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World Polio Day

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Today, 24 October 2012, is World Polio Day:

World Polio Day (October 24) was established by Rotary International over a decade ago to commemorate the birth of Jonas Salk, who led the first team to develop a vaccine against poliomyelitis. Use of this inactivated poliovirus vaccine and subsequent widespread use of the oral poliovirus vaccine developed by Albert Sabin led to establishment of the Global Polio Eradication Initiative (GPEI) in 1988. Since then, GPEI has reduced polio worldwide by 99%.

The polio eradication effort has made huge inroads towards eliminating polio from the planet. In 1988 it was estimated that there were a total of 350,000 cases of poliomyelitis (probably an underestimate); as of this writing there have been 174 cases in 2012. India has been polio-free for over one year, a remarkable achievement. Only three countries have never seen a break in wild poliovirus transmission: Afghanistan, Nigeria, and Pakistan. Continued surveillance and extensive immunization efforts will be needed to remove the last pockets of the disease from these countries.

Because only 1% of poliovirus infections lead to paralytic disease, the 174 polio cases in 2012 translates to 17,400 infections. The virus clearly has the ability to circulate undetected, which can be a problem if surveillance drops. Another thorny issue is how to deal with immunization once the disease has been eradicated. The vaccine strains used for immunization revert to virulence during replication in the human intestine, with the consequence that immunized individuals shed virulent poliovirus into the environment. These viruses will pose a threat to non-immune individuals. How long these vaccine-derived neurovirulent strains will persist in the population is unknown. Therefore we cannot simply stop vaccinating against polio once the disease is eradicated. It will likely be necessary to immunize globally with the non-infectious inactivated poliovirus vaccine until vaccine derived polioviruses are no longer detected.

Related: World Polio Day

Can India remain polio-free?

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global polio 2012India has been free of polio for over one year. This is a remarkable accomplishment, considering that just 30 years ago the country recorded 200,000 cases of the disease annually, or one every three minutes. With polio endemic in two neighboring countries, Pakistan and Afghanistan, and in the more distant Nigeria (figure), can India remain free of the disease? According to Shahnaz Wazir Ali, the Pakistani Prime Minister’s focal person for polio, there is little risk for export of the virus from Pakistan:

The likeliness of polio virus being exported to India from Pakistan is very low, and historically, it has not happened. Those who travel from India to Pakistan are mostly adults. There are rarely any babies. So the chances are low.

It is correct that polio has not traveled from Pakistan to India during the modern era of virus detection (1980 to the present). However, the same genotypes of types 1 and 3 poliovirus have circulated in both countries, implying sharing of viruses some time in recent history. Therefore Ali cannot conclude that export of virus to India ‘has not happened’.

Poliovirus continues to circulate in Pakistan, which shares a border with India: there were 198 cases in that country in 2011, the most of any in the world, and 16 cases so far in 2012. Remember that most poliovirus infections are asymptomatic, so the number of paralytic cases is far lower than the actual number of infections. The ratio of paralytic cases to infections varies according to the viral serotype: 1:200, 1:1800, and 1:1200 for types 1, 2, and 3 respectively. Furthermore, poliovirus has been known to spread from Pakistan from other countries. An outbreak of polio in Xinjiang, China, in 2011 was caused by virus imported from Sindh, Pakistan. There were 21 paralytic cases caused by poliovirus type 1, over half of which occurred in individuals 19-53 years old. The outbreak was halted by immunization but the region remains at risk for importation from Pakistan.

Poliovirus also continues to circulate in Afghanistan, which lies on the northwestern border of Pakistan. Eighty cases of paralytic disease were reported in this country in 2011, and 7 so far in 2012. It has been difficult to control polio in the southern provinces of Kandahar and Helmand due to ongoing armed activities. There is active migration between the southern regions of Afghanistan and Pakistan which has lead to a steady exchange of polioviruses between the two countries.

There many other examples of polio spread from one country to another in recent years. Following cessation of polio immunization in 2003, virus spread from Nigeria to many countries in Africa as well as to Indonesia. From India poliovirus has spread to Nepal, Angola (2005 and 2007), and Tajikistan and then to Russia (2010), in all cases causing substantial outbreaks of the disease.

The message is clear: poliovirus spreads easily among countries, and it is often spread by infected adults, not children. Because poliovirus infection is frequently asymptomatic, such spread cannot be detected by simply examining travelers for signs of paralysis.

For these reasons I am skeptical of Ali’s reassurance that the virus is not likely to spread from Pakistan to India. If adults mainly travel from India to Pakistan, as she says, they could well be infected and import the virus back home before it is detected. Furthermore, adults could bring the virus to India from other countries where poliovirus continues to circulate, although that is not Pakistan’s concern.

Because Pakistan remains a major reservoir of poliovirus, it is a good sign that the country is acknowledging the possibility that they might export the virus to India. The best way to avoid this scenario would be to intensify their immunization programs and eliminate the virus. Apparently Ali has been speaking with Indian officials to learn how they accomplished this goal:

We got to know what actually took India to become polio-free. We have understood the scale and efforts that we require to make Pakistan polio-free.

 

India polio-free for one year

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Year in polio 2011A year has passed since the last reported case of poliomyelitis in India, which occurred on 13 January 2011 in a two year old girl in Howrah, West Bengal. If no additional cases are reported in the next few weeks (some samples are currently being tested for the virus), then it will mark the first time that India has been polio free for one year.

This achievement represents a remarkable turnaround for India, where control of the disease had for years been extremely difficult. As recently as 2009 there were 741 confirmed cases of polio caused by wild-type virus (as opposed to vaccine-derived virus) in India. The tide turned in 2010 with only 42 confirmed polio cases, and in calendar year 2011 there was just one. That is why the 2011 map marking locations of confirmed wild polio cases in India (see figure) shows only one red dot (paralysis caused by type 1 poliovirus) in the country. The blue dots indicate cases caused by type 3 poliovirus.

The challenge now is to keep India free of polio. The map shows why this will be difficult – there are many red dots (cases of type 1 polio) in neighboring Pakistan and Afghanistan. Poliovirus does not respect national borders – China had been free of polio since 1999, but now there are red dots in that country. That outbreak was imported from Pakistan. Even the polio cases in more distant countries such as Africa constitute a threat. As long as there is polio somewhere, all countries must maintain extensive immunization programs. Whether or not that will happen depends upon money, determination, and allowing immunization campaigns to proceed without interruption.

Once polio was eradicated from the United States, the only poliomyelitis was caused by the Sabin vaccine. Consequently this country switched to the use of inactivated vaccine in 2000. As other countries eliminate the disease, vaccine-associated poliomyelitis will become more prominent. If eradication of polio is achieved, the world will have to switch to using inactivated poliovaccine.

Related:

Wild poliovirus in China
Dreaming of inactivated poliovaccine
Poliomyelitis after a twelve year incubation period
Poliovirus vaccine litigation

 

TWiV 149: Live at ICAAC in the Windy City

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twiv at icaacHosts: Vincent Racaniello, Rich Condit, Mark Pallansch, and Trine Tsouderos

Vincent, Rich, Mark, and Trine discuss science and medicine in journalism and the eradication of poliovirus at the 51st Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC).

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Click the arrow above to play, or right-click to download TWiV 149 (62 MB .mp3, 86 minutes).

Subscribe to TWiV (free) in iTunes , at the Zune Marketplace, by the RSS feed, by email, or listen on your mobile device with the Microbeworld app.

Links for this episode:

Weekly Science Picks

Rich – Parachute use to prevent death (Brit Med J)
Vincent – Ian Lipkin’s Op-Ed on Contagion and a review of the movie (both NY Times)

Listener Pick of the Week

Michael – Bacteria billboard for Contagion (YouTube)

This episdode of TWiV is sponsored by Wiley-Blackwell, the leading scientific publisher of books, scholarly journals, major reference works and databases. This month they are offering 25%-off all Microbiology and Virology books. To take advantage of this offer go to www.wiley.com/go/microbeworld.

Send your virology questions and comments (email or mp3 file) to twiv@microbe.tv, or call them in to 908-312-0760. You can also post articles that you would like us to discuss at microbeworld.org and tag them with twiv.

Wild poliovirus in China

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The World Health Organization has confirmed that cases of poliomyelitis in China were caused by wild poliovirus type 1:

1 SEPTEMBER 2011 – The Ministry of Health, China, has informed WHO that wild poliovirus type 1 (WPV1) has been isolated from four young children, aged between four months and two years, with onset of paralysis between 3 and 27 July 2011. All four cases are from Hetian prefecture, Xinjiang Uygur autonomous region, China. Genetic sequencing of the isolated viruses indicates they are genetically-related to viruses currently circulating in Pakistan. The last WPV case in China was reported in 1999, due to an importation from India. The last indigenous polio case occurred in China in 1994.

Remember that only one in 100 poliovirus infections lead to paralysis. For the four paralytic cases reported in China, there are likely 400 individuals who were infected with the virus but did not display obvious symptoms. This suggests a lapse in immunization coverage in this region. Consequently a polio vaccination campaign is planned for early September, the target being 3.8 million children <15 years of age in the outbreak area.

Pakistan, which shares a border with Hetian prefecture, appears to be the origin of the poliovirus strain causing the outbreak. Wild poliovirus type 1 continues to circulate in Pakistan, causing 76 reported cases of paralytic disease so far this year. This incident emphasizes the need for continued high immunization coverage in all countries until eradication of the virus is achieved. It is estimated that China has achieved >99.5% coverage for the third dose of poliovirus vaccine. In a country of 1.3 billion people, that leaves a large number of susceptible individuals.

TWiV 105: Finches score again

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Hosts: Vincent Racaniello, Dickson Despommier, Alan Dove, and Rich Condit

On episode #105 of the podcast This Week in Virology, Vincent, Dickson, Alan, and Rich review eradication of rinderpest, endogenous hepatitis B virus in the zebra finch genome, and identification of the cell receptor for an extinct retrovirus.

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Click the arrow above to play, or right-click to download TWiV #105 (66 MB .mp3, 92 minutes)

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Links for this episode:

Weekly Science Picks

Dickson – Winged Migration
Alan – Web-accessible shortwave receivers
Rich – Personal Genome Project
Vincent – XVIVO scientific animation

Send your virology questions and comments (email or mp3 file) to twiv@microbe.tv or leave voicemail at Skype: twivpodcast. You can also post articles that you would like us to discuss at microbeworld.org and tag them with twiv.