Longitudinal study of Plasmodium falciparum and Plasmodium vivax in a Karen population in Thailand.
BACKGROUND
Clinical case treatment of malaria infections where Plasmodium falciparum and Plasmodium vivax are sympatric has achieved effective reductions in P. falciparum prevalence and incidence rates, but has been less successful for P. vivax. The high transmissibility of P. vivax and its capacity to relapse have been suggested to make it a harder parasite species to control.


METHODS
A clinical malaria case treatment programme was carried out over a decade in a Karen community composed of seven hamlets on the Thai-Myanmar border.


RESULTS
From 1994 to 2004, prevalence rates of both P. falciparum and P. vivax decreased by 70-90% in six of the seven study hamlets, but were unchanged in one hamlet. Overall, incidence rates decreased by 72% and 76% for P. falciparum and P. vivax respectively over the period 1999-2004. The age-incidence and prevalence curves suggested that P. vivax was more transmissible than P. falciparum despite a greater overall burden of infection with P. falciparum. Male gender was associated with increased risk of clinical presentation with either parasite species. Children (< 15 years old) had an increased risk of presenting with P. vivax but not P. falciparum.


CONCLUSIONS
There was a considerable reduction in incidence rates of both P. vivax and P. falciparum over a decade following implementation of a case treatment programme. The concern that intervention methods would inadvertently favour one species over another, or even lead to an increase in one parasite species, does not appear to be fulfilled in this case.
Department of Tropical Hygiene, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. tmpsh@mahidol.ac.th
Waraphon
W
Phimpraphi
Richard E
RE
Paul
Surapon
S
Yimsamran
Supalarp
S
Puangsa-art
Nipon
N
Thanyavanich
Wanchai
W
Maneeboonyang
Sutthiporn
S
Prommongkol
Samarn
S
Sornklom
Wutthichai
W
Chaimungkun
Irwin F
IF
Chavez
Herve
H
Blanc
Sornchai
S
Looareesuwan
Anavaj
A
Sakuntabhai
Pratap
P
Singhasivanon
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D001769:Blood / Q000469:parasitology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008137:Longitudinal Studies; D016778:Malaria, Falciparum / Q000188:drug therapy* / Q000453:epidemiology* / Q000635:transmission; D016780:Malaria, Vivax / Q000188:drug therapy* / Q000453:epidemiology* / Q000635:transmission; D008297:Male; D008875:Middle Aged; D010961:Plasmodium / Q000145:classification / Q000302:isolation & purification; D015995:Prevalence; D012737:Sex Factors; D013785:Thailand / Q000453:epidemiology