DWARF TAPEWORM 



243 



by the armed head, and become adult. It is said that eggs of 

 this parasite can be found in the faeces within a month after an 

 egg of the preceding generation has been swallowed. Self- 

 infection with these eggs 'rarely occurs, since the eggs will not 

 develop unless acted upon by the gastric juices. There is still 

 room for doubt as to whether an insect is not commonly involved 

 as an intermediate host as in other species of Hymenolepis; in 

 fact, several investigators have found cysticercoids in rat fleas 

 which they ascribed to this 

 species. Ransom thinks 

 there is room for doubt 

 as to whether the larvae of 

 Hymenolepis found in the 

 intestinal villi of rats and 

 mice break out and become 

 mature in the lumen. 



The common presence 

 of this parasite or a variety 

 of it in rats and mice indi- 

 cates that infection in man 

 may occur from accident- 

 ally swallowing the " pills " 

 of these animals infected 

 with the eggs or ripe pro- 

 glottids of the worm. 

 Since a single mouse " pill " 

 might contain hundreds 

 of eggs, each of which 

 could develop into an 

 adult in another rat or 

 mouse, or in man, it is 

 not difficult to understand the great numbers of this worm which 

 are often found in a single intestine. 



The unique life history of this species, if true, makes it subject 

 to entirely different preventive measures from those used against 

 most other tapeworms. Since infection results not from eating 

 bladderworm-infected meat, but probably from swallowing egg- 

 infected faeces, especially the " pills " of mice and rats, and pos- 

 sibly also from swallowing infected insects which are acting as 

 intermediate hosts, prevention consists in the elimination of 



FIG. 91. Rat tapeworm, Hymenolepis dimi- 

 nuta, from house mouse in Oregon. Natural 

 size. 



