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Polio, the Great Crippler, should have been wiped out as easily as smallpox. But 20 years into the eradication battle, the carnage continues.

In recent weeks, the virus slithered over the border from India to Tajikistan, infecting hundreds of children. While emergency vaccination programs roll out there, neighbouring Uzbekistan is reporting dozens of cases and there are growing fears that polio will spread throughout central Asia.

It shouldn't be so. Central Asia was declared polio-free in 2002. And in endemic countries on the continent - India, Pakistan and Afghanistan - there has been significant progress in controlling the disease. Polio, a disease that terrorized Canada in the 1950s, before a vaccine was developed, was supposed to be consigned to the dustbin of history by now. Yet, it remains a Hydra-like foe: No sooner is it driven out of one country than it emerges in others.

With the announcement of a new strategy, which will probably be adopted at the World Health Assembly this month, the question remains: Why won't polio go away? Why the tenacious hold, despite an investment of $8.2-billion (U.S.), a dedicated World Health Organization program and a Herculean human effort that has involved thousands of Rotarians, the world's richest man, Bill Gates, and societal power brokers ranging from Indo-Afghan cricketers through to Muslim clerics.

The reasons are not strictly biological. "Failure to finish the job is due to political and military pressures, along with some religious issues," said Mark Joffe, an associate professor in the division of infectious diseases at the University of Alberta in Edmonton. "But there are some technical challenges too."

To date, there has been a one-size-fits-all approach to tackling polio: Immunize, immunize, immunize. But WHO's new strategy says (once you cut through the cautious bureaucratic language) that efforts must be more geographically focused and politically astute.

For instance, public-health officials have learned over the years that polio behaves differently in Asia and Africa, in part because there are different types of polio circulating. In India, where vaccination rates are high, there is a realization that immunity needs to be bolstered. So new, stronger vaccines have been developed and will be rolled out. What's more, polio can survive in small sub-populations such as the migrant workers in Bihar and Uttar Pradesh, so they will be targeted.

In Nigeria, where half the cases of polio were recorded last year, the emphasis will be on bolstering the number of people vaccinated. To do so will require an investment in education and diplomacy to win over Muslim clerics and their faithful, some of whom charge that polio vaccination is a ruse to sterilize women. A similar effort is required in Afghanistan, where Taliban militants have killed health workers and halted vaccination efforts.

Poliomyelitis, which has been around for millennia, was chosen as a candidate for eradication for three reasons: It crippled children; there were cheap, effective vaccines; and unlike, say, malaria, the virus has no animal reservoir, meaning that once it is defeated in humans, it will disappear forever.

While the disease is highly contagious, it is easy to control, in theory. So when polio-eradication efforts were launched in 1988, public-health officials were convinced they could battle it more readily than smallpox.

However, every person who develops smallpox has visible signs of illness and it's spread only by direct, person-to-person contact.

But polio is a stealthy traveller. It infects water supplies, particularly where there is poor sewage treatment. The virus enters through the mouth - usually via that contaminated water - and invades the nervous system, causing paralysis. "It's an invisible infection. People are well and they can still transmit," said Monika Naus, director of the immunization program at the B.C. Centre for Disease Control.

There are technical issues surrounding immunization as well. "Every time we think we have this battle won, the virus shows another face," said Luis Barreto, vice-president of immunization policy and scientific affairs at Sanofi Pasteur Ltd. Canada.

For a long time, the vaccine was trivalent, meaning that it protected against the three types of polio. It worked well, and type 2 was eradicated. But in endemic countries such as India, researchers saw a Ping-Pong effect: When type 1 was controlled, type 3 infections surged, and vice versa.

The solution has been the creation of bivalent vaccines, which seem to suppress type 1 and type 3 simultaneously. This has resulted in an unprecedented, encouraging drop in polio in India and Nigeria.

The other technical issue is when to move from oral polio, or Sabin, vaccine used in the developing world, to the injectable, or Salk, vaccine used in the developed world. The oral vaccine is easy to use: A few drops go into a child's mouth. But it contains weakened live virus, so in some cases it can cause polio. At the same time, however, those who are immunized shed the weakened virus, meaning that others in the community can be exposed, and hence immunized. The injectable form of vaccine contains no live virus, but it is more expensive and requires health workers trained to give injections. "The oral vaccine is not ideal, but the injectable poses very big challenges," Dr. Barreto said.

Despite these hiccups, there has been tremendous progress. At the outset of the campaign in 1988, there were more than 350,000 cases of polio annually and the disease was present in 125 countries. By 2001, that fell to a mere 483 cases in four countries. But the numbers have crept back up. In 2009, there were a discouraging 1,606 cases in 23 countries.

Trying to wipe out an entrenched disease is a high-stakes gamble, one rarely attempted. "Eradication is the venture capital of public health. It's costly and risky, but the potential payoffs are tremendous," said Bruce Aylward, the Canadian epidemiologist who is co-ordinator of WHO's Global Polio Eradication Program.

The question now is whether, with the frustrating spread of the disease, the eradication program is salvageable. Dr. Aylward has no doubt that it is, but he says the world's ability to control or eliminate polio rests with two camps.

The first is the two countries that are hotbeds of polio: India and Nigeria. "Those governments are both excited to have polio on the ropes," Dr. Aylward said. They have manpower and cash provided by Rotary Club members who have steadfastly supported a "Wipe Out Polio" campaign, and money and technical support from philanthropic groups such as the Gates Foundation, which has invested more than $700-million in polio eradication.

The wild card is the Group of Eight countries, which pick up the tab for most global health initiatives. "The day the G8 decides they are not behind eradication is the day that it's over," Dr. Aylward said.

By 2012, the deadline set in the new strategy, we may see the iconic photo of the last child to contract polio. Or we may have to admit that the Great Crippler is here to stay.

André Picard is The Globe and Mail's public health reporter.

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