PARASITE REACTIONS TO HOST MODIFICATIONS
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on a diet high in animal protein 5 of 10 rats became trichomonad-free, and in
2 others the trichomonads were too few to detect in smears but appeared in cul-
tures. Only one of the control rats fed on a normal diet lost its trichomonads
during the experimental period of 36 days. These data indicate that when rats
infected with T. hominis are fed on a diet high in animal protein, the intestinal
contents become unfavourable for them just as they do for rat trichomonads.
Surveys that are undertaken to determine the incidence of T. hominis in hu-
man populations are usually unreliable. Ordinarily many infections are missed
for reasons I need not discuss at present. Occasionally an infection is noted that
is accompanied by a diarrhoeic condition. A meat diet has been suggested for
the treatment of such cases and a number of instances have been brought to my
attention in which the results were very satisfactory both the diarrhoeic condition
and the flagellates disappearing within a very few days. This suggests that a diet
high in protein renders the intestinal contents of man unfavourable for T. hominis
just as it does in rats.
I wish now to direct your attention to another method of attacking this
problem using another species of intestinal protozoon. For the most part investi-
gators have devoted their time and efforts to attempts to eradicate parasitic or-
ganisms, and comparatively few have endeavoured to improve the environment
in the host in favour of the parasite. No one knows what factors prevent a certain
species of parasite from living in a given species of host, but it might be possible
to modify the host in some very simple way that might even occur in nature so
as to render it susceptible to infection. We have tried to modify rat
hosts so that they can be infected with balantidial ciliates to which they are ordi-
narily resistant in nature. Our procedure was based on the results obtained
in our trichomonad studies. We fed rats on diets either high in carbohydrates
or high in proteins. Balantidia obtained directly from the intestine of pigs, or
grown in culture or taken from experimentally infected rats were inoculated
into rats either intragastrically or intracecally. Eighteen rats fed on a diet con-
sisting of 70 per cent casein failed to become infected. Balantidia were later
eliminated within 8 days from 12 rats that had been experimentally infected, by
feeding them on a diet high in casein. Of 49 rats fed on a diet containing 60 per
cent carbohydrates and 15 per cent casein, 47 bacame positive and retained the
infection for from 1 to 33 days. When the carbohydrates were increased to 85
per cent and the casein reduced to 5 per cent, 27 of 52 rats became infected and
remained infected for from 3 to 44 days. A diet of 93.5 per cent carbohydrates,
however, proved to be the most effective, since 89 of 101 rats became infected when
this ration was used and the balantidia were retained up to 154 days. Further-
more, when rats infected on a 60 per cent or 85 per cent carbohydrate diet were
changed to a 93.5 per cent carbohydrate diet the number of balantidia present
increased markedly within from 5 to 8 days,
K