PARASITISM 817 



host. Fleas and lice that are wingless have a greater problem. Fleas are 

 free-living as larvae and have powerful jumping legs as adults so that 

 they can move rapidly through a considerable distance. Lice cannot 

 move fast and will perish in a short time if removed from the host. 

 They seldom attempt to cross voids between hosts, and rely on body 

 contacts between hosts as a means of transmission. 



Most ectoparasites have no serious problem in transmission. In- 

 ternal parasites, however, are adapted to an environment very different 

 from that outside the host, and must produce stages in the life cycle 

 able to withstand external conditions if they are to infect new hosts. 

 Most intestinal parasites produce resistant spores, cysts or eggs, which 

 pass out in the feces of the host. These stages may survive long periods of 

 exposure and are infective when eaten by the next host. Others require 

 an alternate host that frequently is part of the food chain of the final 

 host. Thus, some tapeworm eggs hatch when eaten by an arthropod 

 host and develop to the next stage, which continues to develop only 

 when the arthropod is eaten by a vertebrate host. In a sense the arthro- 

 pod is used as a means of transmission from the vertebrate's feces to 

 its mouth. Some of the intestinal parasites take an active role in trans- 

 mission. The resistant stages expelled in the feces by hookworms and 

 certain trematodes develop into active stages that seek out the next 

 host and penetrate through its skin rather than waiting to be eaten. 



Parasites of body tissues use two routes of dispersal. Some, such as 

 blood flukes, release stages which make their way into the intestine and 

 pass out with the feces. Their subsequent problems of transmission 

 are the same as those of intestinal parasites. Other release stages into the 

 blood that will survive passage through arthropod bloodsuckers. They 

 usually develop through several stages of the life cycle in these arthro- 

 pods. The use of this route by malarial parasites, trypanosomes and 

 filariae has been described. These parasites avoid the problems of the 

 outside world by remaining inside hosts throughout the life cycle. 



Filarial nematodes release their larvae into the blood stream only 

 during those hours of the day in which the arthropod vectors are active. 

 In regions where the insects bite in the daytime the larvae are found in 

 the blood only in the daytime. Strains of Wuchereria occur on dif- 

 ferent islands in the South Pacific, some of which have diurnal, others 

 nocturnal, insects. The strains of parasites found on the different islands 

 have evolved to conform with these patterns. 



If the transmission stages are passive, or if the sojourn between 

 hosts is at all protracted, the odds that an individual parasite released 

 from one host will successfully arrive at another host are small. To 

 balance these odds many parasites produce tremendous numbers of 

 such stages. Tapeworms, roundworms, acanthocephalans and internal 

 trematodes all produce millions of eggs. Protozoan parasites such as 

 Giardia and Entamoeba produce "showers" of encysted stages. The 

 number of filarial larvae in the blood at the appropriate time of day 

 can be enormous. These adaptations not only assure the survival of the 

 species, but if environmental conditions are such that transmission 



