According to the New Oxford American Dictionary application on my computer, a parasite is “an organism that lives in or on another organism (its host) and benefits by deriving nutrients at the host’s expense.” I think this definition is informative, but its specifics merit further consideration.
When most people think of parasites, they usually think of organisms that live on us – ectoparasites – or in us – endoparasites. These organisms fit well with the definition above, in that parasites can “live on or in another organism.” Common examples of ectoparasites include the animal kingdom’s insect version of a vampire, the tick (here’s a picture of a deer tick, courtesy Wikipedia). Similarly common endoparasites include organisms that inhabit the gut, such as a number of worm-like nematode species (not all nematodes are parasitic, but for the purposes of this entry, I am referring to only parasitic nematodes. For an example of a parasitic nematode, see the CDC website on Ascaris lumbricoides). While ticks and nematodes are very different organisms, they both live on or in another organism; they also both obtain nutrients from their host. As such, and as the definition above tells us, they are parasites.
Besides ticks and nematodes, other organisms that are not often considered parasites – such as H1N1 of infamous flu fame and bacteria like Yersinia pestis, which causes Plague – are parasites as well. Not only do they live in or on another organism, the host, they “[derive] nutrients” from their hosts. One can argue, however, that viruses do not acquire nutrients from their hosts. Rather, they use the hosts to make more virus. But why does a tick take blood from a deer or human? Why does a nematode living in the gut feed on food particles or host tissue present there? What is the end goal? Reproduction. Like all organisms, parasites obtain nutrients with reproduction as the final result. So while a virus does not “eat” in the traditional sense, both viruses and ticks take nutrients from a host for the same purpose. They both therefore qualify as parasites.
In the pursuit of reproduction, the definition above states that parasites survive at the “host’s expense.” How do we define “expense”? A decrease in lifetime reproduction, shorter lifespan, increased nutrient requirements for host survival (because of loss to parasite)? Expense could be all of these things, and more. These many and varied definitions make quantifying a parasite’s affect on its host difficult. Furthermore, taking humans as an example, many infections – such as Ascariasis, caused by Ascaris lumbricoides, mentioned above – are asymptomatic in adults until relatively high parasite loads. Thus, while any parasite presence must affect the host, this presence may not cause enough damage to outwardly affect the host. It may, however, have a measurable effect summed over the lifetime of a host with multiple asymptomatic infections. These nuances complicate the way in which we incorporate a parasite’s effect on a host in a definition of a parasite.
The methods in which parasites further their survival and reproduction using hosts also complicate a definition of a parasite. For example, so far we have been assuming that parasites use hosts to reproduce, and that parasites acquire nutrients in some form from their hosts to this end. But do parasites universally obtain nutrients from their hosts in order to reproduce? Strangler figs (links to Wikipedia), for example, do not grow from the ground up. As a seed, they germinate on a host tree. In their growth, they both grow up and down, in so doing “strangling” the host tree. They do not acquire nutrients from their host, but a good place to grow that’s closer to the canopy than the ground, facilitating their eventual reproduction. We can find another example of a parasite that does not take nutrients from its host in sexual parasites, which mate with their host and then discard that host’s genetic contribution (Lehtonen et al, 2013). The parasite requires the mating in order to reproduce itself, but it does not use the host’s genes. Thus, these organisms are parasites in that they require another organism to reproduce, and in that act of reproduction the host obtains no benefits (because its genes were not used). For an interesting national geographic article on sexual parasitism, click here. While obtaining nutrients from a host represents a very common strategy for parasites, it is a means to an end. Parasites, whether by obtaining food or a place to take root, use the host to complete their life cycle, i.e., reproduce.
Parasites are clearly very difficult to define, and the definition needs to include more than “nutrients” and “host’s expense”. So if I had to define parasite myself, and risk the inevitable failure of that definition to apply to all organisms I wish it in the way that I intend, I would define a parasite in this way: An organism that requires another to complete its life cycle, barring instances of predation, herbivory, and relationships among organisms that provide benefits to both (a mutualism).
My definition is of course extremely broad. Such inclusiveness begs the question: How informative and meaningful is the definition? Some words, like “brevity” and “paragraph,” have specific definitions that will not change in the course of our lifetimes. They provide us concrete meaning and ease of use. Other words, like parasite, are more variable. However, such nebulous words lend themselves to more abstract thinking, thinking that is not so tied down in specifics. It’s a good thing, then, that the definition of “parasite” is, in the words of Pirates of the Caribbean, “More of a guideline.”
Citations (Because who doesn’t love references?)
1. “parasite.” New Oxford American Dictionary, Electronic, 3rd Edition, 2010
2. “Parasites – Ascariasis.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 10 Jan. 2013. Web. 17 May 2013.
3. Lehtonen, J., et al., Evolutionary and ecological implications of sexual parasitism. Trends Ecol Evol, 2013. 28(5): p. 297-306.
4. Switek, Brian. “Hacking Sex – The Odd Lives of Sexual Parasites.” Phenomena: Laelaps. National Geographic, 19 Feb. 2013. Web. 17 May 2013.
5. Pirates of the Caribbean: Curse of the Black Pearl. Dir. Gore Verbinski. Perf. Johnny Depp, Geoffrey Rush, Orlando Bloom, Keira Knightly. The Walt Disney Company, 2003. DVD.