Many developed countries have used syndromic surveillance for the early detection and response to disease of public health importance, especially United States. It is reported there were more than 100 different US health jurisdictions using syndromic surveillance to augment their public health surveillance in 2003 . In Asian countries, Japan, Korea and Taiwan (in China) also developed syndromic surveillance systems for diseases outbreak detection [6, 15, 16]. Their value in early notification of outbreaks has been confirmed in these studies [6, 11–13, 15–19]. However until now syndromic surveillance systems have been mostly established in developed countries or areas. Although developing settings don't have sufficient infrastructure and resources compared with developed areas, we still can find opportunities to develop syndromic surveillance in China.
1 Chinese government attaching great importance on developing an early warning surveillance system
The development of the early warning surveillance system for infectious diseases is a great concern of Chinese government and is highly relevant to national policy and to program makers in China. In 2003, state promulgated the "Regulations on Public Health Emergency" that proposed the requirements of conducting monitoring and early warning job on disease of public health importance . Under the article 19 of the "Law of the People's Republic of China on the Prevention and Treatment of Infectious Diseases" amended in 2004, an early warning system for infectious diseases is required to establish in the whole country .
With the government's emphasis on early warning surveillance systems for infectious diseases, some pilot studies of syndromic surveillance were conducted, which provided valuable experience and evidence. In 2007, Chinese Ministry of Health (MoH) issued 'Acute infectious disease prevention and control strategies'. Chinese MoH planed to develop a comprehensive surveillance system on acute infectious diseases, including surveillance on serious clinical syndromes, unexplained death, drugs and health supplies sales, and school absenteeism, to improve the national capabilities of early warning on acute infectious diseases . Thus, developing and implementing a syndromic surveillance system for infectious disease control has been put forward formally in governmental work plan.
2 Potential data sources for syndromic surveillance in rural China
An important data source for syndromic surveillance system is chief complaints, which can be obtained from outpatient logs in China. Chinese doctors are required to fill out outpatient log when each patient's visiting, which includes the patient's demographic characteristics, clinical presentations and treatment, such as name, age, gender, home address, visiting date, major symptoms, preliminary diagnosis and treatment. The log is primarily used for the management and tracking of the patients and also providing the basic information for infectious disease reporting. With spreading application of computerized hospital information system, the electronic outpatient log will become feasible in the near future.
Besides chief complaints, OTC sales information is another important type of data source for syndromic surveillance because most people experiencing symptoms would purchase OTC drugs for self-treatment before presenting at the health care facilities . Recently, computer based drug sales management system is being increasingly used in retail pharmacy stores in China, even in rural areas. The electronic data of the name and quantity of medications sold per day can be fetched out from this system.
In the developing countries, infectious diseases outbreaks occurred frequently in schools or factories [24, 25], so it is of great importance to collect the data of absenteeism. Absenteeism data has popularly been used to support syndromic surveillance for early detection of disease outbreaks alongside other data sources [26–28]. In primary schools in China, it is mandatory for teachers to keep records of the information of absent students and also track the prognosis of the sick students according to the regulations of Ministry of Education. Once there is a cluster of cases found, the school should report it to the local education and health bureaus timely.
3 Expanding access to modern telecommunication technology and Internet in rural China
The timely transmission of surveillance data is critical for the development of syndromic surveillance system, and the widespread application of modern telecommunication network and technology make the rapid electronic data entry, reporting and analysis possible in resource-limited areas .
Since the SARS epidemic in 2003, the Chinese government invested a large amount of money to built the world's largest Internet-based disease reporting system, called the China Information System for Disease Control and Prevention (CISDCP) . By the end of the year 2007, the system already covered 79.04% of township hospitals, 95.99% of hospitals at or above county level and 100% of Center for Diseases Control (CDC) units in China . This system has greatly improved the timeliness and completeness of infectious disease notification. Although CISDCP doesn't cover village health facilities, the usage of computer and Internet are gradually spreading in village health stations thanks to the implementation of New Cooperative Medical Scheme (NCMS), in which computer and Internet are applied to develop information management system for health insurance .
In remote areas without computers, mobile phones are commonly used among residents, which can be used for data reporting and transfer. For example a research conducted in 2005 revealed that adopters, potential adopters, and non-adopters of mobile phones represented 59.6%, 18.8% and 21.6% of the rural residents in Hubei province, a middle- and low- income province in China . Mobile phones have already been used for infectious disease surveillance in China. In 2008, when the earthquake occurred in Sichuan which caused CISDCP paralyzed for several days, a cellular phone reporting programme was initiated emergently by China CDC for infectious disease reporting .
4 Early warning models and methods are being developed and tested for infectious disease surveillance in China
Automatic and periodic application of algorithm and visualization tools to detect abnormal condition timely is another important advantage of syndromic surveillance system. Exploring and developing early warning models and methods will lay valuable groundwork for the application of syndromic surveillance system in China. Since CISDCP was instituted in April 2004, China CDC has established different models to identify potential outbreaks automatically based on CISDCP and GIS technology. For example, a statistic conceptual model was established using historic surveillance data with movable percentile method by Ma JQ and his colleagues in 2007 . Yin Fei, et al applied prospective space-time statistics for the early detection of measles and bacillary dysentery outbreaks with surveillance data in CISDCP [36, 37]. In the study of Yang WZ and his colleagues, control chart was used to detect outbreaks/epidemics for seven infectious diseases with five-years historical data . All these previous research provided experience and groundwork for the development of syndromic surveillance system in China.
In conclusion, with China's economic development, government is increasingly attaching great importance on the establishment of early warning surveillance system of disease outbreak. With the strong political support we see a window of opportunity to develop and implement a syndromic surveillance system in rural China. Currently the increasing popularity of telecommunication and GIS technology has set up an infrastructure for developing a modern electronic surveillance system. Knowledge and experience obtained from the pilot studies in China and other countries, and the theoretical research of outbreak detection models, have laid the necessary groundwork for developing a syndromic surveillance system in China.