8 RESEARCH IN PROTOZOOLOGY 



Passive (natural) resistance of the parasite. Those stages in the 

 hfe-cycle of protozoan parasites that are able to Hve and reproduce 

 in a host possess a passive or natural resistance. Intestinal protozoa 

 are provided in most cases with a cyst stage which successfully 

 resists conditions in the mouth and stomach whereas, although some 

 trophozoites of intestinal protozoa such as Trichomonas hominis 

 are able to pass through the alimentary canal and reach the large 

 intestine in a viable condition, most of them are destroyed. 



The parasite's method of attack. Each species of protozoan para- 

 site differs in its attack upon the host. Some of them appear to be 

 harmless if present in small numbers, others attack the tissues 

 directly and many species no doubt produce toxic substances. As a 

 rule protozoan parasites live in harmony with their hosts. Very 

 few of them bring about pathological conditions. 



Symptomatology. The symptoms characteristic of human dis- 

 eases due to parasitic protozoa are fairly well known. The pro- 

 tozoa bring about changes in the normal functions of the organs 

 of the host that result in symptoms. Some of these appear at the 

 point where injury is being done whereas others become evident 

 in other parts of the body. The genesis of symptoms in protozoan 

 diseases is open to experimental study but very little is known 

 about the subject. 



Pathogenesis. The pathology of many protozoan diseases is like- 

 wise fairly well known but just how the pathological lesions are 

 produced is still to be determined in many cases. The complete 

 pathogenesis has probably never been worked out for any proto- 

 zoan disease. 



Immunology. Comparatively little is known regarding immunity 

 to protozoan infections. Different species of hosts and different in- 

 dividuals of the same species vary with respect to their resistance 

 to a particular species of parasite. Acquired resistance to infection 

 has been demonstrated in certain cases, e.g., one infection with the 

 parasite of oriental sore gives immunity to this disease. 



Aggressimty. Changes in the parasite as a result of association 

 with a host appear to take place, the aggressivity or invasive 

 powers of the parasite becoming either less or greater. The data 

 on this subject are meager and much work still needs to be done. 



Carriers. Hosts that exhibit no visible symptoms of infection are 

 known as carriers. Some of these hosts, known as convalescent 

 carriers, may have shown symptoms and later recovered. Others, 



