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.
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 animalsThe 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.

Parasite

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.
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

Flea

Flea, common name for small, bloodsucking, wingless insects. Adult fleas, which feed on the blood of their hosts, are surface parasites on the skin of humans and other mammals and, less often, on birds. Fleas are found all over the world. Their eggs are laid under carpets, in the folds of tapestry, in refuse piles, and in other places that provide safety and adequate nutrition. In 6 to 12 days the eggs hatch, becoming larvae with biting mouth parts. After a few days of voracious feeding upon organic refuse, the larvae spin cocoons and enter a pupal stage. The adult flea emerges from the cocoon in a few weeks. Adult fleas, which are slightly more than 0.3 cm (more than 0.1 in) long, have broad, rather flat bodies, short antennae, and piercing and sucking mouth parts; their eyes are either minute or absent. Their long, powerful legs enable them to leap relatively high into the air.
Several flea species infest household pets and domestic animals. The dog flea and the cat flea are two of the most common species, both of which are parasites also on human beings, poultry, and livestock. The human flea, the species frequently found most on people, is distributed throughout the world, but is uncommon in the United States. The dog flea, cat flea, and human flea are all intermediate hosts of a common cat and dog parasite, the cucumber tapeworm. Tapeworm eggs are deposited in fecal matter, and some of these eggs may cling to the hair of the primary host. Fleas swallow the eggs, which then undergo some development in the flea. If an animal or person accidentally swallows an infected flea, an adult tapeworm develops in the new host. The rat fleas, in the Tropics and in Europe, are important carriers of bubonic plague. The sticktight flea is another common pest, noted for its habit of clinging tenaciously to its host. Dog eczema is usually associated with the presence of fleas.
Fleas are controlled by destroying the adults and making breeding places unsuitable for larval life. Adult fleas are destroyed by bathing the host with strong soap and by applying insecticides or petroleum. These agents must be properly used to avoid injury to the infected animal or person.
Scientific classification: Fleas constitute the order Siphonaptera. Dog fleas are classified as Ctenocephalides canis, cat fleas as Ctenocephalides felis, and human fleas as Pulex irritans. The rat flea of the Tropics is classified as Xenopsylla cheopis; the rat flea of Europe as Ceratophyllus fasciatus. The sticktight flea is classified as Echidnophaga gallinacea.

Mite

Mite, common name for some 30,000 species of minute, usually oval-bodied arachnids (see Arachnid). They are worldwide in distribution. Mites resemble ticks in having the head, thorax, and abdomen fused into one unsegmented body, but they are usually much smaller. They often have three pairs of legs in the larval stage and four pairs of legs in the nymph and adult stages. The mouthparts are adapted for piercing. Like most arachnids, mites breathe by means of tracheae (small tubes opening on the surface of the body), and they live in both aquatic and terrestrial habitats. Many are animal parasites; some, which subsist on vegetation, produce galls on plants. They are economically and medically injurious, because they carry diseases affecting livestock and humans.
Among the most important mites are the chigger and the itch mite. The follicle mites, which infest human hair follicles and sebaceous glands, are about 0.025 cm (about 0.01 in) long. The bird mites infest the skins of birds; the chicken mite attacks domestic poultry and produces a form of dermatitis in humans. More than 100 species of freshwater mites inhabit United States lakes and rivers; these animals have fringed legs that they use in swimming. Among other common mites are the so-called red spiders, or spider mites, which spin spiderlike webs; feeding on the undersides of leaves, they destroy many types of plants.
Scientific classification: Mites belong to the order Acari. Chiggers make up the family Trombiculidae. Itch mites belong to the family Sarcoptidae. Follicle mites belong to the family Demodicidae. Bird mites belong to the family Dermanyssidae. The chicken mite is classified as Dermanyssus gallinae. Freshwater mites belong to the family Hydrachnidae. Red spiders, or spider mites, make up the family Tetranychidae.

scabies

INTRODUCTION
Scabies, skin disease caused by a tiny parasite called an itch mite. Scabies causes severe itching and is highly contagious. Itch mites are usually spread by close skin-to-skin contact with an infected person; holding hands is sufficient. As a result scabies spreads easily and rapidly in crowded conditions or where there is a lot of close contact with others, especially in daycare centers and nursing homes. Sexual contact is another means of transmission.
HOW SCABIES DEVELOPS
Scabies occurs when the female mite burrows into the outer layers of the skin, where she deposits eggs and wastes. These deposits irritate the skin and cause an allergic reaction that manifests itself in severe itching and, in many cases, a rash or small bumps on the skin. The mites can infest any part of the body, but they most often burrow into folds of the skin—between the fingers; on the elbow, wrist, or knees; around the shoulder blades; on the buttocks or the penis; or under the breasts in women. The itching and rash generally begin about a month after the infestation. Itching may become more intense at night or after a hot bath. Scratching the itchy skin can lead to sores through which bacteria may enter and cause infection.
The itch mite that most commonly causes scabies is the female of the species Sarcoptes scabiei. This pearly white, microscopic member of the arachnid family, to which spiders also belong, measures less than 0.05 cm (0.02 in) long.
DIAGNOSIS AND TREATMENT OF SCABIES A dermatologist (skin doctor) can usually diagnose scabies by sight. The physician may confirm the diagnosis by scraping a portion of the skin and examining the scrapings under a microscope for the presence of mites or eggs. The treatment for scabies consists of applying a lotion that kills the mites, larvae, and eggs. Several lotions are available, but a lotion containing permethrin is most often prescribed. The lotion must be applied all over the body, not just on the affected area. The physician may also prescribe an antihistamine to control the itching.
The mites that cause scabies can live for a few days in bedding or clothing. To prevent others in the household from developing the condition, it is important to wash in hot water any sheets, towels, or clothes that have come into contact with the infected person. Scabies can recur. Someone treated for scabies has been sensitized by the mite and will react much more quickly if exposed again to the mite.