528 Host-Parasite Relationships 



adherence of a motile organism to the surface of the host's body. The 

 latter condition is not easily distinguished from casual association of 

 a free-living species with a pseudo-host. Many other parasites occur in 

 such body cavities as the mouth and other parts of the digestive tract, 

 the mantle cavity of Mollusca, and the cloaca of aquatic vertebrates. So 

 long as these Protozoa are both harmless and useless, they may be con- 

 sidered endocominensah, or inquilines (14). Endocommensals may be 

 expected in terrestrial as well as aquatic hosts. Endoparasites which 

 participate in symbiosis, an association involving mutual benefits to host 

 and parasite, are known as symbiotes. Parasites which destroy the tissues 

 of their hosts or damage them in other ways may be called pathogens. 

 Such terms as "strict parasite" and "true parasite" have been used in 

 the same sense. 



Although some such terminology is convenient for purposes of discus- 

 sion, there may be practical difficulties in distinguishing symbiosis from 

 endocommensalism or commensals from pathogens. It has even been 

 suggested that in a single host species, a pathogen may occasionally be- 

 come a commensal, or a commensal may sometimes harm the host. How- 

 ever, the former change does not necessarily occur in the usual carrier 

 of a normally pathogenic species. It is quite likely that the carrier shows 

 no obvious symptoms because an effective although incomplete immunity 

 has been developed. 



Commensalism as an evolutionary goal 



The evolutionary aspects of pathogenicity and commensalism have 

 interested many parasitologists. According to one theory, the evolutionary 

 goal of the parasite is adjustment to commensalism, an association which 

 tends to conserve available hosts, and in this sense, favors survival of the 

 well-adapted parasite. This hypothesis implies that endocommensals, as 

 the product of long-continued adaptation to a particular species of host, 

 are phylogenetically older than pathogens invading the same host. Patho- 

 genic species would represent newly acquired parasites which have not 

 had time to evolve into commensals. 



Certain objections to this hypothesis have been discussed by Ball (4). 

 So far as experimental data are suggestive, there is little reason for 

 assuming that mere passage of time is a major influence in the loss of 

 pathogenicity. Packchanian's (75) results with Trypanosoma brucei in 

 Peromyscus have shown that a given parasite may cause experimental 

 infections which range, in different species of hosts, from acute lethal 

 types to chronic infections followed usually by spontaneous recovery. 

 Furthermore, investigations on avian malaria have shown that one species 

 of Plasmodium, upon inoculation into a variety of hosts, may produce 

 lethal infections in one species, malaria of moderate severity in another 



