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Figure 2.3 Schematic diagram by Hankey et al. ( Sinha et al., 1953 ) of relapse patterns
following Korean vivax malaria (upper panel) and tropical frequent-relapse P. vivax
(lower panel). It is important to note the frequent-relapse pattern after a long interval
with Korean vivax malaria.
hospital, there was a clear bimodal pattern in which a long pre-patent
period (∼300 days) only occurred following smaller sporozoite inocula
(more reflective of natural infection) ( Coatney et al., 1950a ; Tiburskaya,
1961 ; Moshkovsky, 1973 ). For the Moscow strain, it required ≥1000 sporo-
zoites to produce illness after a 'short' incubation period of 2 weeks. When
increasing sporozoite doses of the tropical 'Chesson' strain were inoculated,
the incubation period shortened slightly, but even with a single mosquito
bite, there was no evidence for a long pre-patent period. These observations,
and a series of experimental investigations in chimpanzees ( Ungureanu et al.,
1976 ), led Garnham to propose that the ratio of hypnozoites to immediately
developing forms in the Korean P. vivax strain was 999:1 compared with the
Chesson strain, where he estimated the ratio as 50:50 ( Garnham, 1988 ; Bray
and Garnham, 1982 ).
In contrast, the initial inter-relapse intervals of Chesson strain P. vivax
in volunteers, and also P. cynomolgi in Rhesus monkeys, were remarkably
regular although they gradually lengthened with each successive relapse
( Krotoski et al., 1986 ; Schmidt, 1986 ) ( Fig. 2.4 ). Two factors were prob-
ably responsible for gradually increasing inter-relapse intervals. The first is
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