INTRODUCTION TO PARASITOLOGY 



times been considered degenerate crea- 

 tures. 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 ob- 

 tain their nourishment directly thru the 

 body wall, an intestine would be super- 

 fluous. Thus, in the case of parasites as 

 with all other animals and plants, the use- 

 less has been eliminated in the course of 

 evolution. 



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 diffi- 

 cult niches, and it is remarkable how they 

 have succeeded in surviving in them. 

 Parasites have tremendous problems to 

 solve--problems of nutrition, of respira- 

 tion, 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 satisfac- 

 tory. 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. 



In contrast, the reproductive system 

 of parasites is often tremendously devel- 

 oped. Since the chances of an egg or 

 larva leaving one host and infecting an- 

 other 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, Diholliriocephalus 

 laliis, will produce over 4 miles of seg- 

 ments containing 2 billion eggs during a 

 10-year life span, and again the number 

 of these tapeworms is not increasing. 

 Since these tapeworms are hermaphro- 

 dites, 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 



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 in- 

 volves ingestion of particulate food thru a 

 permanent or temporary mouth. Sapro- 

 zoic or saprophytic nutrition (the choice 

 of term depending upon whether the organ- 

 ism is an animal or plant) involves ab- 

 sorption of nutrients in solution thru the 

 body wall. The fourth type of nutrition is 

 that employed by viruses, which synthe- 

 size 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 an- 

 other sense also, to refer to non-patho- 

 genic, 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 



