Poliomyelitis, often called simply as polio or known as infantile paralysis, is caused by infection of the central nervous system by poliovirus.  Typically, the infection irreversibly damages motor neurons causing acute flaccid paralysis (AFP).  As no specific therapeutics is available, prevention by vaccination is the basic strategy against this disease.  Polio is a notifiable category II infectious disease under the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law), which obliges doctors who have diagnosed symptomatic or asymptomatic cases (excluding carriers of vaccine strain) to make immediate notification.  Vaccine-associated paralytic poliomyelitis (VAPP) and poliomyelitis caused by secondary infection of vaccine strain excreted by vaccinees are included in the case definition for notification (see http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou11/01.html).  As AFP is produced by causes other than poliovirus, poliovirus isolation from stool specimens, identification and the genetic analysis of the isolates are the global standard for the surveillance of polio.

Present situation of the global polio eradication program
Since WHO launched the global polio eradication program in 1988, the total polio cases and endemic areas steadily decreased in number, and type 2 wild poliovirus has not been isolated since the last isolation in India in 1999.  Now in 2009, the type 1 and type 3 wild polioviruses are circulating in four remaining polio endemic countries, Nigeria, India, Pakistan and Afghanistan (Fig. 1).  The basic strategy adopted by WHO is to prevent transmission of the wild poliovirus by mass vaccination with the oral polio vaccine (OPV) that is cheap and easy to administer.  Even now, in polio endemic areas and high-risk areas, extensive OPV campaign is being continued.  In spite of these efforts, the program has not progressed significantly further since 2000, which was the WHO�fs target year of the eradication (Fig. 2).  All the above four countries unable to eradicate polio have regional problems.  In 2008-2009, India experienced increased incidence of type 3 wild polio in its northern states (WHO, WER 84: 110-116, 2009).  In 2004-2005, type 1 wild polio originating in Nigeria spread to Sudan, Somalia, Yemen, Indonesia and other countries, resulting in a large-scale polio outbreaks.  All these epidemics were brought under the control (Fig. 2).  In 2008-2009, however, type 1 poliovirus originating in Nigeria spread to Niger, Cote d�fIvoire, Sudan, and Kenya, and type 1 and type 3 wild type polioviruses both originating in India spread to Nepal and Angola, respectively.  Thus, exportation of wild type polio from endemic countries is now a big problem for global eradication of polio (WHO, WER 84: 110-116, 2009 & 84: 133-140, 2009).

Since 2000, polio outbreak caused by vaccine-derived poliovirus (VDPV) has been reported from various parts of the world.  In Nigeria, in particular, type 2 VDPV together with types 1 and 3 wild polioviruses has been circulating for these 4 years (see p. 174 of this issue).  In the WHO Western Pacific Region (WPR), the wild polio ceased to be transmitted since 2000 when the termination of the wild poliovirus transmission was declared.  However, small-scale VDPV outbreaks and imported wild polio cases have been reported.  Therefore, potential of polio outbreak still persists in WPR (see p. 173 of this issue).

Polio surveillance in Japan
In Japan, obligatory notification of polio cases under the Infectious Diseases Control Law and various surveillance activities under the National Epidemiological Surveillance of Vaccine-Preventable Diseases (NESVPD) ensure absence of importation and/or circulation of wild poliovirus and VDPV.  In the polio infectious agent surveillance under NESVPD, poliovirus isolates from polio patients and healthy children have been analyzed every year.  Since 1993, no wild poliovirus has been isolated in Japan (see p. 176 of this issue).  The polio susceptibility surveillance conducted also under NESVPD confirmed antibody prevalence sufficient for preventing spread of wild polioviruses (see p. 178 of this issue).  For introduction of inactivated polio vaccine (IPV), sensitive and accurate polio laboratory surveillance has to be continued.  Development of new polio surveillance methodology including environmental surveillance needs to be considered (see p. 180 of this issue).

Laboratory diagnosis of poliovirus
The basic of the laboratory diagnosis is isolation and identification of poliovirus using cultured cells and intra-typic differentiation, differentiation of vaccine type and non-vaccine type viruses (wild poliovirus and VDPV).  Increasing realization of risk caused by VDPV has necessitated laboratory diagnosis with higher precision, and presently isolates judged as non-vaccine-like polioviruses in the genetic or antigenic analysis are further submitted to nucleotide sequencing of the whole VP1 region.  Isolates having mutations in 1% or more of the region are considered as VDPV that accumulated mutations through considerably long circulation.  Once VDPV is detected, intensive surveillance should be conducted.  If considered necessary, polio-preventing measures such as supplemental OPV vaccination campaign should be conducted.

After attaining the eradication of polio, poliovirus stocked in laboratories becomes the only source of polio epidemic.  For that concern, the laboratories including those in Japan were investigated for their possession of poliovirus, and Japanese government submitted the quality assurance report attached with the list of laboratories possessing wild poliovirus to the Regional Commission for the Certification of Poliomyelitis Eradication in the Western Pacific (see p. 181 of this issue).

Issues to be considered
WHO places the global polio eradication program as the number one priority among infectious disease control programs, and is advancing policies optimizing the vaccination in the polio endemic countries.  However, ending the circulation of wild poliovirus within coming few years is a hard challenge in the present circumstances.  Considering that the polio eradication is not within reach in the immediate future, it is important to continue high quality polio surveillance even in polio free countries including Japan.  So long as OPV is used, VAPP will be continuously reported and risk of polio epidemic caused by VDPV never disappears.  Actually in Japan VAPP cases have been reported intermittently (0-2 cases/year, IASR 29: 200-201, 2008) and VDPV was detected (see p. 176 & 180 of this issue).  Therefore, many countries are replacing OPV with IPV, and Japan also needs to take the same action immediately.  The vaccine combining IPV and DPT being developed in Japan, when approved, can be used as vaccine replacing OPV and probably DPT also.

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