Essential oils

ESSENTIAL OILS

oils have a great potential and are generally considered natural, less toxic. The increasing pressure of abolishing and/or decreasing the use of antibiotics as antimicrobial growth promoters for livestock & poultry calls for alternative solutions to sustain the efficiency of current production. Among the alternatives, essential and free from residues. Essential oils have been proven in numerous in vitro studies to exert antimicrobial effects on various pathogens. The current review touched on the basics of essential oils, and the in vivo effects of essential oils on growth, intestinal microflora, anti-oxidation, immune functionality, meat qualities as well as the possible modes of action in poultry & livestock.

The improper use of antibiotics resulted in the selection of bacteria resistant to antibiotics. One of the solutions is to implement bans of using antibiotics as antimicrobial growth promoters (AGP) for farm animals, which makes it imperative to find effective alternatives to antibiotics to sustain the efficiency of current livestock production. Among the alternatives, essential oils have a great potential. The essential oils are generally considered natural, less toxic, and free from residues when compared with antibiotics.

Essential oils are complex mixtures of volatile compounds produced by living organisms and isolated by physical means only (pressing and distillation) from a whole plant or plant part of known taxonomic origin. The term “essential oils” emerged because “oils” were wishfully believed to be “essential” to life, and have a long history of being used by human for cosmetic and medicinal purposes. The development of essential oils, however, was delayed by the advent of antibiotics in the middle of the 19th century, and was renewed recently. It was estimated that, out of 3,000 known essential oils, 300 were recognized as commercially important and mainly used in the flavors and fragrances market. The global essential oil market is expected to reach 11.67 billion USD by 2022. The aim of the current review was to identify the well-recognized efficacy of essential oils for poultry and livestock as well as the conflicting research findings, whereupon more research efforts could be directed to the inconclusive area to facilitate a better understanding of essential oils.

Basics of essential oils

Essential oils are a sum of constituent volatiles, and thus the effects of essential oils should be a totality of effects of all components and their interactions. However, 2 or 3 components could account for up to 85% of the total mixture compared with the minors, and thereby contribute to the primary property of the mixture. For example, the phenols (thymol and carvacrol) constitute about 80% of the essential oils of oregano, the most widespread species of Lamiaceae family, and are mainly responsible for its antibacterial and antioxidant activities. Besides thymol and carvacrol, ρ-cymene was found as another dominant component of oregano. Although the ρ-cymene is not an effective antimicrobial agent by itself, it could facilitate the transport of carvacrol across the cytoplasmic membrane.

Essential oils account for only a small proportion (usually less than 1%) of the wet weight of plant materials, which makes it imperative to improve the yield of essential oils by continuous developments in relevant fields such as genetic engineering and extraction methods. These developments presented challenges to the concept of essential oils as well as the knowledge of biological activities of essential oils. For example, the steam-distilled essential oils from Origanum vulgare showed a great antibacterial activity against reference strains with a moderate antioxidant activity, while the methanolic extract exhibits no antibacterial activity but a high antioxidant activity, which suggests that the bioactivity of essential oils is indeed based on the method of extraction. In addition, there is a growing part of chemically-synthesized essential oils used in feed industry. Most constituents of essential oils are terpenoids and phenylpropanoids. Phenylpropanoids occur less frequently and less abundantly than terpenoids. The well-known plant families for producing essential oils with medicinal and industrial values include Alliaceae, Apiaceae, Asteraceae, Lamiaceae, Myrtaceae, Poaceae, and Rutaceae. Some representative essential oils include, but not limited to, anise (Apiaceae), oregano (Lamiaceae), cinnamon (Lauraceae), garlic (Liliaceae), thyme (Myrtaceae), black pepper (Piperaceae), and Turmeric (Zingiberaceae).

Essential oils for poultry

 Growth performance

Essential oils are perceived as growth promoters in poultry diets. Animal trial results, however, are considerably variable. These factors relate to the experimental essential oils, animals, diets, and environment.

 Feed utilization

Unlike feed intake, improvements in weight gain and feed conversion ratio dominate the observations. Two well-accepted mechanisms are the stimulation of digestive enzyme secretion and the stabilization of ecosystem of gut microflora, leading to improved feed utilization and less exposure to growth-depressing disorders associated with digestion and metabolism. The positive effects of essential oils on digestive enzyme secretion from pancreas and intestinal mucosal have been reported in many broiler studies. These effects were confirmed by the increased digestibility of nutrients. It is noteworthy that there is an inadequate description of the environmental conditions under which these trials were conducted, and poor hygienic conditions might be instrumental for essential oils to favorably affect the growth performance of broilers.

 Antimicrobial and anticoccidial activity

The antimicrobial activity of essential oils has been explored in many in vitro assays which showed that thymol, eugenol and carvacrol have high antimicrobial activity against pathogenic bacteria such as Escherichia coli and Salmonella typhimurium, both of which are potential risk factors of enteric infections. Thymol, eugenol and carvacrol are structurally similar, and have been proved to exert synergistic or additive antimicrobial effects when combined at lower concentrations. Therefore, it is necessary to unravel the synergistic mechanism to optimize their formulation. Different in vitro methods as well as different pathogens exist for ranking the antimicrobial capacity of essential oil components, which could vary dramatically. In vivo studies, essential oils used either individually or in combination have shown clear growth inhibition of Clostridium perfringens and E. coli in the hindgut and ameliorated intestinal lesions and weight loss than the challenged control birds. One well-known mechanism of antibacterial activity is linked to their hydrophobicity, which disrupts the permeability of cell membranes and cell homeostasis with the consequence of loss of cellular components, influx of other substances, or even cell death. It is of note that Gram-negative bacteria are more tolerant to the actions of essential oil than Gram-positive bacteria due to their hydrophilic constituents in the outer membrane.

Coccidiosis, a common parasitosis disease caused by protozoa of the genus Eimeria, leads to malnutrition and performance depression in poultry. There is an increasing interest in using essential oils against coccidiosis infection. The supplementation of essential oils led to a significant reduction of coccidian oocyst excretion and an alleviation of intestinal lesions in chicks. However, the underlying mechanisms still need to be elucidated.

 Anti-oxidative activity and carcass hygiene

Chicken body antioxidative stability could be improved by essential oils fed a blend of carvacrol, cinnamaldehyde, and capsicum oleoresin to Ross 308 broilers, and found a significant increase in the hepatic concentration of carotenoids and coenzyme Q10 at day 21 of age observed that malondialdehyde concentration in liver, duodenal mucosa, and kidney was significantly decreased by supplementing ginger power and thyme oil to broiler diet.

Poultry products are particularly prone to oxidative deterioration due to their high concentrations of polyunsaturated fatty acids. The reviews showed that thyme oil is effective in retarding oxidant degradation in poultry-derived products, such as meat and eggs. The possible reason might be the antioxidant activity derived from the phenolic OH group which acts as a donor of hydrogen interacting with peroxy radicals during the initial process in lipid oxidation and thereby inhibiting or retarding the hydroxyl peroxide formation. There was a linear relationship between the amount of total phenols and antioxidant capacity of medicinal plants. Rosemary, oregano, and sage of the Labiate family were also reported for their effective antioxidant activities in broiler meat. Carcass hygiene could be improved by essential oils, which should be attributed to their reducing the load of pathogens.A blend of carvacrol, thymol, eucalyptol, and lemon might reduce the Salmonella heidelberg-positive crops and subsequently reduce the cross-contamination in carcass processing.  A blend of caprylic acid and essential oils (trans-cinnamaldehyde, eugenol, carvacrol, and thymol) reduced Salmonella Enteritidis and Campylobacter Jejuni in cecal contents of birds, which indicates a less likelihood of microbial contamination of poultry meat and eggs. An interesting study showed improvement in hygiene conditions in poultry house via air disinfectants using thyme and peppermint oils individually as primary components.