INTRODUCTION TO PARASITOLOGY 



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 consider- 

 ably 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, para- 

 sitic kissing bug sucking blood out of a 

 man. And a mosquito is just as much a 

 predator as the kissing bug. The distinc- 

 tion is one of degree. As Elton (1935) put 

 it, "The difference between a carnivore 

 and a parasite is simply the difference be- 

 tween living upon capital and income, be- 

 tween the burglar and the blackmailer. 

 The general result is the same although 

 the methods employed are different." 



TYPES OF PARASITISM 



There are several types of parasitism. 

 Parasitism itself is defined as an associa- 

 tion between two specifically distinct or- 

 ganisms in which one lives on or within 

 the other in order to obtain sustenance. 



SvDibiosis is the permanent associa- 

 tion of two specifically distinct organisms 

 so dependent upon each other that life 

 apart is impossible under natural condi- 

 tions. The relation between many ter- 

 mites and their intestinal protozoa is 

 symbiotic. The termites eat wood, but 

 they cannot digest it; the protozoa can di- 

 gest wood, turning it into glucose, but 

 they have no way of obtaining it; working 

 together, the termites ingest wood par- 

 ticles, the protozoa break the cellulose 

 down to glucose, and the termites then 

 digest the glucose. Lichens furnish an- 

 other example of symbiosis. They are 

 composed of certain species of algae and 

 fungi living together. 



Many insects, ticks and mites have 

 symbiotic bacteria and rickettsiae. The 



symbiotic organisms are found either in 

 special cells, the mycetocytes, in modi- 

 fied parts of the Malpighian tubules, or 

 in special organs, the mycetomes. It is 

 significant that, among blood-sucking 

 arthropods, symbiosis occurs in those 

 which live on blood thruout their life cy- 

 cles (ticks, lice, bedbugs, kissing bugs, 

 tsetse flies, hippoboscid flies) but not in 

 those in which only the adults suck blood 

 while the larvae are free-living (fleas, 

 mosquitoes, phlebotomines, tabanids and 

 stable flies). Blood lacks some metabo- 

 lites which the arthropods are unable to 

 synthesize themselves and for which they 

 depend on their symbiotes. These meta- 

 bolites appear to include vitamins of the 

 B group and probably other substances as 

 well (Buchner, 1953; Koch, 1956; Weyer, 

 1960). 



Mutualism is an association of two 

 organisms by which both are benefited. 

 It differs from symbiosis in that it is not 

 obligatory for both partners. One exam- 

 ple often cited is that of a sea anemone 

 living upon the back of a crab. The anem- 

 one is benefited by being moved to new 

 hunting grounds and by obtaining morsels 

 of food torn off by the crab, while the crab 

 is protected by the bulk and stinging ten- 

 tacles of the anemone. Another marine 

 example is that of the scorpion fish of 

 Indo-Malaya. It lives on the bottom of the 

 sea, where it lies in wait for passing fish. 

 It is covered with a crust of hydroids 

 which camouflage it so that it can seize 

 its unwary prey more easily. The hydroids 

 presumably benefit by being moved to new 

 sources of food and by being provided with 

 a dwelling-place. However, since they 

 can live other places beside the scorpion 

 fish's back, their relation is mutualistic. 



Another example of mutualism, and 

 one closer to us, is the relationship be- 

 tween ruminants and the cellulose-digest- 

 ing bacteria and other micro-organisms 

 in their rumens. The latter are furnished 

 a favorable home by their hosts and aid 

 them by breaking down cellulose to usable 

 compounds. The rumen-dwelling bacteria 

 which produce B group vitamins and thus 

 make an outside source of them unneces- 

 sary for ruminant nutrition probably 



