Poliomyelitis is caused by an enterovirus, poliovirus. Infection involves the gastrointestinal tract, and may progress to the nervous system, resulting in paralysis. Acute flaccid paralysis (AFP) occurs in less than 1% of infections. More than 90% of infections are asymptomatic, with a minor illness characterised by fever, headache, malaise and nausea/vomiting occurring in about 10%. The maximum extent of paralysis is usually reached within three to four days of disease onset. Any paralysis still present after 60 days is likely to be permanent.44
Vaccine-associated paralytic poliomyelitis (VAPP) is acute flaccid paralysis due to a Sabin-like poliovirus (i.e. a virus similar to that used in the Sabin live attenuated oral poliovirus vaccine (OPV)). A vaccine derived poliovirus (VDPV) is defined as having 1%–15% nucleic acid sequence variation from the prototype Sabin strain. The variation is due to long-term (more than one year) virus replication after administration of OPV. The virus replication may occur in an individual with an immunodeficiency (iVDPV) or through sustained person-to-person transmission in areas with low OPV coverage (circulating or cVDPV). VDPVs not clearly assigned to either of these categories are known as ambiguous VDPVs (aVDPV).173
See Appendix 6 for pre 2004 definition
National definition from January 2004:11
Confirmed cases require laboratory definitive evidence and clinical evidence. Probable cases are also notifiable and require clinical evidence and that the case not be discarded as non-polio acute flaccid paralysis by the Polio Expert Committee.
a) Laboratory definitive evidence
b) Clinical evidence
Hospitalisations and deaths
The ICD-10-AM/ICD-10 code A80 (acute poliomyelitis) was used to identify hospitalisations and deaths.
Note: This code includes VAPP and specific codes for indigenous and imported wild-type poliovirus infection. Sequelae of poliomyelitis (ICD-10 code B91) were not included in these analyses.
Notifications, hospitalisations and deaths
No notifications or deaths were recorded for poliomyelitis in 2003, 2004 or 2005. From July 2002 to June 2005 there were 60 hospitalisations with a diagnosis of acute poliomyelitis. Of these, one was coded as VAPP, seven as acute non-paralytic poliomyelitis and 14 as other/unspecified acute paralytic poliomyelitis. The remaining 38 hospitalisations were coded as acute unspecified poliomyelitis. The majority of separations recorded as acute poliomyelitis were in older persons (92% aged 50 years and over). The number of hospitalisations reported as acute poliomyelitis decreased over the period with 39 cases in 2002/2003, 20 in 2003/2004 and only one case in 2004/2005. Only five hospitalisations were recorded as having a principal diagnosis of poliomyelitis (all from New South Wales in the period 2002/2003).
It is unclear exactly when the last case of locally acquired poliomyelitis occurred in Australia. The last laboratory-confirmed case was in 1967. Three clinically compatible cases were notified in 1972; however, no additional information is currently available.174 Australia and the Western Pacific Region were declared polio-free in October 2000.175 The most recent case of VAPP was reported in 1995.176,177 As there have been no reports of indigenous wild-type poliovirus transmission in Australia for at least 30 years, the hospitalised cases reported here are almost certainly not missed notifications of acute wild-type polio infection. Some hospitalisations could represent cases of AFP where poliomyelitis could not be excluded, but most are likely to be adults with late effects of poliomyelitis rather than acute cases, as indicated by the age distribution of the hospitalisations. The most recent hospitalisation data, with only one case coded as acute polio unspecified in an elderly male in 2004/2005, is encouraging in relation to possible improvements in coding practices. The hospitalisation case coded as VAPP in 2002/2003 would be worthwhile investigating, although it could be a coding error.
Although Australia has been declared polio free, achieving high quality AFP surveillance remains an important challenge. Such surveillance is required to detect any imported cases of wild-type polio infection, cases of VAPP, and outbreaks of circulating vaccine-derived polioviruses (cVDPV). In Australia, surveillance of AFP in children under 15 years of age is co-ordinated through the Victorian Infectious Diseases Reference Laboratory in collaboration with the Australian Paediatric Surveillance Unit (APSU). AFP cases are notified and stool specimens are referred to the Australian National Poliovirus Reference Laboratory for testing for polio and other enteroviruses. Cases are referred to the Polio Expert Committee for a determination as to the cause of the AFP. In 2004, Sabin-like poliovirus types 1 and 2 were isolated from a child presenting with AFP. However, the polioviruses were determined to be an incidental finding with the AFP determined as due to infant botulism.178 In 2005, three Sabin-like polioviruses (two isolations of PV3 and one of PV2) were isolated from three patients with AFP, with all considered to be incidental isolations in patients recently vaccinated with OPV.179 A recent review of all specimens referred for testing from AFP cases during 1996–2004 identified no cases as due to polio, with enterovirus 71 now the most common viral cause of AFP. Infant botulism is an important differential diagnosis.180 Three cases of notified AFP were due to infant botulism in 2005.181 AFP cases notified between 1995 and 1999 have also been reviewed, with Guillain-Barré syndrome and transverse myelitis identified as the most common diagnoses.182 In 2005, these two diseases continued to be the primary diagnoses responsible for presentations with AFP.181
The WHO target for surveillance of AFP (one notified case of AFP per 100,000 children aged less than 15 years) has only been intermittently achieved in Australia (in 2000, 2001 and 2004). The WHO target of faecal sampling from 80% of AFP cases has never been achieved, with the 40% sampling rate achieved in 2004 the highest to date (previously 24%–36% and falling to 19% in 2005).178,179 Variability in the adequacy of AFP reporting amongst the populous states of Australia has been noted, perhaps relating to the degree of clinician awareness of AFP surveillance protocols locally, which in turn may be related to the differing structure of public health networks and laboratories in different states.183 A capture-recapture study, comparing AFP surveillance reports with hospital records, has indicated that, in Victoria, failure to reach the WHO target rate of one per 100,000 is due to under-reporting rather than to local variation in AFP rates.184
The global aim to eradicate polio by 2000 has proven elusive. During the period 2002–2005, there was a resurgence of poliovirus 1 and importation into 21 previously polio free countries, due to postponement of eradication activities in Nigeria in 2003 and, to a lesser extent, importations from India.185 In eight countries, wild poliovirus transmission was not sustained, whereas in the other 13 countries, sustained transmission occurred. Four countries (Indonesia, Somalia, Sudan, and Yemen) had outbreaks of over 100 polio cases. Countries where transmission was not sustained had a median three-dose vaccination coverage of 83%, compared with only 52% in those where transmission occurred, underscoring the ongoing importance of high polio vaccine coverage even in countries where polio has been locally eradicated.185 Control strategies for affected countries have involved large scale supplementary immunisation activities and underscored the critical role of sensitive AFP surveillance.
In 2006, endemic transmission of wild-type poliovirus is now constrained to four countries (an all-time low): Afghanistan, India, Nigeria and Pakistan.186 However, 10 countries have reported polio cases due to importations in 2006.187 The Global Polio Eradication Initiative Strategic Plan 2004–2008188 has four main objectives. The most urgent is the rapid interruption of polio transmission in the remaining endemic countries. This requires high level political and resource commitments and, since 2005, the use of monovalent oral poliovirus vaccine (OPV) in order to provide sufficiently high levels of immunity.186 The other three objectives underscore the need to ensure adequate surveillance (Objective 2: Achieve certification of global polio eradication); to move away from OPV to inactivated poliovirus vaccine (IPV) once the risk of VAPP exceeds that from wild poliovirus (Objective 3: Develop products for the global OPV cessation phase); and to integrate poliovirus control and maintenance strategies into routine disease control structures (Objective 4: Mainstream the Global Polio Eradication Initiative).
In 2003, the Australian Government recommended, but did not fund, the use of IPV in place of OPV.76 From November 2005, IPV became a funded part of the routine childhood immunisation schedule in Australia with doses given at 2, 4 and 6 months and 4 years of age. This policy was facilitated by the availability of combination IPV vaccines and removes the risk of VAPP while providing protection against polio importations.189 With the replacement of OPV with IPV in Australia, incidental detection of polioviruses in faecal specimens should no longer occur. Future poliovirus isolations will, therefore, require full investigation.179 An important goal in the diagnosis of all AFP cases is the exclusion of an imported wild or vaccine-associated poliovirus as the cause. The high frequency of arrivals from countries where poliovirus remains endemic and where cVDPV cases have been recently documented (such as China),190,191 by Australian travellers, visitors and refugees, make such importations a distinct possibility. The likelihood of local transmission following importation will be dependent upon the vaccination coverage locally and living conditions primarily relating to the likelihood of faecal contamination of the water supply. Such contamination remains a possibility in rural and remote areas of Australia.192 Travellers should be reminded to ensure that they are vaccinated against polio.193