominal muscles or rib cage, or bacterial-associated mechanical or functional obstruction, inflammation, or other damage to the gastrointestinal tract wall. Additionally, extra-abdominal conditions can mimic intra-abdominal pain [105]. Hence, the pharmacological properties (antibacterial, anti-inflammatory, antioxidant, analgesic, and antiulcer activities, [Table 1]) of C. rotundifolia and its constituents, astragalin and β-amyrin, could explain its effectiveness for combating livestock abdominal pain and associated inflammation.
The use of P. punctulate, mixed with eggs as a cast, for bone fracture ([Table 1]) could be supported by the analgesic effect of a diterpene, isolated from its leaf exudate and stem extracts, that may lessen the pain associated with a bone fracture. The suitability of this method will depend on the type of fracture. In simple closed fractures, casting may be enough to stabilize the bones and allow healing, and the analgesic action of P. punctulate may support the animalʼs wellbeing. However, this will not be enough to heal the bones in open and complicated fractures, and other measures are needed.
The use of P. undulata as insect repellent is substantiated by the insecticidal activity of the essential oil of P. undulata aerial part [66], methanolic extract [67], and the compound carvotanacetone [68], which is also found as a major constituent (91.4%) of the Yemeni P. undulata leaves essential oil [106]. On the other hand, no data were available on the insecticidal activity of Soqotraen Boswellia species and the latices of Aloe species and E. arbuscula. However, related Boswellia species such as Boswellia carterii essential oil, containing α-thujene (69.16%), α-pinene (7.20%), and α-phellandrene (6.78%) as the major components were found to exhibit insecticidal activity against both the pulse beetle Callosobruchus chinensis and C. maculatus
[107]. Moreover, α-pinene was reported to act as an insecticide, especially on mosquitoes such as Culex pipiens causing paludisme and Nile fever vector or dengueʼs vector, Aedes aegypti
[56]. Consequently, we suggest that the insecticidal activity of smoke from burning dead wood of Soqotraen B. dioscoridis could be attributed to α-thujene (9.3%) and α-pinene (8.3%), identified in the essential oil of the bark [108]. Similarly, related Aloe species such as fresh and dried A. arborescens leaves extracts (containing water-soluble tannins) were reported to exert in vitro acaricidal activity against Rhipicephalus (Boophilus) microplus engorged females [109], and the latex of some Euphorbiaceae species (containing diterpenes such as phorbol and its derivatives) showed toxicity against insects
[86]. Thus, it is worth investigating the insecticidal activity of these plants.
In addition to the medicinal uses, the household processed leaves (dried thick paste of boiled leaves) of Cyphostemma digitatum is utilized in Yemen for a variety of culinary purposes (e.g., as a spice and food flavoring in salads and some Yemeni dishes) for generations without any noticeable side effects or acute or chronic toxicity [62]. Likewise, the beverage made of boiled leaves of C. digitatum with salt is used as an appetite stimulant for livestock, presumably because of its flavoring effect (acidic and salty taste). Moreover, the leaves of C. digitatum could be of nutritional value for the livestock due to its content of vitamins C, E, provitamin A, carotenoids, and phenolic compounds ([Table 1]).
In addition to the bioactivity, it is essential to consider the safety of the studied plants. The constituents of Aloe species latex (1,8-dihydroxy-anthraquinones) and A. bracteolate (aristolochic acids) were reported to possess some serious toxicological properties ([Table 1]). Despite the toxicological activities of Aloe latex constituents, the use of the latex as a laxative in human medicine has not been prohibited, but it was restricted in dosage and period of treatment for cases of occasional constipation and when no laxative effect can be attained through diet change or the use of bulk-forming products [23]. Moreover, it has been reported that by considering the minimal use of Aloes in veterinary medicine (very infrequent, short-term treatment of individual animals only) and the limited absorption of the active principles after oral administration, the risk from veterinary use of Aloes to
the consumer of foodstuffs of animal origin is considered negligible [5].
Fenugreek is included in the list of GRAS (generally regarded as safe) when used as spice or flavoring and is also classified as AHPA class 2b, not to be used during pregnancy (this classification is based on relatively high doses used for therapeutic purposes in contrast to lower amounts generally used in cooking and has not been associated with its use as a spice) [80]. Studies in animal models suggest a low acute toxic potential of fenugreek seeds by oral route at 2 and 5 g/kg bw in mice and rats, respectively, and intraperitoneal route at 0.65 and 3.5 g/kg bw in mice and rats, respectively [81]. As described in cattle [80], myopathy may be caused by feeding cattle solely on fenugreek straw, leading to intoxication or an induced deficiency state [110].
The literature showed controversial results regarding fenugreek seedsʼ teratogenic, antifertility, and neurotoxic effects, as well as resulting histopathological changes in vital organs (e.g., liver), hematological parameters, and clinical biochemistry. Several human and animal studies revealed teratogenic effects from congenital malformations to death; testicular toxicity and antifertility effects in male animals associated with oxidative stress and DNA damage; and antifertility, anti-implantation, and abortifacient activity in female animals. These effects have been suggested to be the consequences of a saponin compound of fenugreek seeds. In addition, the studies suggest that antifertility effects, hormonal disturbances, and adverse effects during pregnancy can be due to long-term daily use of at least 100 mg/kg bw of animal [81].
On the other hand, the prenatal oral exposure of 250, 500, and 1000 mg/kg/day of furostanol saponin glycoside (75.23%) and low molecular weight galactomannans based standardized fenugreek seed extracts were found safe and devoid of maternal and embryo-fetal toxicity [82], [83]. Additionally, no effect on the fertility, and fetusʼs growth and development was observed in pregnant rats treated with fenugreek seed at 5 or 20% of the diet. Moreover, 75 mg/kg of trigonelline administered orally to pregnant and nonpregnant female rats showed no antifertility or abortifacient effects.
Although several studies have shown fenugreek to be neuroprotective, accumulating evidence suggests that fenugreek seeds may have neurodevelopmental, neurobehavioral, and neuropathological side effects [81]. Several studies claim the lack of histopathological changes in vital organs (e.g., liver), hematological parameters, and clinical biochemistry caused by fenugreek [81], [84]. However, several cellular and molecular alterations have been noted in animal models studies, especially in some vital organs (liver, kidney), hematological parameters, and blood biochemistry (e.g., a significant increase of hematocrit and reduction of serum total protein levels) by a daily oral administration of alcoholic extract 200 mg/kg rat bw for 4 weeks; A necrotic effect on the liver and the kidney tissues was observed at doses of 100,150, or 200 mg/kg of aqueous extract orally administered twice a week
for 4 weeks to rats; crude saponins treatment to Hisex-type chicks caused necrosis of hepatocytes with lymphocytic infiltration and epithelial degeneration of renal tubules [81]. Due to multiple beneficial health properties of fenugreek seeds (e.g., antidiabetic effect, hypocholesterolemic influence, antioxidant potency, digestive stimulant action, and hepatoprotective effect) and its usage as a spice for seasoning and a flavoring agent and in comparatively larger quantities in making soups and pancakes [78], the reported toxicological effects should be taken seriously. Further research is needed to clarify these serious side effects and demonstrate their mechanisms of action. However, different extraction methods can lead to a different spectrum of constituents. Since the seeds were fed without extraction and no adverse effects in target animals were described, the risk of ethnoveterinary use may be limited.
The local use of C. procera latex in Yemen to treat swelling was documented without mentioning any side effects [3]. However, the local administration of the latex was reported to induce an intense inflammatory response. In addition, the dried latex was found to produce significant inflammation and hyperalgesia in different models of inflammation in rats, and hence, the dried latex-induced inflammation was used to investigate the anti-inflammatory and analgesic activity of anti-inflammatory drugs [85], [111]. No side effects were reported for E. arbuscula latex, although related Euphorbiaceae species were found to contain diterpenes derivatives (e.g., phorbol derivatives with skin inflammatory and tumor-promoting activities) [86]. Therefore, further research of these remedies is needed to illustrate their effectiveness and safety.
Moreover, we emphasize the need to raise the livestock ownerʼs awareness of the harmful side effects of herbal remedies and the appropriate way to use them either by limiting the dosage and period of treatment, avoiding their use during pregnancy and lactation, or by using safer alternatives.
