16,1 Haughwout and Horrilleno: Intestinal Parasites 61 
job that would make the hookworm campaign seem like child’s 
play in some respects; for, to begin with, we have no treatment 
of proved efficacy against Trichuris, and the eggs of the organism 
will remain viable for five years. Nematode infections are prob- 
ably more or less self-limited, and it is likely that in the lapse 
of time the original race would die out, a given host would be 
purged of his infection and everything would be serene for the 
patient. The eggs, however, may have five years to live. Fur- 
thermore, our observations over a period of years confirm us 
in the belief that the supply of worms in the intestine is kept 
pretty constant, and that reinfection with new and vigorous 
strains occurs with regrettable frequency. With the helminths 
we have met in this series, with the exception of Oxyuris, auto- 
infection fortunately does not have to be considered. 
With the protozoa the proposition is a little different. The 
life cycles of the species inhabiting the intestinal tract are not 
perfectly known except that the question of intermediate hosts 
and exogenous development seem not to be regulating factors. 
Forms such as Coccidium, Eimeria, and Isospora, which, of 
course, did not occur in this series, require at least a number of 
hours for sporozoite development before they become infective, 
but their cysts are extraordinarily resistant to unfavorable en- 
vironmental conditions—much more so than the cysts of the 
flagellates and intestinal ameebe. Cysts of the latter are prob- 
ably infective as soon as they leave the intestine, and it seems 
probable that in a large proportion of cases they represent reju- 
venated strains of the organism, imbued with all the vitality 
and potentiality for harm which characterized the strains that 
originally infested the host. 
In groups of people confirmed in the habit of feeding them- 
selves with their fingers and whose habits at stool are not above 
reproach, one is left to speculate as to how many cases are truly 
chronic and how many represent the working out of a vicious 
circle that includes the mouth and anus of a single individual. 
Dobell and Stevenson(15) have cited cases of Entameeba histo- 
lytica infection running courses of from sixteen to thirty-four 
years. But there we have a tissue parasite that in the normal 
course of events passes very little of its time in the lumen of 
the intestine and of whose conduct in the tissue we have only 
imperfect knowledge. Matthews(39) cites a case of Entameba 
coli infection that seems to have been limited to about one year, 
but he also cites James’ case (in a negro) that ran six years; 
but there the factor of auto-infection obtrudes itself. 
