Multiplicity of Plasmodium falciparum infection in asymptomatic children in Senegal: relation to transmission, age and erythrocyte variants.
BACKGROUND
Individuals living in malaria endemic areas generally harbour multiple parasite strains. Multiplicity of infection (MOI) can be an indicator of immune status. However, whether this is good or bad for the development of immunity to malaria, is still a matter of debate. This study aimed to examine the MOI in asymptomatic children between two and ten years of age and to relate it to erythrocyte variants, clinical attacks, transmission levels and other parasitological indexes.


METHODS
Study took place in Niakhar area in Senegal, where malaria is mesoendemic and seasonal. Three hundred and seventy two asymptomatic children were included. Sickle-cell trait, G6PD deficiency (A- and Santamaria) and alpha+-thalassaemia (-alpha3.7 type) were determined using PCR. Multiplicity of Plasmodium falciparum infection, i.e. number of concurrent clones, was defined by PCR-based genotyping of the merozoite surface protein-2 (msp2), before and at the end of the malaria transmission season. The chi2-test, ANOVA, multivariate linear regression and logistic regression statistical tests were used for data analysis.


RESULTS
MOI was significantly higher at the end of transmission season. The majority of PCR positive subjects had multiple infections at both time points (64% before and 87% after the transmission season). MOI did not increase in alpha-thalassaemic and G6PD mutated children. The ABO system and HbAS did not affect MOI at any time points. No association between MOI and clinical attack was observed. MOI did not vary over age at any time points. There was a significant correlation between MOI and parasite density, as the higher parasite counts increases the probability of having multiple infections.


CONCLUSIONS
Taken together our data revealed that alpha-thalassaemia may have a role in protection against certain parasite strains. The protection against the increase in MOI after the transmission season conferred by G6PD deficiency is probably due to clearance of the malaria parasite at early stages of infection. The ABO system and HbAS are involved in the severity of the disease but do not affect asymptomatic infections. MOI was not age-dependent, in the range of two to ten years, but was correlated with parasite density. However some of these observations need to be confirmed including larger sample size with broader age range and using other msp2 genotyping method.
Vafa
Manijeh
M
Department of Immunology, Stockholm University, S-106 91 Stockholm, Sweden. manijeh@imun.su.se
Troye-Blomberg
Marita
M
Anchang
Judith
J
Garcia
André
A
Migot-Nabias
Florence
F
D000017:ABO Blood-Group System; D000818:Animals; D002353:Carrier State / Q000097:blood / Q000453:epidemiology* / Q000469:parasitology; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004912:Erythrocytes / Q000145:classification; D005955:Glucosephosphate Dehydrogenase Deficiency / Q000150:complications; D006801:Humans; D016778:Malaria, Falciparum / Q000097:blood / Q000150:complications / Q000453:epidemiology* / Q000469:parasitology; D009154:Mutation; D010270:Parasite Egg Count; D010963:Plasmodium falciparum / Q000235:genetics / Q000302:isolation & purification*; D011156:Population Density; D012423:Rural Health; D012621:Seasons; D012675:Senegal / Q000453:epidemiology; D012805:Sickle Cell Trait / Q000150:complications; D017085:alpha-Thalassemia / Q000150:complications
D000017:ABO Blood-Group System