Introduction
Public health surveillance is defined as “ongoing systematic collection, analysis, and interpretation of data essential to the planning, implementation, and evaluation of public health practices, closely integrated with the timely dissemination of these data to those who need to know.” [
1] The data will be used for public health actions, such as outbreak detection, intervention planning, and implementation.
The ideal infectious surveillance system should be an integrated system, including clinical and epidemiological information, pathogen analysis, as well as information for vectors, environments, geography, ecology, climate, etc. A laboratory-based surveillance system is an essential and fundamental component of all infectious disease prevention and control programs. Laboratory services for infectious disease are indispensable and a needed capability for the early detection and containment of disease outbreaks to prevent national and international spread of disease.
Early detection of infectious disease revealed by a laboratory-based infectious disease surveillance system will result in immediate public health intervention and reduction in the numbers of illnesses and deaths. The Centers for Disease Control and Prevention (CDC) reports to the national health administration authority as soon as it receives information about the pathogen, infectious source, vector and transmission route. This allows the health authority and public to take precise actions guided by scientific evidence. However, false information misleads the authorities and the public to take wrong actions [
2]. A recent example was that cucumbers were suspected as the infectious source of Shiga toxin-producing
Escherichia coli O104:H4 in June 2011 when a larger German outbreak was threatening the entire world [
3,
4].
The laboratory-based infectious disease surveillance system in developed countries
Developed countries increasingly look to strengthen their national surveillance and response especially using laboratory-based surveillance. The United States (US) has built a strong infrastructure for pathogen surveillance. The CDC in the US works with the Department of Agriculture (USDA) and the Food and Drug Administration (FDA) laboratories, and with public health laboratories in states, cities, and counties to expand laboratory capacity in order to prepare and respond to outbreaks, terrorism incidents, or other emergencies. Currently, the primary surveillance programs or projects in the US include the Emerging Infection Program (EIP), the Laboratory Response Network (LRN), the WHO Global Salm-Surv (GSS) collaboration, the National Antimicrobial Resistance Monitoring System (NARMS), the Network on Antimicrobial Resistance in
Staphylococcus aureus (NARSA), Active Bacterial Core Surveillance (ABCs), National Tuberculosis Surveillance System (NTSS), FoodNet, VetNet, OutbreakNet, PulseNet, and others [
5-
8]. However, these systems are not integrated into a single, comprehensive national laboratory-based surveillance system for infectious disease.
The European Union (EU) has special geographic characteristics with free movement of people and goods across borders. Each of the member states in the EU had its own infectious disease surveillance system and practice. Data often are not comparable. In 2005, EU established the European Centre for Disease Prevention and Control (ECDC). ECDC has emphasized the construction of laboratory capacity in the member states, and has worked to ensure that every country has national reference level laboratory services available, either directly or indirectly, enabling all countries to confirm diagnosis, isolation and further characteristics of pathogens. In Japan, the current surveillance system has two different frameworks of mandatory reporting for severe diseases and sentinel reporting for milder diseases [
9].
The infectious disease surveillance system in China
A national system for infectious disease surveillance in China was established in 1959 and was gradually expanded into three components: the national disease reporting system, nationwide disease surveillance points, and surveillance network for specific infectious diseases [
10]. Many individual infectious disease surveillance systems were established, including influenza, encephalitis B, meningitis, cholera, epidemic hemorrhagic fever, plague, and leptospirosis.
After the 2003 SARS epidemic, the Chinese government improved the disease surveillance system and built an internet-based disease reporting system, the China Information System for Disease Control and Prevention (CISDCP). Hospitals and clinics can immediately and directly report disease using the internet. This allows public-health officials to obtain real-time information, immediately identify disease outbreaks, and implement needed containment strategies [
11].
China’s real-time web-based infectious disease reporting system is a distinguished achievement. However, many aspects of the current Chinese infectious disease surveillance system do not meet the demand for timely outbreak detection and identification of emerging infectious disease. Many infectious disease cases are clinically diagnosed without laboratory evidence, which results in many cases being misdiagnosed. Laboratory confirmation diagnosis and laboratory-based surveillance are weak or unavailable for some infectious diseases. And therefore, communication between laboratory-based surveillance and epidemic surveillance is not sufficient [
12].
The PulseNet International
Molecular typing of bacterial pathogen isolates has become an essential component of epidemiologic investigations of infectious disease. The method used by the PulseNet laboratory is pulsed-field gel electrophoresis (PFGE) that analyzes the restriction fragment patterns of chromosomal DNA of almost all bacterial species. The restriction enzymes selected usually have rare or less restriction sites on the bacterial chromosomal DNA resulting in larger fragments that cannot be separated using regular agarose gels but can only be separated using PFGE methodology. This results in a unique fingerprint image that can be documented digitally and analyzed by computer. Since each isolate has only a single PFGE pattern, the PFGE patterns of isolates from various sources are compared. If the two isolates share same PFGE pattern, they are considered identical or from the same source. Further epidemiological studies are needed to confirm the laboratory findings. If the PFGE patterns from two isolates differ, they are definitely not related, and are considered from different sources. This is the principle for PFGE technology used in infectious disease laboratory-based surveillance and investigations (Fig. 1). Although there are several typing methods for bacterial isolates available, PFGE was selected as the best, and it has been proven globally [
8].
Fundamental to PulseNet is that all participating laboratories use the same PFGE method with identical reagents and follow identical protocols for the same bacterial species to archive information sharing. More importantly, all the persons who perform those experiments are trained with the same teaching materials, knowledge, and operating procedures. Therefore using PulseNet, we can share information generated by the same method performed by various people from various laboratories in various countries. In the past, each laboratory used their own protocols for molecular typing and designation of patterns, and the results could not be compared with those of another laboratory or with another person in the same laboratory. This created misleading information greatly diminishing the power of molecular typing methods for disease surveillance and outbreak investigations.
In addition to identical reagents, protocols, and parameters, PulseNet laboratories are required to use similar instruments to ensure comparable data can be generated. The electrophoresis instruments used by PulseNet laboratories include CHEF-DRII, CHEF-DRIII, or CHEF-Mapper (Bio-Rad Laboratories, Hercules, CA). After electrophoresis, the gels are stained with ethidium bromide and PFGE patterns are digitized in a TIFF format (uncompressed .tif file) using image acquisition equipment capable of 768 × 640 pixels or higher resolution. We use Molecular Analyst Fingerprinting Plus with Data and BioNumerics software designed for PulseNet data analysis [
13]. Use of standardized PFGE typing methods allows isolates to be compared from different parts of the country, or from different countries in the world, enabling recognition of nationwide and international outbreaks attributable to a common source of infection. This is a powerful weapon to meet the challenge of global transmission of infectious disease. Routine analyses of bacterial isolates received by public health laboratories using the PFGE method will lead to identification of outbreaks not readily recognizable by other means, locally, nationally, as well as internationally.
In 1995, the US CDC established PulseNet US [
14]. Within a very short period of time, PulseNet has grown rapidly. The PulseNet network is now replicated in different ways in Canada, Europe, the Asia Pacific region, and Latin America [
15-
20]. These independent networks work together in the PulseNet International family allowing public participants to share molecular epidemiologic information in real time and enabling rapid recognition and investigation of multi-national bacterial disease outbreaks [
18]. Many outbreaks have been monitored by the PulseNet international laboratory family in recent years such as the
Vibrio cholerae outbreak that occurred in Haiti in 2010.
PulseNet China
Chinese scientists have been successful in the surveillance of infectious diseases using laboratory-based surveillance. The restriction pattern analysis method and PFGE was used for investigation of the outbreaks caused by
Vibrio cholerae in the 1980s and
Escherichia coli O157:H7 in 1999 [
21]. Because the instruments, reagents, protocols were not standardized, it had taken a long time to obtain information. And, many scientists had not recognized the importance or the power of using standardized methods and protocols at that time. Too many discussions without action resulted in no standard molecular typing methods for laboratory-based surveillance for bacterial infectious disease.
To improve the laboratory-based surveillance system and to share data on bacterial infectious diseases, scientists from China CDC have participated in PulseNet International meetings and China became a member of PulseNet Asia Pacific in 2001. After several years in preparation PulseNet China was established in 2004 (Fig. 2). PulseNet China is coordinated by the National Institute for Communicable Disease Control and Prevention (ICDC), China CDC. ICDC provides support for provincial and local laboratories that participate in PulseNet China through training, technical meetings, advocacy, research grants, information dissemination, and quality assurance.
Many goals have been fulfilled since PulseNet China was established: a laboratory network was formed, a molecular subtyping database was constructed, standardization of subtyping techniques, protocol evaluation and practice in response to outbreaks was achieved. PulseNet China has become a national pathogen surveillance platform for all bacterial infectious disease. Through October 2011, the training of molecular subtyping techniques for pathogenic bacteria has been performed in all provincial CDCs in mainland China; a total of 18 provincial CDCs and one municipal CDC were qualified to join the PulseNet China network.
PulseNet China learns from PulseNet US but differs. First, PulseNet US is only for foodborne pathogens such as Salmonella species and Shigella species. PulseNet China is for all bacterial pathogens, e.g., Yersinia species, Leptospira species, Streptococcus suis, and many others. In addition, PulseNet China is being used in combination with the multi-lcous sequencing typing (MLST) method for outbreak investigations, and is being improved by developing genome-wide typing methods. Third, we use PulseNet China as a platform to establish future laboratory-based surveillance systems for all bacterial infectious diseases in China.
Outbreak investigation experiences of PulseNet China
As soon as PulseNet China was born in 2004, it has contributed to many outbreak investigations in China. It has been recognized as the most important recent advance in China’s food safety system. It will also begin to allow epidemiologists to identify disseminated outbreaks in China, which in the past would have gone unnoticed [
22].
2005 Neisseria meningitidis group C outbreak in China
N. meningitidis is the leading cause of bacterial meningitis and septicemia in children and young adults in China. After a national immunization campaign was initiated in 1982, no countrywide epidemic has occurred; morbidity rates of meningococcal disease in China has remained relatively stable at 0.2-1.0 per 100 000 during the past two decades. Serogroup A meningococci are responsible for more than 95% of cases, whereas serogroups B and C cause only sporadic cases. A sudden increase in the number of cases due to serogroup C meningococci occurred in 2003 in Anhui Province, and was transmitted to other provinces during 2004-2005. Because we had several years of experience in PFGE analysis, we developed PFGE protocols for
N. meningitidis in a very short time. When 106 serogroup C isolates from 12 provinces were analyzed using PFGE, 89 of them showed an identical PFGE pattern, indicating the same pulse type of
N. meningitidis was widely spread in Anhui Province (Fig. 3). We selected 28 strains for MLST analysis and identified ST-4821 which was unique and responsible for the serogroup C meningitis outbreaks in China. Phylogenetic analysis showed the ST-4821 complex has global public health significance having a unique clone that emerged recently with the potential to cause global epidemics [
23] (Fig. 3).
2005 Streptococcus suis outbreak
S. suis is a good example for the role of PFGE and MLST in an acute outbreak of infectious disease with significant national and international public health significance.
In the summer of 2005, an important outbreak of acute disease in humans caused by
S. suis serotype 2 was reported in Sichuan, China. 215 cases were reported, with 68 diagnosed as streptococcal toxic shock-like syndrome with 39 deaths. Streptococcal toxic shock syndrome is a well-defined syndrome, usually associated with Group A streptococci, and primarily associated with super-antigens. Why did so many cases suddenly appear? Has this pathogen mutated? Many questions were asked by the public and academic community. We analyzed 101 isolates using the PFGE method and only two pulse types were identified. Pulse type I was found in 100 of the 101 isolates, including those from human patients and diseased pigs from Sichuan and four other provinces (Fig. 4). This indicated pulse type I was the primary cause of this human outbreak and the causative agent was clonal. When the MLST method was used, 99 of the 101 isolates were identified as ST7, including all 92 isolates from human patients and diseased pigs collected during the Sichuan outbreak in 2005, as well as another two isolates from the Jiangsu outbreak in 1998 (one from a human patient and another from a diseased pig). In addition, the isolates from Jiangxi (one human and 3 pig isolates) and Guangdong (one human isolate) obtained during the summer of 2005 were also assigned to ST7. One human isolate from a patient in Guizhou (strain GZ1) showed a slightly different PFGE pattern and was identified as ST1. Our conclusion was the ST7 is one emerging unusual clone of
S.suis which had spread across China. We then showed this new clone possesses higher virulence capacities to cause disease. The PFGE and MLST data clearly answered the question of why the Sichuan outbreak of
S. suis was so different from others: it was caused by
S. suis ST7, a new variant that emerged possessing high virulence [
24,
25] (Fig. 4).
Emergence of Shigella flexneri X variant
Shigella is an excellent example showing PFGE and MLST analysis can provide new insight and understanding for the prevention and control of a prevalent but neglected bacterial pathogen.
Shigella spp. are the causative agents of shigellosis with
S. flexneri serotype 2a being the most prevalent in China. Epidemiologic surveillance in Henan Province found a new serotype of
S. flexneri X variant appeared in 2001 and replaced serotype 2a in 2003 as the most prevalent serotype. The serotype also became the dominant serotype in other provinces in China. PFGE analysis showed 154 pulse types in 655
S. flexneri isolates analyzed and identified 57 serotype switching events (Fig. 5). This suggested the
S. flexneri epidemics in China were caused by a single epidemic clone, ST91, with frequent serotype switches to evade infection induced immunity to serotypes the population was exposed to previously. The clone has also acquired resistance to multiple antibiotics [
26] (Fig. 5). The importance of this finding is not to be understated. Generally serotypes are used as markers of a clone that can be in error in this epidemic due to serotype switching. This was well documented and allowed the spread of this new distinct strain. Further, this has sobering implications for vaccine development as the foundation of the success of a vaccine is based on O-antigens that require the temporal stability of the principal serotypes [
27].
The future for the laboratory-based bacterial infectious disease surveillance system in China
As the Chinese economy develops, China needs a strong, integrated infectious surveillance system using modern technology. The China Information System for Disease Control and Prevention (CISDCP) is not enough. Without pathogen information, case number reporting cannot meet the challenge of emerging and re-emerging infectious diseases.
PulseNet China should be intergraded into the CISDCP
More importantly, PulseNet China should be updated to meet the challenge. It should include newly developed and advanced surveillance technologies such as Remote Sensing (RS), Global Position Systems (GPS), Geographical Information Systems (GIS), as well as surveillance information for vectors, climate, and antibiotic resistance. All of these have not been adapted for real-time infectious disease analysis.
With regard to pathogens, PFGE is well-practiced. However, as more isolates are analyzed, the database for those images will be huge. PFGE pattern comparisons require higher quality images and additional analysis. Therefore, sequence-based technology is needed. MLST is excellent in most cases for identifying mutated pathogens, however, as more genome sequences of isolates are completed, genome-wide typing methods should be developed in the near future.
With globalization, a strengthened infectious disease network surveillance network at the national and global level has become an essential public health instrument. Through network surveillance, laboratories can supply and exchange information with the internal and external community to detect, monitor, and contain the spread of disease. Therefore, we should move away from the current single disease laboratory-based surveillance system. All surveillance systems for infectious disease should be integrated into the China Information System for Disease Control and Prevention. Those activities could be combined into one integrated system and take advantage of united knowledge, skills, experience, and information for the same goal. When the website of CISDCP is visited for any infectious disease, the users can read the case numbers, geographic location, epidemiological information, clinical information, antibiotic resistance, as well as the PFGE pattern of the isolate, MLST types, and vectors. And the users can find isolates with identical PFGE patterns to determine if it is a sporadic case, or has potential to become an outbreak. With such a system, we can detect early potential outbreaks globally [
18].
Higher Education Press and Springer-Verlag Berlin Heidelberg
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