Parasites Part One: An Introduction

Author: Brittany Maule

Creepy, crawly, monsters that are sucking away life from the inside: the image that comes to mind for some people when thinking of parasites. While some, okay a lot of parasites do look scary, their function in ecosystems can be vital. In this two-part blog post, I will highlight some background on parasites and talk about some of the well-known and not well-known examples, and then dive into why they are so important to understand from an ecosystem standpoint.

 Parasitism falls under the umbrella of symbiotic relationships, or essentially any relationship between two organisms living together. Parasites benefit at the detriment of another organism¹. The organism the parasite negatively affects is called the host. This definition engulfs a wide range of organisms that can be considered parasites: anything from plants you can see with the naked eye, to a tiny, one-celled organism that can be carried around by a mosquito.

A parasitic plant? Think again! Moss only use trees for structural support, not nutrients

Obtaining Food:

Because this group is so diverse, parasites have a lot of different ways in obtaining nutrients from their host to make sure they grow and survive. One well known example is a group of parasites called tapeworms that can grow up to several meters while living in the human gut. Because these organisms do not have a digestive tract, they absorb our digested food directly across their bodies². Another example, perhaps not as well-known is organisms from the genus Cymothoa. These “tongue-eating” parasites enter through fish gills and eventually drain blood from the fish tongue until it takes the tongue’s place. In the mouth the parasite eats blood and other mucus from the fish for nutrients³. NOVA has an awesome video explaining more about this specific parasite here: The Tongue-Eating Parasite

Clownfish with parasitic isopod. Copyright Lea Lee. http://www.flickr.com/photos/critter71/

Moving Around:

A lot of parasites use different hosts to help them move where they need to be and to move their life cycle forward. Parasites have different stages in their life cycle which often can only be completed in specific hosts. The hosts where the parasite becomes sexually mature and can reproduce is called the definitive host. Any host where the parasite is going through asexual stages of its life cycle is called an intermediate host. Sometimes humans are definitive hosts such as with the tapeworm mentioned above, and sometimes humans can accidentally host intermediate stages of parasites. An example of this is the parasites that cause “swimmer’s itch” from the genus Trichobilharzia. Below is an example of one of these parasites through its life cycle⁴

In this example, snails are the intermediate host, and water birds are the definitive hosts. Humans can accidentally harbor the intermediate host, but swimmer’s itch only causes a rash from the larval stages of the parasite burrowing into the skin.

You may be thinking at the end of this that parasites are all doom and gloom, and while they do make trouble for a lot of organisms, they influence ecosystems in a lot more ways than you might think! Plus, they have some of the most interesting ways in reproducing and obtaining nutrients. Part two coming soon will explore how parasites may be much more important in ecosystems than we once thought.

Additional Resources:  

1.       Centers for Disease Control and Prevention. 2016. “About Parasites” Available from: https://www.cdc.gov/parasites/about.html

2.       Cox, F.E.G., 2002. “History of Human Parasitology” Clin Microbiol Rev. 15(4): 595–612.

3.       National Geographic. 2013. “Tongue-Eating Fish Parasites Never Cease to Amaze.” Available from: http://phenomena.nationalgeographic.com/2013/02/28/tongue-eating-fish-parasites-never-cease-to-amaze/

4.       Centers for Disease Control and Prevention. 2013. “Cercarial Dermatitis” Available from: https://www.cdc.gov/dpdx/cercarialDermatitis/