INFECTION WITH ANIMAL PARASITES 135 



with the invaders. That natural immunity against infection with 

 certain animal parasites may exist is shown by the prevalence of certain 

 infections among man, and their absence among lower animals, or vice 

 versa. 



The aggressiveness of animal parasites is in general probably even 

 greater than that of most bacteria, and a more or less extensive infection 

 apparently occurs in all cases in which the parasite had made successful 

 invasion, some multiplying in the blood-stream (malaria, relapsing 

 fever, trypanosomiasis, Texas fever, filariasis), others in the lymph- 

 stream (filariasis), and others in the tissues (syphilis, trichiniasis, 

 amebiasis) without much opposition on the part of the host. Whether 

 these factors are due to the aggressive forces of the parasites which neutra- 

 lize the defenses of the host, or whether they are due to the hardiness of 

 the parasites and a lack of defense on the part of the host, is not known, 

 but probably the latter is generally the case. 



As with bacteria, animal parasites show a well-marked selective 

 affinity for certain tissues, as the malarial plasmodium for red blood- 

 corpuscles, trypanosomes and spirochetes for blood plasma, trichina 

 for voluntary muscle, various parasites for the intestinal canal and even 

 for certain portions of the intestinal tract, others for the lung, etc. 



Production of Disease. Comparatively little is known regarding 

 the formation of toxic products on the part of the animal parasite. 

 Some, as, e. g., the Treponema pallidum and spirochete of relapsing 

 fever, probably cause disease largely through the production of toxins, 

 especially of the intracellular variety. The chill, fever, and sweat of 

 malaria suggest the liberation of toxic products coincident, or nearly so, 

 with segmentation and rupture of the plasmodium. The late symptoms 

 of sleeping sickness and the whole course of relapsing fever are strikingly 

 similar to the bacterial toxemias. The metabolic products of all animal 

 parasites are probably injurious in some manner and to some degree. 

 The pathogenicity of others is due, in part at least, to mechanical 

 blocking of vessels, as with the filaria, trypanosomes, and malarial 

 organisms ; others (hookworm) abstract blood or consume food material 

 in the intestine, as the intestinal parasites; and others, as migrating 

 foreign particles with irritating secretions, produce local inflammatory 

 changes. 



Nevertheless, we know comparatively little of the offensive factors, 

 and still less of the immunologic defensive factors, operative during the 

 course of infections with animal parasites. With the development of a 

 technic for the cultivation of animal parasites in vitro similar to that 



