Alternative to Antibiotics Used in Sheep Production

Abstract

Sheep play a significant role in agriculture, serving as a primary source of meat, milk, and wool fiber. They constitute a unique class of animals distinguished by their specialized digestive organs. As our population increases, there is a high demand for sheep meat and products from developed and developing countries. In livestock production, disease poses a significant challenge, and the use of antimicrobial and antiparasitic drugs facilitates the control of infections, thereby improving animal welfare, health, and productivity. The use of antimicrobials in sheep farming has become prevalent and has led to antimicrobial resistance. This chapter will focus on the alternatives to antimicrobials used in sheep and how they benefit sheep health and production.

1. Introduction

One of the biggest issues facing development in the upcoming years will be the ability of the world’s food markets to meet the demands of a fast-rising population. The increasing demand for safe, quality foods is driven by exponential population growth. It has been estimated that the human population is expected to exceed 9 billion by 2050 [1] and peak at 9.73 billion in 2064 [2]. Livestock plays a pivotal role in the economy and sustenance of numerous communities, with sheep emerging as the most prevalent livestock species [3, 4]. The growing demand to increase animal protein foods such as meat, milk, and eggs is premised on the increasing world population, whose need for safe, nutritious, and high-quality foods must be met. The current production systems and appliances utilized in livestock farming and veterinary medicine show numerous hindrances and drawbacks to producing sufficient animal foods without compromising environmental health.

Sheep are essential livestock and companion animal species. Sheep (Ovis aries) have been raised for wool, meat, milk, skin, and other beneficial products for humans. Sheep meat is consumed worldwide for its nutritional value and flavor. They provide significant contributions to the agricultural sector of developed and developing countries. Sheep are bred in many countries but are primarily found in countries with challenging climatic conditions. More than 1400 breeds of sheep are reported worldwide [5]. As of 2017, the United States Department of Agriculture (USDA) reported 101,387 sheep farms across the country, housing over 5.3 million sheep, with 90% of the farms having less than 100 sheep. Large- and small-scale sheep producers utilize pasturelands; smaller producers utilize feedlots and pastures, whereas large sheep producers utilize pasturelands [6]. Some of the sheep breeds found in the United States are listed in Table 1. Sheep breeds have various advantageous characteristics and have acquired a worldwide distribution through adaptation to diverse environments, the capacity to utilize inferior quality feed for animal protein, and genetic improvement under different production systems.

Breed name	Purpose	Year introduced in the US
American Blackbelly	Hunting and Hair	—
Awassi	Milk and Wool	2012
Babydoll Southdown	Pets, Wool, and Meat	—
Barbados Blackbelly	Hair	1904
Black Welsh Mountain	Black fleece (wool)	1973
Bluefaced Leicester	Long Wool	1980
Booroola Merino	Fine Wool and Prolific	—
Border Cheviot	Medium Wool and Meat	1838
Border Leicester	Long Wool and Meat	1920
California Red	Wool and Meat	1970
California Variegated Mutant	Wool	1900
Charollais	Terminal Sires and Meat	—
Clun Forest	Wool and Meat	1970
Columbia	Medium Wool and Meat	1900
Coopworth	Long Wool	1970
Cormo	Fine Wool	1976
Corriedale	Meat and Wool	1914
Cotswold	Long Wool	1831
Debouillet	Fine Wool	—
Delaine Merino	Fine Wool	1800
Dorper	Hair	1990
Dorset	Medium Wool	1885
East Friesian	Milk	1993
Finnsheep	Medium Wool and Prolific	1968
Gotland	Wool	2003
Gulf Coast Native	Medium Wool	—
Hampshire	Medium Wool and Meat	1860
Herdwick	Wool	2008
Hog Island	Medium Wool	1700
Icelandic	Meat Wool and Milk	1993
Ile de France	Terminal Sires and Meat	—
Jacob	Wool	1900
Karakul	Wool	1900
Katahdin	Wool	—
Kerry Hill	Medium Wool	2006
Lacaune	Dairy	1996
Leicester Longwool	Long Wool	1700
Lincoln	Long Wool	1800
Montadale	Medium Wool	1930
Navajo Churro	Colored	1600
North Country Cheviot	Medium Wool	1944
Oxford	Medium Wool (Terminal Sire)	1846
Painted Desert	Hair (Exotic)	—
Panama	Medium Wool	1912
Perendale	Long Wool	1950
Polypay	Medium Wool (Prolific)	1970
Racka	Long Wool	2005
Rambouillet	Fine Wool (Dual Purpose)	1800
Rideau Arcott	Medium Wool	1990
Romanov	Terminal Sire	1980
Romney	Long Wool	1904
Royal White	Hair	1990
Santa Cruz	Medium	—
Scottish Blackface	Wool	—
Shetland	Wool	—
Shropshire	Medium Wool (Terminal Sire)	1855
Soay	Colored	—
SAMM	Fine Wool	1999
Southdown	Medium Wool	1820
St. Augustine	Hair	1990
St. Croix	Hair	—
Suffolk	Medium Wool	1888
Targhee	Medium Fine	1926
Teeswater	Long Wool	1996
Texel	Medium Wool (Terminal Sire)	1990
Tunis	Medium Wool	1799
Valais Blacknose	Long Wool	2018
Wensleydale	Long Wool	1990
Wiltipoll	Hair	—
Wiltshire Horn	Hair	1600

Table 1.

List of breeds of sheep in the United States.

Source: https://:www.sheep101.info/201/breedsAZ.html.

1.1 Use of antibiotics in sheep production

Since the middle of the twentieth century, antibiotics have been widely employed to increase livestock productivity and, as a result, lower the price of animal protein to support food security. Despite this increase, sheep production has faced several losses due to infections by parasites and other health-related problems faced by ruminants. Most producers have relied on the use of antibiotics to treat these infections. Antibiotics have reduced the cost of animal-derived protein worldwide, thereby significantly improving the quality of life for billions of people. Several antibiotics used in sheep production include cydectin, fenbendazole, oxytetracycline, penicillin, lasalocid, sulfadimethoxine, decoquinate, aminoglycoside, lincomycin, macrolides, fluoroquinolones, etc. Nevertheless, despite their enormous benefits, the unintentionally growing issue of antibiotic resistance impedes the use of antibiotics in animal production. Antimicrobial resistance (AMR) is an emerging global threat to public health. The World Health Organization (WHO) considers AMR a major threat to global public health. Antibiotics are essential to livestock production to prevent a major drop in the health and productivity of sheep that would affect the security of the human food supply. To remedy this and stop the loss of livestock, it is imperative to create novel approaches and instruments that enhance animal well-being and productivity, like the way antibiotics were once employed. The need for effective alternatives to antibiotics in food-producing animals due to the development of resistant bacteria and the accumulation of antibiotic residue is discussed in this chapter.

2. Methodology used for sheep studies

Several phenotypic data are collected and analyzed following treatments with several alternatives to antibiotics. Body weight is collected using a weighing scale. The body condition score is measured on a scale of 1–5 by physically examining the sheep’s body [7]. Packed cell volume, used as an indicator of anemia, is determined from blood samples in sheep. The FAMACHA (Faffa Malan Chart) scoring system was used to clinically evaluate anemia in sheep. It is a useful tool that farmers can use to determine which animals require treatment. Fecal samples are collected from sheep to determine the number of fecal eggs. For molecular studies, protein analysis is conducted by enzyme-linked immunosorbent assay (ELISA), a molecular technique used to quantify and detect specific antigens or antibodies. The presence and level of secretion of proteins were determined using ELISA [7]. The detection and expression of genes were determined using real-time polymerase chain reaction (PCR).

3. Alternatives to antibiotics used in sheep production

Small ruminant production has been a growing industry in the United States due to demographic changes and the high demand for grass-fed livestock. Globally, healthy sheep are crucial for the long-term success of the sheep industry. In the United States, sheep are raised on pasture, making them more susceptible to the impacts of climatic factors. There have been growing concerns and safety issues surrounding the use of antibiotics for sheep production, leading many countries to seek different alternatives. Alternatives to antibiotics have been the subject of extensive research over the last 20 years. The widely used approach for treatment is drug therapy or the use of anthelmintics. The measures used to reduce parasite infection include reducing stock density and maximizing pasture to reduce parasite numbers [7]. Several alternatives have been proposed to combat parasitic infection and maintain the health status of sheep. The most widely researched alternatives include prebiotics, probiotics, phytobiotics/plant extracts, and essential oils. Understanding these alternatives will aid in designing immunomodulatory strategies to induce an immune response in sheep.

3.1 Prebiotics

According to the Food and Agriculture Organization (FAO), prebiotics are defined as nondigestible substances that benefit the host by selectively stimulating the favorable growth of several beneficial bacteria. Prebiotics are bypass carbohydrate materials that positively directly or indirectly influence the host’s intestinal microbes’ ecology through the proliferation of the gut’s beneficial bacteria, such as anaerobes, Lactobacilli, and Bifidobacteria, by competitive exclusion mechanisms. Research has shown that prebiotics can reduce the use of antibiotics in sheep production. Prebiotics as a feed additive in ruminants have been shown to balance the microflora in ruminants and are a very effective way to combat diseases. Several research studies have used prebiotics to improve ruminant performance, as shown in Figure 1. Supplemented diets with prebiotics have been reported to improve sheep’s dry matter, organic matter, and crude protein digestibility [9]. Another study by Soliman [10] showed that prebiotic supplementation positively impacted rumen parameters, digestibility coefficients, growth performance, and economic efficiency of growing lambs. It has also been reported that prebiotics enhanced total tract digestion and digestible energy in lambs [11]. Also, it has been shown that the supplementation of prebiotics increased the daily body weight gain in lambs [12]. Another study by Lettat et al. [13] showed that prebiotic supplementation could stabilize ruminal pH and lower the risk of subacute ruminal acidosis in livestock fed with a high-energy diet.

Figure 1.

Classification of prebiotics. Source: Author own development. Data source: Ref. [8].

3.2 Probiotics

The gastrointestinal tract (GIT) is known to have a highly diverse microbiota that includes bacteria, viruses, archaea, and protozoa [14]. Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Research has shown that bacteria constitute the dominant group in the gastrointestinal tract (GIT), with at least 500 known bacterial species [15]. Probiotics are microbial substances capable of stimulating the expansion of another microorganism. They are living microbial feed additives that improve the host animals’ intestinal microbial balance and offer health values by establishing a better-suited microbial population by transforming the equilibrium of beneficial and harmful microorganisms. Probiotics work by inhibiting pathogen adherence, producing antimicrobial components, competitively excluding pathogenic microbes, enhancing barrier function, lowering luminal pH, and modulating the immune system, among other mechanisms [16].

Gastrointestinal nematode (GIN) parasite is a critical parasite that affects sheep. Gastrointestinal nematode infections are considered the most important limiting factor in ruminant production systems worldwide, resulting in substantial economic losses to producers [7, 17]. The resistance of this parasite to most anthelmintic drugs poses a significant economic challenge for sheep farming and remains a major obstacle that must be addressed [18]. The damage inflicted by the parasite leads to the loss of substantial blood volume in affected animals, resulting in severe anemia, weakness, nutritional deficiencies, body weight loss, and, in numerous cases, death. Among the parasites that affect sheep, Haemonchus contortus is a highly pathogenic, blood-feeding nematode of small ruminants and a significant cause of mortality worldwide [19]. This parasite has affected lambs, especially during weaning. Previous studies have shown that lambs are at the most significant risk of developing disease due to a relaxed and immature immune system, thereby exposing them to infection [20, 21]. Several anthelmintics have been used to manage the health consequences faced by these parasites, which has led to resistance.

The role of probiotics in ruminants is to influence the intricate microbial population of the rumen, which degrades ingested feed materials and can indirectly enhance the immune system to fight parasitic infection, as reviewed in the chapter “Probiotics and ruminant health” [22, 23]. Studies by Sanders et al. [21] showed that treatment with paraprobiotics reduced the parasitic load of sheep infected with Haemonchus contortus. Another study by Alimi et al. [24] showed that the treatment with kefir, a probiotic, inhibited Haemonchus eggs in sheep. Their results suggested that kefir is a potential tool to control haemonchosis in sheep.

The effects of probiotics have generally led to improved digestibility by increasing enzyme activity in the GIT. Mass et al. [25] reported that applying Bacillus amyloliquefaciens improved calcium absorption. At the same time, improved interactions in the gut with Lactobacillus acidophilus led to more dry matter intake, daily feed conversion efficiency, and apparent digestibility of nutrients. Probiotics can increase the ruminants’ performance by increasing feed digestibility in ruminants and by enhancing the animal’s overall immune system [26]. The immunomodulatory properties of probiotics in sheep have been reported by several studies (as shown in Table 2). They have also been shown to reduce acidosis, a common rumen disease in sheep. Studies by Dagnaw Fenta et al. [50] showed that a combination of probiotics and rumenotorics could be used as promising therapeutics to treat acidosis in sheep. Studies conducted by Han et al. [51] also found that oral administration of Saccharomyces cerevisiae and two strains of Diutina rugosa enhanced the diversity of ruminal microbiota and reduced sheep inflammation and metabolic acidosis. Similarly, Singh et al. [52] showed that using a multistrain probiotic reduced acidosis by lowering the pH and concentration of lactic acid in the rumen.

Probiotic	Effect on Sheep/Lamb	Reference
Bacillus subtilis B-2998D, B-3057D, and Bacillus licheniformis B-2999D	Increased body weight gain, improved intestinal microbiota, strengthened the immune system, and maintained normal physiological processes	[27]
B. subtilis	Significantly increased the total weight gain, microbial abundance, and diversity	[28]
Combination of Lactobacillus acidophilus and Bacillus subtilis	Improved the activities of superoxide dismutase and glutathione peroxidase. Increased growth hormones and immunoglobulin G levels, volatile fatty acids, and acetate	[29]
Bacillus subtilis, Lactobacillus casei, and Saccharomyces cerevisiae	Increased dry matter, organic matter, crude protein, and neutral detergent fiber intakes. Increased blood urea nitrogen, weaning weight, average daily weight gain, and improved health status in lambs.	[30]
Combination of Lactobacillus plantarum HM-10 and Lactobacillus casei HM-09	Regulated intestinal metabolites (SCFAs) and improved the tenderness of meat.	[31]
Lactobacillus acidophilus and a mix of Bifidobacterium animalis subsp. lactis, and Bifidobacterium longum subsp. longum	Improved immune function and the fatty acid profile of the meat	[32]
Combination of Lactobacillus sporogenes and Saccharomyces cerevisiae	Improved growth performance indices (average daily gain, growth rate, and total weight gain). Increased plasma total protein, glucose, urea nitrogen, and aspartate aminotransferase	[33]
Lactobacillus acidophilus, Bacillus subtilis, Bacillus licheniformis, and Enterococcus faecium	Improved nutrient digestibility, daily gain, feed conversion, and economic efficiency	[34]
Combination of Bacillus subtilis, Lactobacillus acidophilus, and Ruminococcus albus/g	Increased digestibility of crude dry matter, organic matter, and neutral detergent fiber, improved meat quality	[35]
A mixture of Bacillus subtilis, sorbitol sodium, vitamin B1, and glucose	Increased body weight and height at withers, circumference of chest, body proportion, and anamorphosis indices. Increased hemoglobin and red blood cells	[36]
Ruminobacter amylophilus, Fibrobacter succinogenes, Succinovibrio dextrinosolvens, Bacillus cereus, L. acidophilus, E. faecium, and S. cerevisiae	Decreased the area, length, and number of eggs from the recovered Haemonchus contortus parasite and improved fecal consistency, promoting a beneficial effect on the gastrointestinal health of lambs	[37]
Pediococcus spp.	Improved lamb growth and maximized economic efficiency of production.	[38]
Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium thermophilum	Improved milk yield with minimal effect on blood plasma metabolite contents and enzyme activities	[39]
Lactobacillus sp., S. cerevisiae	Decreased concentrate consumption and increased energy utilization efficiency. Decreased the meat cholesterol content of local sheep.	[40]
Pediococcus. acidilactici and Pediococcus. pentosaceus	Improved dry matter intake, growth performance, feed conversion ratio, and nutrient digestibility during pre- and postweaning periods.	[41]
Lactobacillus casei HM-09, Lactobacillus plantarum HM-10	Increased intramuscular fat and improved the composition of fatty acids by modulating adenosine 5′-monophosphate-activated protein kinase (AMPK) signaling pathway	[42]
Bacillus amyloliquefaciens H57	Influenced animal behavior (feed consumption) and altered the native rumen community structure or function	[43]
Lactiplantibacillus plantarum	Improved blood lipid parameters, expression of lipid metabolismrelated genes, tail fat metabolites, and volatile flavor compounds	[44]
Allium mongolicum Regel	Improved meat quality without the occurrence of pathological kidney and liver lesions.	[45]
Saccharomyces cerevisiae (NCDC-49)	Improved metabolic and killing activity of phagocytes. Significantly increased γ-globulin levels, lysozyme and ceruloplasmin activity, and proliferative response of blood lymphocytes	[46]
Enterococcus faecalis EF1-mh, Bacillus subtilis BS1-ql, and Lactobacillus sakei LS-qi	Increased intestinal slgA, reduced mRNA expression of toollike/MyD88/NF-kB/MAPK	[47]
Saccharomyces boulardii	Significantly increased IL-10 levels	[48]
Bacillus toyonensis, Saccharomyces boulardii	Increased IgG, lgG1, and IgG2 Increased mRNA transcript of IFNγ, 1 L2, and Bcl6 genes	[49]

Table 2.

Immunomodulatory effects of probiotics strains in sheep.

Probiotics are vital in solving food production problems by replacing antibiotic use. However, there is a need to identify the optimum condition for a probiotic to survive, colonize, expand, and render its effects to the hosts. Also important are detailed dosage-dependent studies to confirm the organism’s identity using molecular testing at a reference laboratory.

3.3 Phytobiotic/plant extracts

Incorporating plant extracts as an alternative to antibiotics in sheep production is a tangible application of the One Health concept, promoting the well-being of animals, humans, and the environment in an integrated and interconnected manner. Phytobiotics encompasses a spectrum of natural bioactive compounds from plants, including herbs, spices, essential oils, and oleoresins [53]. Plant components and extracts are often relatively cheap, widely available, natural, and nontoxic. Phytobiotics have various effects, including antibacterial, antioxidant, and anti-inflammatory properties, gut microbiota alteration, intestinal barrier and nutrient absorption, and immunological modulation, which are advantageous to improving animal growth performance, health, and meat quality [54, 55, 56, 57, 58, 59]. Peptides that have antibacterial activity against Pseudomonas aeruginosa are among the antimicrobial substances found in plants [60]. The efficacy of phytobiotics can largely be attributed to the diverse constituents present in different plant parts. These constituents, ranging from terpenoids and phenolics (like tannins) to glycosides, alkaloids, flavonoids, and glucosinolates [61], collectively contribute to the holistic benefits observed in animal performance and could be employed in the sheep diet.

For instance, a study by Hashemzadeh et al. [62] showed that supplementation of a photogenic-rich herbal mixture (rosemary, cinnamon, turmeric roots, and clove buds) promoted feed intake, enhanced serum and liver antioxidant status, and improved growth performance of heat-stressed feedlot lambs. Phytobiotics have been employed in the rumen microbiota in grazing sheep, as seen in a study by Zhang et al. [63], where Cistanche deserticola in the sheep feed influenced the alteration of rumen flora abundance and regulated rumen fermentation, thereby improving production efficiency. Like Cistanche deserticola, tea saponin (TS) is another phytobiotic showing potential health benefits for sheep. Many tea plants belong to the class of pentacyclic triterpenoid glucoside compounds (Camelliaceae). Goel and Makkar [64] reported that TS inhibits protozoa by altering cell membrane integrity. Tea saponin’s biological qualities can be employed to restrict methane production [65], lower rumen protozoan numbers, and regulate rumen fermentation patterns [34, 66]. To back this up, Liu et al. [67] found that dietary TS supplementation improved organic matter (OM), nitrogen, neutral detergent fiber (NDF), and acid detergent fiber (ADF) digestibility while lowering daily methane emissions in crossbred ewes by 8.8%.

As a growth promoter, Yaxing et al. [68] evaluated the effects of Allium mongolicum Regel (AM) and its extracts on sheep’s growth performance, carcass parameters, and meat quality. They reported that supplementing AM and its extract, most significantly, increased average daily gain (ADG) and decreased feed conversion ratio (FCR) while enhancing the meat quality of the sheep.

The continuous use of antiparasitic drugs against gastrointestinal nematodes (GINs), which are the principal parasites that impact sheep production and health, incurring economic losses in grazing systems worldwide, could ultimately lead to resistance. Therefore, using phytobiotics (herbs) as anthelmintics could be a better alternative against these parasites [69]. Medicinal plant extract, rich in secondary compounds, has been shown to reduce GIN infection [70]. Mejia-Delgadillo et al. [71] reported that treating Peptasan reduced ewes’ nematode counts (mostly Trichostrongylus, Haemonchus, and Oesophagostomum). The plant extract improved weight gain and feed efficiency in lambs over a 28-day feeding period without negatively affecting ruminal fermentation.

Moharam & Kamal [72] investigated the blood antioxidative status of ewes in late pregnancy and early after parturition and examined the anthelmintic effects of black seeds (Nigella sativa) and Ginger (Zingiber officinale). They found that supplementing the ewes’ diet with black seeds and ginger during late pregnancy and early postpartum dramatically reduced malondialdehyde (MDA), a marker of oxidative stress, and improved antioxidant activity in serum and erythrocytic hemolysates. The effectiveness of black seeds and ginger powder was assessed by counting eggs per gram of gastrointestinal nematodes, and fecal analysis demonstrated a substantial decrease in eggs per gram compared to the control group. Several studies have been conducted using ginger as an alternative to antibiotics. Mostafa et al. [73] reported that ginger has antischistosomal activities, while Lin et al. [74] recorded that ginger has a larvicidal effect against Anisakis simplex. Another study by Pathak [75] revealed that supplementing ruminants’ diets with condensed tannins or proanthocyanidins was effective against various developmental stages of gastrointestinal nematodes (GINs) and reduced gastrointestinal parasitic load.

Saparova and Zubova [76] researched the development and evaluation of the effects of novel environmentally safe herbal remedies derived from a combination of extracts from Leuzea carthamoides and Echinacea purpurea on the complex economic traits of pregnant ewes. Their results showed that lambs of the Edilbaevskaya breed positively affected the physiological status and productive qualities of ewes from phytobiotic additives added to the diet. Lambs’ body weight increased, as well as disease resistance. It was suggested that the extracts be added to the diet of pregnant ewes to help the farm transition to a high-yield, antibiotic-free environment.

One of the significant attributes of phytobiotics/plant extracts is their capacity to contribute essential nutrients to livestock while concurrently stimulating the endocrine system, thereby facilitating nutrient metabolism. Furthermore, certain phytobiotics/plant extracts are pivotal in reducing microbial toxins by stabilizing the microbiome [77], consequently diminishing inflammation. This redirection of resources toward growth promotion, rather than immune modulation, results in a more efficient allocation of protein production [78, 79].

3.4 Essential oils

Essential oils contain concentrated and complex mixtures of volatile nonpolar compounds extracted from plant sources. Essential oils are produced worldwide for flavor and pharmaceutical applications [80]. They are widely used for their natural antibacterial, anti-inflammatory, antioxidant, antifungal, and antispasmodic effects [81]. Essential oils represent one of the most promising alternatives to antibiotics in reducing some of the problems associated with the sheep industry.

In grazing ruminants worldwide, gastrointestinal nematodes (GINs) continue to be the most common parasites that cause illness, especially in sheep and goats [7, 82]. Gastrointestinal parasites are responsible for most economic and productive losses around the world. Previous studies conducted by Strabac et al. [83] reported that essential oils could be used as an anthelmintic agent in sheep farms, which affected gastrointestinal parasites. Also, previous studies conducted by Camurca et al. [84] reported using essential oil to control gastrointestinal nematodes in sheep. Mesquita-Sousa reported the use of essential oil in the treatment of sheep infected with gastrointestinal nematodes. Their report shows that essential oils reduced the shedding of nematode eggs by 78%. Despite all the positive reports on using essential oils in sheep production, a better understanding of the mechanism of action is needed, especially when combined with other drugs. da Silva et al. [85] evaluated the effect of orange essential oil on parasitic infection in lambs. They reported that essential oils reduced the severity of the parasitic infection and constantly diminished pasture contamination of gastrointestinal parasites when used with other methods.

Terpenes and terpenoids are the most prevalent components in plant essential oils. Ferreira et al. [18] showed the in vitro and in vivo anthelmintic activity of Thymus vulgaris essential oil extracted from plants effectively inhibited Haemonchus contortus in sheep. Their results show that Thymus vulgaris essential oil has the potential to contribute to the development of new drugs and even herbal medicines, increasing treatment options for sheep.

Numerous studies show that essential oils have bioactivities, such as selective antibacterial activity, ruminal methane emission inhibition, ruminal propionate proportion enhancement, and bypass protein, to the intestine [86]. It has been shown that the addition of essential oil to the diet of sheep improves fiber digestibility. Based on some of these reports, it can be suggested that essential oils could be used as a diet supplement to improve the health status of sheep.

4. Conclusion

The demand for sheep products continuously increases as our population increases. It is essential to look into these alternatives to antibiotics that will provide the animal with beneficial effects for health and production in the host animal by modulating the host immune system. These studies provide a deep insight into the different alternatives to antibiotics used in sheep production. Treatment with prebiotics, probiotics, phytobiotics/plant extracts, and essential oil is promising and could be used in designing immunomodulatory strategies to induce immune responses in sheep.

Conflict of interest

The authors declare no conflict of interest.