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Rapid assessment of malaria transmission using age-specific sero-conversion rates.

Stewart L, Gosling R, Griffin J, Gesase S, Campo J, Hashim R, Masika P, Mosha J, Bousema T, Shekalaghe S, Cook J, Corran P, Ghani A, Riley EM, Drakeley C - PLoS ONE (2009)

Bottom Line: A pilot study, conducted near Moshi, found SCRs for AMA-1 were highly comparable between samples collected from individuals in a conventional cross-sectional survey and those collected from attendees at a local health facility.Both malaria parasite prevalence and sero-positivity were higher in Korogwe than in Same.MSP-1(19) and AMA-1 SCR rates for Korogwe villages ranged from 0.03 to 0.06 and 0.07 to 0.21 respectively.

View Article: PubMed Central - PubMed

Affiliation: Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, UK.

ABSTRACT

Background: Malaria transmission intensity is a crucial determinant of malarial disease burden and its measurement can help to define health priorities. Rapid, local estimates of transmission are required to focus resources better but current entomological and parasitological methods for estimating transmission intensity are limited in this respect. An alternative is determination of antimalarial antibody age-specific sero-prevalence to estimate sero-conversion rates (SCR), which have been shown to correlate with transmission intensity. This study evaluated SCR generated from samples collected from health facility attendees as a tool for a rapid assessment of malaria transmission intensity.

Methodology and principal findings: The study was conducted in north east Tanzania. Antibodies to Plasmodium falciparum merozoite antigens MSP-1(19) and AMA-1 were measured by indirect ELISA. Age-specific antibody prevalence was analysed using a catalytic conversion model based on maximum likelihood to generate SCR. A pilot study, conducted near Moshi, found SCRs for AMA-1 were highly comparable between samples collected from individuals in a conventional cross-sectional survey and those collected from attendees at a local health facility. For the main study, 3885 individuals attending village health facilities in Korogwe and Same districts were recruited. Both malaria parasite prevalence and sero-positivity were higher in Korogwe than in Same. MSP-1(19) and AMA-1 SCR rates for Korogwe villages ranged from 0.03 to 0.06 and 0.07 to 0.21 respectively. In Same district there was evidence of a recent reduction in transmission, with SCR among those born since 1998 [MSP-1(19) 0.002 to 0.008 and AMA-1 0.005 to 0.014 ] being 5 to 10 fold lower than among individuals born prior to 1998 [MSP-1(19) 0.02 to 0.04 and AMA-1 0.04 to 0.13]. Current health facility specific estimates of SCR showed good correlations with malaria incidence rates in infants in a contemporaneous clinical trial (MSP-1(19) r(2) = 0.78, p<0.01 & AMA-1 r(2) = 0.91, p<0.001).

Conclusions: SCRs generated from age-specific anti-malarial antibody prevalence data collected via health facility surveys were robust and credible. Analysis of SCR allowed detection of a recent drop in malaria transmission in line with recent data from other areas in the region. This health facility-based approach represents a potential tool for rapid assessment of recent trends in malaria transmission intensity, generating valuable data for local and national malaria control programs to target, monitor and evaluate their control strategies.

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Current sero-conversion rates and clinical malaria incidence rates in the IPTi placebo cohort for each health facility: (a) for MSP-119 and (b) for AMA-1.Vertical bars indicate the 95% CI for SCR and horizontal bars indicate the 95% CI for malaria incidence. Fitted lines represent linear regression plots. R2 values for MSP-119 and AMA are 0.78 and 0.91, respectively.
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pone-0006083-g006: Current sero-conversion rates and clinical malaria incidence rates in the IPTi placebo cohort for each health facility: (a) for MSP-119 and (b) for AMA-1.Vertical bars indicate the 95% CI for SCR and horizontal bars indicate the 95% CI for malaria incidence. Fitted lines represent linear regression plots. R2 values for MSP-119 and AMA are 0.78 and 0.91, respectively.

Mentions: The estimated SCR for each health facility, including both the current estimate and the historical estimates for those in Same, for each antigen, along with the respective estimates of EIR, are shown in Table 3. SCRs for the different antigens were correlated (Pearson correlation coefficient r = 0.88, CI 0.78–0.97). Excluding from the analysis any individuals who were positive for malaria infection by RDT did not affect the SCR estimate. In Korogwe, the SCRs calculated using all samples were 0.04 (CI 0.037–0.044) for MSP-119 and 0.126 (CI 0.115–0.138) for AMA-1 (Table 3). The corresponding values when RDT positive samples were excluded from the analysis were 0.037 (0.033–0.041) for MSP-119 and 0.114 (0.103–0.126) for AMA-1. SCRs were not significantly different when calculated for patients or accompanying individuals (Table 4). Our estimates of SCR demonstrate that there is considerable heterogeneity in transmission across the study site and suggest that the current upper estimates of transmission (i.e. the upper 95% confidence limit) in the study area range from less than 0.1 to more than 30 infectious bites per person per year. Estimated SCR and EIR equivalents show a trend for transmission to be higher in communities closer to the Indian ocean, as we have documented previously [27]. In the Same dispensaries our SCR estimates indicate a 2-fold (Same District Hospital) to ∼20-fold (Kisiwani) decrease in transmission in recent years. Finally, for each health facility, current SCR values were highly correlated with clinical malaria incidence rates among infants in the placebo (untreated) cohort of the contemporaneous IPTi study conducted at the same sites [26] (Pearson correlation coefficient MSP-119 r = 0.78, p<0.01; AMA-1 r = 0.91, p<0.001) (Figure 6a & 6b).


Rapid assessment of malaria transmission using age-specific sero-conversion rates.

Stewart L, Gosling R, Griffin J, Gesase S, Campo J, Hashim R, Masika P, Mosha J, Bousema T, Shekalaghe S, Cook J, Corran P, Ghani A, Riley EM, Drakeley C - PLoS ONE (2009)

Current sero-conversion rates and clinical malaria incidence rates in the IPTi placebo cohort for each health facility: (a) for MSP-119 and (b) for AMA-1.Vertical bars indicate the 95% CI for SCR and horizontal bars indicate the 95% CI for malaria incidence. Fitted lines represent linear regression plots. R2 values for MSP-119 and AMA are 0.78 and 0.91, respectively.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2698122&req=5

pone-0006083-g006: Current sero-conversion rates and clinical malaria incidence rates in the IPTi placebo cohort for each health facility: (a) for MSP-119 and (b) for AMA-1.Vertical bars indicate the 95% CI for SCR and horizontal bars indicate the 95% CI for malaria incidence. Fitted lines represent linear regression plots. R2 values for MSP-119 and AMA are 0.78 and 0.91, respectively.
Mentions: The estimated SCR for each health facility, including both the current estimate and the historical estimates for those in Same, for each antigen, along with the respective estimates of EIR, are shown in Table 3. SCRs for the different antigens were correlated (Pearson correlation coefficient r = 0.88, CI 0.78–0.97). Excluding from the analysis any individuals who were positive for malaria infection by RDT did not affect the SCR estimate. In Korogwe, the SCRs calculated using all samples were 0.04 (CI 0.037–0.044) for MSP-119 and 0.126 (CI 0.115–0.138) for AMA-1 (Table 3). The corresponding values when RDT positive samples were excluded from the analysis were 0.037 (0.033–0.041) for MSP-119 and 0.114 (0.103–0.126) for AMA-1. SCRs were not significantly different when calculated for patients or accompanying individuals (Table 4). Our estimates of SCR demonstrate that there is considerable heterogeneity in transmission across the study site and suggest that the current upper estimates of transmission (i.e. the upper 95% confidence limit) in the study area range from less than 0.1 to more than 30 infectious bites per person per year. Estimated SCR and EIR equivalents show a trend for transmission to be higher in communities closer to the Indian ocean, as we have documented previously [27]. In the Same dispensaries our SCR estimates indicate a 2-fold (Same District Hospital) to ∼20-fold (Kisiwani) decrease in transmission in recent years. Finally, for each health facility, current SCR values were highly correlated with clinical malaria incidence rates among infants in the placebo (untreated) cohort of the contemporaneous IPTi study conducted at the same sites [26] (Pearson correlation coefficient MSP-119 r = 0.78, p<0.01; AMA-1 r = 0.91, p<0.001) (Figure 6a & 6b).

Bottom Line: A pilot study, conducted near Moshi, found SCRs for AMA-1 were highly comparable between samples collected from individuals in a conventional cross-sectional survey and those collected from attendees at a local health facility.Both malaria parasite prevalence and sero-positivity were higher in Korogwe than in Same.MSP-1(19) and AMA-1 SCR rates for Korogwe villages ranged from 0.03 to 0.06 and 0.07 to 0.21 respectively.

View Article: PubMed Central - PubMed

Affiliation: Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, UK.

ABSTRACT

Background: Malaria transmission intensity is a crucial determinant of malarial disease burden and its measurement can help to define health priorities. Rapid, local estimates of transmission are required to focus resources better but current entomological and parasitological methods for estimating transmission intensity are limited in this respect. An alternative is determination of antimalarial antibody age-specific sero-prevalence to estimate sero-conversion rates (SCR), which have been shown to correlate with transmission intensity. This study evaluated SCR generated from samples collected from health facility attendees as a tool for a rapid assessment of malaria transmission intensity.

Methodology and principal findings: The study was conducted in north east Tanzania. Antibodies to Plasmodium falciparum merozoite antigens MSP-1(19) and AMA-1 were measured by indirect ELISA. Age-specific antibody prevalence was analysed using a catalytic conversion model based on maximum likelihood to generate SCR. A pilot study, conducted near Moshi, found SCRs for AMA-1 were highly comparable between samples collected from individuals in a conventional cross-sectional survey and those collected from attendees at a local health facility. For the main study, 3885 individuals attending village health facilities in Korogwe and Same districts were recruited. Both malaria parasite prevalence and sero-positivity were higher in Korogwe than in Same. MSP-1(19) and AMA-1 SCR rates for Korogwe villages ranged from 0.03 to 0.06 and 0.07 to 0.21 respectively. In Same district there was evidence of a recent reduction in transmission, with SCR among those born since 1998 [MSP-1(19) 0.002 to 0.008 and AMA-1 0.005 to 0.014 ] being 5 to 10 fold lower than among individuals born prior to 1998 [MSP-1(19) 0.02 to 0.04 and AMA-1 0.04 to 0.13]. Current health facility specific estimates of SCR showed good correlations with malaria incidence rates in infants in a contemporaneous clinical trial (MSP-1(19) r(2) = 0.78, p<0.01 & AMA-1 r(2) = 0.91, p<0.001).

Conclusions: SCRs generated from age-specific anti-malarial antibody prevalence data collected via health facility surveys were robust and credible. Analysis of SCR allowed detection of a recent drop in malaria transmission in line with recent data from other areas in the region. This health facility-based approach represents a potential tool for rapid assessment of recent trends in malaria transmission intensity, generating valuable data for local and national malaria control programs to target, monitor and evaluate their control strategies.

Show MeSH
Related in: MedlinePlus