INTRODUCTION TO PARASITOLOGY 



chete, Treponema, the rickettsiae, Ana- 

 plasma, Eperythrozoon, Haemobartonella 

 and Cowdria, and the viruses of hog chol- 

 era, duck hepatitis and yellow fever are 

 stenoxenous. 



5. 7% of its mammals. If all these pos- 

 sible hosts were to be examined, one 

 might expect to find some 3500 species of 

 Eimeria in mammals and 34,000 in chor- 

 dates. 



An euryxenous parasite is one which 

 has a broad host range. Among the coc- 

 cidia, members of the genus Isospora 

 are often euryxenous. So are most try- 

 panosomes, most Plasmodium species 

 (but not those affecting man), and many 

 species of Trichomonas. Most trema- 

 todes are euryxenous, as are Trichinella, 

 Dracunculns and Dioctophyma among the 

 nematodes. Fleas, chiggers and many 

 ticks are euryxenous. Most parasitic 

 bacteria are euryxenous; examples are 

 most species of Salmonella, Escherichia, 

 Brucella, Erysipelothrix and Listeria. 

 Among euryxenous rickettsiae are 

 Rickettsia, Coxiella and Miyagawanella 

 psittacii. Among euryxenous viruses are 

 those of rabies and many encephalitides. 

 Leptospira and Borrelia are euryxenous 

 spirochetes. 



The use of these two terms, however, 

 may be deceptive. There exist in nature 

 all intergrades between them, and all we 

 have done has been to pick out the two 

 extremes of a continuum and give them 

 names. 



Actually, the host range of most 

 parasites is broader than generally sup- 

 posed. The fact is that most animal 

 species have not been examined for para- 

 sites. For example, the genus Eimeria 

 is one of the commonest and best known 

 among parasitic protozoa. Becker (1956) 

 listed 403 species, of which 394 were 

 from chordates and 202 from mammals. 

 This is quite impressive, especially to 

 someone who wishes to study their tax- 

 onomy. However, according to Muller 

 and Campbell (1954), there are 33,640 

 known living species of chordates and 

 3552 of mammals. Some hosts have 

 more than one species of Eimeria, but 

 some coccidian species occur in more 

 than one host. Assuming that these more 

 or less cancel out, we can calculate that 

 Eimeria has been described from only 

 1. 17% of the world's chordates and from 



So far only the qualitative aspect of 

 the host range has been discussed. How- 

 ever, altho a parasite may be capable of 

 living in more than one host, it is much 

 more common in some hosts than in 

 others. The principal hosts of a parasite 

 are those hosts in which it is most com- 

 monly found. The supplementary hosts 

 are those of secondary importance, and 

 the incidental hosts are those which are 

 infected only occasionally under natural 

 conditions. To these should be added 

 experimental hosts, which do not normally 

 become infected under natural conditions 

 but which can be infected in the laboratory. 

 This last category may include both inci- 

 dental and supplementary hosts and also 

 hosts never infected in nature. 



In order to take into account this 

 quantitative aspect of the host-parasite 

 relationship, the terms quantitative host 

 spectrum or quantitative host range are 

 used. These give the amount of infection 

 present in each infected species. 



Several factors affect the quantita- 

 tive host spectrum. One is geographic 

 distribution. The natural quantitative 

 spectrum may be quite different in one 

 locality than in another. The species of 

 animals present may be different, or the 

 incidence of infection may be different. 

 For example, a number of nematodes 

 parasitize both domestic and wild rumi- 

 nants. However, since the wild rumi- 

 nants of North America and Africa are 

 not the same, the quantitative host spec- 

 tra of the same parasites on the two con- 

 tinents are different. The spectrum is 

 still different in Australia, where there 

 are no wild ruminants but where wild 

 rabbits are susceptible to infection with 

 a few ruminant nematodes. 



A second factor is climate. Many of 

 the same host species may be present in 

 different areas but climatic conditions in 

 one area may prevent or favor a para- 



