INTRODUCTION TO PARASITOLOGY 



Site's transmission. For instance, the 

 common dog hookworm in most parts of 

 the United States is Ancylostoma caninum, 

 but in Canada it is Uncinaria slenocephala. 

 This is due to a difference in temperature 

 tolerance of the free-living larval stages. 



Local conditions such as ground cover 

 are also important. If the vegetation is 

 open so that the sunlight can get down to 

 the surface of the soil where a parasite's 

 eggs, cysts or free-living stages are 

 found, survival will be much less, trans- 

 mission will be reduced and the numbers 

 of affected hosts will be fewer than if the 

 vegetation is thick and protective. Or the 

 kinds and numbers of parasites in a herd 

 of animals confined to a low, moist pas- 

 ture may be quite different from those in 

 a herd kept on a hill pasture or on drylot. 



A fourth factor is that of the distribu- 

 tion of acceptable intermediate hosts. 

 Trypanoso7na briicei occurs only in Africa 

 because its tsetse fly intermediate hosts 

 occur only there. The fringed tapeworm 

 of sheep, Thysanosoma actinioides, is 

 found in the western United States but not 

 in the east despite the fact that infected 

 sheep have repeatedly been introduced 

 onto eastern pastures. A suitable inter- 

 mediate host does not occur on these 

 pastures, so the parasite cannot be trans- 

 mitted. 



A fifth factor is that of chronologic 

 time. The quantitative host spectrum may 

 be quite different in the same locality at 

 different periods, particularly if an erad- 

 ication campaign has been carried out in 

 the interim. Echinococcosis is a case in 

 point. At one time it was extremely com- 

 mon in the dogs, sheep and people in Ice- 

 land, but it has now been eradicated. 

 Gapeworms were once common in poultry 

 in the United States, but as the result of 

 modern poultry management practices 

 they are now exceedingly rare in chickens 

 and turkeys, altho they are not uncommon 

 in pheasants. 



A sixth factor is that of the ethology 

 or habits of the host. A species may be 

 highly susceptible to infection with a par- 

 ticular parasite, yet natural infections 



may seldom or never occur. The habits 

 of the host may be such that it rarely 

 comes in contact with a source of infec- 

 tion even tho both exist in the same local- 

 ity. For example, wild mink in the mid- 

 western United States are not infrequently 

 infected with the lung fluke, Paragonimus 

 kelUcotti. It is easy to infect dogs with 

 this fluke experimentally, yet it is ex- 

 tremely rare in midwestern dogs. The 

 reason is that dogs rarely eat the cray- 

 fish which are the fluke's intermediate 

 host. 



Because of these factors, we must 

 speak of «a///rrt/ and potential host spec- 

 tra. The latter term refers to the abso- 

 lute infectability of potential hosts and not 

 to the natural situation. The natural host 

 spectrum is an expression of the actual 

 situation at a particular time and place. 

 The two spectra may be quite different, 

 and of course the natural one will vary 

 considerably, depending on the circum- 

 stances. The complete host spectrum has 

 not been worked out for any parasite, and 

 to do so would be a very time-consuming 

 process. However, it will have to be 

 done, at least for the more important 

 parasites, before we can fully understand 

 their ecology and the epidemiology of the 

 diseases they cause. 



Certain parasites and diseases occur 

 in man alone, others in domestic animals 

 alone, and others in wild animals alone. 

 Still others, including some important 

 ones, occur in both man and domestic 

 animals, man and wild animals, domestic 

 and wild animals, or in all three. A 

 knowledge of their host relations is im- 

 portant in understanding their ecology and 

 epidemiology. 



A disease which is common to man 

 and lower animals is known as a zoo)iosis. 

 Zoonoses were redefined in 1958 by the 

 Joint WHO FAO Expert Committee on 

 Zoonoses as "those diseases and infec- 

 tions which are naturally transmitted be- 

 tween vertebrate animals and man" (World 

 Health Organization, 1959). Less than 20 

 years ago it was said that there were 50 

 zoonoses, but in the above report the 

 World Health Organization listed more 



