INTRODUCTION TO PARASITOLOGY 



13 



Geographic Distribution . Some para- 

 sites, particularly those of man and his 

 domestic animals, are worldwide in dis- 

 tribution, but others are much more re- 

 stricted. But even a widely distributed 

 species may be much more prevalent in 

 one region than another. Many factors 

 are responsible, some of which have al- 

 ready been discussed (pp. 7-8). A para- 

 site which originated in a particular place 

 in a particular host species may never 

 have been introduced into some other lo- 

 cality or host where it could develop per- 

 fectly well. It may have been introduced 

 but may have died out because a suitable 

 vector was lacking or because the climate 

 was not suitable. The ox warble has not 

 been able to establish itself in the south- 

 ern hemisphere because the reversal of 

 seasons has prevented it from completing 

 its life cycle. 



Whenever domestic animals are in- 

 troduced into a new region, there is a 

 good possibility that they will pick up 

 some of the parasites of their wild rela- 

 tives there. The parasite spectrum of 

 cattle in Africa differs from that in North 

 America, both of these differ from the 

 spectrum in Europe, and all three differ 

 from the spectrum in Australia. Wild 

 animals, too, may acquire parasites 

 from domestic ones or from other wild 

 species. Hence, the parasite spectrum 

 of animals in zoos may be quite different 

 from that in their normal habitat, and the 

 success of an attempt to introduce a new 

 game bird or mammal into a region may 

 depend in part on the parasites and dis- 

 eases that it encounters. 



The importance of wildlife as a para- 

 site reservoir for domestic animals is 

 well illustrated by the report of Longhurst 

 and Douglas (1953) on the interrelation- 

 ships between the parasites of domestic 

 sheep and Columbian black-tailed deer in 

 the north coastal part of California, 

 where the two live on the same range. 

 They found in their survey of 63 sheep 

 and 81 deer that 1 species of trematode, 

 5 of cestodes, and 13 out of 18 species 

 of nematodes were common to both 

 hosts. 



Origin of Parasitism . Parasites 

 originated from free-living ancestors. 

 The process probably began soon after 

 the first living forms appeared. The 

 change from a free-living to a parasitic 

 habitat has taken place many times in the 

 course of evolution. It has occurred as 

 new major groups appeared, it has taken 

 place independently many times in each 

 group, and it is undoubtedly still occur- 

 ring. Once established, the parasites 

 evolved along with their hosts. 



In some cases, the parasites first 

 invaded the host thru the integument, like 

 Pelodera and related rhabditid nematodes. 

 In other cases, the parasites were swal- 

 lowed along with their host's food. Para- 

 sites with life cycles involving two or 

 more hosts became established first in 

 one host, and later on developed their 

 more complicated life cycles. The try- 

 panosomes, for instance, were originally 

 gut parasites of insects and only later be- 

 came blood parasites of vertebrates. 



Preadaptation was necessary for 

 parasites to become established. They 

 must have had the ability to survive and 

 reproduce in the host before they entered 

 it. By far the great majority of free-liv- 

 ing forms which entered the alimentary 

 canal of some larger animal were killed 

 and digested, but some of them were able 

 to resist this process and a few were able 

 to live there. Some of the factors involved 

 have already been discussed (p. 10). 



Economic Importance . Parasites 

 are responsible for heavy economic losses 

 to the livestock industry. These are due 

 in part to death, but even more important 

 are the losses due to illness, reduced 

 growth rate, decreased meat, milk, egg 

 and wool production and, in working ani- 

 mals, loss of working energy. It is im- 

 possible to quantitate these losses accu- 

 rately, but rough estimates can be made. 

 The U. S. Department of Agriculture 

 (1954) made such an estimate for losses 

 in agriculture during the ten-year period, 

 1942-1951. The figures on parasite losses 

 in Table 1 are taken from this publication. 

 Further details are given in the publication 

 itself and by Schwartz et al. (1955). 



