INTRODUCTION TO PARASITOLOGY 



11 



developed life cycles in which the larvae 

 also multiply. 



In the parasites with high reproduc- 

 tive rates, infection is left largely to 

 chance. Many other parasites, however, 

 have developed life cycles in which chance 

 is more or less eliminated. In these, the 

 reproductive rate is low. The larva of 

 the sheep ked, Melophagns ovinus, devel- 

 ops to maturity in the body of its mother 

 and pupates immediately after emerging. 

 The pupa remains in its host's wool. The 

 female tsetse fly, too, produces fully de- 

 veloped larvae. The tropical American 

 botfly, Derinaiobia honiinis, captures a 

 mosquito and lays her eggs on it. These 

 hatch when the mosquito lights to suck 

 blood, and the larvae enter the host. 



Morphological and developmental 

 modifications are the most obvious ones, 

 but biochemical ones are even more im- 

 portant. How do parasites survive in 

 their hosts without destruction? What 

 keeps those which live in the intestine 

 from being digested along with the host's 

 food? Why is it that morphologically sim- 

 ilar species are restricted to different 

 hosts which themselves may be morpho- 

 logically quite similar? 



The second question has been an- 

 swered by saying that the same mecha- 

 nism operates which prevents the hosts 

 from digesting themselves, that the para- 

 sites protect themselves by producing 

 mucus or that mucoproteins in their in- 

 tegument protect them, that they secrete 

 antienzymes, or that the surface mem- 

 brane of living organisms is impermeable 

 to proteolytic enzymes. However, much 

 more research must be done before a 

 satisfactory answer can be given. An- 

 swers given to the first and third ques- 

 tions are vague. Compatibility of host 

 and parasite protoplasm is invoked, but 

 all this does is put a name to the beast. 

 The question of how this compatibility is 

 brought about remains unanswered, and a 

 great deal of biochemical and immuno- 

 chemical research must be done before it 

 can be answered (see Becker, 1953; Read, 

 1950; von Brand, 1952). 



Injurious effects of parasites on their 

 hosts . Parasites may injure their hosts 

 in several ways: 



1. They may suck blood (mosquitoes, 

 hookworms), lymph (midges) or exu- 

 dates (lungworms). 



2. They may feed on solid tissues, either 

 directly (giant kidney worms, liver 

 flukes) or after first liquefying them 

 (chiggers). 



3. They may compete with the host for 

 the food it has ingested, either by in- 

 gesting the intestinal contents (asca- 

 rids) or by absorbing them thru the 

 body wall (tapeworms). In some cases 

 they may take up large amounts of cer- 

 tain vitamins selectively, as the broad 

 fish tapeworm does with Vitamin B12. 



4. They may cause mechanical obstruc- 

 tion of the intestine (ascarids), bile 

 ducts (ascarids, fringed tapeworm), 

 blood vessels (dog heartworm), lymph- 

 atics (filariids), bronchi (lungworms) 

 or other body channels. 



5. They may cause pressure atrophy 

 (hydatid cysts). 



6. They may destroy host cells by grow- 

 ing in them (coccidia, malaria para- 

 sites). 



7. They may produce various toxic sub- 

 stances such as hemolysins, histoly- 

 sins, anticoagulants, and toxic prod- 

 ucts of metabolism. 



8. They may cause allergic reactions. 



9. They may cause various host reac- 

 tions such as inflammation, hyper- 

 trophy, hyperplasia, nodule forma- 

 tion, etc. 



10. They may carry diseases and para- 

 sites, including malaria (mosquitoes), 

 trypanosomosis (tsetse flies), swine 

 influenza (lungworms), salmon poison- 

 ing of dogs (flukes), heartworms (mos- 

 quitoes) and onchocercosis (blackflies). 



11. They may reduce their hosts' resis- 

 tance to other diseases and parasites. 



A great deal more could be said about 

 this subject. Additional information is 

 given in the symposium on mechanisms of 

 microbial pathogenicity of the Society for 

 General Microbiology (Howie and O'Hea, 

 1955). 



