Laboratory Surveillance for Wild and Vaccine-Derived Polioviruses --- Worldwide, January 2008--June 2009

The Global Polio Laboratory Network (GPLN) isolates and characterizes polioviruses from fecal specimens of persons with acute flaccid paralysis. The network is coordinated by the World Health Organization (WHO) and includes 144 laboratories in 97 countries. Data from the network are used to guide the Global Polio Eradication Initiative by confirming polio cases, detecting and determining the origin of importations, identifying vaccine-derived polioviruses (VDPVs), and documenting the circulation of wild polioviruses (WPVs). This report updates previous reports (1) and summarizes GPLN activities and detection of WPVs and VDPVs during January 2008--June 2009. During this period, GPLN tested 247,794 fecal samples from 127,566 acute flaccid paralysis cases, from which 14,279 (5.8%) poliovirus isolates (vaccine-related and WPV) were detected, including 4,280 (1.7%) WPVs from 22 countries. GPLN laboratory capacity and capabilities remain an integral part of surveillance for polioviruses and efforts to eliminate polio from the remaining areas of circulation.

Laboratory Network Performance

GPLN provides timely information on significant changes in the global epidemiology of WPV circulation, to ensure the program can respond quickly to importations and changing serotype or geographic distribution by adjusting vaccination tactics. This is accomplished through isolation of polioviruses from stool specimens and characterization by intratypic differentiation (ITD) of the isolates. ITD uses a combination of two laboratory procedures that first separates all poliovirus isolates into the categories of wild and vaccine related, and a second procedure that screens vaccine-related viruses for possible VDPVs (1).

WHO administers an annual laboratory accreditation program for all GPLN facilities to evaluate compliance with recommended technical and operating procedures, performance in proficiency tests, and achievement of targets for accurate and timely results. The quality assurance and accreditation program was modified* in 2008 to accommodate a revised test algorithm† for more rapid poliovirus detection. These changes were implemented initially in 52 (36%) GPLN laboratories in the WHO regions of Africa, Americas, Eastern Mediterranean, and South-East Asia. Overall, 136 (94%) GPLN laboratories were fully accredited by WHO. Six provisionally accredited laboratories met required standards for accurate results but had some performance deficiency (e.g., not meeting target times in ≥80% of results for isolation or reporting). Two nonaccredited laboratories failed the proficiency test and, while resolving performance concerns, are testing samples in parallel with accredited reference laboratories. Targets for timely reporting of poliovirus isolation results were met in all six WHO regions (Table 1), and five of those regions provided ≥80% of ITD results within 60 days of paralysis onset in acute flaccid paralysis cases. In the Western Pacific region, 77% of results were reported within 60 days in the first half of 2009, compared with 40% in 2008.

GPLN tested 247,794 fecal specimens sent to the laboratories from investigations of acute flaccid paralysis cases, from which 14,279 polioviruses and 46,462 nonpolio enterovirus isolates were detected during January 2008--June 2009. Acute flaccid paralysis surveillance consists of the notification and investigation of all persons aged <15 years with onset of acute flaccid paralysis or any person of any age with suspected poliomyelitis. As part of the case investigation, two stool specimens are collected at least 24 hours apart and within 14 days of the onset of paralysis. In addition to testing specimens from acute flaccid paralysis cases, the laboratories also tested 20,277 specimens from other sources, including contacts of acute flaccid paralysis cases, environmental specimens, and healthy children. From these specimens, 149 WPVs were detected, all linked to known endemic regions.

During January 2008--2009, a 6% overall increase in GPLN workload occurred compared with the January 2007--June 2008 period (1). In polio-endemic regions, laboratory workload increased 21% for Africa and 5% for South-East Asia, and decreased 13% for the Eastern Mediterranean region. In polio-free regions, workload decreased 1% for the Western Pacific, and increased 2.5% for Americas and 7.7% for the European region. To improve timeliness in obtaining ITD results among the 44 GPLN laboratories in the polio-endemic regions, ITD testing capacity was increased, from 16 laboratories having this capacity in mid-2006 to 24 at the end of 2008. During 2009, five more laboratories are being upgraded.

Detection and Determination of WPVs and Transmission Links

GPLN detected 4,280 WPVs in acute flaccid paralysis specimens from 22 countries during January 2008--June 2009 (Tables 1 and 2). In 12 countries, only WPV type 1 (WPV1) was detected; in one country only WPV type 3 (WPV3) was detected, and in nine countries, both WPV1 and WPV3 were detected. Overall, 2,582 (60%) WPVs were found in 17 African countries, 476 (11%) WPVs in three Eastern Mediterranean countries, and 1,222 (29%) WPVs in two South-East Asia countries. No indigenous WPV type 2 (WPV2) has been found anywhere in the world since 1999 (2).

Comparative analysis of the nucleotide sequence of the VP1 region of the viral genome (VP1 nt sequence) allows genotype identification and determination of transmission links based on genetic relatedness. Only four WPV genotypes have been detected globally since 2005. One genotype each of WPV1 and WPV3, designated West Africa B (WEAF-B), is indigenous to Nigeria. One genotype each of WPV1 and WPV3, designated South Asia (SOAS), is indigenous to Afghanistan, India, and Pakistan. WPVs circulating in Afghanistan and Pakistan are distinct from those circulating in India. Transmission in these four remaining polio-endemic countries accounted for 3,635 (85%) of all reported WPVs during 2008--2009.

Transmission in the 18 nonendemic countries accounted for 641 (15%) of the reported WPVs during 2008--2009 (Table 2). In 14 of these countries, only WEAF-B genotypes were detected; in three countries (Angola, Democratic Republic of the Congo, and Nepal), only SOAS WPV genotypes were detected, and in one country (Central African Republic), both genotypes were detected. WEAF-B WPV1 and WEAF-B WPV3 genotype viruses both were found in Benin, Chad, Niger, and Sudan; in all but Sudan, this was the result of importations of WPV1 and WPV3 in 2007--2009 originating from Nigeria (3). WPV1 in Sudan represented continuation of an outbreak from 2004. WPV3 in Sudan and Central African Republic represented importations from Chad. Only WEAF-B WPV1 was detected in 10 countries: Burkina Faso, Côte d'Ivoire, Ethiopia, Ghana, Guinea, Kenya, Liberia, Mali, Togo, and Uganda. SOAS WPV1 was found in Central African Republic, SOAS WPV3 genotype was found in Nepal, and SOAS WPV1 and WPV3 genotypes both were found in Angola and Democratic Republic of the Congo. Since 2005, SOAS WPV has been imported into Angola three times. SOAS WPV1 from the first importation in 2005 was not detected in Angola during 2008--2009; however, it continued to circulate in Democratic Republic of the Congo in 2008 after being introduced there in 2006 and spread further to Central African Republic in 2008. WPV1 in Angola during 2008--2009 represented continuation of transmission that began with a second importation of virus from India in 2007. An importation in 2008 of a SOAS WPV3 was exported to Democratic Republic of the Congo in 2008 and subsequently Central African Republic, where it has resulted in continued circulation in all three countries in 2009. SOAS genotypes from Pakistan and Afghanistan were not detected in any other countries.

Single importations of WEAF-B WPV1 related to polioviruses circulating in Sudan and SOAS WPV1 related to poliovirus in Uttar Pradesh, India, were detected in 2008 in sewage samples in Egypt. During 2008--2009, 32 WPV3 and three WPV1 isolates were detected in 33 (14%) of 234 specimens collected in Mumbai, India. The WPV in Mumbai sewage was linked genetically to virus found in acute flaccid paralysis cases in Bihar in 2007, and at least two of these introductions led to apparent local transmission, as indicated by multiple WPV1 and WPV3 detections in sewage for approximately 8 months and 1 year, respectively.

Detection of Vaccine-derived Polioviruses

GPLN screens for vaccine-derived polioviruses (VDPVs) among detected Sabin-like polioviruses (4). During January 2008--June 2009, 9,999 Sabin-like viruses from acute flaccid paralysis cases were screened, and 457 (4.6%) were classified as VDPVs (Table 3). Field evaluation of a new real-time reverse transcription--polymerase chain reaction (rRT-PCR) assay developed at CDC (5) began in 10 network laboratories during 2008. This included retrospective testing of approximately 4,100 Sabin-like polioviruses reported since 2006 and prospective testing of Sabin-like polioviruses identified during 2008--2009 with VP1 nt sequences analyzed when required. This testing identified several VDPVs, including some viruses from Democratic Republic of the Congo (Table 3), provided more rapid test results for the current VDPV outbreaks, and offered reassurance that additional VDPVs were not missed in the laboratories.

Reported by: Polio Eradication Dept, World Health Organization, Geneva, Switzerland. Div of Viral Diseases and Global Immunization Div, National Center for Immunization and Respiratory Diseases, CDC.

Editorial Note:

GPLN has improved the efficiency of poliovirus testing and reduced reporting times by 50% since 2007. Technology transfer of ITD assays has been accomplished even in resource-poor settings in polio-endemic regions. These virus testing improvements enable more rapid implementation of responsive supplementary immunization activities (SIAs)§ targeted at areas of confirmed WPV circulation after importations (3). Ongoing activities to improve testing efficiency and diagnostic procedures benefit the Global Polio Eradication Initiative by providing more accurate and timely information of poliovirus circulation, while conserving limited resources.

GPLN continues to provide critical information to support situational assessments in key endemic countries, including documenting the extent and pathways of polio transmission. For example, a decline in the total number of WPVs detected during January 2008--June 2009 compared with January 2007--June 2008, and a reduction in the number of WPV3 isolates, have provided evidence of progress toward poliovirus eradication in India. This contrasts with a net increase in WPVs detected in the three other polio-endemic countries: Nigeria, Pakistan, and Afghanistan. The situation in Nigeria is a particular concern because of continued WPV1 transmission, expanded transmission of WPV3, and continued cVDPV2 transmission during 2006--2009. Progress to date in improving implementation of SIAs in Nigeria¶ and addressing weaknesses in routine oral poliovirus vaccine coverage in Nigeria has not sufficiently reduced population immunity gaps to all three poliovirus serotypes.

WPV was imported into polio-free areas during 2008--2009, predominantly from Nigeria and India. In addition, WPV has spread into polio-free countries via intermediate countries that failed to interrupt outbreaks resulting from imported WPV. GPLN has helped document the transmission of WPV for periods of more than 6 months in Angola, Burkina Faso, Chad, Côte d'Ivoire, Sudan, and Democratic Republic of the Congo that have spread WPV to other countries. Until WPV transmission is interrupted globally, all countries should maintain high levels of polio vaccination coverage and acute flaccid paralysis surveillance, including timely reporting of laboratory results, to minimize the risk for and effects of WPV importations.

Acknowledgment

This report is based, in part, on contributions by staff members of GPLN laboratories.

References

1. CDC. Laboratory surveillance for wild and vaccine-derived polioviruses---worldwide, January 2007--June 2008. MMWR 2008;57:967--70.
2. CDC. Apparent global interruption of wild poliovirus type 2 transmission. MMWR 2001;50:222--4.
3. CDC. Wild poliovirus type 1 and type 3 importations---15 countries, Africa, 2008--2009. MMWR 2009;58:357--62.
4. CDC. Update on vaccine-derived polioviruses---worldwide, January 2006--June 2007. MMWR 2007;56:996--1001.
5. Kilpatrick DR, Yang C-F, Ching K, et al. Rapid group-, serotype-, and vaccine strain-specific identification of poliovirus isolates by real-time reverse transcription PCR using degenerate primers and probes containing deoxyinosine residues. J Clin Microbiol 2009;47:1939--41.

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Date last reviewed: 9/3/2009