PARASITISM 



803 



harmless. It is transmitted from individual to individual by rat fleas. 

 Unfortunately rat fleas occasionally bite man, and in this way transmit 

 the disease to a host in which its effects are devastating. Fleas can 

 transmit typhus fever, tularemia, undulant fever and other diseases as 

 well as the plague. The human louse will transmit typhus, but the dis- 

 ease kills both the humans and the lice. In regions where lice are 

 abundant the spread of typhus can reach epidemic proportions. During 

 World War I louse-borne typhus killed at least 3,000,000 men. The 

 common tick Dermacentor andersoni (Fig. 39.5) carries more pathogens 

 than any other parasite, including those that produce spotted fever, 



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1 %.^J , v-^ . 



;^^X^"^^SSSSx^'?S:S§SSiii:f$iSS5S$SSSSs!^^ 



Head louse 



Pubic lou-Se 



Body louse 



Figure 39.3. Anoplura. The three varieties o£ human lice. The head louse, 

 Pediciilus hunianus var. capitis, and body louse, P. h. var. corporis, are interfertile 

 varieties of one species that rarely interbreed because one lives on the head, laymg 

 eggs on the hairs, while the other hves on the clothed portion of the body, laying 

 eggs in the clothing. The pubic louse, Phthirus pubis, lives in the pubic region and 

 occasionally in the armpits. (After Patton and Evans.) 



Figure 39 4. Siphonaptera. Life cycle of the rat flea, Xenopsylla cheopts^ ^g?^ 

 fall o the ground and hatch into free-living larvae. These feed on debris, eventually 

 pupate ani emerge as adults that seek out the proper host. (Adult after Chandler; 

 others after Patton and Evans.) 



