POST 00874E : WHAT IS A POLIO CASE?

POST 00874E : WHAT IS A POLIO CASE? 5 January 2006 _____________________________________ From a recently published article, Anthony Battersby (mailto:[email protected]) from the UK, asks some questions about polio that many of you may also have, unless quite familiar with polio eradication. Christopher Maher (mailto:[email protected]) from WHO provides detailed answers. _____________________________________ Dear Claude I was interested to read in the New Scientist that: "OPV virus was isolated from 5 Amish children in Minnesota, none of whom had symptoms. From genetic analysis of the virus, it appears to have derived from vaccine administered somewhere about 2 years ago". This was in the November 19th edition of NS yet to date no reference has been made to these cases on the WHO polio web site, they do not appear on the monthly total by country. So when is a case of polio not a case of polio? Do there have to be symptoms to qualify for being a case of polio? The implications of this episode seem to be rather serious. These cases appear to coroborate the episode in Slovakia earlier this year which was dismissed out of hand. The US has not, I think, used OPV for about 5 years so how come 5 Amish children, probably not widely travelled, come into contact with the virus. Does this mean that polio virus can actually circulate unnoticed for years and if so how will it be possible to certify a country free of polio? It is especially worrying that the index child was immune-deficient. Does this mean that unimmunised HIV/AIDS children especially in Southern Africa can be harbouring the virus asymptomatically? This may be of interest to Technet readers and I would be very glad to have answers to these questions. Anthony -------------------------------------- Claude, Let me clarify the issues raised by Anthony. The short answer is that, yes, a case of polio does have to be symptomatic. Confirmed cases of polio are those that are reported as acute flaccid paralysis, or through some other mechanism in the few countries that do not have AFP surveillance systems, that subsequently have wild poliovirus (or vaccine derived poliovirus) isolated from stool specimens. At this stage of the programme in all infected countries, if the case itself does not have adequate specimens (i.e. two specimens taken within 14 days of onset of paralysis) specimens are taken from five close contacts of the case. If any of these contacts are positive for wild poliovirus, then the case is also confirmed. In the absence of cases, the presence of virus detected from other sources is also significant. Both Egypt and India have made use of environmental (sewage) sampling, under conditions where the areas and populations being sampled were very well understood, and where there were concerns about the quality of AFP surveillance. Where wild virus has been detected through this sampling, the area has been considered as infected, whether or not cases where being detected. In our experience, however, good AFP surveillance is more consistent and more informative than environmental surveillance and therefore is the gold standard. The situation Anthony refers to is that of a vaccine derived poliovirus from an immunodeficient child (referred to as an iVDPV). While circulating vaccine derived poliovirus (cVDPV), such as caused the outbreak in Haiti and the Dominican republic, are detected through AFP surveillance, iVDPVs sometimes are not. An immunodeficient person may not be paralysed despite harbouring a poliovirus for some time, and the virus may be detected as part of the regular work up done on immunodeficient patients. The child in question in Minnesota most likely picked up a Sabin virus from another child who had been exposed to vaccine virus outside the US; as the child had been in clinics several times in contact with other immunodeficient children, one possibility is that the virus was passed on there. The concern raised by Anthony that this virus could have been circulating for a very long period without detection is unfounded; the extent of genetic change in the virus demonstrated that it could not have been circulating in the US undetected since OPV was discontinued, since it was much too "young" geneticaly. It has been known for quite some time that immunodeficient people can harbour vaccine virus for long periods. Anthony's contention that when these episodes are detected they are "dismissed out of hand" is not correct, and demonstrates an ignorance of the programme. Any detection of a vaccine derived poliovirus is treated seriously and investigated as thoroughly as possible; the episode he refers to, in Slovakia, was the subject of very intensive investigation by WHO EURO, including wide use of environmental sampling and review of AFP surveillance quality. There is a difference in terms of programmatic response, however, between detecting a virus that has clearly been circulating, and detecting a virus from an immunodeficient child for which there is little or no evidence of circulation. The detection of virus in the close contacts of the child in Minnesota is not evidence of circulation; ordinary OPV virus passes on to close contacts from recently immunised children. With respect to people infected with HIV and their capacity to harbour poliovirus for long periods, this is not an issue. Long term carriage of poliovirus occurs with primary immunodeficiency, not the type of immunodeficiency caused by HIV. In the period after wild poliovirus has been eradicated, the risks to a polio-free world will be from containment failures (i.e. virus escaping from laboratories or vaccine manufacturing facilities), and from the continued use of OPV, which we know can give rise to VDPVs. The first risk is being managed through a detailed process of containment of both wild and vaccine polioviruses, which is part of the requirement for certification of a polio-free world. The second risk can only be removed through the cessation of the use of OPV at an appropriate time after the world becomes polio free. I hope this clarifies the issue for TECHNET members. There is plenty of information about these subjects in the form of published papers, reports, etc, many of them available through the WHO and CDC polio web sites. If you have any further questions, just let me know! Chris ______________________________________________________________________________ ________________________________ Visit the TECHNET21 Website at http://www.technet21.org You will find instructions to subscribe, a direct access to archives, links to reference documents and other features. ______________________________________________________________________________ To UNSUBSCRIBE, send a message to : mailto:[email protected] Leave the subject area BLANK In the message body, write unsubscribe TECHNET21E ______________________________________________________________________________ The World Health Organization and UNICEF support TechNet21. The TechNet21 e-Forum is a communication/information tool for generation of ideas on how to improve immunization services. 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