India 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 Ministerâ€™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 â€˜has not happenedâ€™.
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 Aliâ€™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 Pakistanâ€™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.