Traditional Latvian herbal medicinal plants used to treat parasite infections of small ruminants: A review

Numerous treatment agents offering prophylaxis against livestock parasites are commercially available. However, because of increasing antiparasitic drug resistance, the increased popularity of environmentally friendly lifestyle choices, and organic farming, there is more demand for new alternatives to livestock anthelmintic control strategies and medications. It is important to develop antiparasitics that are safe, effective, inexpensive, and environmentally safe. Local, traditional herbal plants such as tansy, mugwort, wormwood, and heather may serve as treatments for intestinal parasites of sheep. This overview provides knowledge of traditional Latvian plants with antiparasitic activities to establish a database for further research to develop new herbal antiparasitic drugs.


Introduction
Small ruminants, particularly sheep and goats, occupy an important ecological and economical niche in Latvian agriculture. Among 1.9 million inhabitants, there are 120,726 sheep and 3467 official sheep farms registered [1]. Sheep are kept in Latvia for numerous reasons. Most important, ruminants convert poor foodstuffs into important products such as milk, meat, and wool for human consumption [2]. Moreover, poor, overgrown, and wet fields serve as reservoirs for parasites of sheep such as Fasciola hepatica, Paramphistomum spp., Heamonchus contortus, Trichostrongylus axei, Trichostrongylus colubriformis, Trichuris ovis, and Monezia expansa [3] that cause widespread infections of sheep [4]. Diverse endo-or ectoparasites affect sheep; and their adverse effects on health, production, and welfare are robustly documented [5,6]. For example, Trichostrongylidae infection is among the major challenges to ovine health management because of deleterious disease-caused effects and emerging antiparasitic drug resistance to available manufactured veterinary anthelmintics worldwide [4,7].
Numerous prophylactic agents are commercially available. However, because of increasing resistance to anthelmintics and increased popularity of environmentally friendly lifestyle choices and organic farming, new alternatives to current parasite control and treatment strategies are urgently required. Therefore, medicinal products that are safe, effective, inexpensive, and safe for the environment must be identified. For example, newer treatment strategies employ locally harvested herbal plants. This modern branch of traditional ethnoveterinary pharmacology, similar to phytotherapy, is based on knowledge passed down through generations. This subdiscipline of pharmacology lost popularity during the intensive industrial pharmacological era; however, it is now regaining interest. Unfortunately, we lack compelling scientific evidence supporting the beneficial effects of herbal medications.
Here, we aimed to review the past and present status of traditional herbal medicinal products historically used in Latvian traditional (folk) medicine for their anticipated antiparasitic effects in comparison to the available scientific literature, particularly related to sheep. This overview attempts to consolidate scientific knowledge of traditional Latvian plants with antiparasitic activities to assist future research.

Methods
We searched Scopus, Web of Science, EBSCO, and PubMed for relevant articles published between January 1980 and September 2020. Other publications were sourced from references in individual articles. Relevant articles were selected after reading their titles and abstracts, and full text was acquired if this H. contortus feeds through sucking blood from the abomasum mucosa, which induces local inflammation of the mucosa at the parasite's attachment site. An adult nematode can ingest 0.05 µl of blood per day, leading to anemia of the host. Further, a load of >10,000 adult nematodes in a lamb potentially causes fatal blood loss, and sheep may suffer from the toxic effects of metabolic end-products derived from larval and adult parasites [22,23].
In sheep, the main clinical signs of parasitic gastroenteritis are anemia, delayed growth, anorexia, and weight loss. Hypoalbuminemia frequently occurs in highly infested animals, contributing to edema of the lower jaw and diarrhea [9]. Abnormal gastric secretion occurs in highly infested sheep, which is characterized by increased levels of the secreted gastric hormone gastrin and the proenzyme pepsinogen (a proenzyme) in the systemic circulation [15]. Together, growth retardation, weight loss, clinical illness, veterinary services, treatment expenses, and lost production and income cause significant economic losses.

Worm Control Practices in Ruminants
Numerous types of worm control practices are employed worldwide. For example, a diet supplemented with high amounts of protein, amino acids such as methionine and leucine, and rumen-protected protein boost immunity, inhibit parasite proliferation, maintain agricultural and commercial production, and reduce fecal egg counts during infection [24][25][26]. The commercial vaccine "Barbervax," which was released in Australia, achieves efficacy in field trials; however, the production of its recombinant subunits was unsuccessful [27,28]. Copper oxide wire particles initially used as a mineral supplement are effective and safe for treating H. contortus infection of weaning lambs [29,30]. In Latvia and Baltic states, pharmaceutical deworming is the most frequently used management strategy.
Macrocyclic lactones, which are the main drugs currently used in veterinary practice to treat gastrointestinal helminths, include avermectins and milbemycins; benzimidazoles, including albendazole, febantel, and fenbendazole; tetrahydropyrimidines, including pyrantel and morantel; and cyclic depsipeptides, including emodepside, piperazines, and praziquantel [31]. The Latvian Food and Veterinary service drug register lists 658 prescription-only veterinary antiparasitic formulations, among which 21 are licensed as anthelmintic medications for treating sheep as follows: Ivermectin, levamisole, albendazole, closantel, and monepantel [32]. Ivermectin (a macrocyclic lactone), albendazole (a benzimidazole), or both are most frequently used by Latvian sheep farms.
The rates of anthelmintic resistance in the lower temperature zones such as the Nordic countries, and possibly the Baltic states, are generally lower compared with those in warmer climates [6,41,42]. Unfortunately, anthelmintic resistance is directed against most classes of commonly used broad-spectrum anthelmintics. Resistance of gastrointestinal nematodes against benzimidazoles, imidazothiazoles, tetrahydropyridines, and macrocyclic lactones occurs in sheep [6,14,[42][43][44]. A study using fecal egg-count reduction tests found that anthelmintic resistance in the Baltic state is 27.8% in Lithuanian sheep flocks [11,35]. Macrocyclic lactones, benzimidazoles, and levamisole were used to treat gastrointestinal nematodes in 68.6%, 27.5%, and 3.9% of cases, respectively. A pilot study conducted in Estonia detected resistance against benzimidazoles and macrocyclic lactones [45]. Unfortunately, data are not available regarding anthelmintic resistance in Latvian sheep farms, which inspired the present investigation.

Plants Used in Traditional (Folk) Medicine in Latvia
Latvian culture retains a strong folk memory that frequently contributes to an understanding of veterinary pathogens. For example, knowledge of diverse home remedies and the medicinal properties of plants is historically orally communicated through families, communities, and between generations. Recent research shows the importance of such folk traditions. For example, a summary of local plants used in Latvia during the 19 th Century was obtained from the Archives of Latvian Folklore. The collection comprises songs, poems, and books with 1900 records on the use of medicinal plants. According to these records, 211 plant genera representing 71 families were used by indigenous Latvian people during the 19 th Century. Possible therapeutic uses of local plants and their components, including individual dosages and descriptions of routes of administration, are described in this research [46,47].
Although human medicine commonly employs plants, very little data are available regarding ethnoveterinary medicine in Latvia. For example, a study conducted in Europe did not include reports of herbal medicine applied to animals [48]. The geographical location of Latvia and its consequently temperate climate supports and promotes an abundance of diverse flora. An impressive variety of local plants is, therefore, available for use in herbal medicine, particularly in its veterinary subdiscipline.
Herbal drugs were extensively used worldwide as anthelmintics before the introduction of modern broad-spectrum pharmacological agents [49]. Further, certain modern anthelmintics are derived from naturally occurring plants or their synthetic analogs [50]. Research performed to develop effective drugs from herbal compounds [51][52][53] identified drugs subsequently shown to be economically efficient, with minimal adverse effects [52].
Here, we reviewed more than 128 scientific articles on ethnoveterinary medicinal plants used in countries that share Latvia's representative flora. We selected and summarized plants with suspected antiparasitic/anthelmintic properties naturally occurring in Latvia. We identified 82 plant species and 32 plant families that met the inclusion criteria. The most widely represented plant family was Asteraceae -18 species, followed by Apiaceae -7, Lamiaceae -6, Fabaceae -4, Ranunculaceae -4, and Poaceae and Salicaceae -3 representatives each. Other families included one or two representatives. The Asteraceae family is the most common medicinal plant found outside of Latvia, with a similar distribution of families [48].
Despite a large number of plants with medicinal properties for human and veterinary medicine, these properties are greatly affected by the chemical composition of each plant, particularly by secondary plant metabolites. Such compositions may vary depending on climate, country, soil properties, season, and other environmental conditions. The fact that a plant has anthelmintic activity in one part of the word does not mean that it will be the same in another. The chemical composition and properties of a plant are affected by harvesting, storage, the components of a plant, and the type. Most frequently, the entire plant is used, or various combinations of plant extracts [54]. Further, it is important to choose the appropriate extraction solvent according to the solubility's of secondary metabolites of a target plant. Water and methanol are most frequently used solvents. Some believe ethanolic extracts are better, because it is easier for them to enter the parasite's body through transauricular absorption [3].
Two broad study types are used to determine a plant's properties -in vitro and in vivo. Each study has advantages and disadvantages. In vitro studies are less expensive, simultaneously analyze multiple plants, and are reproducible [55]. Further, in vitro studies allow the investigation of specific parasites and their lifecycles. The egg hatch (eggs), larval development (larvae), and larval motility (adults) tests are typically performed. In vivo studies are lengthier, only investigate one plant at a time, and are difficult to replicate. An in vitro study should initially be performed, and the plants that exhibit the best effect should be selected for subsequent in vivo studies. Often, the in vitro and in vivo results for the same plant differ. Further, the outcomes can be influenced by an animal's internal factors, and plant species exert clinical effects that vary depending on the digestive system of the host, for example, ruminant versus a monogastric animal [54][55][56].
Polyphenolic tannins, which are of considerable medical interest as well, possess an astringent taste and are used in the food and pharmaceutical industries. Tannins are classified as hydrolyzable (gallic acid and ellagic acid) or condensed (proanthocyanidins). The medicinal properties of tannins are as follows: Anthelmintic, antibacterial, antidiarrheic, antiviral, antihepatotoxic, antihypertensive, anti-ulcer, anticancer, and antioxidant (condensed tannins only) [43,78,79].
Alkaloids are heterocyclic nitrogen-containing compounds, representing an undoubtedly important group of secondary metabolites. Approximately 20,000 alkaloids have been isolated, mainly from plants, but occur as well in microorganisms, marine organisms, and terrestrial animals. Alkaloids are classified according to their molecular backbones or according to their botanical origins. The most important functional groups include indole alkaloids, isoquinoline alkaloids, tropane alkaloids, steroidal alkaloids, pyridine, and pyrrolizidine alkaloids [80]. Pharmaceuticals derived from natural alkaloids include commonly used analgesic and cardiovascular agents, chemotherapeutic and antimalarial agents, centrally acting sedatives and stimulants, smooth muscle relaxants, and others. Important examples include atropine, cysteine, scopolamine, cocaine, quinine, morphine, codeine, ephedrine, reserpine, ergotamine, vinblastine, vincristine, and caffeine [76,79,[81][82][83].
Tanacetum vulgare L., or tansy, is a perennial herb produced by genera of the Asteraceae family. The plant has a specific smell; and wild tansy, which is widely distributed across Latvia, is commonly mentioned in Latvian folk medicine. Tansy exerts healing effects, including anti-inflammatory, antioxidant, antibiotic, and cytotoxic effects [88,89]. Tanacetum balsamita L. (customary), Tanacetum parthenium (L.) Sch. Bip. (syn. Pyrethrum parthenium (L.) Sm., or feverfew are cultivated in Latvia. Despite its use as a typical folk medicine plant, there are few scientific articles or studies that focus on tansy's antiparasitic effects. For example, T. vulgare is ineffective on the eggs and larvae of H. contortus [90].
Recent studies show that tansy comprises numerous polyphenols. Spectrophotometric analysis of tansy flowers reveals gentisic, caffeic, chlorogenic, p-coumaric, and ferulic acids; hyperoside,  isoquercitrin, rutin and quercitrin, patuletin, luteolin, kaempferol, and apigenin [89]. The total concentrations of phenol range from 46 mg to 50 mg gallic acid equivalents (GAE)/g. Polyphenolic compounds (phenolic acids) extracted from tansy possess antioxidant activities, although the antibacterial activities of its essential oil are low [99,100]. Further, methanol is the best extractant for polyphenols, compared, in descending order, with ethanol, water, isopropanol, and acetone. The yields of phenolics range from 12 mg to 63 mg GAE/g of the dried herb. Water serves as the most efficient extractant for preparing flavonoids [101]. Artemisia vulgaris L., or mugwort, is a very large (50-170 cm tall) herbaceous plant. Another widely distributed mugwort of the Asteraceae family grows in diverse habitats in Latvia. The plant possesses antileishmanial, antimalarial, antibacterial, antifungal, anthelmintic, and anti-inflammatory properties [102][103][104]. Mugwort comprises compounds with significant chemical polymorphisms and contains terpenes, a monoterpenoid, which contributes to the plant's odor, which is the basis of its insecticidal activity, therefore, employed as an insecticide [105,106].
Artemisia is extensively studied, and numerous medicinal plants are derived from species of this genus. A review of different plants from this genus confirms their high concentrations of tannins and condensed tannins, which contribute to their antioxidant effects [107]. Artemisinin is a sesquiterpene lactone, which is produced by the Artemisia genus, is important for treating malaria [108]. Studies of the antibacterial effects of essential oils of Artemisia show that 1,8-cineole and alfa-thujone in the oil isolated from the aerial components of mugwort achieve good and nonselective activity against Escherichia coli, Salmonella Enteritidis, Pseudomonas aeruginosa, Klebsiella pneumonia, Staphylococcus aureus, Candida albicans, and Aspergillus niger. The diminished activity of essential oil from A. vulgaris root is likely explained by its reduced content of 1,8-cineole and thujone. A. absinthi essential oil contains a higher content of beta thujone and achieves higher antimicrobial effects, except when used to treat infections caused by S. aureus or E. coli. [109].
A. absinthium L., or wormwood, which belongs to the Asteraceae family, similarly emits a specific wormwood odor, is widespread throughout Latvia; and folk medicine considers wormwood to have medicinal effects. Leaves and the aerial components of the plant are most commonly used in Europe [47,49]. Artemisia spp. found in Latvia are as follows: A. campestris L. (field wormwood), A. annua L. (annual mugwort), A. austriaca Jacq., A. maritima L. (sea wormwood), A. sieversiana Ehrh. ex. Willd., and A. verlotiorum Lamotte (Chinese mugwort). These rarely occur Latvia. In contrast, A. dracunculus L. (estragon) is the only one among these species that is cultivated as a herb and does not grow in the wild. A. abrotanum L. possesses medicinal properties.
A. absinthium serves as a source of numerous conventional and traditional medicines [113]. The most important chemical constituents in wormwood include flavonoids (santonin) and sesquiterpenes (artemisinin), which exert antiparasitic activities [105,114]. Artemisinin is a "parent drug" of antimalarial agents, and its derivatives have been used since 1980 [104]. These products are toxic to animals and induce neurotoxicity at high doses. A. absinthium essential oil contains thujone (α and β) and exerts antimicrobial and antifungal effects [3]. A. absinthium oil possesses antispasmodic, antiseptic, antiparasitic, and  Available at www.veterinaryworld.org/Vol.14/June-2021/18.pdf insecticidal properties [114,115]. Aqueous and ethanolic extracts of A. absinthium are effective against gastrointestinal nematodes in sheep, particularly against H. contortus [57]. However, a recent study of wormwood as a dietary supplement did not detect a significant effect on lambs infected with H. contortus [115]. A partial antiparasitic effect was detected in ruminants infected with Eimeria spp. [57]. Studies of the chemical composition of Artemisia absinthium identified and characterized different groups and individual active components of plants and essential oils. The essential oil, in minimal amounts, is detected using macro-and microscopic tests, assessed as absinthin and resorcinol [116]. The essential oil of A. absinthium contains 1, 8 cineol, camphor, linalool, alpha-and beta thujone, 4-terpineol, borneol, α-cardinol, absinthin, artemetin, myrcene, and other mono-and sesquiterpenes. Its essential oil is used as an anti-ulcer drug, an anticarcinogenic agent, an analgesic, an immunomodulator, an antimalaria, an antiseptic, an antibacterial, an anthelmintics, a larvicide, and a pesticide, although it may be toxic under certain circumstances [99,113,117].
C. vulgaris (L.), hill or heather, is a perennial plant of the Ericaceae family that grows in nutrient-poor soils and is often found in pine forests. In Latvia, heather, which is a valuable nectar-producing plant with medicinal properties, is the only species of the genus that grows in Latvia. The main chemical components of the plant that potentially engage parasites are tannins. C. vulgaris (L.) has low nutritional value, and only goats use it as forage. When goats were fed heather, only a mild anthelmintic effect is observed. Reduced numbers of released T. colubriformis egg are found, although heather increases immunity to parasites [119]. When heather is fed to experimentally infected goats, it is associated with a reduction in the establishment of T. colubriformis larvae and a significant decrease in nematode egg excretion by the host [122]. The aerial components of heather have been studied during different growth stages; however, heather's medicinal properties are mainly described in traditional medicine, the plant's pharmacologic potential is a focus of current investigations.
Phenolic compounds are found in varying amounts throughout C. vulgaris. The highest total concentrations of phenolic compounds, tannins, flavonoids, hydroxycinnamic acids, and proanthocyanidins are detected in leaves and roots during all growth stages, except during flowering. During flowering, the highest concentrations of flavonoids, proanthocyanidins, and anthocyanins are detected in flowers. Heather herb possesses antioxidant activity, and its antibacterial effects are mediated by the leaf, stem, rhizome, and root extracts. Leaf and stem extracts inhibit the infectivities of Gram-positive and Gramnegative bacteria, and roots are active against Bacillus subtilis, and extracts of the rhizome are active against E. coli [122]. Further, combinations of the active substances and amounts differ according to the season and are higher during growth. The total tannin concentrations in heather range from 22 mg to 36 mg TAE/g dw, with a higher proportion of the condensed form versus the hydrolysable form [123].
Aqueous extracts of inflorescences of heather contain carbohydrates, protein, lipids, fructose, glucose, organic acids, and fatty acids. Phenolic compounds identified in C. vulgaris extract are myricetin-3-O-glucoside and myricetin-O-rhamnoside. More polar extracts with more phenolics exhibit stronger antioxidant and antibacterial effects, whereas n-hexane and ethyl-acetate extracts achieve better anti-inflammatory and cytotoxic effects [124]. Liquid chromatography-mass spectrometry and 1 H-nuclear magnetic resonance analyses of heather identified the fatty acids as follows: Linolenic (35%), linoleic (27%), and palmitic (21%); a high content of fiber and carbohydrates (75%); α-tocopherol quercetin, kaempferol, and myricetin derivatives as well as procyanidins. The total amounts of phenolic compounds (TPCs) are higher in hydroalcoholic extracts. These extracts exert good antibacterial effects without causing adverse effects [125]. Photospectrometric analysis (Folic-Ciocalteu) revealed the presence of TPCs at concentrations ranging from 67.55 mg GAE/g to 142.46 mg GAE/g and flavonoids at concentrations ranging from 42.11 RUE/g to 63.68 mg RUE(Rutin equivalent)/g (using aluminum chloride) [125]. Another study demonstrates antioxidant properties [126]. The phenolic compound quercetin likely accounts for heather's calming effects [127,128].

Conclusion
The emergence of helminths resistant to the most frequently used antihelmintic drugs requires urgent action to identify alternative methods. Herbal plants have huge potential for this purpose, as they are relatively inexpensive, safe, and effective. We, therefore, focused here on the typical Latvian herbal plants tansy, wormwood, mugwort, and heather as the focus of further in vitro studies to determine the effects of these plant extracts on strongyloid parasites.

Authors' Contributions
LK: Conceptualized the aim of the study, designed, supervised, and corrected the manuscript. AK, DK, RS, DB, and LK: Equally conceived the work with literature and drafted and reviewed the manuscript. All authors have read and approved the final manuscript.

Acknowledgements
This study was carried out in the framework of LLU (Latvia University of Life Sciences and Technologies) internal project "Antiparasitic drug resistance in Latvian sheep farms" (No. LLU P12) and with implementation of the research project "Development of herbal plant containing medical extracts with anti-parasitic effect" funded by the Latvia Ministry of Agriculture and Rural Support Service program LAD16.2 project: The support for pilot projects and the development of new products, practices, processes and technologies. The authors are grateful to Polina Skliarevitch for helpful comments on a draft manuscript.

Competing Interests
The authors declare that they have no competing interests.

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