On the trail of polio viruses

A child is given an oral polio vaccine
during a vaccination drive in Niger, West
Africa.

Shelina Moonsamy isolates a wild polio
virus for genetic sequencing at South
Africa’s National Institute for
Communicable Diseases.
(Images: Irin News)

Like a forensic detective, Nicksy Gumede-Moeletsi, a virologist who heads the sequencing section at the Polio Molecular Unit of South Africa’s National Institute for Communicable Diseases (NICD), tracks her “criminal”, in this case a virus, by studying variations in genotypes instead of fingerprints.

The sequencing unit in South Africa is one of seven centres in the world tracing the origin of polio outbreaks. Gumede-Moeletsi traced a 2005 outbreak in Angola – which had remained polio-free since 2001 – to a polio strain prevalent in India by determining the genetic constitution of the polio virus.

From Angola, the Indian virus strain travelled south to neighbouring Namibia, north to the Democratic Republic of Congo (DRC) and from there reached the Central African Republic in 2008.

Polio, a highly infectious non-curable disease that can cause total paralysis in a matter of hours, is easily spread by contact from person to person. Flies can also passively transfer the polio virus from faeces to food.

According to the Global Polio Eradication Initiative, spearheaded by the World Health Organisation, Rotary International, the US-based Centres for Disease Control and Unicef, most people infected with polio virus have no signs of illness, and are unaware that they are infected.

The disease can be prevented by orally giving children aged under five a live weakened Sabin polio vaccine, named after its developer Albert Sabin, or by a trained health worker injecting the inactivated Salk polio vaccine.

Uncovering the route along which a virus spreads is critical. Polio outbreaks can be caused either by a wild polio virus type, or a vaccine-derived polio virus (VDPV).

On rare occasions, strains of the live virus contained in the oral polio vaccine given to children are known to have changed, reverting to a form with the capacity to circulate, and which can cause paralysis in humans.

Gumede-Moeletsi said it was essential for a country experiencing an outbreak to know what it was up against, and sequencing helped detect the possibility of a new strain, as well as determining whether it was a wild polio virus or a VDPV.

Sequencing also helped to find gaps in the affected country’s immunisation coverage. For example, if sequencing picked up a mutated form of the virus that seemed newer than a strain prevalent before the country’s last vaccination drive, it might indicate that the coverage of the immunisation programme had not been that effective, said Dr Adrian Puren, deputy director of NICD.

Finish the job

The polio detective work has also been used to actively encourage Afghanistan, India, Pakistan and Nigeria – the only four countries in which polio is still endemic – “to finish the job of complete eradication”, said Mark Pallansch, who heads the polio laboratory at the Centres for Disease Control. He has worked on the polio eradication programme since 1985 and has traced frequent outbreaks to a virus originating in the four countries.

In August 2003, radical Muslim leaders in three northern states of Nigeria suspended polio immunisation campaigns after rumours that the vaccine was laced with agents causing Aids and sterility.

A new outbreak, originating in the state of Kano, subsequently occurred and infected previously polio-free areas of Nigeria as well as eight formerly polio-free countries in West and Central Africa.

The strain then travelled from Sudan to Saudi Arabia, “probably during Haj”, the annual Muslim pilgrimage to Mecca, Pallansch said. In 2005 it spread to Yemen as well as Indonesia – a country that had been polio-free for 10 years.

These outbreaks underline the importance of being vigilant. “You need a good surveillance system and a regular immunisation programme,” said Pallansch. “As long as polio is endemic, even in one country, everyone has to keep up with their immunisation programme.”

He pointed out that Nigeria’s neighbour, Cameroon, had managed to keep the disease from developing into an outbreak with an effective immunisation campaign. This was in contrast to its other neighbours, such as conflict-ridden Sudan.

Polio can be eradicated, “if you have the will,” said Pallansch. “If a developing country like Bangladesh, with a huge population and challenges, can do it, anyone can do it.”

The threat posed by VDPV is mounting: in 2008 Gumede-Moeletsi picked up 18 cases in DRC and four in Ethiopia. She is concerned about the threat posed by VDPV, which is mounting. In 2008 Gumede-Moeletsi picked up 18 cases in DRC and four in Ethiopia.

Her concern supports calls for wider use of the inactivated polio vaccine, which eliminates the risk of VDPV as it does not contain live virus, once wild polio has been eradicated. But according to Sona Bari of WHO, “Only the oral polio vaccine has been proven to stop polio circulating within a community.” So the debate on which vaccine to use continues.

Polio facts

  • As long as a single child remains infected, children in all countries are at risk of contracting polio.
  • Between 2003 and 2005, 25 previously polio-free countries were reinfected by new imports of the virus.
  • One in 200 infections leads to irreversible paralysis, usually in the legs.
  • Among those paralysed, 5% to 10% die when their breathing muscles become immobilised.

Do you have queries or comments about this article? Email Mary Alexander at marya@mediaclubsouthafrica.com

Source: Irin News

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