Biology Reference
In-Depth Information
2010
) and uncomplicated febrile illness (
Ross and Thomson, 1911
;
Kitchen, 1949b
) through to severe and fatal malaria (
Barcus et al., 2007
;
Rodriguez-Morales et al., 2008
;
Tjitra et al., 2008
;
Kochar et al., 2009
,
2010
;
Andrade et al., 2010
;
Alexandre et al., 2010
;
Yadav et al., 2012
;
Lacerda et al., 2012
).
2. PRE-PATENT AND INCUBATION PERIODS
Kitchen in 1949 reviewed the pre-patent period (time from infec-
tion to first appearance of parasites in peripheral blood) and incubation
period (time from infection to first fever >100 °F [37.8 °C]) in 428 natu-
rally induced (mosquito-borne) infections in adults with nine strains of
P. vivax
. The mean pre-patent and incubation periods were 12.2 (range
8-23) and 13.4 (range 8-31) days, respectively (
Kitchen, 1949a
). The
incubation periods were similar to those (13.6 days for each strain) seen
with the Madagascar (mean 13.6 days) (
James, 1931
) and McCoy (mean
13.6 days) (
Boyd, 1941
) strains. In contrast to
P. vivax
strains of tropical
origin, prolonged incubation periods are often seen with temperate-zone
strains (
Hankey et al., 1953
;
Coatney et al., 1971
), including a mean of
282 days for a Rumanian strain (
Shute, 1939
); 22% of patients infected
with Russian strains had prolonged incubation periods ranging from 254
to 360 days (
Nicolajev, 1939
).
In a primary
P. vivax
infection of a non-immune host, the first fever
occurs at very low parasite densities and can occur up to two (
Boyd,
1938
) to three days (
Kitchen, 1949a
) before parasites are detectable in the
peripheral blood. In a low-endemicity area of Thailand, 4/151 (2.6%) of
P. vivax
-infected children presenting to a primary care centre with fever,
did not have parasites detected on their first film and were only detected
on a subsequent film within the following week (
Luxemburger et al.,
1998
). In both adults and children,
P. vivax
has a lower pyrogenic thresh-
old (the parasite density required to evoke a fever) compared to
P. falci-
parum
, with median parasitaemia in uncomplicated vivax malaria lower
than that seen in uncomplicated falciparum malaria (
Ross and Thom-
son, 1911
;
Kitchen, 1949a
). There have been similar findings in endemic
areas, with the pyrogenic threshold for
P. vivax
(181/µl) in Thailand
being eightfold lower than that for
P. falciparum
(1460/µl) (
Luxemburger
et al., 1996
). Fever thresholds rise with time: in untreated pauci-immune
adults, parasite densities by the time of natural termination of fever are
rarely <500/µl (
Boyd, 1938
).
