II. Parasites

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A. The Ubiquitous Organisms

Parasites are everywhere. There are far more kinds of parasitic than nonparasitic organisms in the world, and organisms that are not parasites are usually hosts, harboring parasites within and upon them. Their diversity in size, complexity, and life cycles is truly astonishing, ranging from single-celled amoebas responsible for diarrheal distress to the multicellular tapeworms that can grow to 20 feet in humans and 100 feet in whales. Table 1 illustrates examples of different types of parasites.

Table 1. Examples of parasites

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B. The Parasitic Way of Life

The parasitic way of life is highly successful and is one form of a living relationship called symbiosis, which involves a relationship between two organisms that live together in close association, often with mutually beneficial results. When a symbiont actually lives at the expense of the host—that is, uses nutrients required by the host—then it is called a parasite. Some parasites live their entire mature lives within or on the host and are totally dependent on their hosts for nutrients; these types of parasites are called endoparasites. Others, such as fleas or mosquitoes, only visit for a meal—they eat and run (or fly) and are termed ectoparasites. Parasites can be unicellular protozoa, such as the plasmodia that live inside red blood cells and are the causative agent of malaria, the major infectious disease in the world today.

Most parasites, however, are multicellular. Parasitic worms include tapeworms, which live in the digestive tract and cause anemia, and schistosomes, which inhabit the veins of the bladder or intestines and are the causative agents of the disease schistosomiasis. Nematodes, another form of worm, are responsible for heartworm in dogs and blindness in humans. Arthropods, such as fleas and ticks, are temporary parasites that visit the host for frequent or occasional feedings and can also act as vectors (carriers) of parasites that cause disease. Parasitic fungi, including mushrooms, molds, and mildews, feed on plants or animals. By this definition of parasite, certain bacteria and viruses can be considered to live as such, but conventionally the term parasite refers only to eukaryotic organisms—that is, organisms that have a nucleus and other subcellular organelles.

Parasites can be transmitted in a variety of ways: in contaminated food or drinking water, by swimming in lakes and rivers, or in the feces or saliva of an insect or other animal. Table 1 indicates modes of transmission for several parasites.

A parasite is often associated with damage to the host (see Table 1). A parasite may harm its host in any number of ways: by mechanical injury, such as boring a hole in it; by eating or digesting and absorbing its tissues; by poisoning the host with toxic metabolic products; or simply by robbing the host of necessary nutrients. Most parasites inflict a combination of these conditions on their hosts. Of course, the parasite is only trying to survive, taking from its environment—its host—what it needs to sustain its life processes so that it can reproduce. Parasites do not have evil intent, any more than bacteria or viruses do.

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C. Prevention and Treatment

The treatment of parasitic diseases reflects the great diversity of parasites. Many unicellular parasites can be treated with drugs, such as quinine for malaria and arsenic derivatives for sleeping sickness (which is caused by the unicellular parasite Trypanosoma brucei). Tapeworms and nematodes can also be treated with drugs that interfere with their metabolism. Although much progress has been made in developing vaccines against three major parasitic diseases of humans —malaria, leishmaniasis, and schistosomiasis—to date, no truly effective vaccines are available that protect against parasitic diseases. Interestingly, vaccines protective against several protozoan and nematode parasites are available for animals. One reason for this may be that the conditions for testing vaccine efficacy in animals are less stringent than for humans, leading to a more rapid identification of effective vaccines.

Although no effective vaccines for any human parasitic disease exist yet, good hygiene, clean drinking water, sanitary facilities, and effective programs of insect control can prevent many parasitic diseases. A simple change of habits, such as staying out of lakes and rivers or sleeping under mosquito netting, could prevent many serious, debilitating diseases.