Definitive & Intermediate Hosts in Parasitic Life Cycles

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  • 0:03 What Is a Parasite?
  • 0:38 Definitive Hosts
  • 1:36 Intermediate Hosts
  • 2:49 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Parasites utilize hosts in many different ways. In this lesson, we'll look at two different kinds of hosts, definitive and intermediate, and how they support different stages of the parasite life cycle.

What Is a Parasite?

You've likely heard of parasites because these are organisms that live on or in another organism, called a host. Most parasites cause trouble for their hosts because they cause disease or some other form of damage, so we try to stay away from them if at all possible! Not all parasites are the same though. Some of them have to live with their host for their entire lives, while some only live with their host for certain life stages, such as fertilization, sexual reproduction, or growth. For this lesson, we're going to look at two specific types of hosts called definitive and intermediate.

Definitive Hosts

For an organism to be considered a definitive host, the parasite needs to sexually reproduce while living on or in that host. Tapeworms are parasites that use a definitive host as a place to reach maturity and reproduce sexually. Tapeworms don't spend their entire lives in one host, though. They will be laid as eggs in the grass, something like a sheep will eat that grass and become infected, and then perhaps something like a wolf will eat the sheep and also become infected. What a journey for those little tapeworms!

You can think of a definitive host as the ''final'' host for the parasite. When you grow up and reach maturity, you will likely settle down in a house and maybe even raise a family. Before then you may have bounced around for a while, moving from one apartment to another, but once you settle in this ''final'' place, you set up shop for good. Another example of a definitive host is the mosquito, which is where the parasite that causes malaria will reproduce sexually to create the next generation of that parasite.

Intermediate Hosts

Now, if that host instead supports a developmental stage of a parasite's life, then we call it an intermediate host. This type of host serves as a vector for the intermediate stage of the parasite. You can think of the intermediate host like the UPS delivery person who cares for the package along the way, either passing it on to another delivery driver or leaving it at its final destination, the definitive host.

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