INTRODUCTION TO PARASITOLOGY 



infestation to parasitism by external para- 

 sites. 



The term life cycle refers to the de- 

 velopment of a parasite thru its various 

 forms. It may be simple, as in an organ- 

 ism which multiplies only by binary fis- 

 sion, or it may be extremely complex, 

 involving alternation of sexual and asexual 

 generations or development thru a series 

 of different larval forms. A )>ionogenetic 

 parasite is one in which there is no alter- 

 nation of generations. Examples of this 

 type are bacteria, flagellate protozoa 

 such as Trichomonas , nematodes such as 

 Ascaris and Ancylostoma, and the ecto- 

 parasitic fish trematodes of the order 

 Monogenorida (= Monogenea). A hetero- 

 genetic parasite is one in which there is 

 alternation of generations. Examples of 

 this type are malarial parasites and coc- 

 cidia, in which sexual and asexual gen- 

 erations alternate, the endoparasitic 

 trematodes of higher vertebrates of the 

 order Digenorida (= Digenea), in which 

 there may be several larval multiplicative 

 stages before the adult, and the nematode, 

 Strongyloides, in which one generation is 

 parasitic and parthenogenetic while an- 

 other is free-living and sexual. 



Depending on their type, parasites 

 may live in only one or in a number of 

 different types of hosts during the course 

 of their normal life cycles. A monoxen- 

 ous parasite has only one type of host-- 

 the definitive host. Examples are coc- 

 cidia, amoebae, hookworms, fish trema- 

 todes, horse bots, streptococci and most 

 pox viruses. A heteroxenous parasite 

 has two or more types of host in its life 

 cycle. Examples are the malarial para- 

 sites, most trypanosomes, trematodes of 

 higher vertebrates, filariae, tapeworms, 

 the rickettsiae, yellow fever virus and 

 various encephalitis viruses. 



These two pairs of terms are inde- 

 pendent of each other. Parasitic amoebae 

 and hookworms are monogenetic and 

 monoxenous. Filariid and spirurid nema- 

 todes are monogenetic and heteroxenous. 

 Strongyloides and most coccidia are het- 

 erogenetic and monoxenous. Malarial 

 parasites and trematodes of birds and 



mammals are heterogenetic and hetero- 

 xenous. 



Another group of terms deals with 

 host range, i.e. , the number of host spe- 

 cies in which a particular parasite may 

 occur. These parasites can be either 

 monoxenous or heteroxenous, monogen- 

 etic or digenetic. Indeed, there may be 

 a difference in host-restriction between 

 the definitive and intermediate hosts of 

 the same parasite. For example, the 

 blood fluke, Schistosoma japoniciini, can 

 become adult in a rather wide range of 

 mammals, but its larval stages will de- 

 velop in only a few closely related species 

 of snails. 



The term, monoxenous parasite, is 

 used by some authors for a parasite which 

 is restricted to a single host species. 

 Such parasites undoubtedly exist, but they 

 are fewer than our present records indi- 

 cate. The human malarial parasites were 

 once thought to be monoxenous in this 

 sense of the word, but they have more re- 

 cently been found capable of infecting 

 apes, and it is now known that chimpan- 

 zees in West Africa are naturally infected 

 with P. malariae, the cause of quartan 

 malaria in man (Garnham, 1958). Many 

 species of coccidia are also known from 

 but a single host, but for the most part 

 closely related wild hosts have not been 

 examined nor have cross transmission ex- 

 periments been attempted with them. Be- 

 cause of this and because of the confusion 

 arising between this usage of monoxenous 

 and the one defined above, this usage 

 should be avoided. 



A stenoxenoiis parasite is one which 

 has a narrow host range. Among the 

 coccidia, members of the genus Eimeria 

 are generally stenoxenous, as are the 

 human malaria parasites and cyclophyl- 

 lidorid tapeworms. Many nematodes such 

 as the hookworms, nodular worms, fila- 

 riids and spirurids tend to be stenoxenous. 

 Both biting and sucking lice are steno- 

 xenous, and many are even limited to 

 specific areas on their host. Relatively 

 few bacteria are stenoxenous, but Strep- 

 tococcus agalactiae, Mycobacterium 

 leprae, Vibrio, Mycoplasma, the spiro- 



