A parasite is a creature that lives off of another creature or person and uses their resources, often their own body to survive. This is similar but distinct from a ‘symbiotic’ relationship that some creatures share wherein both organisms benefit in some way from the pairing. In a parasitic relationship only the parasite benefits to the detriment of the ‘host’ (in the third scenario ‘commendalism’ the only one animal benefits but to no expense for the other). Some large water mammals for example share a symbiotic relationship with birds who bathe on their backs and find themselves safe from predators and with access to parasites on the hippo which they can eat. This then gives them a food supply while clearing the hippopotamus of dirt and parasites. Hippopotami will even open their mouths to allow birds inside to hunt.
Meanwhile in a parasitic relationship the pairing is not mutually beneficial. An example of a parasite for example is head lice, which infest a creature’s hair and suck blood from the scalp. This damages the host’s scalp and leads to blood loss and itching, while providing a habitat for the head lice. This is one example of a human parasite. Parasites meanwhile are also distinct from the predator/prey relationship as the host is kept alive in order to supply continued food/habitat rather than a quick solution.
Many friendships could be considered ‘symbiotic’ and some examples in zoos and captivity have shown unusual pairings of animals that become attached and provide. In Haller Park in Kenya, a turtle (Mzee) and hippo calf (Owen) became so attached that they would sleep together, wash together and follow each other around the grounds. This relationship could then be considered symbiotic though it is not a typical example. In many ways the relationship between a man and a dog could be considered symbiotic in that the dog is protected and fed by the human but in return provides the human with stress relief (the extensive health benefits of petting dogs have been exhaustively studied) and protecting their home from intruders. This was even more the case when humans first started domesticating animals where cats for example were used to kill mice and other infestations on boats and dogs were used for hunting. Even farmyard animals could be considered examples of symbiosis.
However symbiosis in humans is more commonly used to describe the existence of ‘friendly’ bacteria in our bodies which help to keep us in good health. This is known as ‘endosymbiosis’ as the symbiotic bacteria lives inside us. One example is lactobacilli which is a type of lactic acid bacteria that exists in the vagina and gastrointestinal tract and is symbiotic in that they help inhibit the growth of harmful bacteria.
Similarly the definition for parasites can also be quite blurry and can be used to describe some of our own day to day relationships in regular parlance. While the term is generally used to describe creatures that live off our resources and damage our health, it could also be used to describe a human baby which lives off of a mother’s foods inside their womb doing damage in the process. Similarly while some would consider pets symbiosis, others might consider them an example of a parasite. Particularly caged animals which live off human resources without giving anything ‘back’ as such. Even a one-sided relationship with another human could be considered an example of a human parasite (quite literally).
Examples of human parasites that we normally think of as ‘parasites’ in the traditional sense and that require treatment however are: head lice, tics, helminths, protozoa and leaches among others. Fungal infections meanwhile could also be considered parasites as something that grows off of human flesh and uses the nutrients in our bodies to survive.
Here the distinction can be made again between parasites that exist inside and outside of the human body with those living inside posing a more serious threat. Here creatures such as helminths – worms that live inside our stomach, intestines and other organs – will grow up to metres inside our bodies feeding off our food and resulting in malnutrition, malabsorption, jaundice, diarrhea and vomiting. Eventually such an infestation could be lethal. Similarly, while much smaller and less dangerous, single-celled protozoa will also live inside the intestines and feed off of the stools resembling microscopic jellyfish. Both of these will be diagnosed by examination of the stools and will be treated using anti-parasitic medication.
In the traditional sense parasitism is a highly successful mode of life with over half of all organisms on the planet having at least one parasitic phase. Symbiosis too is a highly successful strategy for life on Earth and has even caused ‘co-evolution’ to occur where animals evolve not individually within their species but as one large network. As biologist Dorion Sagan states ‘life did not take over the globe by combat, but by networking’. Symbiotic intelligence has also been used to describe ‘collective intelligence’ within species, also a highly valuable asset.
Interestingly as humanity has progressed we have begun to consciously control and choose our symbiotic and parasitic relations with other animals and organisms on the planet in order to get more from our resources. The domestication and farming techniques described above are examples of this, as is the use of horses and mules for transport. This is partly what has begun to blur the line between what precisely should be considered a ‘symbiotic’ relationship.
Another example of more considered and conscious use of symbiosis is the existence of ‘probiotic’ yogurts and medications which include lactobacilli for its health benefits. Here you choose to drink a living organism for its ability to stave off those that aren’t wanted.
Human parasites have also been used for their potential benefits, thereby turning a once parasitic creature into a symbiotic one. For example, in some areas it has been known for women to purposefully ingest tape worm into the stomach (tapeworm being a form of helminth) so that it would eat any of their food and thus help them to remain thin while feeling full. Anti-parasitic medication would then be consumed in order to kill off the parasites once they reach a certain size or become dangerous.
Leeches too have long been consciously used for their health benefits. A leech will attach itself to its host and use its sharp teeth in order to break the skin, before using a powerful suction to drink the blood. The ‘medicinal’ leech has been used since the 18th century for ‘bloodletting’ when it was believed that withdrawing large quantities of blood was a helpful cure for disease. Today however they are still used in order to help the discolouration of black eyes, and after surgery to help with the reattachment of skin flaps in order to promote healing and blood flow in the skin. Any bacteria that causes disease or infection could be considered a parasite, but now using new genetic modification techniques to use bacteria and viruses that have been essentially ‘neutralised’ in terms of their harmful effect as a delivery system for medicine and genetic alterations.
In fiction, human parasites and symbionts are largely portrayed in imaginative manners. The ‘liquid’ alien ‘symbiote’ known as ‘Venom’ in Spiderman for example attaches itself to any living host to give them enhanced strength and power in exchange for a body that it can coat, control and use to survive. In the manga series ‘Guyver’ characters similarly where a suit of ‘biological’ alien armour. In Superman ‘Parasite’ is a creature that saps the life force of super-powered characters in order to gain their strength for a short time. Meanwhile the recent blockbuster hit ‘Avatar’ features many symbiotic relationships in which the inhabitants of the planet Pandora connect with the native animal-life via a tendril in order to gain control and a ‘shared consciousness’ of sorts.
As we continue to develop technology and in particular genetic modification techniques, the role of symbiosis might begin to change and come more and more to the forefront of medicine and perhaps transhumanism.