A parasite is an organism that depends on another organism, known as a host, for food and shelter. As an example, tapeworms live in the digestive system of a large variety of animals. The tapeworms have no digestive system of their own, but absorb nutrients through their skin from partially digested food as it passes through the host.
A parasite usually gains all the benefits of this relationship. In contrast, the host may suffer from various diseases, infections, and discomforts as a result of the parasitic attack. In some cases, however, the host may show no signs at all of infection by the parasite.
The life cycle of a typical parasite commonly includes several developmental stages. During these stages, the parasite may go through two or more changes in body structure as it lives and moves through the environment and one or more hosts.
Arthropod: A phylum of organisms characterized by exoskeletons and segmented bodies.
Definitive host: The organism in which a parasite reaches reproductive maturity.
Helminths: A variety of wormlike animals.
Intermediate host: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.
Nematodes: A type of helminth characterized by long, cylindrical bodies; commonly known as roundworms.
Protozoa: Single-celled animal-like microscopic organisms that must live in the presence of water.
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.
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. More parasitic species are known than nonparasitic ones. Parasites affect just about every form of life, including nearly all animals, plants, and even bacteria.
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, the Dutch scientist Anton von Leeuwenhoek (1632–1723) was perhaps the first person 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. The stress placed on the organism's resources can affect its growth, ability to reproduce, and survival. This stress can sometimes lead to the host's premature death. Parasites, and the diseases they cause and transmit, have been responsible for tremendous human suffering and loss of life throughout history. The majority of parasitic infections occur within tropical regions and among low-income populations. However, almost all regions of the world sustain parasitic species, and all humans are susceptible to infection.
An infectious disease, or infection, is a condition that results when a parasitic organism attacks a host and begins to multiply. As the parasite multiplies, it interferes with the normal life functions of the host more and more. The host begins to feel ill as a symptom of the parasite's invasion and activities. In many cases, the host's immune system (which fights foreign bodies in the body) may be able to respond to the parasite and destroy it. In many other cases, however, the parasitic infection may over-whelm the immune system, resulting in serious disease and even death.
Until a century ago, infections were the primary means of human "population control" worldwide, often killing enormous numbers of people in epidemics of diseases such as bubonic plague and typhoid fever. Even today, infections actually cause more deaths during war and famine than do actual injuries and starvation. Fortunately, many infectious diseases can now be treated by means of antibiotics and other drugs and by a variety of preventative methods.
Almost all infections contracted by humans pass from other humans or animals. Some infections originate from outside the body, among them a cold from kissing someone with a cold; rabies from a dog bite; hepatitis B from a contaminated needle entering the bloodstream; hepatitis A from germs transferred from fingers to mouth after touching a dirty toilet seat; measles, mumps, and the flu from tiny moisture particles that exit the mouth and nose when a person sneezes, coughs, or talks; syphilis from an infected sex partner; tetanus from a soil-contaminated wound; salmonella from ingesting undercooked eggs, meat, and poultry; and many diseases ranging from the relatively innocent to the fatal—such as gastroenteritis, cholera, and dysentery—from drinking or bathing in contaminated water.
Endogenous (caused by factors within the organism) infections occur when the host's resistance is lowered, perhaps by malnutrition, illness, trauma, or immune depression. Weakening of the host's immune system may permit normally harmless organisms already present in or on the host or in the environment to cause illness.
The major types of organisms that cause parasitic infections include species of protozoa, helminths or worms, and arthropods.
Protozoa. Protozoa are single-celled organisms that carry out most of the same physiological functions as more complex organisms. More than 45,000 species of protozoa are known, many of which 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 diseasecausing organisms.
Intestinal protozoa occur throughout the world. They are especially common 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. A cyst consists of a protozoan encased in a protective outer membrane. The membrane protects the organism as it travels through the digestive tract of a previous host. Once inside a new host, the parasite develops into a mature form that feeds and reproduces.
Amebic dysentery is one of the most common parasitic diseases. It often afflicts travelers who visit tropical and subtropical regions. The condition is characterized by diarrhea, vomiting and weakness. It is caused by a protozoan known as Entamoeba histolytica.
Another protozoan that causes severe diarrhea is Giardia lamblia. This organism was originally discovered by Leeuwenhoek and has been well-publicized as a parasite that can infect hikers who drink untreated water.
Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa are typically transmitted through another organism, called a vector. A vector is an organism that carries a parasite from one host to another host. In many cases, the vector is an invertebrate, such as an insect that itself feeds on a host and then passes the protozoan on through the bite wound. Some of the most infamous of these protozoa are the ones that cause malaria and African sleeping sickness.
Helminths. Helminths are wormlike organisms including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Leeches are also helminths and are considered ectoparasitic, since they attach themselves to the outside skin of their hosts.
One of the most infamous nematodes is Trichinella spiralis. At one stage of its life cycle, this nematode lives in the muscle tissue of animals, including swine. Eventually, these organisms make their way into the intestinal tissue of humans who happen to ingest infected, undercooked pork.
The largest parasitic roundworm, common among humans living in tropical developing countries, is Ascaris lumbricoides. This roundworm can grow to a length of 35 centimeters (15 inches) within the small intestine of its host.
A parasitic roundworm that affects dogs is Dirofilaria immitus, or heartworm. This worm infects the heart tissues and eventually weakens
the cardiac (heart) muscles to the point of failure. If left untreated, heart-worm can kill a dog.
Tapeworms are a class of worms characterized by their flat, segmented bodies. The segments hold both male and female reproductive organs, allowing self-fertilization. Segments 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 stomach lining with hooks or suckers on their head.
Common tapeworms that attack humans are Taenia saginata, Taenia solium, and Diphyllobothrium latum. These parasites use intermediate hosts, such as cattle, swine, and fish respectively, before entering the human body. Parasites such as these infect an intermediate host organism while in a early developmental form. But they do not grow to maturity until they have been transmitted to the final host.
In the case of Taenia species, for example, tapeworm eggs are passed into cattle or swine through infected soil. They develop into an intermediary
stage 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, which are passed on when consumed by humans. The tapeworms develop into adults that attach to the intestinal lining of the host.
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 skin. These muscles allow the worm to expand and contract its shape and, thus, move its body. Flukes usually have an oral sucker, sometimes ringed with hooks. They use the sucker to attach themselves to the host's tissues.
Some of the most infamous flukes are species of the genus Schistosoma that cause the often-fatal disease known as schistosomiasis. 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 bloodstream 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 exterior skeletons and segmented bodies. Examples include the crustaceans, insects, and arachnids. The arthropods 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.
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 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 diseases are malaria, yellow fever, filariasis, elephantiasis, and 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 Onchocerciasis (which causes river blindness), sandflies can carry leishmaniasis and kala-azar, and tsetse flies can carry the trypanosomes that cause sleeping sickness. Livestock, such as horses and cattle, can be infected with a variety of botflies and warbles that 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 livestock. Lice commonly live among the hairs of their hosts, feeding on blood. Some species are carriers of 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.
Another prominent class of arthropods that contains parasitic species is the arachnids. Included in this group are spiders, scorpions, ticks, and mites.
Mites are very small arachnids that infest both plants and animals. One common type of mite is the chigger, which lives in grasses. As larvae, they may 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. The adult female tick 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 diseases include Rocky Mountain spotted fever, Colorado tick fever, and Lyme disease.
Many parasitic infections can be treated by a variety of medical procedures, such as the use of antibiotics. The best way of controlling infection, however, is prevention. Scientists have developed and continue to test a number of drugs that can be taken as a barrier 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.