The term “self medication” comes from human medicine. It is defined as “The consumption of a substance, without physician input, to compensate for any medical or psychological condition.”
Parasitism is one of the greatest disease problems in grazing livestock. Controlling parasites with drugs is challenging, particularly in recent times due to the rise in drug-resistant internal parasites. Evidence suggests that parasitized animals in the wild use natural plant secondary compounds as anti-parasitic agents. Can parasitized domestic sheep and goats also learn to use such compounds? If the answer is yes, they could learn to self-medicate with them and eat plant secondary compound-rich vegetation, either on rangeland or pasture, when needed, while having other nutritious and safe forages available to meet their nutri-tional requirements.
Wild animals apparently use medicinal herbs to treat illness. Unfortunately, the information available on self-medicative behavior in wild species is often anec-dotal. Controlled experiments in the wild may be constrained by animal welfare and preservation issues.
Based on his studies on primates, Michael A. Huffman defined a set of conditions to separate self-medication from normal feeding in the wild: (1) the animal should show signs of illness; (2) it should seek a substance that is not part of its normal diet and has no nutritional value; (3) the animal’s health should im-prove; (4) laboratory analysis of the substance should establish that enough active ingredients have been ingested to bring about the changes observed.
Effects of Parasitism
The first step of establishing self-medication should be that an animal shows signs of illness. Parasitism results in obvious signs of disease such as diarrhea, high body temperature, passivity, etc. Subclinical parasitism can also challenge the infected animal by reducing production, even when no signs of the in-fections are observed.
The most significant effect of gastrointestinal parasites on herbivores is a depression in food intake, which varies from 6% to 50% depending on the nu-trients in the diet. Anorexia may result from pain and discomfort from parasites or a consequence of hormonal changes due to disruption in the gastroin-testinal tract. Thus, anorexia is one of the first signs suggesting herbivores sense their parasitic burdens.
Impaired protein metabolism
Gastrointestinal parasites also impair protein metabolism. Worms cause damage to the lining of the gastrointestinal tract, which causes increased plasma leakage and sloughing of protein from the bowel. Parasitized animals also rely on their immune system to fight parasites and activating the immune system is costly in terms of protein metabolism.
Infusions of casein (milk protein) into the stomach improves the rate of protein retention of parasitized animals. Lambs infected with larvae of Trichostrongylus colubriformis and given a choice between two foods with different protein contents increase intake of protein-rich foods, which may help counter the losses of protein due to parasitism. Parasitized animals grazing a mixed grass-clover pasture increase the proportion of N-rich clover in their diet rela-tive to non-parasitized sheep. Thus, herbivores may also sense a parasitic infection through their increased need for protein.
Disrupted nutrient absorption
Internal parasites may also disrupt absorption and retention of other nutrients besides nitrogen such as minerals and vitamins. For instance, helminths interfere with the absorption of vitamin B12 and enhance cobalt deficiencies, which could also trigger signals that an herbivore is infected with internal parasites.
Toxins produced by internal parasites may also help the animal sense parasitism. For instance, parasite-derived molecules can activate some cytokines, which in turn negatively impact cells and their metabolism in the host animal.
Besides being aware of their parasitic burdens, a second step of self-medication is that after eating or using a certain medicinal plant, herbivores should experience relief from the upset or discomfort caused by the parasites. Animals are more likely to learn about the benefits of a medicine when they expe-rience illness or discomfort and then experience a medicine that leads to recovery.
How is self-medication knowledge acquired?
Individual foraging behaviors are mainly acquired by learning from social models, first from mother, then from peers, and from individual post-ingestive experiences. In the wild, the kinds of substances monkeys rub on their fur vary from site to site. Even when the same substances are available at several sites, the items a particular group uses for rubbing is likely to be different, suggesting a socially transmitted pattern of use. Leaf swallowing behavior in chimpanzees as a means of physically expelling intestinal parasites appears to have its origins in the opportunistic feeding behavior of some “pioneering” individuals.
The spread of self-medicative behavior within the group seems to be influenced by social models.
Unfortunately, learning from mom and the needs of her offspring may be in conflict. Mamber goats teach their kids to avoid intake of tannin-rich P. len-tiscus, but P. lentiscus has strong deworming properties. In fact local goat breeders tether parasitized animals close to P. lentiscus bushes. Therefore, a young parasitized mamber goat must learn on its own – sensing the benefits of eating P. lentiscus while parasitized and relying on its own experiences – that is contrary to mother’s teaching.
All the conditions listed above have been demonstrated for wild chimpanzees infected with internal parasites. Chimpanzees suffering from parasite-related diseases eat the bitter pith of the plant Vernonia amygdalina, which contain anti-parasitic compounds and is eaten by chimps at doses high enough to combat parasites.
Other plants selected by chimpanzees have medicinal effects to combat internal parasite at the doses commonly consumed:
1. Limonoids in Trichilia rubescens have antimalarial activity;
2. Thiarubines in Aspilia species have anti-parasitic and antibiotic properties;
3. Methoxypsoralen in Ficus exasperata is a strong antibiotic.
Some medicines found in nature for various parasites
Considerable attention has been given recently to the anthelmintic (deworming) properties of plant secondary compounds such as tannins, alkaloids and terpenes in plants consumed by livestock.
Livestock feeding on tannin-containing herbaceous species such as sulla, sainfoin, and Sericea lespedeza or browse, such as heather, a number of acacia species, and lentisk, have lower fecal egg counts than those eating plants of similar quality, or the same rations without tannins. Tannins impair larval es-tablishment and decreases reproduction of internal parasites.
Substituting concentrates with cassava hay – containing 13% hydrogen cyanide – resulted in impressive reduction of Eimeria oocysts excretion in Vietnam-ese goats. Coccidiosis is also alleviated in goats when they eat the fruits of Melia azedarach that contain both tannins and limonoids.
Neem extract reduces feeding activity of the tick larvae and reduces molting by 60%. When lambs infected with ticks eat azadirachtin, a neem limonoid, in sufficient amounts, it reaches the peripheral blood, decreasing blood feeding by ticks, and often kills egg-laying ticks after detachment.
Alcohol extracts of neem tree bark (Meliaceae) reduce blood parasites in rats. Limonoids in T. rubescens have antimalarial activity in chimpanzees. Further-more, chimpanzees may eat soils to enhance the biological actions of T. rubescens. They ingest a certain soil shortly before or after eating T. rubescens. This par-ticular soil (dominated by kaolinite) improves the anti-malarial activity of T. rubescenswhen when eaten together. Lastly, repeated sampling of the bitter-tasting anti-malarial agent chloroquine by malaria-infected mice reduced blood parasite and death in these animals.
When parasitized herbivores are faced with a choice between manure-contaminated and non-contaminated areas in a pasture, they avoid ma-nure-contaminated areas. Non-parasitized animals avoid manure as well, but this behavior is exaggerated in parasitized animals. Avoidance occurs even when parasite-rich pastures are high in nutrients.
When infected animals are forced to graze contaminated pastures they graze further from the soil surface than non-parasitized animals thus minimizing parasite intake. Horses grazing in highly stocked pastures exhibit a “latrine behavior”, whereas horses grazing on rangelands, where the parasite risk is lower, defecate randomly while grazing.
The salivary glands of ticks secrete molecules to decrease the ability of the host sensing the infestation. Many species of wild cattle live in tick-infested are-as, but tick loads are usually kept to very low levels, primarily by frequent self-grooming. Do ticks trigger self-grooming, inferring that cattle are able to identify infestations? Ample evidence from many studies on antelope in Africa strongly supports the idea that grooming bouts occur in response to an in-ternal cue that initiates grooming bouts at periodic intervals, resulting in removal of ticks before they attach and begin to feed.
An increased exposure to ticks, however, is also associated with increased grooming. The trigger for grooming may be histamine or the absorption of tick saliva released at biting. In other words, herbivores are aware of their tick burdens, although this awareness explains only a small part of their grooming behavior. The larger the herbivore, the less frequent are grooming bouts. Animals are well able to sense the presence of fleas, but grooming activity may be too energy-expensive to combat fleas.
Collectively, this information suggests herbivores are “aware” of the presence of parasites infecting their bodies and have a number of strategies to self-medicate to reduce the effects and strength of the infestation.
Pasture as Pharmacy:
Tannins, an Example of
based on material at www.extension.usu.edu/behave
Sheep grazing in a pasture seem like the definition of a simple, pastoral scene. But that pasture is more than just a sheep dining room, it’s a pharmacy too, where sheep can be taught to select their own medicine.
Dr. Juan Villalba, associate research professor of foraging behavior in the Department of Wildland Resources at Utah State University, and his colleagues have found that sheep struggling with gastrointestinal nematode infection purposely select foods that act as medicines to help alleviate what’s hurting in-side.
Tannins are secondary compounds produced by plants for their own purposes. Among other things, tannins can provide self-defense against foraging her-bivores; can attract pollinating insects for reproductive purposes; can help resist dehydration and nutrient stress, and can provide protection from ultra-violet radiation.
They also have an astringent effect that sheep usually don’t like. They reduce the amount of forage sheep consume as well as inhibit protein digestibility. But at the right doses they also reduce the number of parasites in a sheep’s gut. A series of papers released by Villalba describe how animals with parasites prefer plants with secondary compounds.
Tannins, sodium bicarbonate, polyethelene glycol, and sericea lespedeza are just some of the compounds that help ward off ailments such as internal para-sites, Villalba says.
“What the animals do is like what we humans do when we have an upset stomach or heartburn,” he explains. “We seek to lower our discomfort, but when it’s done, we don’t keep selecting the medicine. The animals seek to return to a state of homeostasis or internal stability, so when the medicine is no longer needed, they have specific behaviors that respond to the end of a particular discomfort.”