Definitive & Accidental Hosts in Parasitic Life Cycles

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

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.

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

Definitive hosts are commonly used by worm-like or insect-like parasites. Humans are the definitive host for pinworms, whip worms, and certain kinds of tapeworms. Other tapeworms use dogs as their definitive hosts (and fleas as a temporary or intermediate host).

Trypanosoma brucei is a protozoan known to cause sleeping sickness in humans, but actually uses the tsetse fly as its definitive host, as that is where it develops into maturity and reproduces.

The definitive host for a Plasmodium, the genus of parasite that causes malaria, is the female Anopheles mosquito.

Accidental Host Examples

Sometimes a parasite is found in or on the body of a host where it is not normally found. These hosts are called accidental hosts, and the parasites become temporary or accidental parasites. A host can be both accidental and definitive (or any other type), in other words, the accidental host can still be where the parasite reaches maturity and reproduces, it's just not the usual species.

Accidental hosts have been documented in the human archaeological record. For example, the eggs of a fish parasite were once found in the mummified stomach of a human. Normally fish parasites wouldn't infest or infect a human, but they may have been introduced to the deceased human during his life, by eating infected fish.

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