The concept of surveillance was developed principally for control of the transmission of infections and for the early detection of outbreaks. The main elements of surveillance methods have been described elsewhere. Surveillance is a continuous, systematic process of descriptive information collection, validation, analysis, interpretation, and dissemination for use in planning, and in the implementation and evaluation of public health policies and strategies for the prevention and control of diseases or disease outbreaks [1-3]. The public health problems approached in this way, including acute and chronic diseases and environmental hazards, are diverse, necessitating the development of tools for the timely monitoring of disease trends. Furthermore, surveillance systems must be evaluated regularly, to ensure that they provide valuable information in an efficient manner [4,5].

Efficient disease surveillance systems are the key to the timely detection of early-warning signs potentially signalling the occurrence of disease outbreaks or epidemics. The World Health Organisation (WHO) has highlighted the importance of improving national epidemic surveillance capacities [6,7]. Recently developed innovative tools, such as mobile telephone technology and electronic systems, have facilitated the improvement of surveillance systems, by reducing data processing [8]. However, these systems are mostly implemented in high-income countries [9], as most developing countries are faced with logistic and budgetary constraints, resulting in low-quality surveillance systems based on pen-and-paper methods. In many cases, these low-tech systems provide health institutions with inadequate support, resulting in frequent “health crises” [10]. Moreover, the healthcare infrastructure, laboratory diagnostic capacity, skills and number of physicians in these countries are generally insufficient to deal with emerging diseases likely to cause epidemics. Consequently, delays in raising the alarm often limit the possibility of an effective early response to new, emerging public health problems.

The need for an efficient sentinel surveillance network in Madagascar was highlighted by worldwide infectious disease threats to public health, such as severe acute respiratory syndrome (SARS) in 2003, avian influenza A H5N1 in 2005 and the Chikungunya epidemics observed in the Indian Ocean region in 2006. In addition, the 2005 International Health Regulations stressed the importance of commitment to the goal of global security and asked all member states to establish and implement effective surveillance and response systems, making it possible to detect and contain public health threats of national and international importance. As a result, the government of Madagascar, in partnership with the Institut Pasteur de Madagascar, established 13 fever sentinel sites in 2007, expanding the network to 34 sites by 2011, to improve the timely detection and management of febrile disease outbreaks. Two key attributes of the sentinel surveillance system are monitored continuously: timeliness and data quality. This system was designed to identify outbreaks for which public health interventions may be required early enough for such interventions to be effective.

We report here the indicators, for 2007 to 2011, of the syndromic sentinel surveillance network, presenting spatiotemporal trends, alert detection capability and evaluations of the process on the basis of timeliness and quality data.

The sentinel surveillance network in Madagascar has been described elsewhere [11,12]. Briefly, it includes primary healthcare centres (sentinel sites) from across the country (Figure 1) and is managed by a national steering committee. The network was expanded from 13 influenza-like illness (ILI) sentinel sites in 2007 to 34 sentinel sites in 2011, with the aim of improving geographic coverage and representativeness of the country as a whole (4 sites are located in Antananarivo) (Figure 1). The sentinel surveillance system makes use of syndromic indicators to monitor the occurrence of selected diseases of importance for the country. The main criterion for the inclusion of cases or patients is fever or diarrhoea. For patients with fever, additional screening criteria (based on syndromic case definitions) are used to identify specific syndromes: malaria, ILI, dengue-like syndromes. Standard WHO case definitions are used, to ensure comparability [11,12]. Malaria diagnosis requires biologic confirmation with a positive rapid diagnostic test in patients with fever syndromes.

Cases and patients at the participating sites are identified by trained healthcare personnel participating in the surveillance network on a voluntary basis. One of the key features of the system is the timely transmission of syndromic data, on a daily basis, by coded short message service (SMS) messages sent from mobile phones. Upon reception at the IPM, the data transmitted in this manner are input daily into a specifically designed MS Access® database and analysed as soon as possible after the patients’ initial visit. This results in a turnaround time of 24 hours, from data collection to reception at the IPM, even for data sent from the most remote areas of the country. The data received by SMS include: sentinel site code, date of data collection, total number of outpatient consultations, total number of confirmed malaria cases, total number of ILI cases, total number of dengue-like cases, total number of diarrhoea cases, and the number of consultations by age group. The age groups were those commonly used by the Ministry of Health in Madagascar: less than 1 year, 1-4 years, 5-14 years, 15-24 years, 25 years and over.

Surveillance data are analysed and presented in easy-to-interpret tables and graphs providing the number of cases for each syndrome monitored. In addition, daily and weekly baselines (mean number of cases in the corresponding period of previous years) are calculated for each syndrome and plotted against current observations, to identify early signs of outbreaks triggering alerts. The information is disseminated on a weekly and monthly basis to healthcare staff involved in the network and to the staff of the Ministry of Health (MoH) in Madagascar.

The sentinel surveillance system in Madagascar has two key functions: it provides an early warning of potential threats to public health and it can be used to manage public health programmes, by providing data for malaria indicators, for example. It can rapidly detect unexpected increases in the incidence of fever or diarrhoea syndromes and the biological surveillance associated with the syndromic surveillance programme can then identify the causes of these syndromes.

This system has been described in terms of the methods used [11] and in relation to aspects of influenza surveillance [12,13], such as the spread of the influenza A(H1N1)pdm09 virus [14,15]. During the influenza A(H1N1)pdm09 pandemic, the circulation of this virus in Madagascar was detected and the spread of the virus was followed from October 2009 to March 2010 [14]. We have already highlighted the weaknesses of the routine disease surveillance system in Madagascar, which is based on passive collection and limited capacities for diagnosis outside the capital city. None of the early-warning signs was identified by routine surveillance. Routine surveillance is useful for monitoring long-term programmes, but inappropriate for the timely detection of aberrant patterns. By contrast, syndrome-based near-real time surveillance can detect unusual events more rapidly [15-18]. This timeliness is a key element of the surveillance system and should be evaluated periodically [19].

The evaluation of surveillance systems should promote the most effective use of public health resources, by ensuring that surveillance systems operate efficiently [20]. The sentinel system in Madagascar was clearly simple and rapid, but we found that some process indicators tended to decline over time, due to high staff turnover. The decrease in the number of fever forms received annually, between 2007 and 2011, is one of the weaknesses of this system. The increase in the number of sentinel sites increased the workload of central staff managing the different activities. A lack of co-ordination hindered the training of new healthcare workers entering the network, and changes in practices were discovered only during supervision in the field. Challenges resulting from high staff turnover have also been identified in other countries [6,8]. The indicators used for the continuous assessment of the sentinel network in Madagascar are useful for a rapid, basic internal evaluation, but an external evaluation approach is also required, using CDC guidelines [21], for example, and including economic indicators as an integral part of the surveillance evaluation process [4].

The choice of methods used in the sentinel surveillance system in Madagascar was based on the capabilities of the volunteer healthcare providers and the financial resources available. The Madagascan network has grown over the years and its expansion is probably now limited by the human resources required to manage the network and data analysis. We have found that progressive step-by-step implementation is best, with assessment of the various processes, evaluations of network management capacity and the training of healthcare workers, to make the processes more acceptable.

Despite the results obtained to date, the sustainability of this system remains unclear, although data transmission costs amount to only about 2 US dollars per sentinel site per month. The Madagascan network has been supported by funding from various sources over the years, focusing on different health topics. Self-sustainability is another challenge, as already described [8], and has already been identified as a weakness of this network. We therefore need to focus on the first steps of surveillance system implementation and all system changes. Initial funds targeted arbovirosis, because of the spread of Chikungunya epidemics in Indian Ocean countries in 2006, and influenza, due to the threat posed by avian flu. However, the steering committee subsequently decided to include other diseases associated with febrile syndromes. This policy has been tremendously successful, making it possible for the network to provide epidemiological information not only about arboviruses, but also about malaria and influenza, throughout the country. In 2008, the first human case of Rift Valley fever was detected, by this network, at Taolognaro (in the south of the country), a site used for both syndromic and biological surveillance. For malaria, the network has monitored the shift from control to elimination following the strengthening of malaria prevention and control measures. The usefulness of sentinel networks for influenza detection is well documented and was assessed in the last pandemic period in 2009 [15]. Funding for work on these diseases has improved geographical coverage and made it possible to extend the network over the last five years. This network has become an additional tool for public health decision-making. The syndromic surveillance has been shown to be an effective approach to surveillance and, thanks to the availability of large mobile phone networks throughout Madagascar, the cost of real-time data transmission is low. This surveillance method may also facilitate compliance with the revised International Health Regulations for low-income countries and the aim of the Global Outbreak Alert and Response Network (GOARN) [22].

However, the rapidity with which the system can identify unexpected events, which is seen as an advantage [23], must be weighed against delays in the response. For instance, the time required to conduct investigations and retrieve diagnostic and epidemiological information might negate the advantage of rapid data acquisition, particularly in developing countries, in which it can be hard to find the resources necessary for investigations.

The lack of historical data made it difficult to interpret the syndromic trends at each sentinel centre. One of the challenges in our system is determining epidemiological baselines for each centre, to facilitate the development of better statistical methods and more sensitive alert thresholds, as suggested by several authors [24-28]. Indeed, five years after the establishment of this network, large amounts of data are already available and data analysis methods have identified trends for ILI, malaria and dengue-like syndromes in areas of Madagascar with different climates. We now need to develop spatiotemporal models to increase the sensitivity of the alert detection process. However, limited geographical coverage and limited resources may prevent the detection of some epidemic events by this network.

It is clear that the greatest advantage of this system is the ease with which it can be implemented, thanks to the availability of mobile phones and mobile phone networks. Furthermore the quality of the homogeneous data collected will make it possible to improve the system relative to its principal objective: identifying epidemic events early. We recommend this solution for other African countries, because it performs very well and provides rapid benefits in terms of public health decision-making.