10 



INTRODUCTION TO PARASITOLOGY 



make for the greatest understanding of the 

 organisms we study and of their relations 

 with each other and with their hosts. 



Parasite evolution : Parasites have 

 evolved along with their hosts, and as a 

 consequence the relationships between the 

 parasites of different hosts often give val- 

 uable clues to the relationships of the 

 hosts themselves. Certain major groups 

 of parasites are confined to certain groups 

 of hosts. Sucking lice are found only on 

 mammals. Biting lice occur primarily on 

 birds, but a few species are found on 

 mammals. The monogenetic trematodes 

 are found almost without exception on fish; 

 some of the more highly evolved digenetic 

 trematodes are found in fish, but more 

 occur in higher vertebrates. There is a 

 tendency, too, for the more advanced di- 

 genetic trematodes to occur in the higher 

 host groups. 



One would expect that, as evolution 

 progressed in different host groups, there 

 would develop in each one its own group 

 of parasites. This has often occurred. 

 Thus, of the 48 families of digenetic 

 trematodes listed by Dawes (1956), 17 

 occur only in fish, 8 only in birds, 3 only 

 in mammals, 2 in fish and amphibia, 3 in 

 reptiles and birds, 6 in birds and mam- 

 mals, 1 in fish, amphibia and reptiles, 

 2 in reptiles, birds and mammals, 1 in 

 amphibia, reptiles and birds, 3 in all but 

 fish, and 2 in all five classes of verte- 

 brates. Of the 11 classes of tapeworms 

 recognized by Wardle and McLeod (1952), 

 4 are found only in elasmobranch fish, 3 

 only in teleosts, 1 only in birds, 1 in 

 teleosts, amphibia and reptiles, 1 in tel- 

 eosts, birds and mammals, and 1 in am- 

 phibia, reptiles, birds and mammals. 



This same tendency is apparent even 

 in parasitic groups which are quite widely 

 distributed. For example, many reptiles 

 and mammals (but not birds) have pin- 

 worms of the family Oxyuridae, but each 

 group has its own genera. Iguanas have 

 Ozolaimus and Macrae is, other reptiles 

 have Thelandros , Pharyngodon and sev- 

 eral other genera, rodents have Asfiicu- 

 liiris, Syphacia and Wellcomia, rabbits 

 have Passalurus, equids have Oxyuris, 



ruminants have Skrjabinema, and man and 

 other primates have Enlerobius . 



On the other hand, there are many 

 exceptions to this general rule, and it 

 cannot be used without corroboration as 

 the sole criterion of host relationship. 

 Many fish-eating birds and mammals have 

 the same species of trematodes for which 

 fish act as intermediate hosts. And the 

 fact that the pig and man share a surpris- 

 ing number of parasites is no proof of 

 their close relationship despite their sim- 

 ilarity of character and personality; it 

 simply reflects their omnivorous habits 

 and close association. 



Adaptation to parasitism : Adaptation 

 to a parasitic existence has required many 

 modifications, both morphological and 

 physiological. Locomotion, at least of the 

 parasitic stages, has often become re- 

 stricted. Certain organs and organ sys- 

 tems may be lost. Tapeworms lack an in- 

 testine altho their ancestors presumably 

 had one, and adult trematodes have no eye- 

 spots altho their turbellarian ancestors 

 and many of their larvae have them. 

 Parasitic amoebae have no contractile 

 vacuoles altho their free-living relatives 

 do. 



In contrast, many structures are 

 modified or hypertrophied for the para- 

 sitic life. Many helminths have hooks and 

 suckers to help them hold their position. 

 The protozoon, Gianiia, has turned most 

 of its ventral surface into a sucking disc. 

 The mouthparts of many insects and mites 

 have become highly efficient instruments 

 for tapping their hosts' blood supply. The 

 chigger, which does not suck blood, has 

 developed a method of liquefying its hosts' 

 tissues. The food storage organs of many 

 parasites have been enlarged. Many blood- 

 sucking arthropods which are unable to ob- 

 tain all the nutrients they need from blood, 

 have established symbiotic relationships 

 with various microorganisms and have 

 formed special organs for them. 



The reproductive system of many 

 parasites has been hypertrophied to pro- 

 duce tremendous numbers of eggs. Other 

 parasites, such as the trematodes, have 



