Mechanical disturbances by parasites — Filaria causing elephanti- 

 asis by stoppage of the lymph passages. Abundance of round- 

 worms in chickens sometimes causes plugging of the alimentary 

 canal. 



Production of specific poisons by the parasite — By the hookworm, 

 certain tapeworms, et al. 



Centers of infection produced — Injury to intestinal mucosa, or to, 

 skin. 



Dangers from intermittent parasites — Disease carriers. 



Economic value of parasites — From our viewpoint, not all para- 

 sites noxious — Parasites an important factor in the control of injurious 

 species. Fighting insect pests by the introduction of their para- 

 sites. Hyperparasites of injurious parasites. 



Determination of parasites in the living host — Ectoparasites — 

 Diagnosis usually simple, but cases sometimes complicated by 

 secondary infection. 



Endoparasites — 1. Examination of feces for protozoa, eggs 

 and larvse of intestinal and liver parasites (Hall, '11). 2. Examina- 

 tion of sputum for evidence of lung parasites. 3. Examination of 

 urine for evidences of kidney, bladder, or certain blood parasites. 

 4. Examination of blood, not only for blood parasites but for patho- 

 logical conditions indicative of the presence of certain parasites 

 elsewhere in the body. 5. "Harpooning" for trichinae. 6. Clinical 

 symptoms sometimes sufficient. 



Methods of control of parasites — Must be mainly preventive. 

 Such measures must be based on a knowledge of the life-history of 

 the parasite. 



Destruction of intermediate hosts, when feasible — Mosquitoes 

 for malaria; stray dogs and coyotes for certain sheep parasites, etc. 



Sanitary methods — Cleanliness; sewage disposal; meat inspec- 

 tion in local slaughter houses; thorough cooking of food; inspection 

 of uncooked foods; avoidance of infection through drinking water. 



Origin of parasites — Parasitic habit not a basis of classification — 

 A tendency to regard parasites as constituting a natural group. 

 Older writers grouped various endoparasites together as "helminths." 



