III. Tissue Parasites 
The three species of helminths included here inhabit human body 
tissues in the larval form. The adults of all three are normally in- 
habitants of the intestine of the definitive host. Man may be an 
intermediate host ( Echinococcus ) or both intermediate and definitive 
hosts ( Trichinella ) or simply an accidental host ( Toxocara ). In all 
three charts, the cycle in man has been left unclosed to denote the 
“blind alley” ending of the parasites in humans. 
Trichinella spiralis 
T. spiralis is a nematode parasite which goes through both adults 
and larval stages within a single animal host but two hosts are 
necessary to continue the infection. Except in the tropics, it is 
world-wide in distribution. The cycle is a relatively simple one 
with a short adult life span and a considerable longer larval life. 
The host, which is both the definitive and intermediate host, may be 
any carnivorous or omnivorous animal, but chiefly, man, hogs, rats, 
bears, foxes, dogs, and cats. From the standpoint of man, the hog is 
the primary source of infection, and the life cycle chart has been 
prepared on this basis, including only the most important and basic 
steps. 
It is the general consensus, that the infection is maintained in 
swine primarily through ingestion of the infective larvae in meat 
scraps (usually pork) in uncooked garbage. This swine to swine 
transmission is shown in the life cycle chart. 
The cycle in man is initiated by ingestion of meat (pork) con- 
taining encysted larvae (diagnostic and infective stages). In the 
intestine the liberated larvae mature very rapidly and by the fifth 
day the females begin to deposit larvae, a process which continues 
for about 4 weeks or longer. The males live for a relatively short 
time and usually are passed out soon after fertilization. The young 
larvae reach the tissues by way of the lymphatics and blood. Although 
they are carried to all parts of the body, they ordinarily develop only 
in striated muscle. Encystment may begin at about 3 weeks and 
calcification of the cyst often starts as early as 6 months and is 
usually completed within 18 months. The majority of the encysted 
larvae probably die within 1 to 2 years after infection. 
Toxocara canis (Larva migrans) 
Human infections with larvae of nematode parasites of lower 
animals are called larva migrans. Larva migrans may be caused by 
many different species of parasites and may be either cutaneous or 
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