Explained | Poliovirus detected in sewage water in Kolkata: how worried should we be?

Polio is highly infectious and mostly affects children younger than five. There is no cure, and it can only be prevented by immunization. (Representative image)

The detection of poliovirus, a pathogen responsible for poliomyelitis, a life-threatening and disabling disease eradicated in India since 2014, in the sewage water in Kolkata hit the headlines this week and alarmed many.

The fear is that the disease may be on its way back into the country. But is this justified or far-fetched? We explain it here.

What causes poliomyelitis?

Poliomyelitis, often called only polio, is a serious disease caused by the poliovirus. This virus spreads from person to person and is capable of infecting a person’s spinal cord, causing paralysis.

The majority of people infected get just flu-like symptoms. Less than one in 100, however, go on to have serious symptoms such as paraesthesia (feeling of pins and needles in the legs), meningitis (infection in the covering of the brain and spinal cord) and paralysis.

Paralysis is the most severe symptom associated with polio as it often leads to permanent disability and even death in some cases.

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Poliovirus is very contagious and lives in an infected person’s throat and intestines. It enters the body through the mouth and spreads mainly through contact with the faeces of an infected person but can also spread through droplets from a sneeze or cough of an infected person.

What are the types of poliovirus?

There are three wild types of poliovirus (WPV) – type 1, type 2, and type 3. People need to be protected against all three types of the virus in order to prevent the disease and though there is no cure, the polio vaccination is the best protection.

Type 2 wild poliovirus was declared eradicated in September 2015 globally, with the last virus detected in India in 1999.

Type 3 wild poliovirus was declared eradicated in October 2019. It was last detected in November 2012 in Nigeria. Only type 1 wild poliovirus remains in circulation in some countries such as Pakistan.

There are two kinds of vaccines used to protect against polio disease—oral polio vaccine (OPV) and inactivated poliovirus vaccine (IPV).

Polio vaccination in India

According to senior virologist Dr T Jacob John, who has been associated with the Christian Medical College in Vellore for a long and is known for research work related to polio, India conducts one annual national and two sub-national pulse immunisation campaigns with bivalent (type 1 and 3) oral polio vaccine (bOPV) for all children below five years.

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This is in addition to routine immunization with five doses—totalling 10 to 15 doses per child in different states.

However, John points out, on rare occasions, OPV itself may cause vaccine-associated paralytic polio (VAPP) in vaccinated children (vaccinated VAPP) and unvaccinated child contacts (contact VAPP).

To avoid VAPP, developed countries immunise children with the inactivated poliovirus vaccine (IPV), which is completely safe.

John, however, contests that the administration of OPV in India using a bivalent vaccine is not appropriate. He said that there is still a reason for the administration of type 1 OPV (given that neighbouring Pakistan still has this virus in circulation).

However, since type 3 has been eradicated worldwide, there is no need for it to be administered. It also poses the risk of VAPP, he said.

John has been writing to the Union government and the World Health Organisation to stop using bOPV in India.

Why did alarm bells ring?

The last case of polio was detected in India in 2011 in Howrah, West Bengal, and in 2014, the World Health Organisation declared India polio-free.

However, as India continues to use OPV, which means that the kids vaccinated with it shed the virus in their stool, the virus is sometimes detected in sewage samples which is monitored through the National Public Health Surveillance programme.

Under this programme, sewage samples from 58 sites are investigated every week in 13 states to track pathogens.

The samples found in sewage are either cVDPV (circulating vaccine-derived poliovirus—when the virus has mutated after getting shed in the stool and is capable of infecting others) or iVDPV (immune-compromised vaccine-derived poliovirus—when the vaccine has been given to a child whose immunity is not strong enough to clear the virus completely and the virus has leaked in the sewage through stool).

Officials in the Union health ministry said that it is the detection of cVDPV which is more concerning since it is capable of infecting others while the sample detected now is categorised as type 1 PV.

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The Kolkata story

Government officials told Moneycontrol that the virus sample was detected in sewage water from Shyam Lane in Metiabruz locality of Kolkata in late April this year. It was investigated in detail at the WHO lab in Mumbai and the Centre for Disease Control and Prevention in Atlanta, USA in May.

The genomic investigations at these labs confirmed that the virus sample was a case of type-1 iVDPV.

Virologist Dr Shahid Jameel who is associated with Oxford University and Ashoka University too told Moneycontrol that detecting poliovirus in sewage means very little because the oral polio vaccine used in the pulse polio programme is a live vaccine that is shed in stool.

“So it’s natural for this virus to be found in sewage sometimes,” he said. “The detection of wild-type poliovirus and not the vaccine strain would be of concern”.

John, too, concurred that had it been either type 2, not used in India’s polio vaccination programme or type 3, which is used here but eradicated globally now, it would have been significant from the public health perspective and a cause of major concern.