Parasites and Parasitisms. 5 



contaminated by devouring the carcases, offal, scraps, of other 

 infested animals, or the water or food contaminated by these, 

 becomes the main condition of an outbreak. So with the 

 hundreds of other conditions varying with the parasite, the host, 

 and the environment, the rule is that these, conditions must be 

 changed before we can hope to get rid of the parasitic invasions. 

 But so long as, and wherever, these favorable conditions exist we 

 must be prepared to face' an outbreak of parasitism, and this by 

 reason of the local increase of parasites, which until now, and 

 elsewhere, may have been considered as comparatively harmless, 

 Thus it is that the existence and gravity of a parasitism often de- 

 pends quite as much on the favorable conditions of the environ- 

 ment as on the presence of the parasite. But given a real 

 parasite, with injurious qualities, the aggregation of a large num- 

 ber of the animals that form its normal host, and an environment 

 especially favorable to its preservation and propagation and we 

 must be prepared to meet with an extensive, dangerous and de- 

 structive outbreak. No previous, lengthened period of immunity, 

 and no hi.story of this parasite showing an apparent harmlessness, 

 must be allowed to blind us to the probability of a dangerous 

 increase of such parasites whenever the conditions become in 

 every way favorable. As the potato-beetle can only live and 

 multiply where potatoes are grown, so the parasite of the animal 

 can only increase where there is an abundance of its hosts. And 

 as with the host so with the conditions of the larval existence of 

 the parasite. Both are essential in many cases, and when both 

 are present they may cause outbreaks of which no preceding 

 counterparts can be found. 



For the same reason most parasitisms can be dealt with by 

 changing the condition of the environment, and in this way cutting 

 off the next generation of the parasitic organism. This is usually 

 too much ignored, and treatment is too often confined to the mere 

 exhibition of parasiticides, which, however effectual in preserving 

 the individual animal, does little towards the much more philo- 

 sophic resource of extirpating the parasite. This is the counter- 

 part of the same faultily circumscribed view and action, which 

 expends itself on measures of serum therapy and immunization in 

 the case of contagious diseases and declines to grapple with the 

 far more important and immenssely more economic resort of ex- 



