Background
In Africa, some countries in the North have achieved malaria elimination in the last 30 years (Libya in 1973, Tunisia in 1979, and Morocco in 2010) and the majority of the region is largely regarded as malaria free despite a few disputed residual foci in Algeria and Egypt. Elimination, however, poses a major challenge in the majority of countries in Africa owing to the intrinsically high transmission intensity within each country's national borders [
1] or in the cases of low transmission areas, threats posed by neighbouring high transmission countries [
2], operational constraints posed by fragile health systems that may not reach remote foci of infection [
2], the sophistication of current health information systems to identify all cases of clinical and asymptomatic malaria [
3], and the ability to meet the immediate increases in financing needs that would be diverted from other health problems [
4,
5].
A recent systematic analysis of available national data has defined possible biological threats (including current and historical receptivity of transmission and connectivity to other countries) and operational threats (including political stability, heath system capacity and access to populations) to elimination across 99 malaria endemic countries [
2]. Within Africa the most technically and operationally feasible countries, as judged by a combined ranking, were Botswana, Swaziland, São Tomé and Príncipe, Eritrea, and Djibouti. The Republic of Djibouti was considered the African country with the most favorable combinations of biological risk and operational requirements to reach elimination and its ranking globally was equivalent or better than many countries in the Americas. However, several recent publications have highlighted that despite the fact that 32 countries have declared an elimination ambition [
6] few have defined the true biological feasibility of elimination [
7] and fewer have defined a combined biological and operational feasibility assessment [
6,
8]. There have been limited opportunities for countries to assess their technical and operational strengths and weaknesses for elimination unless undertaken as part of very detailed reviews of large amounts of empirical data as implemented recently in Zanzibar, United Republic of Tanzania [
9].
National Malaria Indicator Surveys (MIS) have been established largely to provide milestone measurements for reaching endemic control ambitions in high to moderate transmission countries, with parasite prevalence as the main transmission indicator. In low transmission areas this indicator is not efficient [
10,
11]. Here we present the results of a recent adaptation of a standard MIS to include a national serological survey in the Republic of Djibouti to define the extent of transmission and explore the possibilities of malaria elimination.
Discussion
We present here the results from the first national MIS in sub-Saharan Africa to include combined measures of
Pf parasitological and serological prevalence among all age groups, undertaken in the Republic of Djibouti.
P. falciparum parasite prevalence was 0.5% and
P. falciparum sero-prevalence to MSP-1
19 and/or AMA-1 was 9.9% (Table
2). Sero-prevalence significantly increased with age and the countrywide
P. falciparum sero-conversion rate (λ) was estimated to be approximately 13 new exposures per 1000 persons per year. Among the 517 individuals who were positive for
P. falciparum antibodies, 46 were children under the age of five years suggestive of recent malaria exposure. Over 76% of these children were from Tadjourah, Dikhil and Obock regions (data not shown). Overall sero-prevalence in the oldest age group was below 15%. In 2008 among 2896 patients with malaria-like symptoms attending government run clinics with laboratory facilities were tested for presence of infection, 119 (4.1%) were reported positive using microscopy and majority of these cases (74%) were seen in Djibouti Ville [PNLP, unpublished data]. The combined PCR validated
P. falciparum RDT positivity and sero-prevalence showed some evidence of between region variations with Djibouti Ville showing the lowest rates (8.1%). These regional differences were overall not statistically significant. This study therefore shows that the risks of
P. falciparum malaria are low across the Republic of Djibouti but majority of clusters surveyed, 102/129 (79%), reported at least one individual who was positive for
P. falciparum antibodies and/or peripheral blood infection (Figure
3). Although between region variations in exposure was not statistically significant, there appeared to be micro-epidemiological heterogeneity in sero-prevalence with 5 clusters located in Dikhil region showing relatively higher sero-positivity (Figure
3). Residence, aridity, distance to major road/railway and coastline, use of ITNs, recent and present fever histories and travel were not statistically significant predictors of infection and/or sero-prevalence (Table
2). Increasing age and decreasing household wealth were the only two variables significantly associated with increasing
P. falciparum infection and sero-prevalence independently and in combination.
Sero-epidemiological studies carried out in Tanzania on AMA-1 and MSP-1
19 showed that the two antigens provided estimates of seroconversion rates that were independently correlated with entomological-based estimates of
P. falciparum exposure in the same areas [
26,
39]. AMA-1 has highly immunogenic properties reflecting recent exposure [
40,
41] while MSP-1
19 antibodies saturate over longer periods of exposure representing cumulative risks [
26]. Combining exposure to both antigens in a single ELISA test is likely to provide a more robust measure of parasite exposure in areas of low parasite prevalence. However interpretation of sero-positivity from optical density values of titres measured through ELISA has an unknown sensitivity and specificity and will depend largely on the approaches taken to classify positivity. Here we elected not to use traditional three standard deviations above established control sera but have used a mixture model method which has previously been used for defining sero-positivity to measles and rubella [
42‐
44]. By using the population itself to define sero-positivity we remove any bias due to population-specific immunoglobin levels, nutritional or co-infection history. However, defining sero-positivity still requires further work to either define their sensitivity, eliminate the need for a cut off or improve approaches to defining it.
From a sampling perspective the largest limitation of the MIS was that it was powered to detect changes in ITN coverage and not parasite exposure. This represents a perennial problem for national MIS across Africa designed to detect increases in coverage measures and not declines in exposure. There is a recommendation however by partners to measure changing parasite prevalence through national surveys every 3-5 years [
45]. Using the period prevalence of parasitaemia and/or sero-positivity and the between cluster variability from the survey undertaken in the Republic of Djibouti in 2008-09 we estimate that one would require testing a total population of over 22,000 to achieve a +/- precision of 5% within a point estimate. As prevalence becomes rare the costs of sampling to define parasite exposure therefore increase substantially.
The field investigations of parasite prevalence included two types of HRP-2 RDTs specific only to
P. falciparum and these, and a random sample of 50 RDT negatives, were validated using PCR resulting in exactly similar results. It is widely held in the Republic of Djibouti that
P. vivax constitutes less than 5% of all malaria infections [
16], however this remains to be investigated properly as vivax infections are often missed during routine clinical microscopy [
46]. We have not established
P. vivax prevalence during the national survey and due to funding constraints we were not able to undertake large-scale PCR-based investigations of vivax and other rare species on the filter paper samples. All neighboring countries connected to the Republic of Djibouti also have
P. vivax transmission.
P. vivax is harder to detect, treat and eliminate [
47]. The national guidelines on malaria case-management for the Republic of Djibouti are not clear on recommendations for the diagnosis and treatment of vivax malaria [
48].
In 2009 the Republic of Djibouti submitted a proposal to the GFATM for five years support to achieve a malaria pre-elimination status by 2015. About 6.6 million USD was approved in June 2009 but the agreement is unlikely to be signed before mid-2011 (personal communication, The Global Fund). Our findings support the biological possibility for elimination reported elsewhere [
2] but do not examine the operational feasibility of elimination. Interestingly the GFATM support from round 6 and round 9 continues to promote universal coverage of ITN [
20,
21]. ITN coverage was 1.4% among children in 2006 [
23] and rose in 2008-09 to 19.6% among children aged less than 5 years, with 41.5% of households owning at least one ITN and 14.4% of the entire population using them the night before survey [
22]. Increasing ITN coverage to over 80% would continue to be a huge resource outlay for little obvious biological benefit where malaria risk is intrinsically low and spatially over-dispersed. A more logical approach might involve targeted vector control with combinations of larval and adult mosquito target interventions in response to improved vector breeding and/or clinical case detection and cartography [
49]. Mathematical models suggest that targeting the minority who contribute the majority to transmission in low transmission settings is likely to yield larger returns [
50‐
52]. Although there was a generally limited clustering of antibody/infection prevalence in Djibouti, there are 4 regions (Tadjourah, Dikhil, Obock and Ali Sabieh) which contribute to the majority of cases. However a migration from blanket coverage to targeted intervention is predicated on a robust and timely surveillance system transitioning from routine clinical treatment with incomplete monthly and quarterly reporting to a model based on daily or weekly aggressive, infectious disease surveillance supported by public awareness campaigns and universal parasitological testing. Malaria is not a notifiable disease in Djibouti and case reporting is embedded within the routine health information system which suffers from perennial under-reporting and data quality problems typical of most health information systems in Africa [
53‐
55]. Surveillance precedents already exist in the Republic of Djibouti for polio and cholera and new infectious disease surveillance initiatives are being developed to track West Nile and Dengue viral infections [Amar Abdo, personal communication]. Malaria surveillance also needs to become an up-regulated disease reporting system with the ability to detect, intervene and eliminate residual foci of malaria infections as and when they emerge. The round 9 proposal includes 0.48 million USD to strengthen surveillance but deciding on how best to develop and implement malaria surveillance systems for elimination has a much weaker evidence-based platform than ITN delivery [
3].
Although travel histories in the last six months were not associated with
P. falciparum infection or sero-prevalence in this study, human population movement is likely to be a significant challenge to elimination in the Republic of Djibouti. Several authors have highlighted the role played by the rail route to Ethiopia in the recent re-emergence of malaria transmission in Djibouti [
14,
56,
57]. Between 700,000 and 1.4 million passengers use this rail route each year [
58]. Residents of Djibouti Ville often migrate during the months of May-September to avoid the excessive heat to Hargeisa in Somaliland or Dire-Dawa in Ethiopia. The road from Djibouti Ville to Addis via Galafi has recently been upgraded with funding from the World Bank and the European Union to increase transportation of goods and people between countries [
58]. Djiboutian armed and police forces train in Ethiopia and are stationed as part of UN peace keeping in Cote D'Ivoire. Thousands of foreign military personnel are stationed in Djibouti Ville and engage in regional travel. Therefore the significance of human population movement for the risks of imported infectious diseases, including malaria, should not be under-estimated and may be an important contributor to disease hot spots in the country. Although recent reports show significant reductions in malaria infection prevalence in Ethiopia [
59], Eritrea [
60], Somalia [
61] and Yemen [
62], the connectivity between the Republic of Djibouti and these countries demonstrates the importance of constantly reviewing the origin and destination risks against regional human population movement to define and contain importation risks [
63]. Use of mobile phone data to track population movements internally and externally [
63]; screening arriving passengers for malaria at the border entry points and recording of their detailed travel history; and redesigning household survey sampling and questionnaires to capture detailed travel histories of individuals tested for malaria infections are possible approaches to assessing the impact of human population movement on malaria in Djibouti. Work is ongoing to assess the theoretical impact of human population movement on malaria transmission in Djibouti using a combination of travel history data from surveys; air passenger origin and destination data; and estimates of malaria risk of origin and destination [Deepa Pindolia, Personal Communication].
Conclusion
This study demonstrates the potential of adapting standard national MIS to collect information not only on the prevalence of malaria infections but also serological markers of exposure which are important in determining transmission in areas of low endemicity [
26]. The financial cost associated with the materials required to collect dry blood spots for serology and subsequent analysis were relatively small. The additional survey time as a result of collecting the samples on the filter papers appeared minimal. However, it was not possible to conduct a full cost-effectiveness analysis of this adaptation of a standard MIS as this was beyond the scope of this study. The biggest challenge, perhaps, is less to do with actual cost of including a serological component in standard MIS but the expertise required to implement ELISA to investigate exposure. This serological method for malaria is highly specialised and currently implemented largely by research groups. Potentially, however, training of technicians at national public health laboratories in malaria ELISA, as was done in Djibouti for this study, could mitigate this constraint.
The results of the survey support the apparent low case reporting of
P. falciparum in the Republic of Djibouti and at face value the country may appear to be poised for elimination, a state enjoyed across the country between 1910 and the 1970's [
15,
16].
P. falciparum elimination would be possible if surveillance and response to foci of infections were substantially improved drawing on experience and systems established for polio and other rare, notifiable infectious diseases. Detailed investigations into the extent of
P. vivax transmission are also needed for a comprehensive understanding of the possibility of elimination [
47]. This would require careful planning through a feasibility analysis as undertaken recently in Zanzibar [
9] and long-term uninterrupted political support and financing [
8]. At present the national malaria control programme has managed to secure some funding to support salaries from the GFATM round 6 funding for a short period but no funds since 2008 to implement any activities [Hawa Guessod, Personal Communication]. The signing and releasing of funds from round 9 has yet to materialize although it is anticipated that a larger national survey will be undertaken in 2012 during which an RDT that can detect both
P. falciparum and
P. vivax will be used in addition to collecting blood samples on filter papers for multi-species PCR and ELISA [Hawa Guessod, Personal Communication]. This and the proposed future studies will substantially improve our understanding of malaria transmission in Djibouti, but without a carefully defined plan, staff or funding elimination of malaria in Djibouti remains only a biological possibility. The threshold of parasite prevalence seen as the benchmark for deciding whether to move on to malaria elimination or sustain low endemic control is seen as 1% parasite prevalence [
3,
11]. But to detect this level of prevalence requires large and expensive survey samples. The threshold for serological markers remains unclear although the absence of exposure among children under the age five years would be a good indicator that a country is approaching pre-elimination [
3]. An alternative equivalent index is case incidence of 1 per 1000 persons at risk [
3,
26]. To reliably estimate this index, there is need for accurate passive and active case detection data over several years [
11]. This requires a properly functioning national malaria surveillance system based on a quality assured diagnostic capacity to provide the accurate information on whether Djibouti has achieved the biological threshold for elimination.
Acknowledgements
The Malaria Indicator Survey was implemented by the Programme National de Lutte Contre le Paludisme of the Ministry of Health of the Republic of Djibouti with funding support from the Global Fund and the United Nations Children's Fund. Technical assistance was provided by the World Health Organization's Eastern Mediterranean Regional Office of (WHO-EMRO) and the Wellcome Trust, UK. We also wish to acknowledge Bart Faber and Ed Remarque of the Biomedical Primate Research Centre, Rijswijk, The Netherlands, for providing the antigen used in this study. AMN is supported by the Wellcome Trust as a Research Training Fellow (#081829). CKM is supported by a Wellcome Trust grant to the Kenya Major Overseas Programme (#084538). MJM is supported by a Wellcome Trust grant (#088634). RWS is supported by the Wellcome Trust as Principal Research Fellow (#079080). CJD acknowledges the support of the Wellcome Trust (#078925) which also supports JC. The authors are grateful to the support and diligence of the 32 field staff notably the supervisors Abdallah Mohamed, Abdillahi Ayeh, Mohamed Abakari, Abdoulkader Garad, Mohamed Moussa, Idriss Farah and Mahad Ibrahim and to Viola Kirui and Deepa Pindolia for help with data cleaning, preparation and map production. We thank Dr Tharcisse Barihuta for his advice and facilitation of the survey. The authors would like to thank Mohamed Abakari, Ubah Moussa, Abdinour Sudan for their help in preparing the ELISA samples at the Laboratoire National Sante Publique. The authors are also grateful for comments on earlier drafts of the manuscript from Dr Emelda Okiro.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
AMN was responsible for study design, data cleaning, analysis, interpretation, drafting and production of the final manuscript. MBM was responsible for the serological analysis of samples in the laboratory and data assembly and contributed to the final manuscript. CKM was responsible for the analysis of the optical density data, interpretation of results and contributed for the final manuscript. MAO, HHG, IAA contributed to the survey design, data assembly and cleaning and contributed to final manuscript. MN was responsible for the polymerase chain reaction analysis of all positive and selected negative samples and contributed to the final manuscript. JC and CJD were responsible for the development of ELISA methods, interpretation of results and contributed to final manuscript. MJM contributed to the statistical analysis of the optical densities, was responsible for the PCR of positive and negative samples and contributed to the final manuscript. RWS was responsible for overall scientific management, analysis, interpretation and preparation of the final manuscript. All authors read and approved the final manuscript.