HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



proximately by the host-parasite reactions. For example, 

 if a parasite is pathogenic and lethal for a certain host 

 the association is supposed to be recently acquired 

 whereas the absence of symptoms indicates a long pe- 

 riod of consociation. Frequently carriers are spoken of as 

 reservoirs since they are storehouses for the organisms 

 that are responsible for the spread of the parasite to new 

 hosts. In certain cases the parasite is infective both to 

 man and to lower animals and one or both kinds of hosts 

 may serve as reservoirs. Very little is known at present 

 regarding the conditions underlying the carrier state. 



(2) Latency. Similar in certain respects to the car- 

 rier condition is the state known as latency. When para- 

 sites are present in a host but do not make themselves 

 manifest, they are said to be latent. Latency, however, 

 does not necessarily require the dissemination of the par- 

 asites by the host. In some cases a host may be parasi- 

 tized and show no symptoms ; in other cases a host may 

 recover from symptoms but still harbor parasites; both 

 types of conditions may be included under the term 

 latency. Certain changes in host or parasite may bring 

 on symptoms in a host that had never previously exhib- 

 ited evidences of infection ; such a case might be consid- 

 ered one with an extended incubation period. 



(3) Relapse. Symptoms may appear in a host that 

 had previously shown symptoms but had apparently re- 

 covered; such a reappearance of symptoms is known as 

 a relapse, if the latent period is short, and a recurrence, 

 if the latent period is long. Relapses may be induced in 

 certain infections by definite stimuli but the physiologi- 

 cal bases for this have not been determined. 



38 



