visceral depending on the body area affected and the parasite spe- 
cies concerned. Cutaneous larva migrans is commonly due to filari- 
form larvae of the dog and cat hookworms. The cycle in the normal 
hosts is the same as that for human hookworms (included in the 
intestinal helminth series). In man the larvae are unable to proceed 
further than the cutaneous layers in the region of penetration. The 
principal agent of visceral larva migrans appears to be the dog 
ascarid, Toxocara canis . The life cycle of T . canis is included here 
as representative of the larva migrans group. 
T. canis is an intestinal nematode having a life cycle similar to 
Ascaris lumbricoides, the human ascarid species. It is cosmopolitan 
in distribution. The one-celled egg (diagnostic stage) is passed in 
the feces of the dog and undergoes development in the external 
environment to the embryonated stage (infective stage). Upon in- 
Igestion by the normal host, these embryonated eggs hatch in the 
intestine and the liberated larvae undergo a lung migration before 
maturing in the lumen of the intestine. 
Man becomes an accidental and abnormal host through ingestion 
of the embryonated eggs. In the human intestine, the eggs hatch and 
the larvae penetrate into the mucosa and the circulation. However, 
since they are not in a normal host they do not complete the lung 
migration but rather are filtered out in various organs, chiefly the 
liver. They remain immature and eventually die in the tissues. The 
infection is more common in children than in adults and is character- 
ized by a persistent high eosinophilia. 
Echinococcus granulosus 
Echinococcus spp. ( E . granulosus and E . multilocularis) are the 
etiologic agents of hydatid disease in man. E, granulosus is widely 
distributed in temperate and subtropical regions and other areas 
where sheep, cattle, and hogs are raised. E , multilocularis is preva- 
lent in southern Europe, Russia, Alaska and neighboring territories. 
Only the larval stages infect humans and the hydatid cysts may be 
found in various tissue, chiefly liver and lungs. Like most cestodes, 
the normal life cycle of Echinococcus involves two hosts, definitive 
and intermediate. The adults are found in the intestines of various 
carnivora, especially dogs and foxes, and larval development occurs 
in sheep, cattle or swine ( E . granulosus) or in rodents ( E . multilocu- 
laris), The life histories of the two species are similar (except for 
the choice of intermediate host) so only that of E, granulosus is 
represented here. 
The eggs of the worm (diagnostic stage) are passed in the feces 
of the definitive host. These are ingested by the intermediate host 
in which the infective larvae (hydatid cysts) develop. The larval 
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