Relapse - Advances in Parasitology - page 126

in Europe while vector densities are still low would be an epidemiologi-
cal pointer. There has been remarkably little recent interest in this question
despite its obvious importance. The greatest uncertainty is the epidemiol-
ogy of relapse phenotypes in the Indian sub-continent, because the sub-
continent harbours the majority of P. vivax in the world and clearly has both
phenotypes.
Prospective studies from India over the past 25 years have recorded
relapse rates following chloroquine treatment of between 8.6% (Orissa) and
8.9% (Madhya Pradesh) and 40.1% (Delhi) ( Adak et al., 2001 ; Adak et al.,
1998 ; Gogtay et al., 2000 ; Saifi et al., 2010 ). In Mumbai, 19 out of 150
patients with vivax malaria and treated with chloroquine only followed for
1 year had a relapse (17 within 6 months) ( Gogtay et al., 2000 ). Reinfection
was considered unlikely. Higher rates were reported from the Delhi area
( Adak et al., 2001 ; Adak et al., 1998 ), where the authors concluded 'that the
P. vivax population in northern India is polymorphic. Group I is the tropi-
cal or Chesson strain type of frequent relapsing P. vivax with a short period
of latency between the primary attack and the first relapse, which is similar
to other South East Asian strains such as those from Thailand and Vietnam.
Figure 2.7 The temporal pattern of recurrences of Plasmodium vivax malaria in patients
with acute vivax malaria treated in Kolkata ( Kim et al., 2012 ). The recurrent infections
are divided into genetically homologous relapses (shown in red) and genetically heter-
ologous recurrences (shown in grey). The date of enrolment is shown as day 1. Heter-
ologous recurrences occurring after 120 days were considered as reinfections allowing
back-extrapolation of the proportion of all such recurrences, which were suspected rein-
fections (point A) and thus by subtraction, the proportion that were suspected relapses.
The cluster of late genetically homologous relapses (green circle) presumably represent
relapses of the long-latency P. vivax phenotype. For interpretation of the references to
colour in this igure legend, the reader is referred to the online version of this topic.