Oral polio vaccine (Photo credit: quilty2010)
Polio eradication may be entering a new phase in India where incidence of the disease has become so faint, it’s sometimes undetectable. Yet polio is still very much there and capable of spreading. Sewage in New Delhi tested positive for poliovirus six straight weeks recently. (Polio is usually spread by oral contact with water contaminated by the feces of people infected by the virus.) But no cases of polio have been reported in the entire state of Delhi for more than a year. How can there be circulating virus and no cases?
Only about one in 200 polio infections results in paralyzing disease. But these cases must be detected to be counted and to prevent further spread. Across India, more than 20,000 people comprise the detection network, in theory at least one to every block of every district in the country.
But in the states of Uttar Pradesh and Bihar, one of the world’s largest polio redoubts, the challenges of detecting polio cases are immense. The sheer reproductive force of the region astonishes: 500,000 children are born every month in Uttar Pradesh. Poverty, illness and death are likewise extreme in degree while infrastructure is scant. And many people are highly mobile, migrating far and wide, all of which makes detecting polio cases from this oceanic flux difficult. And it is getting harder.
Frequent, massive vaccination campaigns have beaten polio down. Uttar Pradesh hasn’t seen a case since April and only six in total this year. But the disease is cropping up elsewhere. New cases were reported in the state of Jharkhand, which borders Bihar, but also in Maharashtra on the other side of the country. And a spectacular outbreak ignited in Tajikistan a few months ago. 452 people became infected with a poliovirus traced back to Bihar. A smaller outbreak occurred in Nepal, again originating in neighboring Bihar.
But even giant outbreaks don’t threaten polio eradication. Make it rain oral polio vaccine and new outbreaks can be fairly reliably extinguished, albeit at significant cost. The problem facing eradication efforts is that the gauges read “zero” when clearly the actual number of polio cases sits above zero. As the World Health Organization put it, new cases in India last week and the sewage samples in New Delhi are “evidence of ongoing, low-level [poliovirus] transmission in the country.” (Wild Poliovirus Weekly Update, 11 August 2010).
The level of transmission has dropped to where it can’t always be seen but it remains high enough to sustain the cycle of infection. The good news is that strenuous vaccination efforts have driven cases down. The bad news is that polio is not eradicated and now might fly too low to be picked up by the existing detection network, hindering efforts to stamp the virus out for all time.
Today the Bill and Melinda Gates Foundation invited proposals to address exactly this problem. According to the foundation, “as eradication nears the signal provided by paralytic disease will be eventually lost; new methods to monitor poliovirus circulation are increasingly necessary.” The foundation identified other obstacles to eradication that need to be addressed but the overarching theme was for what it described as the “poliovirus endgame.”
How close is this endgame? The steep fall in cases evokes guarded optimism from Steve Wassilak at the Center for Disease Control, which is part of the Global Polio Eradication Initiative. In email, he said:
We, the interested parties, do need to avoid over-weighing any decline in cases as a clear path to zero, given the ups and downs in the past, particularly in India. However, we are squarely now in the high season and the total number of cases is at an historical low…
The outlook for 2010 appears good, although according to Dr. Yash Paul, a pediatrician in Jaipur, India, “In India high incidence of polio starts June onwards, so we shall know the likely polio scenario in end August” because of reporting lags. Also, one good year doesn’t automatically beget another. While 2005 saw just 66 cases, 2006 brought over 600 cases while 2007 produced more than 800.
Wassilak pointed out that the passing of the high season for polio could snap the few remaining chains of transmission. Or not. “[A]n alternative,” Wassilak hypothesized, “is can mobile populations be sustaining/contributing to transmission but cases of paralysis among these populations be missed?” The sewage tests in New Delhi, at Swaran Cinema, might support this hypothesis if the moviegoers are from neighboring Uttar Pradesh.
Other known problems obstruct eradication. Notably, the oral polio vaccine quite often fails to elicit immunity. WHO began addressing this in April. (See Polio Eradication: Harder Than it Looks.) Similarly, the Gates Foundation, in its invitation for polio eradication proposals today, included investigating why “vaccines have shown reduced efficacy in children living in certain resource-poor environments.”
A newer potential problem is re-infection among persons who were earlier protected by prior immunizing exposure. Such re-infections might be helping sustain the poliovirus cycle in areas with long-running eradication efforts—a vicious circle.
Finally, India is not the only endemic source of polio. Nigeria, Pakistan and Afghanistan also have never interrupted transmission, although a decade ago, 20 countries fell in this category. Unfortunately, transmission has become re-established in several nations in Africa and is unlikely to be stopped in those places by the end of the year, a revised goal for the eradication effort which originally was supposed to conclude in 2000.
Still, the potential is there for this be the turning point.
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Related:
Gates Seeks to Close Out Polio in Nigeria (June 7, 2010)
Heavy Lifting: Raising Health Beyond Polio's Reach (May 13, 2010)
Wall Street Journal: Pulling the plug on polio eradication? (April 26, 2010)
Polio Eradication: Harder Than it Looks (April 14, 2010)