666 INTESTINAL PROTOZOA AND HOST-PARASITE RELATIONS 



doubt large numbers of protozoan parasites, not only characteristic human species 

 but also species that live in lower animals, are frequently ingested by man, but are 

 unable to establish themselves within the human host because of some type of natural 

 resistance. On the other hand, certain protozoan parasites possess means of resist- 

 ing these natural defenses of the host and, on this account, are successful in their at- 

 tempts to grow and multiply within the digestive tract and thus bring about an infec- 

 tion. 



CHANGES IN HOST AND PARASITE DURING AN INFECTION 



Several of the intestinal protozoa of man are known to injure the host, thus bring- 

 ing about the production of symptoms and often leading to diseased conditions that 

 may result in the death of the host. On the other hand, other intestinal protozoa 

 are supposed to be harmless commensals that live in the lumen of the intestine on 

 bacteria and other food substances; these species when present in small numbers ap- 

 parently do not injure the host to any appreciable extent. It is conceivable that 

 enormously large numbers of them may bring about pathological conditions. The 

 symptoms characteristic of the various diseases due to parasitic protozoa are for the 

 most part well known but the genesis of these symptoms is still to be elucidated. 

 Likewise, the injuries due to pathogenic species of intestinal protozoa have frequently 

 been described in detail, but how these injuries are brought about is still in many ■ 

 cases in some doubt. Practically nothing is known regarding immunity to intestinal 

 protozoan infections. 



That the parasite itself may undergo changes during residence in the host has been 

 suggested by various investigators. Thus it is supposed that a strain may become 

 more aggressive, especially if it succeeds in passing from host to host in rapid suc- 

 cession. It has been suggested also that an immunity to the defenses of the host may 

 be built up by the parasite. Our knowledge of this subject, however, is too slight to 

 make a discussion of the problems involved profitable. 



HOST-PARASITE ADJUSTMENTS DURING AN INFECTION 



Cessation of symptoms in protozoan infections does not mean that the invading 

 parasites have been eliminated from the host. On the contrary, in most cases the 

 organisms remain in the body, perhaps for years, although too few in numbers to 

 bring about the production of symptoms. A host in this condition is known as a 

 "carrier." Such hosts pass infective stages of the organism in their feces and are thus 

 largely responsible for the spread of infection. This carrier condition is due to some 

 adjustment between host and parasite which brings about a sort of equilibrium. The 

 parasite is said to be latent and, when the defenses of the host are weakened sufficient- 

 ly, may succeed in increasing so rapidly in numbers as to bring about a relapse. 

 Whether the host is sufficiently injured during the carrier period to exhibit observable 

 symptoms is a problem that is at present in dispute, 



HOST-PARASITE SPECIFICITY 



Most of the human intestinal protozoa are rather rigidly limited in their habitat 

 to the digestive tract of man. Dogs, cats, and other lower animals can be infected 

 with Endamoeba histolytica but apparently very seldom become infected in nature. 



