Characterising temporal trends in asymptomatic Plasmodium infections and transporter polymorphisms during transition from high to low transmission in Zanzibar, 2005-2013.
BACKGROUND
Improved understanding of the asymptomatic malaria parasite reservoir is a prerequisite to pursue malaria elimination efforts. We therefore characterised temporal trends and transporter polymorphisms in asymptomatic Plasmodium infections during the transition from high to low transmission in Zanzibar.


METHODS
Healthy individuals participating in cross-sectional surveys conducted 2005-2013 were screened for asymptomatic malaria by PCR. Complexity/diversity of infection and transporter polymorphisms were assessed in Plasmodium falciparum positive samples. Symptomatic samples were included for comparison of polymorphisms in 2013.


RESULTS
PCR-determined parasite prevalence declined from 21.1% (CI95% 17.4-24.9) to 2.3% (CI95% 1.7-2.9) from 2005 to 2013. P. falciparum remained the predominant species; prevalence was highest in children and young adults aged 5-25 years. Parasite densities and complexity of infection, but not population genetic diversity of P. falciparum, decreased from 2005-2009. pfcrt 76T (99.2-64.7%, p < 0.001) and pfmdr1 86Y frequencies (89.4-66.7%, p = 0.03) decreased over time. Pfmdr1 (a.a.86,184,1246) YYY and YYD haplotypes were more frequent in asymptomatic than symptomatic infections in 2013 (p < 0.001).


CONCLUSIONS
There is a declining, albeit persistent, reservoir of parasites present at low-densities in asymptomatic individuals in Zanzibar. This study revealed important characteristics of the remaining parasite population, including intriguing temporal trends in molecular markers associated with antimalarial resistance, which need to be further investigated.
Malaria Research, Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden. Electronic address: ulrika.morris@ki.se.
Ulrika
U
Morris
Malaria Research, Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden.
Weiping
W
Xu
Zanzibar Malaria Elimination Programme (ZAMEP), Ministry of Health, Zanzibar, Tanzania.
Mwinyi I
MI
Msellem
Department of Medicine, University of California San Francisco, CA, USA.
Alanna
A
Schwartz
Zanzibar Malaria Elimination Programme (ZAMEP), Ministry of Health, Zanzibar, Tanzania.
Ali
A
Abass
Malaria Research, Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden; Department of Medicine, Kungälv Hospital, Kungälv, Sweden.
Delér
D
Shakely
Malaria Research, Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden; Zanzibar Malaria Elimination Programme (ZAMEP), Ministry of Health, Zanzibar, Tanzania.
Jackie
J
Cook
Malaria Branch, Centers for Disease Control and Prevention, Atlanta, GA, USA.
Achuyt
A
Bhattarai
Health Metrics at Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden; School of Public Health, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa.
Max
M
Petzold
Department of Medicine, University of California San Francisco, CA, USA.
Bryan
B
Greenhouse
Zanzibar Malaria Elimination Programme (ZAMEP), Ministry of Health, Zanzibar, Tanzania.
Abdullah S
AS
Ali
Malaria Research, Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden.
Anders
A
Björkman
Department of Medicine, Solna, Karolinska Institutet, Stockholm, Sweden.
Gabrielle
G
Fröberg
Malaria Research, Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden; Department of Public Health Sciences, Karolinska Institutet, Stockholm, Sweden; Centre for Clinical Research Sörmland, Uppsala University, Sweden.
Andreas
A
Mårtensson
D000293:Adolescent; D000328:Adult; D058345:Asymptomatic Infections; D002648:Child; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D005260:Female; D005838:Genotype; D006801:Humans; D007223:Infant; D016778:Malaria, Falciparum / Q000453:epidemiology* / Q000469:parasitology*; D008297:Male; D026901:Membrane Transport Proteins / Q000235:genetics*; D008875:Middle Aged; D059208:Parasite Load; D010963:Plasmodium falciparum / Q000145:classification / Q000235:genetics*; D020641:Polymorphism, Single Nucleotide; D015995:Prevalence; D015800:Protozoan Proteins / Q000235:genetics*; D013636:Tanzania / Q000453:epidemiology; D055815:Young Adult
D026901:Membrane Transport Proteins; D015800:Protozoan Proteins
Antimalarial drug resistance markers; Asymptomatic; Low transmission; Molecular surveillance; Plasmodium