Introduction to Parasitology

Parasitology is the science which treats of parasites. The word "parasite" is derived from the Greek and means, literally, "situated beside". It was used by the ancient Greeks originally for people who ate beside or at the tables of others, and referred both to sycophants or hangers-on and to priests who collected grain for their temples. While the social meaning of the term has been partially retained, it has been given a new connotation by scientists. Parasites are defined as organisms which live on or within some other living organism, which is known as the host. Parasitism is the association of two such organisms.

Parasites may be either animals or plants - viruses, rickettsiae, spirochetes, bacteria, yeasts, fungi, algae, mistletoe, dodder, protozoa, helminths, arthropods, molluscs, and even certain vertebrates such as the cuckoo. The general principles of parasitology apply to all. However, in this book we shall deal primarily with animal parasites, leaving the plant parasites to textbooks of microbiology.

In our everyday thinking we consider that animals can live in three main habitats - land, fresh water and sea water. A fourth habitat is the parasitic one, which is quite different from the other three. As a matter of fact, there are quite a few different parasitic habitats, each with its own characteristics. Parasites are found in the lumen of the intestinal tract, on the outside of the body, in the skin, in various tissues, in the blood plasma, inside different types of cells, and even inside cell nuclei.

Parasites have arisen from free-living animals. Some parasites closely resemble their free-living relatives, but others have undergone structural changes which make them more suited to their changed environment. Since these changes have in many cases been the loss of some power which their free-living relatives possess, parasites have some-times been considered degenerate creatures. The opposite is true. Parasites are highly specialized organisms. Those powers which were unnecessary, they have lost. For instance, the adults of most parasitic worms have relatively little ability to move around. But they don’t need it. Too much activity might even lead to their reaching the point of no return and being discharged from their host’s body.

As another example, tapeworms have no intestinal tract. But, since they obtain their nourishment directly thru the body wall, an intestine would be superfluous. Thus, in the case of parasites as with all other animals and plants, the useless has been eliminated in the course of evolution.

In contrast, the reproductive system of parasites is often tremendously developed. Since the chances of an egg or larva leaving one host and infecting another are very small, the numbers of eggs produced must be very large. Many parasitic worms produce thousands of eggs a day. The female of Ascaris suum, the large roundworm of swine, lays about 1, 400, 000 eggs per day (Kelley and Smith, 1956). Assuming that she lives 200 days, which is not an excessive life, she will have laid 280 million eggs in her lifetime. Since the number of Ascaris in the world is staying more or less the same, we can conclude that on the average only two of these eggs will produce adult worms - a male and a female. The chance of any particular egg ever becoming a mature worm is thus about 1 in 140 million, which is much less than a man’s chance of being struck by lightning.

The broad fish tapeworm of the dog, man and other animals, Dibothriocephalus latus, will produce over 4 miles of segments containing 2 billion eggs during a 10-year life span, and again the number of these tapeworms is not increasing. Since these tapeworms are hermaphrodites, each egg can become an egg-laying worm, but its chances of doing so are a hundred times less than those of the Ascaris egg. Parasites are continually being confronted by odds of this sort and are continually surmounting them.

Life of one sort or another seems to have flowed into every possible niche. Parasites live in some of the most difficult niches, and it is remarkable how they have succeeded in surviving in them. Parasites have tremendous problems to solve - problems of nutrition, of respiration, of excretion, of getting from one host to another - and the different and often ingenious ways in which different parasites have solved these problems are amazing. Some of their adjustments are almost perfect; others are less satisfactory. In general, we may say that the more satisfactory the solution, the more abundant are the parasites. The rare ones are the less successful ones.

We can think of parasitism as related basically to the solution of the problem of nutrition, and we can think of the other problems as somewhat secondary. This is obviously an incomplete and defective view, but nevertheless it has some value.

Living organisms have four general types of nutrition. Holophytic nutrition is typical of plants; it involves synthesis of carbohydrates by means of chlorophyll. Holozoic nutrition is animal-like; it involves ingestion of particulate food thru a permanent or temporary mouth. Saprozoic or saprophytic nutrition (the choice of term depending upon whether the organism is an animal or plant) involves absorption of nutrients in solution thru the body wall. The fourth type of nutrition is that employed by viruses, which synthesize their proteins directly from the host’s amino acids and do not have a true body wall during their parasitic phase.

The terms saprophyte and saprophytic are often used by bacteriologists in another sense also, to refer to non-pathogenic, non-parasitic organisms. The terms saprozoite and saprozoic are also similarly used with reference to free-living animals, but much less frequently.

Coprozoic or coprophilic organisms are animals which live in feces. They may be either saprozoic or holozoic or both, and are sometimes mistaken for true parasites.

Parasites resemble predators in some respects; indeed, one grades into the other. In general, we think of predators as larger than or as large as their prey, while we think of parasites as considerably smaller. A lion seizing an antelope is a predator, as is a spider capturing a fly. But there is a distinction only in size of prey between a predatory assassin bug capturing another insect and sucking out its juices and the closely related, parasitic kissing bug sucking blood out of a man. And a mosquito is just as much a predator as the kissing bug. The distinction is one of degree. As Elton (1935) put it, "The difference between a carnivore and a parasite is simply the difference between living upon capital and income, between the burglar and the blackmailer. The general result is the same although the methods employed are different."