Self-medication is well documented among Chimpanzees. Chimps with severe intestinal infections, either of roundworms or protozoas, chew on the "bitter pith" of certain bushes, especially a shrub called Vernonia amygdalina or bitter leaf that is widely used by human healers:
When ingesting the pith from young shoots of V. amygdalina, chimpanzees meticulously remove the outer bark and leaves to chew on the exposed pith, from which they extract extremely bitter juice and residual amounts of fiber. The amount of pith ingested in a single bout is relatively small, ranging from portions 5-120 cm long by 1 cm in width. The entire process, depending on the amount ingested, takes anywhere from less than 1 to as long as 8 minutes. Mature conspecifics in proximity to individuals chewing Vernonia bitter pith (or leaf swallowing, described below) show no interest in ingesting the pith. Infants however, have on occasion been observed to taste the pith discarded by their ill mothers. Thus, from a very young age individuals in a group are exposed to the plant-eating behaviors and to the plants and their context of use.
In general, when an individual chews the bitter pith of V. amygdalina, that individual is in ill health, as evidenced by diarrhea, lethargy, weight loss, and nematode infection. In two cases recorded in detail, recovery from such symptoms was evident 20-24 hours after the individuals chewed the bitter pith. In one of these cases, the eggs per gram (EPG) of feces level of an O. stephanostomum infection could be measured; it was found to have dropped from 130 to 15 in 20 hours. Seven other individuals, monitored over the same period, had O. Stephanostomum infections but were not observed chewing bitter pith; these individuals did not register such a dramatic drop in EPG. In these seven control cases, O. stephanostomum EPG levels actually increased over time. The rise in EPG levels represents the overall trend for increased reinfection by O. stephanostomum at the beginning of the rainy season. The ingestion of the bitter pith of Vernonia appears to affect nodular worm reproductive output and provide relief from symptoms of related gastrointestinal upset.Other chimps swallow, whole, certain very fuzzy or prickly leaves, which seem to sweep worms out of their intestines. It is fascinating to think how this knowledge might have arisen and spread.