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Immunology => Histocompatibility Groups => Parasite
Parasite
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.
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.
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.
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.
PARASITES OF ANIMALS
Animals are infected by many parasites including protozoans, worm parasites, and arthropod parasites such as mites, ticks, and fleas. Veterinarians diagnose these parasites in or on pets by checking the animal for visible parasites or by examining blood, tissue, or waste products under a microscope. Common worm parasites of dogs and cats include hookworms, roundworms, and tapeworms.
Hookworm infection occurs when larvae in the soil penetrate the pet's skin, move into the bloodstream, and eventually travel to the intestine. Adult worms mature in the wall of the intestine and feed on blood from the intestinal lining, sometimes causing serious anemia. Roundworm infections of dogs and cats occur when these pets eat microscopic worm eggs present in the soil. The eggs develop larval stages in the intestine and some of these larvae penetrate the intestinal wall, move into the lungs, are coughed up and reswallowed, and once again enter the small intestine where they mature into 10 to 15 cm (4 to 6 in) worms. Roundworms compete with the pet for food and may cause malnutrition.
While the roundworm enters its host by ingestion and the hookworm enters by active penetration of the skin, the heartworm enters its dog host with the help of a mosquito vector. Microscopic larvae known as microfilariae enter the blood along with mosquito saliva when an infected mosquito bites a dog. The larvae travel via the blood stream to the heart and develop into sexually mature male and female heartworms. They grow 5 to 10 cm (2 to 4 in) in length infesting the heart's chambers and lodging in the veins that enter the heart.
PARASITES OF HUMANS
Humans are subjected to numerous protozoan, worm, and insect-related parasites. Two of the most damaging human parasites are the protozoan Plasmodium that causes malaria and the flatworm Schistosoma that causes schistosomiasis. There are an estimated 400 million to 600 million cases of malaria each year and 200 million cases of schistosomiasis worldwide.
In malaria, the infective larval stage of the Plasmodium protozoan is transmitted to humans by the bite of a female Anopheles mosquito. The larvae undergo asexual reproduction in the liver producing a cyst that releases new larval stages into the blood stream. Larvae invade red blood cells and reproduce, eventually rupturing the blood cells. Upon rupturing, a toxin is released that causes the chills and fever that are the characteristic symptoms of malaria patients. Drugs such as chloroquine can be used to prevent infection in the blood. Mosquito control by use of repellents and pesticides is also helpful in preventing spread of the parasite.
Humans are infected with Schistosoma when they enter water containing infected snails. The larval stages of this flatworm develop in the tissues of infected snails and eventually release fork-tailed cercariae into the water. The cercariae penetrate human skin, lose their forked tails, enter the blood, and migrate to major veins in the liver, intestine, or urinary bladder. Within about six weeks of infection, the juvenile worms develop into sexually mature adults measuring 1 to 2 cm (0.4 to 0.8 in) in length. The males and females mate and produce microscopic eggs, some of which migrate to the liver and cause a condition known as cirrhosis. Other eggs move into the intestine and are passed out in the feces. When untreated human sewage enters waters containing the snail hosts, the eggs hatch and start a new cycle.
Preventive measures include the use of boots and gloves or special ointments to block penetration of the larvae into the skin. Molluscicides (drugs that kill snails) are used to kill infected snails but they often kill other important fish and invertebrate species. The drug praziquantel has proved effective in killing Schistosoma in humans, although some people experience adverse side effects.
PARASITES OF PLANTS
Similar organisms that parasitize animals also infect plants. Fungi cause the majority of plant diseases. Although they typically feed on dead organic matter, fungi can also feed directly on living tissues. The fungus that causes Dutch elm disease is often carried from tree to tree by beetles. It attacks and eventually kills by blocking water flow through the plant. Protozoans such as phytoflagellates can parasitize milkweed plants. Bacterial plant diseases include fire blights, certain soft rots, and citrus canker. The bacteria that cause these diseases destroy tissue or block the passage of water through the plant. Numerous viruses, such as the tobacco mosaic virus, also attack plants. Certain insects and worms, particularly nematodes, parasitize the roots, stems, and leaves of plants. They secrete chemicals that induce plant cells around the parasite to rapidly divide and produce large growths known as galls. Galls formed by the root knot nematode can cause serious physical damage to the roots of important crops including tomatoes and potatoes.
Some higher plants feed on other plants and cause them harm. One group known as hemiparasites, or water parasites, absorbs water and nutrients from their plant hosts. Witchweed is a hemiparasitic seed plant that damages sugarcane, corn, and other grasslike crops by attaching itself to the host's roots and absorbing minerals and water, eventually killing the host. Mistletoe, another hemiparasite, parasitizes broadleaf trees including ash, maple, walnut, birch, and some conifers. Mistletoe roots bore into the host's branches in order to draw out water and nutrients. Birds eat mistletoe berries, which pass through their digestive tracks, are excreted, and sometimes stick to a tree branch where they produce a new mistletoe bush.
True plant parasites lack chlorophyll and cannot photosynthesize. These plants must obtain carbohydrates as well as minerals and water from their plant hosts. True plant parasites include dwarf mistletoe, which primarily parasitizes conifers; dodder, which parasitizes important agricultural crops such as alfalfa, clover, sugar beets, and woody perennials such as olive trees; and broomrape, which causes extensive damage to tomato crops.
PARASITOLOGY
There are many research areas in parasitology practiced by different types of specialists. Microbiologists and virologists primarily work with parasitic bacteria, rickettsiae, and viruses. Plant pathologists work with fungi, nematode parasites of plants, and other plant parasites. Animal parasitologists work with parasitic protozoans, worm groups, and arthropod parasites. Those who specialize in parasitic protozoans are called protozoologists whereas those who study parasitic worms are called helminthologists. Others who examine parasitic insects of humans are called medical entomologists.
Parasitologists who describe new species of parasites are known as systematic parasitologists. Parasite immunologists study ways in which hosts can reject parasites and they also attempt to develop vaccines against parasites. A growing area of parasitology is ecological parasitology including mathematical and computer modeling that predicts how parasites behave in wild populations.
Parasitologists in pharmaceutical industries develop drugs to prevent, control, and eradicate plant and animal parasites. Many parasitologists work to discover the complex life histories of animal and plant parasites whose life cycles remain partially or completely unknown.
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