Parasites we’re going to look at two

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.

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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.Let’s revisit the tapeworm’s journey again. Remember how it started off in the grass as an egg, was eaten by a sheep, and then eaten by a wolf? If the wolf is the definitive host where the tapeworm sexually reproduces, then the sheep serves as an intermediate host where the tapeworm has a chance to grow and develop.

The protozoan parasite Toxoplasma gondii, which causes the disease toxoplasmosis, is another example. This parasite makes use of organisms, like birds and rodents, as intermediate hosts but uses cats as their definitive host. Keep in mind that just because a host is the intermediate host doesn’t mean that the parasite doesn’t cause it harm. For example, a human who becomes infected with Toxoplasma gondii can become very sick as the disease takes hold and causes problems in the brain, skeletal muscle, and eyes.

Lesson Summary

Parasites come in all shapes, sizes, and forms, but one thing they have in common is that they live on or in another organism called a host.

If the parasite sexually reproduces while utilizing the host, we call it a definitive host.If the parasite moves around from host to host, it will also likely encounter at least one intermediate host, which serves as a vector for the parasite. Much like a package being delivered to your front door, the parasite is hosted during growth and development stages but does not reach sexual maturity in this host. It does so once the delivery truck drops it off at the door of the lucky definitive host.


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