Parasites

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Parasites

A parasite is an organism that depends on another organism, known as a host, for food and shelter. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection . The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation . There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria .

The study of parasites

Parasitology is the study of parasites and their relationships with host organisms. Throughout history people have coped with over 100 types of parasites affecting humans. Parasites have not, however, been systematically studied until the last few centuries. With his invention of the microscope in the late 1600s, Anton von Leeuwenhoek was perhaps the first to observe microscopic parasites. As Westerners began to travel and work more often in tropical parts of the world, medical researchers had to study and treat a variety of new infections, many of which were caused by parasites. By the early 1900s, parasitology had developed as a specialized field of study.

Typically, a parasitic infection does not directly kill a host, though the drain on the organism's resources can affect its growth, reproductive capability and survival, leading to premature death. Parasites, and the diseases they cause and transmit, have been responsible for tremendous human suffering and loss of life throughout history. Though the majority of parasitic infections occur within tropical regions and among low-income populations, most all regions of the world sustain parasitic species, and all humans are susceptible to infection.

Though many species of viruses, bacteria, and fungi exhibit parasitic behavior and can be transmitted by parasites, scientists usually study them separately as infectious diseases. Types of organisms that are studied by parasitologists include species of protozoa , helminths or worms, and arthropods .

Protozoa

Protozoa are one-celled organisms that are capable of carrying out most of the same physiological functions as multicellular organisms by using highly developed organelles within their cell . Many of the over 45,000 species of known protozoa are parasitic. As parasites of humans, this group of organisms has historically been the cause of more suffering and death than any other category of disease causing organisms.

Intestinal protozoa are common throughout the world and particularly in areas where food and water sources are subject to contamination from animal and human waste. Typically, protozoa that infect their host through water or food do so while in an inactive state, called a cyst, where they have encased themselves in a protective outer membrane and are released through the digestive tract of a previous host. Once inside the host, they develop into a mature form that feeds and reproduces.

Amebic dysentery is one of the more common diseases that often afflicts travelers who visit tropical and sub-tropical regions. This condition, characterized by diarrhea, vomiting and weakness, is caused by a protozoan known as Entamoeba histolytica.

Another protozoan that causes severe diarrhea, but is also found in more temperate regions, is Giardia lamblia. Among Leeuwenhoek's discoveries was G. lamblia, which is a now well-publicized parasite that can infect hikers who drink untreated water in the back country.

Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa are typically transmitted through another organism, called a vector, which carries the parasite before it enters the final host. Often the vector is an invertebrate, such as an insect, that itself feeds on the host and passes the protozoan on through the bite wound. Some of the most infamous of these protozoa are members of the genera Plasmodium, that cause malaria ; Trypanosoma, that cause African sleeping sickness ; and Leishmania, which leads to a number of debilitating and disfiguring diseases.

Helminths

Helminths are worm-like organisms of which several classes of parasites are found including nematodes (roundworms ), cestodes (tapeworms), and trematodes (flukes). Leeches, of the phylum Annelid, are also helminths and considered as ectoparasitic, attaching themselves to the outside skin of their hosts.

Nematodes (roundworms)

Nematodes, or roundworms, have an estimated 80,000 species that are known to be parasitic. The general morphology of these worms is consistent with their name; they are usually long and cylindrical in shape.

One of the most infamous nematodes is Trichinella spiralis, a parasite that lives its larval stage encysted in the muscle tissue of animals, including swine, and make their way into the intestinal tissue of humans who happen to digest infected, undercooked pork.

The largest parasitic roundworm, common among humans living in tropical developing countries, is Ascaris lumbricoides. This roundworm can grow up to 14 in (35 cm) in length within the small intestine of its host.

One roundworm, Enterobius vermicularis, or pinworm, actually thrives in more temperate climates. This relatively small roundworm is not limited to humans living in relative poverty, but is known to infect the well-off just as easily.

Adult worms of Wuchereria bancrofti live in the blood and lymph of the host. Elephantiasis , characterized by extreme enlargement of a host's extremities, is the rare but dramatic result of the host body's defensive reaction to the presence of the worms.

Known to dog owners Dirofilaria immitus, or heart-worm infection, if left untreated, can kill a dog as the worms infect the heart tissues and eventually weaken the cardiac muscles to the point of failure.

Cestodes (tapeworms)

Cestodes, or tapeworms, are a class of worms characterized by their flat, segmented bodies. The segments are called proglottides and hold both male and female reproductive organs, allowing self-fertilization. Proglottides that contain fertilized eggs break off or dissolve, passing the eggs out of the host. Adult tapeworms typically reside in the intestinal tract of vertebrates , attaching themselves to the mucosal lining with hooks or suckers on their scolex, or head. They do not possess a digestive tract, or alimentary canal , of their own, but absorb nutrients through their tegument, or skin, from partially digested food as it passes through the host.

Common tapeworms that frequent humans are Taenia saginata, Taenia solium, and Diphllobothrium latum. These parasites use intermediate hosts—cattle, swine, and fish respectively. Many parasites infect an intermediate host organism while in a developmental form, but they do not grow to maturity until they have been transmitted to the final or definitive host. In the Taenia species, the eggs are passed into cattle or swine through infected soil . They develop into an intermediary stage, called a cysticercus, that embeds in the muscle and connective tissue of the animal. Infected animals that are processed for meat but improperly cooked still harbor the parasite and pass the cysticerci on when consumed by humans. The cysticeri develop into adult tapeworms that attach to the intestinal lining of the host. The cysticerci of T. solium can, themselves, cause medical complications. Instead of developing immediately into adult tapeworms, these cysticerci can migrate to any organ of the body, commonly ending up in the muscles or brain . A serious infection in the brain can lead to mental complications, including seizures and personality changes.

Trematodes (flukes)

Trematodes, or flukes, are another class of helminths that have parasitic species. Adult flukes are typically flat, oval-shaped worms that have a layer of muscles just below the tegument, or skin, that allow the worm to expand and contract its shape and, thus, move its body. Flukes usually have an oral sucker on their anterior end, sometimes ringed with hooks, that is used to attach themselves to the host's tissues.

The life cycle of a typical trematode begins when eggs, that are passed out of a previous host's digestive tract, find themselves in fresh water. The ciliated larval form, called miracidia, emerge from the eggs and swim until they find the appropriate species of their intermediate host: usually a snail. The miracidia penetrate the snail and change into another form, called sporocysts. The sporocysts undergo further changes resulting in yet another form of the parasite called cercariae, which burrow out of the snail and pass into the water again. A cercaria has a flagella-like tail that helps it swim through the water in search of its final host, typically a mammal or avian species. The cercariae make contact with the skin of a host and burrow in. Host animals may also become infected with flukes by ingesting meat, usually fish or crustaceans, that are harboring the cercariae in an encysted form, called metacercaria, within their tissues. In either form, once inside the final host the parasite moves through tissues or the blood to the desired organ, often the intestine or liver, where it matures into a reproducing adult, starting the cycle again.

Clonorchis sinensis, a fluke common in the Far East, is a trematode that uses fish as one of its intermediate hosts and fish-eating mammals , including humans, as a final host. The adult C. sinensis flukes eventually make their way through the bile ducts to the liver of the host.

Another fluke that uses both a snail and a second intermediate host is Paragonimus westermani. Freshwater crabs and crayfish can harbor P. westermani metacercaria that, in turn, may be consumed. The adult of this parasite makes its way into the lungs of its host.

Some of the most infamous flukes are species of the genus Schistosoma that cause the often fatal schistosomiasis. The cercariae of these flukes infect human hosts directly by burrowing into the skin of a person wading or swimming in infected water. One species, S. mansoni, enters the blood stream as an immature worm and can be carried through various organs, including the lungs and heart, before maturing in the liver.

Arthropods

Arthropods are organisms characterized by exoskeletons and segmented bodies such as crustaceans, insects and arachnids . They are the most diverse and widely distributed animals on the planet . Many arthropod species serve as carriers of bacterial and viral diseases, as intermediate hosts for protozoan and helminth parasites, and as parasites themselves.

Insects

Certain insect species are the carriers of some of humanity's most dreaded diseases, including malaria, typhus , and plague. As consumers of agricultural crops and parasites of our livestock , insects are also humankind's number one competitor for resources.

Mosquitoes , are the most notorious carriers, or vectors, of disease and parasites. Female mosquitoes rely on warm-blooded hosts to serve as a blood meal to nourish their eggs. During the process of penetrating a host's skin with their long, sucking mouth parts, saliva from the mosquito is transferred into the bite area. Any viral, protozoan, or helminth infections carried in the biting mosquito can be transferred directly into the blood stream of its host. Among these are malaria, yellow fever , W. bancrofti (filariasis and elephantiasis), and D. immitis (heartworm).

Flies also harbor diseases that can be transmitted to humans and other mammals when they bite to obtain a blood meal for themselves. For example, black flies can carry river blindness, sandflies can carry leishmaniasis and kala-azar, and tsetse flies (found mainly in Africa ), carry the trypanosomes that cause sleeping sickness. Livestock, such as horses and cattle, can be infected with a variety of botflies and warbles that can infest and feed on the skin, throat, nasal passages and stomachs of their hosts.

Fleas and lice are two of the most common and irritating parasitic insects of humans and our livestock. Lice commonly live among the hairs of their hosts, feeding on blood. Some species are carriers of the epidemic inducing typhus fever. Fleas usually infest birds and mammals, and can feed on humans when they are transferred from pets or livestock. Fleas are known to carry a variety of devastating diseases, including the plague.

Arachnids

Another prominent class of arthropods that contains parasitic species is the arachnids. Though this group is more commonly known for spiders and scorpions, its parasitic members include ticks and mites .

Mites are very small arachnids that infest both plants and animals. One common type is chiggers, which live in grasses and, as larva, grab onto passing animals and attach themselves to the skin, often leading to irritating rashes or bite wounds. Scabies are another mite that causes mange in some mammals by burrowing into the skin and producing severe scabs, lesions, and loss of hair.

Ticks also live their adult lives among grasses and short shrubs. They are typically larger than mites, and it is the adult female that attaches itself to an animal host for a blood meal. Tick bites themselves can be painful and irritating. More importantly, ticks can carry a number of diseases that affect humans. The most common of these include Rocky Mountain spotted fever, Colorado tick fever, and the latest occurrence of tick-borne infections: Lyme disease .

Control of parasites

Most parasitic infections can be treated by use of medical and surgical procedures. The best manner of controlling infection, though, is prevention. Scientists have developed and continue to test a number of drugs that can be taken as a barrier, or prophylaxis, to certain parasites. Other measures of control include improving sanitary conditions of water and food sources, proper cooking techniques, education about personal hygiene, and control of intermediate and vector host organisms.

Resources

books

Brown, Harold, and Franklin Neva. Basic Clinical Parasitology. Norwalk, CT: Appleton-Century-Crofts, 1983.

Noble, Elmer, and Glenn Noble. Parasitology: The Biology of Animal Parasites. Philadelphia: Lea and Febiger, 1989.

Schmidt, Gerald, and Larry Roberts. Foundations of Parasitology. St. Louis: Times Mirror/Mosby College Publishing, 1989.

Warren, Kenneth, and John Bowers, eds. Parasitology: A Global Perspective. New York: Springer-Verlag, 1983.

periodicals

Jaenike, John. "Behind-the-Scenes Role of Parasites." Natural History June 1994: 46-48.

Moore, Janice. "The Behavior of Parasitized Animals." Bio-Science (February 1995): 89-96.

Tilton, Buck. "Don't Drink the Water." Backpacker (February 1994): 50-55.

Jeffrey R. Corney

KEY TERMS

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Arthropod: —A phylum of organisms characterized by exoskeletons and segmented bodies.
Cestodes: —A class of worms characterized by flat, segmented bodies, commonly known as tapeworms.
Definitive host: —The organism in which a parasite reaches sexual maturity.
Helminths: —Term to define various phyla of worm-like animals.
Intermediate host: —An organism infected by a parasite while the parasite is in a developmental form, but does not sexually mature.
Nematodes: —Characterized by long, cylindrical bodies, commonly known as roundworms.
Protozoa: —One-celled organisms.
Trematodes: —A class of worms characterized by flat, oval shaped bodies, commonly known as flukes.
Vector: —Any agent, living or otherwise, that carries and transmits parasites and diseases.