Many with. These hosts are called accidental hosts

Many parasites need a host at some point in their life cycle.

In this lesson, we’ll discuss how parasites utilize two different types of hosts: definitive hosts and accidental hosts.

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Parasitic Life Cycles

They’re the stuff of science fiction or horror: picture slimy worms, or tiny insects, looking for a way to crawl into your body and take over. They seem out to get us, but often the reality is a little different.Parasites are organisms that benefit from using a part or the whole of another organism.

In doing so, parasites harm the other organism, or host, in some way. Organisms that engage in parasitism, the act of being a parasite, can harm a host in a number of ways. Some parasites take nutrients from their hosts, while others derive protection, and still other parasites can actually control their hosts through chemical and hormonal sabotage, leading to the host’s death while the parasite lives on.

Types of Parasites and Hosts

By definition, all parasites use a host at some point. Parasites themselves are categorized in many ways, including obligate (require the host for all their lives), temporary (only needs a host for a little while, like a tick), or facultative (doesn’t need a host but if one comes around they’ll make use of it).

However, for the purposes of this lesson, we will only discuss the hosts, which are categorized by how they take part in the cycle of the parasite. Hosts can be definitive, temporary, vectors, or reservoirs, or they can accidentally be any of these. Let’s look deeper into definitive and accidental hosts.

Definitive and Accidental

Definitive hosts are hosts in which the parasite reaches maturity and, often, reproduces. Parasites usually target a specific species for this purpose.

However, some parasites may find themselves with a host that they would not normally associate with. These hosts are called accidental hosts or incidental hosts. Accidental hosts can be any kind of host. Accidental definitive, accidental temporary, etc. Let’s take a look at some examples.

Definitive Host Examples

What are some examples of a definitive host? All known viruses are really obligate parasites, since they mature inside the host and hijack the cellular machinery of their host to reproduce. For example, the definitive host for the bovine leukemia virus is a cow.But what about true living organisms, like bacteria? Humans are definitive hosts for many bacteria, including Pneumococcus, group B streptococcus, and mycoplasma bacteria. Many vertebrate animals act as definitive hosts for these and other bacteria.

The malaria parasite uses a mosquito as its definitive host
A common tick, which can infest accidental hosts

It’s estimated that a large percentage, up to 70%, of documented human parasites are actually accidental, making humans the accidental host, since we often ingest parasites either directly or from our food. Then again, even a walk in the woods can expose us to accidental parasites. Ticks, for example, are blood-sucking arachnid pests that would normally infest deer and other forest animals, but will feed on humans as their accidental host.

Lesson Summary

Parasites require a host to gain nutrients, protection, or even as a part of their life cycle.

Some organisms that engage in parasitism cannot survive without a host, while others aren’t as needy.Definitive hosts are hosts in which a parasite reaches maturity and often reproduces if possible. Humans are very often examples of definitive hosts.

Some obligate parasites that infect humans include viruses, pneumococcus bacteria, group B streptococcus bacteria, and mycoplasma bacteria.A big examples of definitive hosts are humans, which can harbor viruses, bacteria, and worm-like parasites like the pinworm, whip-worm, and some tapeworms. Plasmodium; the malaria parasite requires both humans and the mosquito, but its definitive host is the mosquito.Accidental hosts, also known as incidental hosts, are those that end up infested by a parasite that is not normally meant to be associated with that host. Accidental hosts can be any kind of host. Examples of accidental hosts include humans with fish parasites (likely a result of diet), and ticks, which can feed on humans as accidental hosts, rather than their normal deer or forest animal hosts.


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