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Author: Benjamin C. Moeller, PhD, DABT
Consulting Editor: Dionne Benson, DVM, JD
Chapter 10 Marijuana
Marijuana comes from the Cannabis sativa plant.
The plant has a long history of use by humans for recreational, medicinal, religious, and industrial purposes.
Marijuana is also known as pot, weed, grass, hemp, hashish, and many other names.
Cannabis sativa has very distinctive leaves, with five to seven long leaflets on each stem.
The plant also produces small green flowers, also called buds, and fruit that is oval and flat (see Fig. 10.1).
The two major active compounds contained in the marijuana plant are CBD and THC.
CBD products are widely available in many states as oils, creams, serums, and other products and may contain THC.
THC is the compound responsible for producing psychoactive effects.
The concentration of THC is variable, depending upon growing conditions, plant genetics, which parts of the plant were ingested, whether the plant was fresh or dried, processing after harvest, and time between harvesting of the plant and use (see Fig. 10.2)
Exposure by ingestion of plant material or inhalation of smoke produced by burning the plant may be harmful to equines.
The systems most affected by exposure to marijuana in horses are the neurological and GI systems.
Prohibited substance under USEF, FEI, AQHA, and ARCI regulations.
Mechanism of Action
Not specifically known in horses.
In dogs, THC is a CB1 and CB2 receptor antagonist.
The receptors for CB1 are located in the CNS and, when exposed to THC, may cause adverse effects on the mind and body.
Colic may result due to irritation of the lining of the GI tract after ingestion of marijuana.Fig. 10.1. Marijuana bud (Cannabis sativa).Source: Photo courtesy of Seth Wong, Industrial Laboratories, Wheat Ridge, CO.Fig. 10.2. Ground marijuana plant material.Source: Photo courtesy of Seth Wong, Industrial Laboratories, Wheat Ridge, CO.
Toxicokinetics
Marijuana is absorbed through the GI tract after ingestion or absorbed through the respiratory system after inhalation of marijuana smoke.
Distribution is not specifically known in equines but in dogs, marijuana is highly distributed to liver, kidney, brain and fat.
Metabolism is not specifically known in equines but in dogs it occurs primarily in the liver. Metabolites of marijuana do cross the blood–brain barrier.
Elimination is not specifically known in equines but is through the urine and feces (through bile) in small animals.
Toxicity
Not specifically known in equines.
Toxic dose depends on many variables, including amount and route of exposure as well as potency of the marijuana ingested or inhaled.
Systems Affected
Nervous system – CNS depression or other abnormal behavior in horses. In dogs may include ataxia, disorientation, and coma. CNS stimulation occurs in some dogs.
GI system – colic.
Risk Factors
Marijuana use by an owner/trainer/handler may expose an animal to it.
Consumption of marijuana plant parts (either deliberately or accidentally) or inhalation of second‐hand smoke.
Historical Findings
Some horses may be deliberately exposed to marijuana.
Owners may not admit possible exposure to an animal as they may be hesitant to disclose marijuana possession or use.
Some animals may experience exposure in pastures or in baled hay or other feeds or supplements.
Location and Circumstances of Poisoning
Marijuana use by humans is widespread, and therefore cases can occur anywhere in the country.
Many states have legalized recreational use of marijuana, making exposure to animals more likely.
Onset of clinical signs is not specifically known in horses, but in humans the onset of effect of THC occurs in 30–60 minutes post ingestion and 6–12 minutes post‐inhalation.
In dogs, clinical signs are typically seen within 30–60 minutes of exposure but may be as soon as 5 minutes or as long as 12 hours post‐exposure.
In humans, depending on the
