Relapse - Advances in Parasitology - page 128

Rio de Janeiro that six of 80 travellers presenting with vivax malaria (who
had returned from the Amazon region and not received chemoprophy-
laxis) had an incubation period of between 3-12 months, and another from
Brasilia describing long latency in three patients suggest that long-latency
forms may coexist with frequent-relapse phenotypes in Brazil ( Brasil et al.,
2011 ; Tauil et al., 2010 ). Long-latency P. vivax has been well documented in
the Central American region.
6. THE EFFECTS OF AGE AND IMMUNITY ON RELAPSE
In malaria endemic areas, such as the north-western border of Thai-
land, the age profile of P. vivax malaria suggests much more rapid acquisition
of immunity than for P. falciparum ( Luxemburger et al., 1996 ). Entomologi-
cal studies suggested similar transmission rates (at least in terms of measured
entomological inoculation rates), so it is likely that relapse contributes to
much of this age difference. It also suggests that relapses are probably par-
tially suppressed in older patients. Thus, both the proportion of infections
which relapse and the number of symptomatic relapses per mosquito sporo-
zoite inoculation decline with age. Immunity, and therefore, age is likely to
be a significant confounder in epidemiological assessments based on passive
case reporting in many P. vivax endemic areas. In many areas, adults are more
likely to present to malaria clinics than children. In India, the peak age of
malaria presentation is often in young adults (and often with a predomi-
nance of young males). Yet in the indigenous population living in trans-
mission areas, a significant degree of immunity should have been gained
(by both sexes) by early adulthood, which reduces the number of relapses.
Usually in endemic areas, it is children who bear the brunt of malaria. This
applies to both falciparum and vivax malaria when malaria is uncontrolled,
but with increasing control, falciparum declines more rapidly than vivax,
and their epidemiology separates. The paucity of data from children may
contribute to the low apparent relapse rates reported from the Indian sub-
continent. Malaria clinic data may not be representative of disease epidemi-
ology in some endemic areas. Studies of children living in the endemic areas
of the Indian sub-continent might reveal higher relapse rates.
7. DRUG EFFECTS ON RELAPSE
Over 80 years ago, Sinton et al. in India provided the first evidence
for the radical curative activity of plasmoquine ( Sinton and Bird, 1928 ;