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 potency and amount of marijuana used, the half‐life can range from about 20 hours to 10 days.
Systems most commonly affected include:Nervous system – depression, ataxia or other abnormal behavior. Signs of CNS stimulation sometimes occur in dogs and may possibly occur in horses.GI – signs of colic may occur.
Synthetic cannabinoids.
Substances with CNS depressant effects (opioids, benzodiazepines or others).
Amphetamines or other substances with CNS stimulant effects.
Careful questioning of the client may reveal exposure to marijuana.
Inspection of pastures or feed material may reveal an exposure source.
THC is detectable by LC‐MS.
In humans, metabolites of THC may be detectable in urine for approximately 3–15 days after using marijuana.
Treatment objectives are to provide supportive care for affected animals and prevent absorption of marijuana. Removing the source of the marijuana is also important.