Acute flaccid paralysis surveillance indicators in the Democratic Republic of Congo during 2008-2014

Introduction The last wild poliovirus (WPV) case in Africa was reported in July 2014, thus underscoring the tremendous progress towards polio eradication worldwide. This study aimed to analyze the results of a seven-year surveillance of Acute Flaccid Paralysis (AFP) in the Democratic Republic of Congo (DRC) and to identify potential gaps that need to be addressed. Methods Epidemiological and virological data obtained from AFP surveillance among AFP cases less than 15 years from January 2008 to December 2014 in DRC were retrospectively considered and analyzed in this study. Results Of the 13,749 AFP cases investigated, 58.9% received at least three doses of oral polio vaccine (OPV), 7.3% never received OPV, while the status of 18.3% was unknown. Analysis of surveillance performances showed that all, but two, indicators were below the required WHO-specified targets. Non-polio enterovirus (NPEV) isolation rate was consistently below the minimum requirement at ≥10% and the proportions of stool specimens that reached the laboratory within 72 hours of being sent were always below 15% (WHO target is ≥80%). Virus isolation and differentiation showed that 1.5% of AFP cases were infected by WPVs, 5.5% by Sabin strains, 0.5% by vaccine-derived polioviruses (VDPVs) and 7.2% by NPEVs. Conclusion Our findings indicate that additional efforts are needed to address the timeliness of adequate stool specimens’ arrival to the laboratory. It remains essential to maintain high polio vaccine coverage and high AFP surveillance standards to ensure rapid detection and containment of either WPV importation or VDPV re-emergence in DRC.


Introduction
Poliomyelitis is a highly infectious disease caused by the polioviruses (PVs) belonging to the species Enterovirus C, genus Enterovirus and family Picornaviridae [1]. Like most other Enteroviruses, PVs are transmitted by the feco-oral route. Poliomyelitis can affect individuals of any age, but primarily involves children aged less than five years. In up to 1% of those infected the virus invades the central nervous system leading to muscle weakness and irreversible paralysis (usually in the lower limbs), often progressing to breathing difficulties, and death [2,3]. In 1988, the World Health Organization (Sabin strains of serotypes 1, 2 and 3) that are able to multiply to high titers in the gastrointestinal tract, thus inducing strong mucosal immunity to PVs [5]. The major disadvantage of OPV is that vaccine strains can replicate and mutate during inter human circulation in under immunized populations, thus leading to the emergence of neurovirulent vaccine derived PVs (VDPVs) [5,6].
Although polio outbreaks continue to occur, the poliomyelitis incidence has drastically decreased by > 99%: from an estimated 350,000 cases in 1988 to 359 cases in 2014 [7]. The number of polio-endemic countries has also decreased from 125 countries in 1988, to only two countries in 2014 (Pakistan and Afghanistan) [7].
AFP is defined as acute or sudden onset of weakness or paralysis of a limb characterized as flaccid (reduced tone) in a child < 15 years of age [8]. AFP surveillance consists of the detection and investigation of flaccid paralysis of new onset in children less than 15 years or any other suspected poliomyelitis case in a person of any age. It has been adopted globally as an essential strategy for monitoring the progress of the polio eradication initiative [9].
Hence, AFP surveillance remains the "gold standard" to identify areas and populations who are at high risk, to assess polio status of a given country, to reveal the need of SIA and to certify the absence of wild PV (WPV) circulation in countries that are no longer reporting poliomyelitis cases [10].  [12]. Moreover, DRC as any other country remains at risk of WPVs re-importation from the remaining polio-endemic reservoirs [13,14].
The Institut National de Recherche Biomédicale (INRB), located at the capital city Kinshasa, is the WHO-accredited National Reference Poliomyelitis Laboratory (NRPL) in DRC. INRB has routinely registered epidemiological and virological data from AFP surveillance since 1997. Regular analyses of data generated from an AFP surveillance system are of great interest for the identification of potential problems to be addressed in order to guarantee timely detection of WPV re-importation or VDPV re-emergence. WHO developed a set of performance indicators to ensure that AFP surveillance is conducted with the required standards, maintained and operated uniformly [8].
This study describes the results of the surveillance of AFP in DRC from January 2008 to December 2014 and identifies the gaps that need to be addressed in order to provide the highest level of confidence in the control and prevention of PV infection in DRC.

Study area and design
The DRC is located in sub-Saharan Africa, bordered by Angola, the South Atlantic Ocean, the Republic of Congo, the Central African Here, a retrospective descriptive study was conducted using AFP surveillance data routinely collected from January 2008 to December 2014 by the INRB at Kinshasa. All AFP cases reported nationwide during the studied period were considered for this study.

Organisation of AFP surveillance in DRC
Data for AFP surveillance is routinely registered in the weekly and coding gene [19].

WHO Accredited National Reference Polio Laboratory
The WHO-accredited NRPL is located at the INRB in Kinshasa, the capital city of DRC. The NRPL follows standardized procedures for i) the isolation of PV and non polio enteroviruses (NPEVs) using RD and L20B cell cultures, ii) the PV identification to confirm WPV cases, iii) the differentiation of the three PV serotypes (1-3), WPVs,

Sabin-like PVs and VDPVs by ITD and iv) the shipment of the suspected WPV and VDPV isolates to a WHO Specialized Polio
Laboratory where sequencing of VP1 gene is performed. The timeliness, accuracy and quality of the NRPL are assessed through proficiency testing for laboratory techniques and annual onsite accreditation review [20].

Indicators of AFP surveillance performance
WHO has established some minimum performance indicators that should be used to ensure that the quality of AFP surveillance is achieved and maintained. In this study, we evaluated the performance of the AFP surveillance system using the following WHO-specified indicators:    Figure 3. The National Expert Committee classified overall 214 (9.1%) cases with inadequate stool specimens as polio compatible while the remaining 2125 (90.9%) AFP cases with inadequate stool specimens were discarded (Figure 3).
The circulation of WPV has been stopped over the country since 2011 [13]. All WPV cases obtained during the study period were genetically characterized and results from this study showed that the most frequently serotype was WPV type 1 while WPV type 2 were not detected (data not shown  (Table 3). Two important indicators of AFP surveillance performance were consistently below the minimum standard throughout the study period. Despite the fact that most specimens arrived at the laboratory in good conditions, the proportion of stool specimens arriving at national level within three days of being sent did not reach 15% which is far below the acceptable minimum of 80% (Table 3). Moreover, the annualized non polio AFP rates that reveal the sensitivity of the system were consistently < 10% throughout the study period (Table 3).

Discussion
The present study reports the incidence of AFP over seven-year (2008-2014) period in DRC. The performance of AFP surveillance over these years was also evaluated. The age distribution of AFP cases confirmed that low age was a risk factor as previously shown [22]. However, the average NPEV isolation rate was consistently below the minimum requirement throughout the study period (Table 2).
These NPEV isolation rates were lower than the 12% and 17.6% reported in Ghana [23] and Iraq [31], respectively. This could be explained by the fact that many NPEV strains belonging to the Enterovirus Cspecies cannot be isolated on RD cells. Previous studies showed that the introduction of HEp-2c cell line in the isolation algorithm significantly increases the NPEV isolation rate in tropical settings [32,33]. Moreover, the timeliness of transportation of the stool specimens is critical for the surveillance system as enteroviruses should survive until the time of analysis for the laboratory to be able to isolate them [34]. According to the WHOspecified national targets for AFP surveillance, at least 80% of stool specimens must arrive at the laboratory within 72 hours of collection [35][36][37]. Results from this study reveal that this target remained out of reach throughout the study period. The proportion of specimens that arrived at the laboratory within 72 hours was consistently less than 15% whereas it was expected to be at ?80%. This low performance for specimen's transportation to the laboratory may be Angola [30]. WPV has virtually disappeared in DRC since 2011 [38], but efforts should be made towards eradication of poliomyelitis caused by neurovirulent VDPVs by maintaining high quality AFP surveillance, supplemented by environmental surveillance which is being initiated. These are essential for timely detection and response to potential WPV re-introduction and VDPVs re-emergence in DRC.

Conclusion
The