Last Updated 12 Feb 2021

Humans and Parasites: Why Can’t We All Just Get Along?

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The definition of a parasite is: “an organism that lives on or in an organism of another species, known as the host, from the body of which it obtains nutriment. ” (Dictionary. com,2012) There are several kinds of parasitic relationships in the world. Mutualism is one of them. This occurs when each member of the association benefits the other. Can humans and parasites have a mutualistic relationship in medicine? Dating back to the B. C. era it has been believed that parasites, most commonly leeches and maggots, were the cure for various maladies.

Leeches at one time were thought to cure everything from obesity to mental illness. In the early 20th century there was no longer a need for parasites with the medical and technological advances we were making. Their popularity has begun to grow since the 1980s and is seen more often in medical practice.

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Thesis Statement

Some parasites, over the course of history, have proven to hold a symbiotic relationship with the human body in medical applications. Prior to the days that we think of as modern medicine, parasites were used for many things in medical practice. One example is leeches.

The use of leeches in medicine started around 2,000 B. C. with the Greek and Roman physicians. During the medieval times they were very prevalent in the household medicine cabinets as they were used on a regular basis to treat many different ailments. (Jaffari, M. 2012) As modern medicine emerged parasites were viewed as a bad thing. Things we did not want inside us or around us. We discovered many ways to eradicate them from our bodies, thus also eradicating them from medicinal use. In more recent years testing is being done especially with the helminth (worm) species of parasites.

These parasites are being used to treat medical conditions such as ulcerative colitis, vascular diseases and allergies, just to name a few, with very interesting results. It’s beginning to seem as if humans and parasites in some applications can benefit each other after all. The use of parasites to treat ailments has been around for centuries. It is thought to date back as far as the Stone Age. The first written reference of this was in a medical poem by Nicander of Colophon (185-135BC). This poem referenced leeching (bloodletting) in particular. It is believed that all ancient civilizations used bloodletting in their medical practice.

In ancient Greece they believed in the Humoral Theory. This theory stated that the body was made up of four humors; these were blood, phlegm, black bile, and yellow bile and you became ill because these humors were out of balance. The leeches were used to keep these humors in balance; thus, keeping the patient healthy. During the early nineteenth century in Europe it was not uncommon to find leeches in people’s medicine cabinets. The use of leeches during this time was very prevalent and they were believed to be almost a “cure all”. Bloodletting became so popular in Europe, that the commercial trade in leeches became a major industry.

France during this era suffered such a shortage of leeches that they had to import 41. 5 million of the parasites. Due to the extremely high demand for this product, the medicinal leech almost became extinct in Europe. When the numbers being harvested from the wild became insufficient, some countries started the practice of leech farming. (El-Awady, 2003) Leeches were not the only parasite that received high acclaim for medical uses. The maggot has been known for its healing ability since the 16th century. In 1917, Dr. William S. Baer made an incredible observation while working as a physician during World War I.

Two soldiers were brought into the hospital, both having compound fractures to their femurs and very large wounds to the abdomen. It was discovered that these two soldiers had been wounded in battle seven days prior but because of the over growth in the area the two men were over looked when the wounded were first recovered. At that time the mortality rate for people with compound fractures was as high as 85%, even with the best medical care. Despite the lack of food and water, being exposed to the elements during those seven days, and the severity of the wounds, the doctor found that the men had no fever, and there was no evidence of sepsis.

When their clothing was removed the doctor discovered that the wounds were filled with blow fly maggots. The maggots were washed from the injuries to find that the exposed bone and tissue were in perfect condition. Cultures were taken with very few bacteria still present. Ten years later Dr. Baer would use this theory on four children with osteomyelitis which is a disease causing inflammation of the bone and bone marrow usually caused by reoccurring bacterial infections. (Dictionary. com, 2012) All four children had had several surgeries for this, all of which had failed to heal.

The doctor obtained maggots from his neighborhood and trying to copy most of the conditions that he observed with the soldiers he attempted his experiment. He used no chemical antibiotics, no iodine to clean the area, surgery was done bare handed and using only water. This way if the wound healed it would be strictly due to the maggots. Dr. Baer continued his treatment of maggot therapy to find that in six weeks the wounds these children suffered from had healed completely. In the spring of 1929 more cases were submitted using this form of treatment with the same results. (Baer, 1931)

During the early to middle 20th century the technological and medical advances we had made took the place of “medieval medicine” and along with it the use of parasites in medical practice. Maggots were removed from medicinal use in the 1930s. With the introduction of new antibiotics and better surgical techniques we found that we had a much better grasp of healing and inhibiting bacterial infections and we no longer required the use of maggots to heal open wounds. Although, in 1989 there were findings that maggot therapy was superior in certain cases to antibiotic therapy for eradicating a bacterial infection.

The first modern clinical studies of maggot therapy were started at the Veterans Affairs Medical Center in Long Beach, CA, and at the University of California, Irvine. The results of these studies showed that the use of maggots today is still an effective and safe treatment for certain types of wounds. The reports also stated that there is no reason to use maggots as a last resort. There are published reports of a limb salvage rate of over 40% in pre-amputation maggot therapy. When this therapy was used even earlier in the course of treatment the results were even more dramatic. Sherman, 2010)  Leeches have always had their place in history as we have seen. Leech therapy was used up until the 1960s when it too was removed from medical practice. Bloodletting of sorts was taken over by modern day phlebotomy. So we again saw no need in using the old practices with the advances we had made. In the 1980s, leech therapy made a big comeback by plastic surgeons that used leeches to relieve venous congestion. If this congestion was not treated, the blood would clot and arteries that bring the blood and nourishments to the tissues would become plugged and the tissues would die.

This is where the leech therapy would come in handy. Once they were applied to the site, the leeches sucked the excess blood, which helped to reduce the swelling in the tissues and promoted healing by allowing fresh, oxygenated blood to reach the area until normal circulation could be restored. The leeches also assisted in the healing by the secretion of an anticoagulant known as hirudin which would keep the blood in the area from clotting. The present day medical world is once more looking towards some parasites as very useful tools in certain areas of medical practice.

The leech’s saliva contains a number of chemical compounds that are very useful in medicine. We already mentioned the anti-coagulant hirudin. This has been studied for use in helping to prevent heart attacks and strokes. The leech naturally secretes a local anesthetic that it uses when attaching to the host to avoid detection. This has proven helpful to lessen pain in some patients. Doctors and patients have seen the benefits of the vasodilator and a prostaglandin secreted by this parasite as it is known to reduce swelling in situations of inflammation.

The leech’s gut harbors a bacterium known as Aeromonan hydrophila which aids in its digestion of ingested blood and produces an antibiotic that kills other bacteria that can cause tissue decomposition. The most present day uses for leech therapy are digital replants, skin grafting, hematomas, and some leeches are being raised specifically for pharmaceutical uses. Leech therapy is proving to be useful for a number of other conditions including osteoarthritis, ophthalmology and dermatology.

As with any treatment there can be side effects. Leeching is no exception. A few of the side effects associated with leech therapy include infection, excess blood loss that may require blood transfusion, and allergic reactions. Science has developed a “clean” leech which helps to lessen the number of bacterial infections. These are leeches specially raised to not carry the normal bacteria that regular leeches carry. As an alternative to real leeches, Scientists at the University of Wisconsin-Madison have developed a mechanical leech.

This invention mimics the action of the live leech allowing for fresh blood to flow through the wound. (El-Awady, 2003) Maggots have also found their niche in today’s medical world. These often thought of as “icky” parasites are used in some wound care applications. This therapy even has a name, maggot debridement therapy or MDT. Maggots are very efficient healers of wounds as we have seen. The reason for this is that the maggots eat only the dead tissues and leave the healthy, living tissues intact. They also excrete compounds which inhibit and may even kill bacteria.

This is extremely useful in areas where there is poor blood supply; these areas are unable to benefit from antibiotic therapy as it is unable to reach the required area in high enough concentrations to do what was intended. The most common uses for maggots today are wound debridement and diabetic foot ulcers. The side effect patients are most at risk for is bacterial infections. As with the leeches scientists are able to raise “clean” maggots, lowering the number of infections. More recently added to the list of helpful parasites has been the hookworm.

These parasites live in your gut and thrive on the nourishment that you take in. Hookworm infection is actually known to have a beneficial effect on health. There are published studies that show hookworms can lessen or even cure allergies such as inflammatory bowel disease, food allergies and even asthma. The saliva that the worms produce changes the immune system and keeps your body from overreacting to the substances that cause the allergies. (Sifferlin, 2012) There are several uses for hook worms being studied presently.

These are; ulcerative colitis, allergies and asthma control. Some of the outcomes have been very positive. Because the worms feed from the same nutrients that you are taking in the largest risk with hookworm infection is anemia. In third world countries this can be a devastating side effect but, in the western world there is less concern about this issue.


Parasitic mutualism occurs when each member of the association benefits the other. Our ancient ancestors seemed to have made good use of the human parasite relationship.

As technology increased we turned from the “old way” of doing things and disregarded any use of parasites in medical practice. But, did our ancient ancestors know something that we are just now starting to grasp once again? With the testing we are seeing more recently it seems that it is possible for these creepy crawlies that make us feel so squeamish could be very helpful to us and our health. Even with the positive results of the use of parasites in medicine there are still many mysteries regarding the symbiotic relationship between parasite and man.

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Explore how the human body functions as one unit in harmony in order to life

Humans and Parasites: Why Can’t We All Just Get Along?. (2017, Mar 07). Retrieved from

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