Drug-Resistant Malaria - libdoc.who.int - World Health Organization

30 I REVIEW
or in vitro testing. The changing status of the parasite from the
peripheral ("RI") to the central ("Rlll") area may similarly be
observed. However, in vitro methods provide better resistance
predictors, since the particular studies may be set up more
adequately to satisfy statistical prerequisites of sample size and
randomization.
Analysis of the small number of in vitro tests that have so
far been done in Asia confirms long-standing in vivo indications
of the westward trend of chloroquine resistance. While at present
more information is available from macro than from micro testing,
the greater operational convenience of the latter promises to
provide more comprehensive data. Recent macro in vitro results
from individuals in Thailand, Burma, Bangladesh, India and Sri
Lanka have been grouped by study locale, and weighted means
calculated from the maturation percentages that occur at the
discriminating dose levels of 0.25, 0.75, 1.25 and 3.0 nanomo1
(0.25; 0.75; 1.25; 3.0 x 1O-6M/l). These weighed means show in
a single figure the degree of parasite susceptibility in an area.
Prediction of development of or increase in level of resistance
may be obtained by. charting the geographical flow of resistance
into receptive areas, or from repetitive studies in fixed foci.
The geographical range of resistance in South-East Asia in
1978-1980, in terms of these weighted means, dimishes towards the
west, from the high levels of 70 to 97 observed throughout
Thailand, through the somewhat lower levels around 65 in Burma and
27 to 63 in Bangladesh and north-east India, to a borderline SIRI
situat ion across central India where the means range from 23 in
Orissa westwards to 5 in Gujarat. In Sri Lanka in 1980 the mean
was 4, indicating hypersensitivity. In some areas of Thailand,
tests carried out in 1977 gave weighted means of 30 to 45; by
1980 in nearby, epidemio1ogica11y similar places the weighted
means had reached values of 85 to 91.
In order to indicate the direction of flow of ch10roquineresistant
P.falcipartnn, and determine changes in the degree of
resistance, it is l.ecessary to increase geographical coverage of
in vitro testing o.erations and design and to undertake repetitive
testing in selected indicator areas. Only with such information
will the rational planning and deployment of appropriate containment
measures be possible.
The model propounded above depends essentially on the
application of Mendel's law, given Rand S. It is therefore
somewhat surprising that RIll is relatively rarely seen while RI
and RII responses seem to remain more prevalent even in the oldest
known f·')ci. This observation is not readily explained although
some of the reasons could be differences in the biological
advantages between moderately and highly resistant parasites and