In arm-in-cage studies, testing must be conducted
with insertions at limited intervals, with a new batch
of mosquitoes for each test, because continuous exposure
may cause mosquitoes to fatigue or may induce
prolonged blockage of their antennal chemoreceptors,
both of which will prevent further biting.
Conducting tests of a repellent in which the arm is
inserted into the cage at fixed intervals, however, has
some obvious limitations. A repellent might stop
working between the removal of the arm and the
subsequent insertion, but the failure would not be
detected until the next scheduled insertion, causing
an inflated measure of the duration of protection
provided by that repellent. In our study, the greatest risk of overestimation of complete-protection times
would affect the repellents that were tested with
once-hourly insertions into the cage. According to
our protocol, however, hourly insertions were only
used by subjects who found that a repellent initially
protected them for more than four hours. Only the
two highest-concentration DEET-based repellents
in our study (20 percent and 23.8 percent DEET)
qualified for once-hourly insertions by some of the
subjects, and the range of protection these repellents
afforded (180 to 360 minutes) is consistent with previously
published reports of the efficacy of DEET.
15,16
Any rounding errors resulting from the intervals between
insertions into the cage would also tend to
overestimate the efficacy of the other repellents we
tested, and 11 of the 12 non-DEET products still
had mean complete-protection times of less than 23
minutes.