International Note - Acute flaccid paralysis surveillance: a global platform for detecting and responding to priority infectious diseases


Following the 1988 World Health Assembly resolution to eradicate poliomyelitis, the number of polio-endemic countries has decreased from > 125 in 1988 to only six countries considered polio-endemic by the end of 2003. As part of the eradication strategies, a sensitive global surveillance system was established that captures a high proportion of acute flaccid paralysis (AFP) cases in children aged <= 15 years of age, with virologic testing of stool specimens to detect cases of paralytic poliomyelitis. As AFP surveillance systems matured, countries increasingly applied the AFP structure and system to detect other priority diseases, mostly measles and other vaccine-preventable diseases. In the World Health Organization (WHO) Region of the Americas, fever and rash surveillance was added to capture measles and rubella. In the WHO African Region, AFP surveillance provided the nucleus to develop an "Integrated Disease Surveillance and Response" (IDSR) system for priority infectious diseases. This report describes the current status of global AFP surveillance and its expansion to include reporting of measles and other vaccine-preventable diseases, and explores the extent to which expanded AFP surveillance systems could be used as a platform for a global network of disease detection and response.

The AFP surveillance system

Any disease eradication initiative relies on highly sensitive surveillance to guide programmatic action. This is especially important for polio eradication, since only one out of 200 infections with poliovirus results in clinically apparent paralytic disease. To identify and eliminate the remaining pockets of wild poliovirus transmission, surveillance must detect and investigate as many cases of paralytic poliomyelitis as possible.

AFP surveillance relies on a standard case definition of the "acute flaccid paralysis" syndrome which, by definition, includes all cases of paralytic poliomyelitis. The quality of AFP surveillance is monitored using standard indicators to measure the sensitivity of AFP reporting, the completeness and timeliness of specimen collection, and the quality and proficiency of laboratory support. The non-polio AFP rate (Table 1), a unique attribute of AFP surveillance, allows AFP reporting completeness to be compared among and within countries, since a non-polio AFP case of 1 or more per 100 000 population of < 15 years of age indicates sensitive surveillance.

AFP surveillance depends on the following main system elements: (i) immediate reporting and investigation of AFP cases; (ii) routine monthly negative reporting ("zero" reporting even if no AFP case is seen) from all health facilities; and (iii) weekly active surveillance visits to priority health facilities and other reporting sites likely to see AFP cases. AFP surveillance systems in industrialized and many middle-income countries are integrated into existing disease surveillance systems and are run by national health staff. Developing countries with fewer resources receive considerable external technical and funding support for AFP surveillance; in these countries, a network of surveillance medical officers (SMOs) was put in place to ensure that the system performs adequately. Linkages between public health workers and clinical doctors are established through frequent activities to inform and sensitize clinicians. In countries with weak health systems, efforts are made to involve the informal health sector (traditional healers), communities and community informants in reporting AFP cases.

AFP systems were established even in countries and areas affected by conflict, or those in post-conflict and other complex emergency situations such as in Afghanistan, Angola, Democratic Republic of the Congo, Somalia and southern Sudan.

As of mid-2004, AFP surveillance systems were operating in 198 of 215 countries and territories in the world, with surveillance data reported weekly from all countries to WHO at the regional and global level. Field AFP activities are supported by a three-tiered global polio laboratory network (Map 1), which operates in all six WHO regions and consists of 145 laboratories: 123 at the national level, 15 regional reference laboratories and seven global specialized laboratories. Network laboratories process stool samples from AFP cases to perform virus isolation, serotyping, intratypic differentiation and genomic sequencing. A WHO-sponsored laboratory accreditation programme ensures high-quality laboratory performance, and 96% of network laboratories were fully accredited by WHO in 2003. The surveillance system is able to cope with very large workloads: in 2003, almost 35 000 AFP cases were reported globally, with testing of adequate stool specimens from 86% of all AFP cases (Table 1).

The Global Commission for the Certification of Poliomyelitis Eradication has specified that the interruption of wild poliovirus transmission in a WHO region can only be certified following a period of 3 years without isolation of wild poliovirus from an AFP case, in the presence of high-quality, "certification-standard" AFP surveillance - a standard that all WHO regions have successfully reached and maintained through continued monitoring and fine-tuning of surveillance quality. Of the six WHO regions, three have already been certified polio-free: the WHO Region of the Americas in 1994, the Western Pacific Region in 2000, and the European Region in 2002. Certification-standard AFP surveillance has been maintained in all endemic and most non-endemic countries of the WHO African Region, Eastern Mediterranean Region and South-East Asia Region (Table 1).

Table 1. Structure and performance of global acute flaccid paralysis (AFP) and measles surveillance systems, by WHO region, 2003

External technical and funding support for AFP surveillance is provided by the international polio partnership. Of the more than US$ 98 million provided for AFP surveillance in 2003 by the partnership, US$ 47 million was used for surveillance activity costs (including the laboratory network, transport and communication, meetings), and US$ 51 million funded more than 2 700 international and national staff (Table 1) to support AFP surveillance and, where appropriate, supplementary immunization activities.

Expansion of AFP surveillance

Measles surveillance. Globally, more than two thirds of countries with AFP systems report that surveillance for measles has benefited considerably from AFP surveillance (Table 1). However, the intensity of measles reporting - as measured by the proportion of suspect cases that are tested - and the way in which AFP systems have influenced measles reporting vary across regions according to programme goals (i.e. measles elimination vs. mortality reduction).

Comprising 690 laboratories, the Global Measles Laboratory Network (Map 1) was developed along the lines of the Global Polio Laboratory Network. The network's primary roles are confirmation of suspected measles cases using IgM testing and genetic characterization of measles viruses. Measles laboratories have utilized much of the polio laboratory infrastructure; housed at the same institutions, they use similar systems for specimen transport, data management, communication and reporting of results. Network laboratories routinely test measles-negative sera for rubella, and have processed more than 60 000 serum specimens from suspected measles cases in 2003. Measles laboratories also perform serological diagnosis of yellow fever in countries in Africa and Latin America where yellow fever is prevalent (Map 1).

Figure 1. Global vaccine-preventable disease laboratory network

Global vaccine-preventable disease laboratory network

Case-based measles surveillance with laboratory confirmation of suspected cases is fully implemented in the WHO Region of the Americas, where it has been instrumental in monitoring the successful interruption of endemic measles virus transmission. This system is now being implemented to include rubella surveillance in support of the regional goal to eliminate rubella and congenital rubella syndrome by 2010. Throughout the Americas in 2003, 95% of suspected measles cases had a blood specimen tested. In the other WHO regions, the proportion of measles cases tested ranges from 1.2% to 29%.

In 28 of 46 countries in the WHO African Region, casebased measles surveillance with laboratory confirmation has been introduced following nationwide, measles vaccination campaigns targeting all children aged 9 months to 14 years, which were conducted to reduce overall measles mortality. Measles surveillance activities were included in the terms of reference of polio-funded SMOs.

As part of the global measles mortality reduction strategy, case-based measles surveillance has been supported by an international coalition of partners: the Measles Initiative.

Other diseases. In addition to surveillance for measles and rubella, AFP surveillance systems have also been used to report other vaccine-preventable diseases, such as neonatal tetanus (NT). In countries where NT is a public health problem, it is estimated that current reporting captures < 5% of NT cases. Unlike measles, there is no laboratory confirmation for NT. Reporting efficiency of NT has been considerably increased by including NT during the active case search for AFP cases in health facilities. This allows the programme to monitor progress towards NT reduction and to target immunization response activities.

In west and central Africa and in endemic areas of Latin America, the AFP surveillance system now encompasses reporting of and laboratory testing for yellow fever (Map 1).

AFP surveillance provided the nucleus around which the IDSR system in the WHO African Region was built. Several countries around the world have expanded the roles of their AFP SMOs to include support for strengthening routine immunization services, outbreak investigation and emergency response. In 2003, SMOs and polio and measles laboratory personnel assisted in the detection and investigation of outbreaks of cholera, dengue, haemorrhagic fevers, malaria, meningitis, Rift Valley fever and severe acute respiratory syndrome (SARS).

Editorial note. Adoption of the global polio eradication goal required that a truly global AFP surveillance system was established, in all countries and reporting units, worldwide, including in countries in difficult circumstances and those affected by conflict.

As the AFP surveillance system matured, countries and regions became increasingly interested in utilizing this system to report other priority diseases. Currently, two thirds of countries with AFP systems globally (Table 1) are utilizing the AFP system for the surveillance of other vaccine-preventable diseases, particularly for measles reporting. AFP surveillance staff and infrastructure continue to provide crucial support for the investigation of and response to other epidemic-prone diseases.

The 2003 SARS outbreak, the possibility of renewed influenza pandemics and the importance of the early detection of and response to outbreaks of other emerging or re-emerging infectious diseases all highlight the need for a more comprehensive global disease detection system. The urgency of such a system is underscored also by continuing efforts to restructure the International Health Regulations as a framework for the containment of global public health risks. Currently, a variety of systems exist for the global detection of existing diseases such as influenza, Haemophilus influenzae type B (HiB), hepatitis, HIV and of emerging infectious diseases such as SARS. However, none of these existing systems, which are often based on sentinel sites and focus on a single disease, approaches the global reach or reporting efficiency of the AFP system.

The following attributes of AFP surveillance allow the reliable monitoring of progress towards eradication and facilitate a timely immunization response to virus detection: (i) a globally standardized approach, particularly the centralized and transparent quality control mechanisms (e.g. monitoring of quality indicators for sensitivity and specificity, accreditation of polio laboratories); (ii) the reporting of a syndrome - AFP - rather than of a disease; (iii) the rapid and reliable laboratory confirmation of cases, as well as genetic characterization of identified virus strains; (iv) weekly reporting from all levels, down to the district and even community level, where needed; (v) a combination of routine "zero" reporting from facilities when no cases are detected with active surveillance visits to priority reporting sites; (vi) continued training of staff in case investigation, data management and basic epidemiology; (vii) regular supervision an devaluation of the system at the country, regional and global level; (viii) efficient linkages between national and international field, epidemiological and laboratory staff, to allow weekly feedback of surveillance performance and results from subnational to the national and international level; and (ix) response and control with an effective and standardized vaccination strategy.

To date, diseases successfully incorporated into AFP systems, or using the AFP infrastructure, such as measles, share common traits: a well-defined syndromic presentation, relative ease of specimen collection for laboratory confirmation, strong international commitment for control and elimination, resulting in adequate external funding to ensure high-quality surveillance and a continued focus on using surveillance data for targeted control activities. The most obvious way to maintain and expand the existing AFP and measles reporting systems is to add the reporting of other vaccine-preventable diseases, although care should be taken not to jeopardize polio eradication goals by overburdening the system.

Most of the key attributes of AFP surveillance characterize the AFP system as a very attractive platform on which a truly global, rapid system to detect and respond to other priority infectious diseases could be built. The global polio and measles laboratory network could expand into a global network of public health laboratories for priority infectious diseases. AFP systems, particularly in resource-poor and underserved countries and areas, have repeatedly been instrumental in detecting and responding to outbreaks of other diseases. The opportunity to build on AFP surveillance is unique also because of the existing long-term commitment of the international community to polio eradication: current support for the AFP system represents an important component of a much larger investment by the international polio partnership to polio eradication, considered as a global public good. To protect this investment, and given the current programme status and of envisioned timelines to global interruption of wild poliovirus transmission and certification, high-quality, appropriately funded AFP systems will need to continue to function for a minimum of 5 years.

Successful expansion to detect other priority diseases would require: (i) a focused approach to disease selection; (ii) careful consideration of disease-specific specimen collection and reporting needs; and (iii) continuing international commitment to conduct surveillance for the selected disease, to ensure the necessary funding for expanding human resources and laboratory capacity as necessary.

Source: WHO Weekly Epidemiological Record, Vol 79, No 48, 2004.

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