ANIMALS AND THEIR PROTOZOAN PARASITES 9 



known as contact carriers, may never have exhibited symptoms. 

 Most animals are probably contact carriers of one or more species 

 of protozoan parasites since only a very small number of protozoa 

 bring about symptoms. Carriers serve as reservoirs and are respon- 

 sible for the dissemination of the parasites. There is some question 

 as to whether certain species that sometimes produce symptoms 

 attack the carrier host in which they live but not severely enough 

 to bring about symptoms. 



Latency. This is the term applied to conditions in which para- 

 sites are present in the host but do not bring about symptoms. 

 Such a host is always a carrier but may not be disseminating para- 

 sites. Sometimes the term primary latency is used to describe the 

 period from the entrance of the parasite into the host until symp- 

 toms appear when this period is longer than usual. Secondary 

 latency may be used for the period between the primary attack 

 and relapse. 



Relapse. The reappearance of symptoms in a host that has 

 passed through a primary attack constitutes a relapse. If the period 

 since the primary attack is extended the reappearance of symptoms 

 is sometimes known as a recurrence. Latent periods and relapses 

 appear to result from reactions between the host and the parasite 

 involving various changes in resistance and virulence about which 

 we know very little. 



Escape of parasites from the host. Many protozoa attack parts 

 of the host from which natural channels lead to the outside through 

 which escape is easy. For example, intestinal protozoa pass out in 

 the feces of the host, the protozoa of the human mouth are trans- 

 ferred directly from one host to another during kissing and blood- 

 inhabiting protozoa are sucked up with the blood by intermediate 

 hosts. 



Host-parasite specificity. By this term is meant the adaptability 

 of a parasite to one or several species of hosts. When a parasite 

 is able to live in only one species of host it is said to exhibit rigid 

 host-parasite specificity. Host-parasite specificity is said to be weak 

 when a parasite is able to live in several species of hosts. Various 

 terms are used to describe these relations. For example, a host 

 is said to be tolerant if it is easily parasitized by a particular species 

 and is said to be refractory if it is parasitized with difficulty. A 

 parasite is said to be a natural parasite if it frequently occurs in a 

 particular host and a foreign parasite if it does not ordinarily 



