Parasite

I

Introduction

Parasite, organism that lives in or on a second organism, called a host, usually causing it some harm. A parasite is generally smaller than the host and of a different species. Parasites are dependent on the host for some or all of their nourishment. For example, a tapeworm, a flattened worm that lives in the gastrointestinal tract of mammals, lacks an intestine of its own and must absorb predigested food from the intestine of its host. This food is the tapeworm’s only energy source for growth and reproduction. Parasitism affects most life forms, from bacteria infected by the viruses known as bacteriophages, to humans, who are subject to more than 100 parasites known to cause disease.

II

Types and Forms of Parasites

Parasites come in a variety of forms. Many arthropod parasites, including mites, ticks, and mosquitoes, cause a number of debilitating animal and human diseases. Certain plants, including mistletoe and dodder, parasitize other plants to obtain water and nutrients. Microscopic parasites include single-celled protozoans such as amoebas and sporozoa, fungi, and bacteria, which can infect animals and plants. Viruses are entirely parasitic, able to survive and reproduce only within other living organisms.

Parasites that live on the inside of the host’s body are known as endoparasites, while those that live on the outer surface of their hosts are known as ectoparasites. This distinction reflects adaptations made by the parasite to overcome certain barriers to parasitism. For example, when invaded by a parasite, a host often triggers an immune response, a cellular reaction that works to destroy the invader. Parasitic worms, including flatworms (soft-bodied worms, such as tapeworms and flukes) and roundworms (thin, unsegmented worms, also called nematodes) are endoparasites, usually living in the intestines, lungs, liver, or other internal organs of their hosts. These worms have developed adaptations that enable them to avoid the host’s immune response, such as during a developmental stage when they are protected by a cyst wall or an outer surface that constantly changes, thereby making it difficult for the host immune system to target the parasite for attack.

Many ectoparasites have developed structures, such as suckers, hooks, and teeth, which help penetrate the host’s outer surface. Primitive fishes, such as hagfish and lampreys, use suctionlike mouths to attach to the outer surface of other fish and suck out nutrients. Some annelids (segmented worms), such as leeches, are also ectoparasites, using sucking disks to feed on the blood and tissues of vertebrate hosts.

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III

Parasite and Host Relationships

Parasites vary in the ways they use their hosts. Temporary parasites spend only part of their lives in or on their hosts. Ticks, fleas, mites, and other arthropods, for example, attach to hosts and then detach to live as free-living organisms. Ticks normally live in woods and tall grass. To feed they may climb onto a passing dog, sink their mouthparts into the flesh, drink a small amount of blood, and then drop off the host. Most flatworms and roundworms are permanent parasites and live their entire adult lives in their hosts.

Facultative parasites are not dependent on their hosts for survival. Many leeches will feed on the blood or tissues of their hosts, but when released in an aquatic environment survive as free-living organisms. Obligate parasites are totally dependent upon their hosts for survival and will die without their host. A bacteriophage, for instance, would be unable to survive and reproduce if it was removed from its bacterium host.

IV

Life Cycle of Parasites

In order to survive from one generation to the next, parasites have a series of distinct developmental stages and hosts collectively known as a life cycle. Life cycles range from a simple, single host that is home to the larval and adult stages of a parasite, to the more complex life cycles requiring one host for the developmental stage of the parasite and a second host for the adult stage.

Beef tapeworms have a simple life cycle. These worms form cysts in the muscles of cows. When a human eats infected beef that is improperly cooked, the cyst enters the human digestive tract and opens to release a worm that attaches to the wall of the small intestine. The worm absorbs large quantities of nutrients from the intestines, sometimes causing malnutrition in its human host. The adult worm releases eggs that are passed out in the feces where they can infect other animals.

The eye fluke is a good example of a complex life cycle, although many variations of complex life cycles exist. Adult eye flukes live in the eyelids of wading birds and release their eggs into the water when the birds dip their heads underwater to feed. Each egg hatches and releases a microscopic free-living larva called a miracidium. The miracidium must penetrate the skin of a specific species of aquatic snail within a few hours or it will die. Once inside the snail, the miracidium develops into a 1 to 2 mm (0.04 to 0.08 in) long, saclike stage called a redia. The redia feeds on snail tissue and buds off other larval stages through asexual reproduction.

A new larval stage called a cercaria is produced within the redia. The 0.5 mm (0.02 in) long cercaria is a free-living, nonfeeding, short-lived stage that resembles a tadpole. It migrates to the surface of the snail's soft tissue and is shed into the environment. There, it swims and attaches to the surface of a small invertebrate such as a snail, clam, or crab, and forms a cyst. Wading birds feed on these invertebrates and become infected when the cyst wall breaks in the bird’s mouth. The released larva, called a metacercaria, travels through a slit in the back of the bird’s throat and migrates to the bird’s eye. In the bird’s eyelid it develops into a mature adult capable of producing eggs and starting the cycle once again.

Other parasites have life cycles that involve intermediate organisms, or vectors, which carry disease-causing microorganisms from one host to another. The protozoan blood parasite that causes sleeping sickness, or trypanosomiasis, infects humans, cattle, and other animals. It uses the tsetse fly as a vector to carry it from one host to the next. When a tsetse fly bites an infected animal, it picks up the parasite when it sucks blood. When an infected fly bites another animal, the parasite enters the bloodstream and begins to reproduce in the new host.