26 BULLETIN 817, U. S. DEPT. OF AGRICULTURE. 
From the table it appears that the highest percentage of intes- 
tinal infestation with Ascaris is found in pigs between the ages of 
21 and 5 months, about half of which are infested. Beyond this age 
- the percentage of infestation gradually decreases until among old 
hogs over 14 years of age only about one in three is infested. 
The figures relating to the numbers of worms of different sizes 
found are of interest with reference to the question of the age at 
which pigs are most susceptible to infection. We know that about 
24 months is required for Ascaris to reach maturity after the eggs 
have been ingested. We should therefore expect few if any adult 
worms among the pigs of the first age class (1 to 24 months), and 
this is the case, less than 3 per cent of the worms present in these 
young pigs being adult worms. In pigs 24 to 5 months-old on the 
other hand nearly one-half of the worms are mature, in pigs of the 
next class (5 to 7 months) nearly two-thirds of the worms are ma- 
ture, then there is a slight decrease in the proportion’ of mature 
worms in the next two age groups, the proportion again increasing 
in pigs 14 to 4 years old in which over two-thirds of the worms are 
mature. 
It is not known how long an individual Ascaris may live, but in 
view of the rather large size of the worm and its slow growth to 
maturity it is probable that it is long-lived; quite likely it may live 
a year and perhaps much longer. Thus, infested pigs a year or more 
old may as a rule have acquired their worms while still very young, 
no additional infection having occurred after the first few months 
of life. The table, however, shows that pigs continue to be suscep- 
tible to infection as they grow older. In fact, the percentage of 
small worms among pigs 12 to 18 months old is higher than among 
the pigs of the younger groups and even among the oldest pigs (14 
to 4 years old) small worms were occasionally encountered, indicat- 
ing recent infection, a considerable number (nearly one-third) of the 
worms present being of medium size, indicating comparatively recent 
infection (probably within 24 months). The table nevertheless 
seems to show that pigs less than 5 months old are considerably more 
susceptible to infection than older pigs. 
Without attempting further analysis of the figures given in the 
table, we may conclude that among the pigs coming to the Chicago 
market slightly more than half of those less than 5 months old have 
been found to be infested with Ascaris, and that among older pigs 
the frequency of infestation progressively decreased with age until 
among pigs 14 years or more old only about one animal in three was 
infested. Further, the pigs less than 5 months old showed a greater 
average number of worms than older pigs, although as a natural 
result of the slow growth of the parasites they showed relatively 
fewer adult worms. Finally, it may be concluded that young pigs are 
