Polio could paralyse an unvaccinated child if allowed to spread in the UK, an expert has warned.
Officials in London declared a national incident yesterday after poliovirus was discovered in sewage works in the city.
UK Health Security Agency (UKHSA) chiefs are now scrambling to find where it could be in circulation following the discovery.
Samples of the virus were found in Beckton Sewage Treatment Works, which serves four million people in the UK capital.
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One of the most severe symptoms of polio is paralysis, which is caused when the virus attacks nerves in the spine and the base of the brain.
NHS Scotland says that this can affect less than one per cent of cases.
Nicholas Grassly, a Professor of Vaccine Epidemiology at Imperial College London, told the Mirror that it is vital that the virus does not take hold in the country.
He said: "Only one in a few hundred people are paralysed by polio, even in the unvaccinated population.
"So far there haven't been any cases of paralysis reported, but if the virus spreads it will eventually cause paralysis in an unvaccinated child.
"The priority now is to identify where it is."
Prof Grassly said at the moment officials have "no idea" how many people currently have the virus.
It has been detected in sewage several times since February, but Prof Grassly said he believes today's announcement was made because experts are now confident it is spreading.
The last community outbreak in Britain was in the 1970s but health officials insist the current risk to the population is low.
"What we can see from the sewage is that it's been detected on multiple occasions and there's genetic diversity, which suggests there's been local circulation."
He continued: "Hopefully this is something that disappears, but there's a risk that it could spread further.
"We don't want it to reach children who are unvaccinated. Whilst it's not too worrying at this stage, it's of sufficient enough concern for the UKHSA to do this briefing."
Dr Kathleen O’Reilly, Associate Professor in Statistics for Infectious Disease and expert in Polio Eradication, said that detection in sewage had helped prevent the virus spreading in Israel.
Dr O'Reilly said: "These findings suggest that there may be localised spread of poliovirus, most likely within individuals that are not up to date with polio immunisations."
She continued: “The findings are from sewage sampling, as people infected with poliovirus shed virus in their faeces, which can then be detected in sewage treatment plants.
Sewage surveillance for polio, and other pathogens, have been extremely useful to detect emergence of pathogens and respond to these detections early, preventing disease and onward spread. The surveillance in London that identified vaccine-derived poliovirus is such an example.
"Another example is from Israel in 2014 where poliovirus was also detected in sewage samples, and polio cases were prevented through vaccination."
Oral vaccines against polio offered abroad contain an “attenuated” form of the virus - weakened in a laboratory so it cannot cause disease.
These vaccine-derived forms of the virus remain in stool and are occasionally picked up in routine UK testing of sewage samples, which are conducted in London and Glasgow.
The oral vaccines are different to the polio injections offered in the UK which contain a completely inactivated form of the virus.
Prof David Heymann, Professor of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, said: “Vaccine derived polio virus is now present in many countries around the world.
"The virus results from a mutation of the Sabin virus that is a live virus used to vaccinate against polio, and it causes paralysis in some, though the majority of infections are asymptomatic.
“Sabin-virus based vaccine (live oral polio vaccine) is now only being used in countries that are in the active eradication phase of polio eradication – other countries have switched to inactivated polio virus vaccine that cannot mutate.
“The fact that it has been found in sewage in the UK attests to the strength of the surveillance programmes of UKHSA.
"Its presence in the sewage reminds us that polio eradication has not yet been completed in the world. The high vaccination coverage using inactivated polio vaccine in the UK will limit the spread of vaccine derived polio and protect those who have been vaccinated against polio paralysis.”
It has prompted calls to make sure people - particularly children - have been vaccinated for polio.
Dr David Elliman, consultant paediatrician at Great Ormond Street Hospital, said: “Parents sometimes ask why, when diseases are uncommon in UK, or in the case of polio has been eliminated, do we continue to vaccinate against them.
"The answer is that, although we are an island, we are not isolated from the rest of the world, which means diseases could be brought in from abroad.
"The finding of vaccine derived polio virus in sewage proves the point. Although the uptake of polio vaccines is high in UK, there are children who are unimmunised and therefore at risk of developing polio if in contact with this virus.
"The risk is small, but it is easily preventable by the vaccine, which in the UK is killed and so cannot cause the disease. There is no upper age limit for the vaccine. Anyone who is not fully vaccinated against polio should seek advice from their health visitor or general practice."
The risk of transmission throughout the community is currently being established.
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