10 PARASITES OF THE DOMESTIC ANIMALS 



nutriment of its host or feed upon its host's tissues, it is detrimental to 

 the parasite's welfare to destroy its host. To destroy the body of the 

 animal harboring it would mean the sacrifice of the parasite's means of 

 subsistence as well as in most cases its shelter. When the host animal 

 dies its internal parasites die with it, and, if it were not for the previously 

 occurring transmission of their offspring to new hosts, the species would 

 rapidly perish. Serious disturbance or death of the host due to its 

 parasites is usually brought about by their presence in large numbers, 

 in which case there is the operation of numerous pathogenic factors. 

 A fatal termination may follow rapidly, but more often there are afebrile 

 morbid phenomena running a prolonged course. In no case is the victim 

 at once destroyed and wholly or in part devoured. 



The parasite is always smaller and weaker than its host, and in many 

 cases its influence upon the latter is not observable. It may be said in 

 general that the degree of injury will depend upon the following prin- 

 cipal factors: 



Influence Upon the Host 



1. The Number of Parasites Present. — ^A tapeworm or one or two 

 ascarids in the intestines may not produce a noticeable effect upon the 

 host. If these parasites are numerous there may be serious disturbances 

 in the host resulting from the deprivation of nutriment which has been 

 appropriated by the infesting worms, from the toxins which they elab- 

 orate, or a more acute effect may be brought about through obstruction 

 of the bowel by large numbers of the parasites in mass. 

 * 2. Their Location. — An encysted larva of the beef or pork tapeworm 

 in its usual location will do no observable harm to its host, but if it 

 should lodge in the eye or central nervous system it might give rise to 

 serious disorders. As a rule, intestinal parasites are less harmful than 

 those which invade the blood or respiratory tract, while of the external 

 parasites, those which burrow into the integument are more injurious 

 than those living upon the surface. 



3. The Nature of their Food. — Any parasite which feeds upon the 

 tissues of its host is more harmful than one which merely appropriates 

 a share of the latter's ingested nutriment. The blood-sucking worms, 

 when present in considerable numbers, bring about serious depletive 

 disturbances, while such worms as the adult ascarids, nourishing mainly 

 upon the residue of food materials, are, in general, less harmful. Sucking 

 lice, armed with piercing mouth parts, are more disturbing to the animal 

 harboring them than the biting lice which feed upon cutaneous debris 

 and the products of their irritation. 



4. Their Movements. — Serious pathologic conditions may be 

 brought about by the migrations of parasites or their change from a 

 usual to an unusual position. Muscular trichinosis, the collective 



