Hundreds of observations of animals in the wild suggest that they sometimes self-medicate, ingesting plants or other substances with medicinal actions to make themselves feel better. As a veterinarian with decades of experience treating animals that eat all the wrong things, I find this idea intriguing; are these episodes due to coincidence, instinct, or a sophistical knowledge of herbal medicine? And how do they know what would help what ails them?
In a book called “Wild Health”, author Cindy Engel cites hundreds of observations in which elephants travel hundreds of kilometers to find herbs that help with birthing and chimpanzees ingest clay to settle their upset stomachs. Engel’s conclusions are doubtlessly exaggerated, as she seems to believe every claim advanced by folk medicine or herbal practitioners, but many of these observations have held up to scientific scrutiny. If we knew how animals choose their own treatments, maybe it would change how we medicate our pets (and ourselves).
Simple association seems like the place to start in order to understand self-medication. If it feels good, do it. (Pardon me while I self-medicate with my morning cup of coffee!) When a chimp has a stomach ache after eating too much of a fruit that contains a low level of a toxin, eating a several mouthfuls of clay would help coat the stomach and intestine and absorb the toxin, relieving the intestinal distress. (The kaolin in Kaopectate is a form of clay used to settle the stomach, although recent studies suggest that it isn’t very effective) . Regardless of why the primate first tried the “clay cure”, the association of eating clay and feeling better would reinforce this self-medicating behavior. The inverse is known to be true as well: Eating something that makes the animal (or person) sick will often cause an “acquired food aversion” so that the mere smell of the item causes a loss of appetite. This reaction apparently does not require a conscious association between ingestion and nausea, although it is reasonable to assume that an intelligent animal would also remember that they were sick after eating a particular food. Sheep farmers have used thisprinciple to discourage coyotes from attacking their flocks; a dead sheep is laced with lithium (which causes intense nausea and vomiting) and left where coyotes are likely to feed. After getting sick on mutton, the coyotes avoid sheep and concentrate on the rabbits and rodents that form their natural diet.
If an association between eating something and feeling better is an obvious answer, a new question arises: What if the self-medicating animal chooses something that has an unpleasant taste or sensation, but cures some illness days later? Wouldn’t the animal associate the immediate unpleasantness of chewing a bitter herb with illness, rather than cure? My guess is that individual animals vary widely in their willingness to experience something unpleasant in return for “delayed physical gratification”. Many pet owners tell me that their dog or cat takes their medication willingly, as if they knew that it was going to make them feel better. On the other hand, most animals will do their best not to take their drugs, even when it will make them feel better eventually. It is possible that the time that elapses between the medication and the physical improvement is critical; if taking a bitter pill today makes the animal feel better the next day, the association is stronger than if a week of medication is required before improvement. (These same principles apply to humans; a recent study found that nearly half of the people prescribed medications for serious chronic diseases failed to take their medicine. Perhaps they just couldn’t tell that it made them feel better, even though they may have understood the purpose of the medication intellectually).
Another consideration is that the body can sometimes make these associations better than the brain. When mice with a fatal genetic immune disease are given their immune suppressive drugs flavored with the distinctive taste of saccharine for several weeks, then their disease can be controlled with saccharine alone. Apparently the renegade immune cells themselves “remember” the association between the taste of the artificial sweetener and the effect of the drug.
There could also be a cultural component to self-medicating in wild animals. The most credible observations of animals taking their own cures are from elephants and chimpanzees. Not only are these species well-studies (due to their appealing, human-like traits), but they are also highly social animals in which cultural transmission of behaviors is known to occur. If your mom tells you to “take this and you will feel better”, then you do it—even if you are a chimpanzee.
Do our pets show evidence of self-medicating? There are several well-known examples of dogs and cats seeking things that might make them feel better. Cats will often lick bricks or eat cat litter when they are anemic, and we assume that the body “wants” more iron-containing minerals to build up the number of red blood cells. Although this makes some sense in the wild (where poor diet might be a cause of anemia), pets certainly get all the iron they need from their food, and anemia in cats and dogs is almost always caused by other diseases that can’t be helped by eating more iron. But the body doesn’t know that it has Autoimmune Hemolytic Anemia, it just knows that the blood is too thin. Similarly, eating grass is a common behavior when a dog has an upset stomach, but it usually just makes things worse and causes vomiting. In the wild environment, throwing up intensely for a few days might be lifesaving when a wolf eats a bit of toxic, spoiled deer meat, but In domestic dogs purging on grass is likely to delay recover from routine stomach upset. These common behaviors are likely hard-wired physiologic responses shaped by evolution for the wild environment, and not “self-medicating” actions learned by association.
A recent study was done to determine the frequency of the many various causes of stomach upset in dogs. The conclusion was that the great majority of all acute intestinal problems in dogs were due to eating all manner of irritating, toxic, fat-laden, and spoiled materials. Perhaps if they lived in the wilderness there would be less bad things for them to get into, and maybe they could find the right herbs to settle their stomach when they needed it.

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