Parasites get a bad rap. By definition, parasites form a relationship that's characterized by exploitation and dependence. They take advantage of their often-unwitting hosts' hospitality, often leaving them sick and malnourished -- but usually not dead. Some of the creepiest examples of parasites turn their hosts into zombies, and like something out of a horror movie, they keep them alive while consuming them from the inside out.
Parasites aren't all bad, though. In fact, some of them can actually be quite beneficial to humans and other living creatures. Five thousand years ago, doctors in Egypt thought leeches could cure a wide range of ailments, from headaches to flatulence. That might sound crazy now, but we actually still employ leeches to help reattach severed fingers and to treat circulation disorders (and Demi Moore claims that leech therapy keeps her looking young) [source: Nature].
Since the days of ancient Egypt we've learned much more about parasites and the ways they interact with humans and other creatures. As much as we hate to admit it, parasites are just as important to the natural order of things as any other living creatures, and some scientists have posited that removing parasites from modern life can have serious consequences on human health. Humans have long been interested in turning the tables on parasites and getting them to help heal us, and as we get to better understand parasites we are able to use them in farming and other applications.
In this article, we'll take a look at some of the different ways parasites can be beneficial to humans, animals and even plants. And we'll also explore a couple of examples of symbiosis -- when the two species form a mutually beneficial relationship.
Perhaps the No. 1 area in which humans get parasites to do our dirty work is out on the farm. As anyone with a vegetable garden will attest, insect pests can wreak havoc on everything from fruit trees to squash, and they cause billions of dollars in crop losses each year. To deal with insect problems, many conventional farmers opt for the nuclear option, spraying their fields with poisonous insecticides. The problem: Chemicals that are poisonous to tiny creatures like insects are generally bad for humans, too. But one of the few non-toxic weapons that farmers have in the fight against crop-killing bugs is parasites.
Natural biological control isn't exactly a new technology -- it has, of course, been around for millions of years -- but employing beneficial insects to keep pest populations down is a relatively new practice. Some "beneficials" are merely predators, and they simply prey on the insects that are harming the crops. Others are parasites, which have a different and often creepier relationship to their host.
Aphids, small lice-like insects that reside under the leaves of plants, are some of the most harmful pests faced by fruit and vegetable farmers. They can be attacked using insecticides, but natural parasites can often be even more effective, because they have the ability to seek out aphids that might somehow avoid the spray. One such example is the Aphidius ervi parasite, which sneaks up on unwitting aphids, lays an egg in them, and then once the egg hatches, the larvae consumes the aphid from the inside out [source: Arbico Organics]. Now, that's a type of biological warfare we can get behind!
Parasites are often employed as a last resort, like in Thailand in 2010, when a massive infestation of invasive mealy bugs threatened that country's cassava crop [source: Than]. When no other pest-control methods worked, farmers engineered a sting operation, bringing in parasitic Anagyrus lopezi wasps, which, like the aphid parasites, lay their eggs right inside the mealy bugs' bodies. (Notice a theme yet?) Thai officials responded swiftly, and the parasites were very successful in controlling the mealy bug outbreak [source: Bellotti].
Most sane people would never intentionally infect themselves with a parasite -- especially some nasty stomach worm. But what if we told you that some types of parasites could actually help you? In recent years, scientists have discovered that certain parasites have the ability to interfere with autoimmune diseases. Of course, that doesn't mean that just anyone should go out and intentionally infect themselves with stomach worms. But in some select cases where the benefits outweigh the costs, getting a parasite is a legitimate source of medicine.
One of the pioneers of this type of radical parasite therapy research is Tufts University gastroenterologist Joel Weinstock, who had a revelation of sorts when exploring the question of why diseases, from asthma to multiple sclerosis, are on the rise in developed countries but not in undeveloped parts of the world. Weinstock discovered a possible answer: worms [source: Baker].
Weinstock's theory -- which is still being tested and hasn't yet been proven -- is that there's a direct correlation between a lack of intestinal worms and a rise in autoimmune diseases. In developed countries like the United States we've done an excellent job -- some would say too good a job -- avoiding parasitic worms, but we may be paying the price in the form of other, even more harmful diseases.
Weinstock began thinking about helminthic therapy in the early 1990s, when he noticed how prevalent inflammatory bowel disease had become in North America. At the same time, he realized that parasitic worms, or helminths, have a unique effect on their human hosts. Instead of inducing inflammation (the body's normal response to invasion), they actually calm the immune system. According to the theory, because people have lived with helminths through much of history, the human immune system has evolved to fight them, and when worms are removed entirely, the body's immune system turns against itself. Helminthic therapy, or worm therapy, may emerge as a legitimate field of medicine, but it's still very new and few studies have been done to date [source: Velasquez-Manoff].
Parasites have many talents. Some researchers believe that their curative abilities aren't limited to autoimmune diseases, like Crohn's disease and inflammatory bowel disease. Some intestinal worms are also believed to cure allergies, which share some notable characteristics with autoimmune diseases. Some people claim that our old friend the hookworm has the ability to cure everything from allergies to hay fever to asthma -- but your allergies would have to be pretty bad to knowingly infect yourself with blood-sucking worms.
A gentleman named Jasper Lawrence made worm therapy for allergies famous a few years ago. Suffering from debilitating asthma and allergies, Lawrence heard about the theory that hookworms could cure allergies, so he traveled to Africa and walked around with his shoes off in several open-air latrines. After successfully contracting hookworms (and probably a lot of other nasty bugs), Lawrence reported that his allergies had subsided, and he recently told the public radio program Radiolab that he hasn't had an asthma attack or allergy symptoms since his visit to Africa [source: Radiolab].
Convinced that hookworms are the answer to the world's allergies and asthma, Lawrence -- who isn't a doctor -- returned to North America and began shipping orders of hookworms to allergy sufferers, delivered in the form of a patch, for about $3,000 per treatment. But when the Food and Drug Administration caught wind of Lawrence's little side project, he fled to Mexico and then flew to England, where he was born [source: Adams].
Lawrence's story is rather gross and definitely a bit sad, but the underlying fact is that intestinal worms might provide important clues about how allergies work. Because of new research, as well as personal stories like Jasper's, the hygiene theory, -- which states that cleanliness and the lack of childhood exposure to bacteria and parasites leads to increased incidents of allergies and autoimmune diseases -- is gaining wider acceptance [source: Baker]. Several different studies are currently underway to look at how parasites like hookworms might be able to cure allergies and asthma, but nobody has definitively proven that hookworms are the answer.
Humans aren't the only ones that can benefit from parasites; in the animal kingdom there are several examples of two different organisms living in close association with one another. In cases where one organism leaches resources from its host, it's usually defined as a parasite, but inter-species relationships are rarely quite so black and white. Symbiosis is when organisms of different species are able to work together for mutual benefit, transcending the parasite-host relationship.
One of the more famous examples of symbiosis is the red-billed oxpecker, a medium-sized bird found in sub-Saharan Africa. The oxpecker rides on the back of large mammals, like rhinos or water buffalos, feeding on the ticks that are found on their host's back. So, in this peculiar relationship, the oxpecker gets a free ride and a meal, while relieving the host from an unwanted blood-sucking parasite. Everybody wins, right?
Typically, the oxpecker-mammal relationship has been thought of as symbiotic or mutualistic, but a recent study published in the journal Behavioral Ecology showed that while the birds do indeed benefit from their relationship, they don't significantly decrease the number of ticks on their host. And to make matters worse, from pecking at the ticks, the oxpeckers can slow down the healing time of wounds, and they can also remove earwax. Oxpeckers feed on blood (that's of course why they're so interested in ticks), and they have been observed sucking the blood of their hosts and even opening new wounds. Suddenly this relationship is starting to sound a little more one-sided [source: Weeks].
There's one more major factor that should be taken into account, though: When oxpeckers rest on the backs of large mammals, they can serve as an early warning if predators are approaching. So are they parasites after all? Let's just say that oxpeckers have a complicated relationship with their hosts [source: Nature].
If you think the relationship between the oxpecker and water buffalo is complicated, wait until you take a look at the dirt beneath your feet. By definition, a parasite is something that gets nourishment, or in some other way benefits, at the expense of its host. But sometimes the relationship can be mutually beneficial.
Earlier in this article we looked at the ways in which farmers employ parasitic wasps and other insects to be their foot soldiers in the battle against unwanted insects. But many farmers are similarly dependant on certain of types of fungi, yeasts and molds, many of which are parasites that combine to produce nutrient-rich soil at the expense of the plants they feed on. In some cases, fungi and plants form a symbiotic relationship, similar to that of the oxpecker and water buffalo, which benefits both parties.
One such example is mycorrhizal fungi, which can be found at your typical garden supply store. Mycorrhizal fungi colonize the roots of plants, where they absorb mycelium, moisture and carbohydrates from the plant. In turn, they supply the plant with nutrients from the surrounding soil, making it easier for the plant to absorb essential minerals and also helping to protect it from harmful pathogens. The symbiotic relationship between mycorrhizal fungi and plants is one of the most prevalent on Earth, as it exists in more than 90 percent of all vascular land plants [source: New York Botanical Garden].
The phenomenon isn't limited to fungi, though. Rhizobium, a common type of soil bacteria, forms a similar relationship with soybean plants. By itself, the soybean plant can't fix nitrogen, so it depends on the rhizobium bacteria, which is found in the roots of the plant, to fix atmospheric nitrogen and make it available to the plant. In return, the bacteria, like mycorrhizal fungi, receive carbohydrates, which the plant produces via photosynthesis.
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Author's Note: 5 Reasons Parasites Are Beneficial to the Earth
Like maggots and mold, parasites are one of the things that naturally inspires a gag reflex, and rightly so. But I was actually quite pleased when I got this assignment, because I had read about some of the research on hookworms and allergies previously, and have always been curious about it. As a lifelong allergy sufferer, I've always wondered why my body conspires to torture me when the seasons change, but no matter how bad it gets, I don't think I'd ever get up the courage to do what Jasper Lawrence did.
- Adams, Tim. "Gut instinct: the miracle of the parasitic hookworm." May 22, 2010. (April 6, 2012) http://www.guardian.co.uk/lifeandstyle/2010/may/23/parasitic-hookworm-jasper-lawrence-tim-adams
- Arbico Organics. "Natural Pest Control for Gardens, Greenhouses & Farms." (April 6, 2012) http://www.arbico-organics.com/category/beneficial-insects-predators-parasites
- Baker, Billy. "His parasite theory stirs a revolution." The Boston Globe. Dec. 31, 2007. (April 6, 2012) http://www.boston.com/news/science/articles/2007/12/31/his_parasite_theory_stirs_a_revolution/
- Jabr, Ferris. "For the Good of the Gut: Can Parasitic Worms Treat Autoimmune Diseases?" Scientific American. Dec. 1, 2010. (April 6, 2012) http://www.scientificamerican.com/article.cfm?id=helminthic-therapy-mucus
- Nature. "Bloody Suckers: Leech Therapy." PBS.org. (April 6, 2012) http://www.pbs.org/wnet/nature/bloodysuckers/leech.html
- New York Botanical Garden. "Hidden Partners: Mycorrhizal Fungi and Plants." (April 6, 2012) http://sciweb.nybg.org/science2/hcol/mycorrhizae.asp
- Radiolab. "Parasites." September, 2007. (April 6, 2012) http://www.radiolab.org/2009/sep/07/
- Than, Ker. "Parasitic Wasp Swarm Unleashed to Fight Pests." National Geographic. July 19, 2010. (April 6, 2012) http://news.nationalgeographic.com/news/2010/07/100719-parasites-wasps-bugs-cassava-thailand-science-environment/
- Velazquez-Manoff, Moises. "The Worm Turns." The New York Times. June 29, 2008. (April 6, 2012) http://www.nytimes.com/2008/06/29/magazine/29wwln-essay-t.html
- Weeks, Paul. "Red-billed oxpeckers: vampires or tickbirds?" Behavioral Ecology. 2000. (April 6, 2012) http://beheco.oxfordjournals.org/content/11/2/154.abstract