- Open Access
Age-specific malaria seroprevalence rates: a cross-sectional analysis of malaria transmission in the Ouest and Sud-Est departments of Haiti
© von Fricken et al.; licensee BioMed Central Ltd. 2014
- Received: 3 June 2014
- Accepted: 26 August 2014
- Published: 14 September 2014
Malaria transmission continues to occur in Haiti, with 25,423 confirmed cases of Plasmodium falciparum and 161,236 suspected infections reported in 2012. At low prevalence levels, passive surveillance measures, which rely primarily on reports from health systems, becomes less appropriate for capturing annual malaria incidence. To improve understanding of malaria transmission in Haiti, participants from the Ouest and Sud-Est departments were screened using a highly sensitive enzyme-linked immunosorbent assay (ELISA).
Between February and May 2013, samples were collected from four different sites including a rural community, two schools, and a clinic located in the Ouest and Sud-Est departments of Haiti. A total of 815 serum samples were screened for malaria antibodies using an indirect ELISA coated with vaccine candidates apical membrane antigen (AMA-1) and merozoite surface protein-1 (MSP-119). The classification of previous exposure was established by using a threshold value that fell three standard deviations above the mean absorbance for suspected seronegative population members (OD of 0.32 and 0.26 for AMA-1 and MSP-1, respectively). The observed seroprevalence values were used to fit a modified reverse catalytic model to yield estimates of seroconversion rates.
Of the samples screened, 172 of 815 (21.1%) were AMA-1 positive, 179 of 759 (23.6%) were MSP-119 positive, and 247 of 815 (30.3%) were positive for either AMA-1 or MSP-1; indicating rates of previous infections between 21.1% and 30.3%. Not surprisingly, age was highly associated with the likelihood of previous infection (p-value <0.001). After stratification by age, the estimated seroconversion rate indicated that the annual malaria transmission in the Ouest and Sud-Est department is approximately 2.5% (95% CI SCR: 2.2%, 2.8%).
These findings suggest that despite the absence of sustained malaria control efforts in Haiti, transmission has remained relatively low over multiple decades. Elimination in Haiti appears to be feasible; however, surveillance must continue to be strengthened in order to respond to areas with high transmission and measure the impact of future interventions.
- Plasmodium falciparum
Over the past decade there has been a renewed interest in eliminating malaria from the island of Hispaniola, with a bi-national strategy recently adopted between the Dominican Republic and Haiti to eliminate malaria by 2020 . Recent reports of emerging chloroquine resistance in Haiti , coupled with increased international aid, present a time sensitive window in which malaria control efforts should be scaled up, before treatment strategies must switch to more expensive combination therapies . Furthermore, only one species of malaria parasite is present in Haiti, Plasmodium falciparum, and the principal mosquito responsible for malaria transmission, Anopheles albimanus, is primarily zoophilic making it a poor vector of disease . Finally there is little chance of malaria being reintroduced into Haiti once it has been successfully eliminated .
Although transmission continues to occur in Haiti, with 25,423 confirmed cases and 161,236 suspected infections reported in 2012 , findings from a 2012 country wide cross-sectional survey administered by Population Services International suggest parasite prevalence rates to be <1% . However, focal transmission has been documented by other studies, with parasite rates in the Artibonite Valley of 3.1% , and parasite rates ranging from 0-34% in the Sud-Est Department , indicating persistent and heterogeneous malaria transmission.
As Haiti gears up for malaria elimination, obtaining sensitive measurements of malaria transmission will be crucial to monitoring the impact of control efforts adopted to achieve this goal . In low transmission settings, there is a tendency to rely on passive malaria surveillance over active surveillance due to budgetary constraints; however, passive surveillance is not as sensitive at accurately capturing malaria incidence, especially in areas with poor health infrastructure like Haiti. To overcome this difficultly, serological markers of malaria have been used to determine malaria exposure rates in low transmission settings, allowing researchers to estimate seroconversion rates (SCR) by modelling the age specific seroprevalence [9–15]. Recently, a study by Arnold et al. examined cross-sectional and longitudinal data from 1991-1998 using merozoite surface protein-119 (MSP-1), and found the SCR to be roughly 2.3% in Leogane, which is located in the Ouest department of Haiti . Estimating malaria transmission by measuring long-lasting antibody responses generated from previous malaria infections also allows the investigation of long-term trends without the estimated seroconversion rates being skewed by seasonal transmission, which is appropriate in this setting since the endemic-epidemic state of malaria coincides closely with rainfall patterns in Haiti [10, 16].
The purpose of this study was to provide valuable information on current trends in malaria transmission in the Ouest and Sud-Est departments of Haiti by analysing data collected in 2013 with ELISA techniques employing more than one P. falciparum specific antigen. This data adds to the current body of literature on malaria in Haiti, while providing policy-makers baseline information on malaria transmission rates in these regions that support the rationale for malaria elimination in Haiti.
Study location and enrollment
Ethical approval to conduct this research was obtained from the Haitian-based Ethical Review Committee, the University of Florida Institutional Review Board, and the Office of Research Protections, United States Army Medical Research and Materials Command.
ELISA protocol and procedures
Serum samples were screened for antibodies against AMA-1 and MSP-119 using an indirect enzyme-linked immunosorbent assay (ELISA). Serum samples of subjects were diluted in 5% non-fat skim milk in phosphate buffered saline (NFSM-PBS). ELISA plates were coated in duplicate with the respective antigen diluted in 5% NFSM-PBS to a final concentration of 1 ml/ml for AMA-1 and 0.5 ml/ml for MSP-1, before overnight incubation at 4°C. The next day, antigen was removed and plates were washed five times with 0.05% tween-20 in PBS-K and then blocked for one hour with 5% NFSM-PBS to reduce non-selective binding. Following additional wash, diluted serum samples as well as positive and negative control sera were plated in duplicate and incubated for two hours at 4°C. Horseradish peroxidase conjugated rabbit anti-human IgG secondary antibody was diluted 1:1,000 in 5% NFSM-PBS and added to the plate. After one hour, plates were washed seven times and treated with 3,3’, 5,5’-tetramethylbenzidine (TMB) substrate solution in the dark for 20 minutes to allow sufficient colour development and stopped with 2 M sulfuric acid.
Determination of seropositive and seronegative population members
Estimation of seroconversion rates from cross-sectional data
The observed cross-sectional seroprevalence was used to estimate the seroconversion rate using a method similar to those previously described [11, 18]. Briefly, an age specific seroconversion model was fit to the prevalence of AMA-1, MSP-1, and AMA-1 or MSP-1 seropositive population members using all participants (aged 2 to 80) and separately for participants less than 20 years of age to estimate the rate of seroconversion (λ). Participants were separated into “age classes” to depict aggregated seroprevalence, with wider age ranges used in older groups, due to the lower number of participants over the age of 20. Since the AMA-1 and MSP-1 responses are long-lasting and the estimation of reversion rates has been suggested to be unreliable with cross-sectional data, a reversion rate (ρ) of zero was used for the final analysis , however non zero reversion rates were also explored. Without seroreversion, the probability of infection (prevalence) at age x was modeled using the equation P(x) = [1 ‒ exp(-λ * x)]. When seroreversion was included in the model, the probability at age x was modeled using the equation P(x) = λ/(λ + ρ) [1 ‒ exp(-(λ + ρ)x)]. The functions were optimized (using R) to give estimated seroconversion/reversion rates and , as well as their standard errors for the calculation of 95% confidence intervals for and . Odds ratios for the probability of having a previous exposure, as determined by a positive ELISA response, were calculated using a simple logistic regression by age category.
Estimation of seroprevalence using AMA-1 and MSP-1
Study population characteristics by site of enrollment
Participants given ELISA
Site of enrollment
Hosana Baptist school
Portail Leogane clinic
Number and prevalence of seropositive participants by age class
2 to 5
6 to 9
9 to 13
14 to 17
18 to 20
21 to 29
30 to 49
Estimation of seroconversion rates for AMA-1 and MSP-1
In an effort to meet the island wide goal of malaria elimination by 2020, the gametocidal drug primaquine (PQ), was added to the malaria national treatment policy for Haiti in 2010 . This treatment policy change places Haiti in a unique position to monitor and quantify the impact single dose PQ administration has on P. falciparum transmission, which could hold valuable information on PQ tolerance and malaria elimination strategies abroad. Findings suggest that these regions have experience a relatively low and constant state of P. falciparum transmission, given the stable increase in seroprevalence by age observed in this study. In samples that had positive responses to either AMA-1 or MSP-1, the estimated SCR of 2.5% (95% CI 2.2%, 2.8%) from this study is slightly higher than the <1% prevalence rate estimate by PSI in 2012 . However, when the seroconversion rates were determined individually from a positive AMA-1 or MSP-1 response, the estimated SCR decreased to 1.6% (95% CI 1.3%, 1.8%) and 1.8% (95% CI 1.6%, 2.1%) for AMA-1 and MSP-1, respectively. The differences in SCR estimates could be a result in variation in individual antibody responses, as suggested by Figure 4, where some seropositive respondents have strong responses to only a single antigen (regions II and III). Trends were also observed in the antibody responses after stratification for age, which could indicate that the duration of AMA-1 and MSP-1 antibody titers are different, as previously suggested . In Figure 3, in 4 out of 5 age groups below 20 years of age the seroprevalence of MSP-1 was higher than the seroprevalence of AMA-1, whereas 2 of 3 of the age groups above 20 show higher seroprevalence of AMA-1 compared to MSP-1. However, in this sample the likelihood of participants having a strong AMA-1 response ( > 0.5 AU) and a MSP-1 response below the threshold (Figure 4, region III) was not significantly different by age (p > 0.1).
The inclusion of a seroreversion rate in the model also slightly increased the estimated seroconversion rates. Due to the cross-sectional nature of this study and the long duration of antibody detection for AMA-1 and MSP-1, it was appropriate to set the seroreversion rate to zero. When seroreversion was included in the model, the seroreversion rates were -0.006 (95% CI -0.016, 0.003) and 0.008 (95% CI -0.005, 0.022) for AMA-1 and MSP-1 respectively. Since both of the confidence intervals for the seroreversion rates include zero and the inclusion of a seroreversion rate in the model had little effect on the estimates of seroconversion, therefore a priori exclusion of a seroreversion rate from the final model was justified in this circumstance. When comparing the estimated seroconversion rates from study participants under 20 years of age, the continuity in the age-specific seroprevalence curve could indicate that over multiple decades, a relatively constant state of low malaria transmission has occurred in these regions, even in the absence of sustained malaria control efforts. Entomological studies investigating the vector competency of A. albimanus mosquito, may better explain this phenomenon, of stable low transmission.
One of the primary limitations of this study was that serum samples were collected using a convenience sample, which limited our ability to infer how this sample population represents Haiti as a whole. Findings may have been skewed by potentially enrolling participants from clinics (n = 203), however, this potential sampling bias was adequately addressed by excluding all malaria RDT positive individuals (5/815) from final analysis. This study also only screened for previous exposure to P. falciparum, although the likelihood of finding other species or mixed infections remains low, given recent reports, and the presence of host protective factors [5, 20]. As with other ELISA protocols, setting a threshold for the classification of a sample as seropositive is subject to interpretation. To validate the method used in this study, thresholds using absorbance values of four and five standard deviations above the suspected seronegative population mean were also evaluated and fell within the calculated confidence intervals for seroprevalence and SCR using only three standard deviations.
As reported cases of malaria in the Dominican Republic have reached a 15-year low of 952 cases , malaria continues to be a major public health concern in Haiti. Findings from this study further support the notion of sustained low low-level transmission in Haiti, while using a highly sensitive technique that could be used to determine malaria transmission elsewhere in Haiti. These data suggests that any efforts to advance malaria control locally have not had much impact over the last five decades, yet neither have the past political upheavals or natural disasters from recent decades resulted in major malaria epidemics.
Future studies should expand seroprevalence methodologies to other departments in order to establish countrywide trends. Research examining barriers to access, protective host characteristics, the extent of heterogeneous malaria transmission in other departments, and vector proficiency could also enhance elimination models in Haiti. Elimination in Haiti appears to be feasible; however, surveillance must continue to be strengthened in order to respond to areas with high transmission, while measuring the impact of future interventions.
The authors would like to extend a special thanks to the dedicated staff at Community Coalition for Haiti and the Christianville Foundation for without their support this study would not be possible. The authors would like to thank Benjamin D. Anderson for technical assistance with the ELISA protocol and Alexander Kirpich for his assistance with the seroconversion model. The antigens used in the ELISA came from the following sources: Plasmodium falciparum yP30P2 PfMSP-119(Q-KNG)FVO/VK1, MRA-53 deposited by DC Kaslow, obtained through the MR4 as part of the BEI Resources Repository, NIAID, NIH and Recombinant PfAMA-1 mixture of 3D7 and FVO & Anti-PfAMA-1 rabbit serum and/or purified IgG, provided by David Narum, obtained from the National Institute of Health Laboratory of Malaria and Vector Research (LMVR), NIAID, NIH, DHHS. This study was funded by the Armed Forces Health Surveillance Center, Global Emerging Infections Surveillance and Response Division to B.A.O. and by University of Florida, College of Public Health and Health Profession funds to M.E.V.
- Clinton Health Access Initiative: The feasibility of malaria elimination on the island of Hispaniola, with a focus on Haiti. 2013, Available at http://globalhealthsciences.ucsf.edu/eliminating-malaria-on-the-island-of-hispaniola%20Google Scholar
- Keating J, Krogstad DJ, Eisele TP: Malaria elimination on Hispaniola. Lancet. 2010, 10: 291-293. 10.1016/S1473-3099(10)70075-X.View ArticlePubMedGoogle Scholar
- Londono BL, Eisele TP, Keating J, Bennett A, Chattopadhyay C, Heyliger G, Mack B, Rawson I, Vely JF, Désinor O, Krogstad DJ: Chloroquine-resistant haplotype Plasmodium falciparum parasites, Haiti. Emerg Infect Dis. 2009, 15: 735-740. 10.3201/eid1505.081063.PubMed CentralView ArticlePubMedGoogle Scholar
- Roberts L: Elimination meets reality in Hispaniola. Science. 2010, 328: 850-851. 10.1126/science.328.5980.850.View ArticlePubMedGoogle Scholar
- WHO: World malaria report 2013. 2013, Geneva: World Health Organization, Available at http://www.who.int/malaria/publications/world_malaria_report_2013/report/en/Google Scholar
- Eisele TP, Keating J, Bennett A, Londono B, Johnson D, Lafontant C, Krogstad DJ: Prevalence of Plasmodium falciparum infection in rainy season, Artibonite Valley, Haiti, 2006. Emerg Infect Dis. 2007, 13: 1494-1496. 10.3201/eid1310.070567.PubMed CentralView ArticlePubMedGoogle Scholar
- Raccurt CP, Cicéron M, Dossil R, Boncy J: Prevalence of Plasmodium falciparum during the rainy season (June-December) in the southeast district of Haiti. Médecine Santé Tropicales. 2012, 22: 435-439.Google Scholar
- Greenwood BM: Control to elimination: implications for malaria research. Trends Parasitol. 2008, 24: 449-454. 10.1016/j.pt.2008.07.002.View ArticlePubMedGoogle Scholar
- Bousema T, Youssef RM, Cook J, Cox J, Alegana VA, Amran J, Noor AM, Snow RW, Drakeley C: Serologic Markers for Detecting Malaria in Areas of Low Endemicity, Somalia, 2008. Emerg Infect Dis. 2010, 16: 392-399. 10.3201/eid1603.090732.PubMed CentralView ArticlePubMedGoogle Scholar
- Cook J, Reid H, Iavro J, Kuwahata M, Taleo G, Clements A, McCarthy J, Vallely A, Drakeley C: Using serological measures to monitor changes in malaria transmission in Vanuatu. Malar J. 2010, 9: 169-10.1186/1475-2875-9-169.PubMed CentralView ArticlePubMedGoogle Scholar
- Drakeley C, Corran P, Coleman PG, Tongren JE, McDonald SLR, Carneiro I, Malima R, Lusingu J, Manjurano A, Nkya WMM, Lemnge MM, Cox J, Reyburn H, Riley EM: Estimating Medium and Long term trends in malaria transmission using serological markers of malaria exposure. Proc Natl Acad Sci U S A. 2005, 102: 5108-5113. 10.1073/pnas.0408725102.PubMed CentralView ArticlePubMedGoogle Scholar
- 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: Rapid assessment of malaria transmission using age-specific sero-conversion rates. PLoS One. 2009, 4: e6083-10.1371/journal.pone.0006083.PubMed CentralView ArticlePubMedGoogle Scholar
- Wilson S, Booth M, Jones FM, Mwaatha JK, Kimani G, Kariuki HC, Vennervald BJ, Ouma JH, Muchiri E, Dunne DW: Age-adjusted Plasmodium falciparum antibody levels in school-aged children are a stable marker of microgeographical variations in exposure to Plasmodium infection. BMC Infect Dis. 2007, 7: 67-10.1186/1471-2334-7-67.PubMed CentralView ArticlePubMedGoogle Scholar
- Cook J, Kleinschmidt I, Schwabe C, Nseng G, Bousema T, Corran PH, Riley EM, Drakeley CJ: Serological Markers Suggest Heterogeneity of Effectiveness of Malaria Control Interventions on Bioko Island, Equatorial Guinea. PLoS One. 2011, 6: e25137-10.1371/journal.pone.0025137.PubMed CentralView ArticlePubMedGoogle Scholar
- Arnold BF, Priest JW, Hamlin KL, Moss DM, Colford JM, Lammie PJ: Serological measures of malaria transmission in Haiti: comparison of longitudinal and cross-sectional methods. PLoS One. 2014, 9: e93684-10.1371/journal.pone.0093684.PubMed CentralView ArticlePubMedGoogle Scholar
- Bonnlander H, Rossignol AM, Rossignol PA: Malaria in Central Haiti: a hospital-based retrospective study, 1982-1986 and 1988-1991. Bull Pan Am Health Organ. 1994, 28: 9-16.PubMedGoogle Scholar
- von Fricken ME, Weppelmann TA, Eaton WT, Alam MT, Carter TE, Schick L, Masse R, Romain JR, Okech BA: Prevalence of glucose-6-phosphate dehydrogenase (G6PD) deficiency in the Ouest and Sud-Est departments of Haiti. Acta Trop. 2014, 135: 62-66.View ArticlePubMedGoogle Scholar
- Corran P, Coleman P, Riley E, Drakeley C: Serology: a robust indicator of malaria transmission intensity?. Trends Parasitol. 2007, 23: 575-582. 10.1016/j.pt.2007.08.023.View ArticlePubMedGoogle Scholar
- Tongren JE, Drakeley CJ, McDonald SL, Reyburn HG, Manjurano A, Nkya WM, Lemnge MM, Gowda CD, Todd JE, Corran PH, Riley EM: Target antigen, age, and duration of antigen exposure independently regulate immunoglobulin g subclass switching in malaria. Infect Immun. 2006, 74: 257-264. 10.1128/IAI.74.1.257-264.2006.PubMed CentralView ArticlePubMedGoogle Scholar
- Weppelmann TA, Carter TE, Chen Z, von Fricken ME, Victor YS, Existe A, Okech BA: High frequency of the erythroid silent Duffy antigen genotype and lack of Plasmodium vivax infections in Haiti. Malar J. 2013, 12: 30-10.1186/1475-2875-12-30.PubMed CentralView ArticlePubMedGoogle Scholar
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