INTRODUCTION TO PARASITOLOGY 



common types of parasitism are the last 

 three. Of these, commensalism is 

 clearly the most desirable, both from the 

 standpoint of the host (which isn't harmed) 

 and of the parasite. Parasitosis, which 

 harms the host, is in the long run harm- 

 ful to the parasite also. By injuring their 

 hosts, parasites harm their environment, 

 and if they are so indiscreet as to kill 

 their hosts, they die too. Parasitiasis 

 is intermediate between parasitosis and 

 commensalism in some cases, but not in 

 all. 



HOST- PARASITE 

 RELATIONS 



Depending on their species, para- 

 sites may live in any organ or tissue of 

 the host; they may live on its surface, or 

 they may spend most of their time away 

 from it. Special terms have been applied 

 to these relationships. An endoparasite 

 is a parasite that lives within the host's 

 body. An ectoparasite is one that lives 

 on the outside of the body. An erratic 

 {oT aberrant) parasite is one that has 

 wandered into an organ in which it does 

 not ordinarily live. An incidental para- 

 site is a parasite in a host in which it does 

 not usually live. A facultative parasite is 

 an organism that is capable of living 

 either free or as a parasite. An obliga- 

 tory parasite is an organism which must 

 live a parasitic existence. A periodic 

 parasite is one which makes short visits 

 to its host to obtain nourishment or other 

 benefits. A pseudoparasite is an object 

 that is mistaken for a parasite. Para- 

 sites may themselves be parasitized by 

 hyperparasites . 



An organism which harbors a para- 

 site is its host. There are several types 

 of host. A definitive host is the host 

 which harbors the adult stage of a para- 

 site. An intermediate host is the host 

 which harbors the larval stages of the 

 parasite. A first intermediate host is 

 the first host parasitized by the larval 

 stages of the parasite. A second inter- 

 mediate host is the host parasitized by 

 the larval stages at a later period in the 

 life cycle. A paratenic or transport host 



is a second (or third) intermediate host 

 in which the parasite does not undergo any 

 development but usually remains encysted 

 until the definitive host eats the paratenic 

 host. 



The vector of a parasite or disease 

 agent is an arthropod, mollusc or other 

 agent which transmits the parasite from 

 one vertebrate host to another. If the 

 parasite develops or multiplies in the 

 vector, it is called a biological vector. 

 If the parasite does not develop or mul- 

 tiply in it, it is called a mechanical vec- 

 tor. 



Intermediate hosts of helminths are 

 biological vectors, but biological vectors 

 are not necessarily intermediate hosts. 

 Indeed, the latter term has no applica- 

 tion to protozoa, bacteria, rickettsia or 

 viruses, none of which have larvae. Mos- 

 quitoes are biological vectors of malaria 

 and of yellow fever, and the tsetse fly is 

 a biological vector of Trypanosoma 

 brucei, for the parasites must develop in 

 them to become infective for the next ver- 

 tebrate host. However, tabanid flies are 

 merely mechanical vectors of Trypano- 

 soma evansi, since the parasites undergo 

 no development in them. 



The terms infection and infestation 

 are used by different people in different 

 ways. The former term originally re- 

 ferred to internal agents of disease, 

 while the latter was used with reference 

 to external harassing agents, including 

 not only ectoparasites but also rodents, 

 pirates and thieves. This usage was cur- 

 rent during the latter part of the nine- 

 teenth century. Later on, it was felt de- 

 sirable to distinguish between parasites 

 which multiplied in their hosts and those 

 which did not. "Infection" was then used 

 for the former type of parasitism, and 

 "infestation" for the latter. This usage 

 was popular for a time, but it was never 

 universally accepted. More recently 

 there has been a trend toward the older 

 usage. Most American parasitologists 

 have accepted it, but most British ones 

 prefer to speak of helminth infestations. 

 In this book infection will be used to refer 

 to parasitism by internal parasites, and 



