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A parasite is an organism that lives in another organism, called the host, and often harms it. It is dependent on its host for survival - it has to be in the host to live, grow and multiply. A parasite cannot live independently. Although a parasite rarely kills the host, in some cases it can happen. The parasite benefits at the expense of the host - the parasite uses the host to gain strength, and the host loses some strength as a result.

Parasites, unlike predators, are usually much smaller than their host. They reproduce at a faster rate than the host.

The word "parasite"comes from the Greek "parasitos", with para meaning "alongside", and sitos meaning "food" - therefore meaning "eating at the side of, as one would when seated at the same table". It was in the 18th century that the word parasite entered the English language as a biologic term; before that it referred to humans, such as a relative or friend who lived at the expense of another person.

There are three main types of parasitic diseases:

•   Protozoa - this parasite is a single-cell organism. Plasmodium, which causes malaria, is an example. A protozoa can only multiply (divide) within the host.

•   Helminths - worm parasites. Schistosomiasi is caused by a helminth. Other examples include roundworm, pinworm, trichina spiralis, tapeworm, and fluke.

•   Arthropods - includes insects and spiders (arachnids). The insect is not the parasite, but it carries it, it is a vector of parasitic diseases.

Types of parasites There are several types of parasites. Below are examples of some.

Ectoparasite - a parasite that lives on the host's surface, examples include some mites and hair and body lice.

Endoparasite - one that lives inside the host, examples include heartworm, tapeworm, and flatworms. One that inhabits the spaces inside the host's body is an intercellular parasite, while an intracellular parasite inhabits the cells of the host's body, these include bacteria or viruses, and they rely on a third organism known as the vector (carrier). The vector transmits the endoparasite to the host. The anopheline mosquito is a vector, it transmits a protozoan of the genus Plasmodium, which causes malaria.

Epiparasite - this one feeds on another parasite, a relationship known as hyperparasitism. A flea which lives on a dog may have a protozoan in its digestive tract, the protozoan is the hyperparasite.

Parasitoid - the larval development takes place in/on another organism, the host usually dies. In this case there are characteristics of predation, because the host dies.

Ascaris round worms are parasites commonly hosted in the intestines of various terrestrial animals, chiefly herbivores. They are typically large worms characterized by a mouth surrounded by three lips. The species Ascaris lumbricoides is probably the most familiar parasite in humans. An almost identical worm, often called A. suum, occurs in pigs.

The intestinal roundworm Ascaris lumbricoides infection in humans follows the ingestion of Ascaris eggs that have contaminated foods or soil. In the small intestine the larvae are liberated and migrate through the intestinal wall, reaching the lungs, where they may produce a host sensitization that results in lung inflammation and fluid retention. About 10 days later, the larvae pass from the respiratory passages into the digestive tract and mature into egg-producing worms, which grow to some 15 to 40 cm (6 to 16 inches) in length, in the small intestine. Serious, even fatal, complications of ascariasis result from the infiltration of the larvae into sensitive tissues, such as the brain, and from the migration of the adult worms into various body structures where they produce abcesses and toxic manifestations.

Hookworm infection begins when the worm is in the larval stage. It penetrates the skin and migrates during its life cycle through the liver and the lungs, and it attaches to the mucosa of the small intestine where it matures. Hookworms deplete the body of nutrients, and a major effect is severe chronic iron-deficiency anemia.

Necator americanus, which ranges in size from 5 to 11 millimetres (0.2 to 0.4 inch), is responsible for about 90 percent of human hookworm infections that occur in tropical and subtropical regions of the world. Ancylostoma duodenale, 8 to 13 millimeters long, is found on all continents but is most prevalent in warm regions. A. braziliense, from 8 to 11 millimetres long, is normally parasitic in dogs and cats; man, however, is sometimes infected by this species in the southern United States, South America, and Asia. A. ceylanicum, normally parasitic in dogs, is sometimes found in man in South America and Asia. A. duodenale, possesses four hooklike teeth in its adult stage, and N. americanus has plates in its mouth rather than teeth.

Both species of hookworm have similar life cycles. The adult worm attaches itself to the mucosal tissue lining the small intestine, where the female may produce several thousands eggs a day, which are passed in the feces. If the fecal matter reaches suitable soil, the eggs are hatched, and the infective, threadlike larvae may penetrate human skin, usually that of the foot, by way of the sweat glands and hair follicles. They then invade the lymph and blood vessels, reach the lungs, and pass up the respiratory tree to reach the mouth, where they are swallowed and sent to the small intestine; there they mature and start a new reproductive cycle. The intestinal organisms are reputedly long-lived, with a lifetime that may span some 10 years.

The symptoms of hookworm disease ordinarily begin with ground itch, an itchy skin irritation caused by the larvae when they penetrate the skin and marked by papules and vesicles that are often located between the toes. In passing through the lungs, the larvae may produce coughing and fever. In the intestine, the mature worm sustains its life by bloodsucking, and persistent feeding by many worms over many years results in secondary anemia.

In heavy infestations (which may involve more than 500 larvae), the general symptoms include pallor of the skin and mucous membranes, fluid retention in the face and extremities, constipation alternating with diarrhea, abdominal tenderness, increased appetite for bulky or strange substances (e.g., clay), delayed puberty and stunted growth, fatigue, dullness, and apathy. Hookworm infestation tends to be constantly widespread in varied regions of the world.

The tapeworm needs two hosts to complete its life cycle:

•   Intermediate host – such as sheep, pigs, cattle, goats, horses, camels, wallabies and kangaroos. Infection begins when the grazing animal eats dog or dingo faeces infected with tapeworm eggs. The eggs hatch in the animal’s gut into embryos (called oncospheres). These embryos penetrate the wall of the intestine and are carried in the bloodstream to vital organs such as the liver, lungs or brain, where they can develop into watery ‘blisters’ called hydatid cysts. These cysts contain around 30 to 40 tapeworm heads (the first segment of the tapeworm). A mature fertile cyst may contain several million such heads.

Definitive host – such as dogs and dingoes. Infection begins when the animal eats offal that contains hydatid cysts. The swallowed cysts burst and the tapeworm heads travel to the gut and attach themselves to the intestine wall. The tapeworms are mature after about six weeks. An adult E. granulosis tapeworm is only six millimetres long. Thousands can inhabit the gut of an infected animal. Each mature worm grows and sheds the last segment of its body about every two weeks. This last segment contains immature eggs. The eggs are passed from the animal’s body in faeces and may stick to the animal’s hair or contaminate the vegetable garden. The eggs are highly resistant to weather conditions and can remain viable for months. The eggs have to be swallowed by an animal (intermediate host) to form hydatid cysts.