To study Bancrofti?s filariasis in localities of the Amazon
region of Brazil, surveys were being made through examination of thick
smears of blood taken between 7:30 and 9:30 p.m. and stained by
Giemsa?s.
Time of collection being much earlier than the period of
maximum microfilaraemia, and the quantity of blood examined quite
small (about 20cumm) it was thought that a substantial proportion of
parasitized individuals might escape detection by the method.
In order to investigate this possibility, a special survey was
made, by examining 968 people of Bel?m, State of Par?, using both the
thick smear of blood taken from 7 to 9:30 p.m. and Knott?s concentration
method, with 2cc of blood collected from 11:30p.m. to 3a.m. The
assumption had previously been that, by using a concentration method
with blood collected during the period of maximum microfilaraemia,
practically 100% of the positives would be detected, and the comparison
of the results obtained by the two methods would give the error to be
expected when using only the first one in general surveys.
The results were as follows:
N? %
People examined . 968 -
Total positives . 139 14.3
Positives by the thick smear of blood,
collected from 7 to 9:30p.m. 121 12.3
Positives by Knott?s, with 2cc of blood
taken from 11:30p.m. to 3a.m. 139 14.3
As shown above, Knott?s technique made with blood collected
in late hours of the night revealed only 2% more positives than
the thick smear of blood taken from 7 to 9:30p.m. The difference between
the percentages of positives obtained by the two methods is not
statistically significant. Thence, the examination of thick smears of blood
collected at the beginning of the night seems to be sufficient for surveys
of Bancrofti?s filariasis in areas where the microfilariae show
nocturnal periodicity, since it yields fairly good results and is much more
practical than taking slides in late hours, or using other methods which
involve the collection of venous blood.
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