Garlic, Allium sativum L.,, is the quintessential herb that has been applied in the horse industry with purely a human prescribing model. Horse physiology has significant differences that are particularly noteworthy in relation to the safe use of garlic. This is one herb in which natural is not synonymous with safe.
Evidence based research cites a number of medical actions for Allium sativum, including hypocholesterolemic, digestive stimulant, antimicrobial and antiplatelet (Braun & Cohen, 2007). However, when considering the use of Allium sativum for horses it would be prudent to consider the mechanisms of action of each of these particular medical actions in context of equine physiology and management practices as opposed to human prophylactic use.
1. Hypocholesterolemic: A randomized, double-blind, placebo-controlled intervention study (humans) reported that aged garlic extract (AGE) supplementation was effective in lowering plasma concentration of total cholesterol. In vitro (rat) research found hydrophilic and hydrophobic compounds of garlic inhibit cholesterol synthesis by deactivating the associated reductase enzyme (Braun & Cohen, 2007) (Yeh & Liu, 2001). The deactivation of the enzyme is implicated in a rare, serious side effect of human statin monotherapy causing rhabdomyolysis (Chang, Staffa, Parks, & Green, 2004).
Potential Adverse Herb-Drug Interactions: Horse owners use herbs to treat specific physiological and behavioural conditions, often without veterinary consultation (Hoffman, Costa, & Freeman, 2009). There is increasing potential for adverse herb-drug reactions and toxicity when relying on human traditional herbal medicine principles and the lack of a Good Manufacturing Code for animal herbs to manage such problems as adulteration, contamination, deterioration and mis-identification.
2. Digestive Stimulant: In vivo (rat) research indicates garlic increases the bile flow rate and digestive enzymes of the pancreas and small intestine (Platel & Srinvassan, 2000) (Ramakrishna Rao, Platel, & Srinivasan, 2003).
Equine Management Practice: Intermittent feeding practices and the feeding of two large feeds a day, traditionally high in grains and consumed rapidly, have been causally linked to the occurrence and severity of equine gastric ulcer syndrome due to the high acid profile (Reese & Andrews, 2009). Noting the herb’s digestive stimulant action, garlic may potentiate the high acid profile.
3. Antiplatelet: Five of six human trials reported a positive effect of garlic to reduce platelet aggregation. Studies have indicated that a polysulfide constituent inhibits both platelet aggregation and calcium ion mobilisation induced by blood coagulation, reducing the cystolic calcium concentrations (Vilahur & Badimon, 2013)
Nutritional secondary hyperparathyroidism: “Millers disease” or “Big head” tends to occur when horses are fed diets that are low in calcium and high in phosphorus (Secombe & Lester, 2012).
4. Antimicrobial: In vitro research has shown garlic has antimicrobial effects against a wide variety of gram-positive, gram-negative and acid-fast bacteria (Braun & Cohen, 2007). Propionate and butyrate volatile fatty acids where increased by garlic in rumen batch fermentation trials (Busquet, Calsamiglia, Ferret, Carro, & Kamel, 2005).
Equine Gastric Ulcer Syndrome: Increased production of butyrate and priopionate volatile fatty acids may contribute to the formation of gastric ulcers (Metayer, et al., 2004).
Adverse effects of garlic and drug interactions are well documented in humans including urticaria and gastrointestinal irritation (Borrelli, Capasso, & Izzo, 2007). An allergen, alliin lyase, has been identified and immunologically characterised (Kao, et. al., 2004). Pearson, Boermans, Bettger, McBride, & Lindinger (2005) found that the potential exists for horses to develop garlic toxicosis when chronically fed garlic. However, they concluded further research was necessary to determine a safe dose that would be associated with beneficial effects.
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Borrelli, F., Capasso, R., & Izzo, A. A. (2007). Garlic (Allium sativum L.): Adverse effects and drug interactions in humans. Molecular Nutrition Food Research, 51, 1386-1397.
Braun, L., & Cohen, M. (2007). Garlic. In L. Braun, & M. Cohen, Herbs & Natrual Supplements. An Evidence-based guide (2 ed., pp. 331-337). Marrakville, NSW, Australia: Elsevier.
Busquet, M., Calsamiglia, S., Ferret, A., Carro, M. D., & Kamel, C. (2005). Effect of garlic oil and four of its compounds on rumen microbial fermentation. Journal of Diary Science, 88(12), 4393-4404.
Chang, J. T., Staffa, J. A., Parks, M., & Green, L. (2004). Rhabdomyolysis with HMG-CoA reductase inhibitors and gemfibrozil combination therapy. Pharmacoepidemiology and Drug Safety, 13(7), 417-26.
Hoffman, C., Costa, L., & Freeman, L. (2009). Survey of Feeding Practices, Supplement Use, and Knowledge of Equine Nutrition among a Subpopulation of Horse Owners in New England. Journal of Equine Veterinary Science, 29(10), 719-728.
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Pearson, W., Boermans, H. J., Bettger, W., McBride, B. W., & Lindinger, M. I. (2005). Association of maxiumm voluntary dietary intake of freeze-dried garlic with Hienz body anemia in hroses. American Jornal Veterinary Research, 66(3), 457-465.
Platel, K., & Srinvassan, K. (2000). Stimulatory influence of selected spices on bile secretion in ras. Nutrition Research , 20(10), 1493-1503.
Ramakrishna Rao, R., Platel, K., & Srinivasan, K. (2003). In vitro influence of spices and spice-active principles on digestive enzymes of rat pancreas and small intestine. Nahrung/Food, 6, 408-412.
Reese, R. E., & Andrews, F. M. (2009). Nutrition and dietary management of equine gastric ulcer syndrome. Veterinary Clinical Equine, 25, 79-92.
Secombe, C. J., & Lester, G. D. (2012). The role of diet in the prevention and management of several equine diseases. Animal Feed Science and Technology, 173(1-2), 86-101.
Vilahur, G., & Badimon, L. (2013). Antiplatelet properties of natural products. Vascular Pharmacology, 59(3-4), 67-75.
Yeh, Y., & Liu, L. (2001). Cholesterol-lowering effect of garlic extracts and organosulfur compounds: human and animal studies. Journal of Nutrition, 131(3), 9895-9935.