Why is polio so hard to beat?

Story highlights

  • In August, Nigeria reported four cases of polio
  • Health officials believe more cases may have gone undetected

(CNN)For just more than two years, Nigerian health officials celebrated a country free of the paralyzing effects of the poliovirus.

The achievement was yet another step toward eradicating polio for good, both in the country and globally, with only two countries -- Afghanistan and Pakistan -- remaining endemic for the disease.
    But in August, celebrations were put on hold with the announcement of four new cases of polio in Nigeria. First, the World Health Organization reported on August 11 that two children had been paralyzed by the poliovirus, and two more reported cases followed in subsequent weeks in Nigeria's northeastern state of Borno. This state is where the militant group Boko Haram is based and has challenged immunization efforts.
    Before August, no cases had been reported in the country -- or on the continent -- since July 2014. Just one more year without polio would have meant the WHO African region could be declared polio-free.
    "We are deeply saddened by the news," Dr. Matshidiso Moeti, the WHO's regional director for Africa, said in a statement at the time.
    The first two cases were in different locations -- one in Gwoza and the other in Jere -- leading officials to fear the likelihood of more being uncovered.
    Now, the government of Nigeria is working with WHO and the Global Polio Eradication Initiative to improve surveillance systems and conduct large-scale polio immunization programs not only in Nigeria but in the surrounding countries of Chad, Niger, Cameroon and the Central African Republic.
    "The overriding priority now is to rapidly immunize all children around the affected area and ensure that no other children succumb to this terrible disease," Moeti said.

    Where did it come from?

    Genetic sequencing of the virus that infected the first two children found that they had a strain, or type, last reported in Nigeria in 2011 -- again in Borno. According to the Global Polio Eradication Initiative, this indicates that the strain has been circulating silently for five years.
    "This is particularly concerning because the virus that has been affecting these children is a virus that has been circulating (undetected)," Dr. Michel Zaffran, the WHO's director of polio eradication, said in an August Facebook Live discussion.
    Polio infection causes paralysis in one in every 200 cases, so there may be many more cases without paralysis that officials are unaware of. "We haven't been able to pick up this virus through the surveillance systems established in country," Zaffran said. Initial symptoms of polio infection include fever, headache, vomiting and pain in the limbs.
    The virus is spread through poor sanitation -- for example, if people don't wash their hands after using the toilet -- or occasionally through contaminated food and water.
    Nigeria has come a long way in controlling polio despite resistance and insecurity in the northeastern states, where Boko Haram operates, but the circulation of myths also reduces the acceptance of immunizations.
    In 2012, the country reported 122 cases of polio -- accounting for more than half the world's total that year -- but numbers dropped to 53 the following year and just six in 2014.

    Will we reach global eradication?

    To date, smallpox remains the only human disease to have been eradicated. The parasitic disease Guinea worm is a close contender to be the second, with only 12 cases reported this year, as of August 31.
    The Global Polio Eradication Initiative launched in 1988, at which point 125 countries were affected by polio and the virus paralyzed 150,000 people each year, according to Zaffran. The initiative's original goal was a polio-free world by 2018, but that's been pushed back slightly to 2019 or 2020.
    Achieving that would mean the last case of polio to be reported anywhere in the world has to be this year, as there must be three years without any cases for an end to the disease to be declared.
    Despite the setbacks in Nigeria, the WHO believes eradication is a possibility as long as the recent outbreak is promptly controlled. "The target may slip if we're not able to interrupt transmission in Nigeria rapidly," Zaffran said.
    Globally, 27 cases of wild poliovirus have been reported in 2016. Fifteen were in Pakistan, where insecurity and mistrust of government services have caused problems.
    "There's been a breakdown of trust between the government and communities," said Sona Bari, a WHO spokeswoman on polio eradication. Bari explained that access to people has improved in Pakistan in recent years thanks to partnerships with local leaders and the use of community vaccinators. But quality services are lacking.

    A global emergency?

    In 2014, the WHO declared polio a public health emergency of international concern. Sixty percent of polio cases at the end of 2013 were due to international travel, and evidence suggested this was aided by adult travelers to affected regions. The disease is completely preventable by vaccination, but states that are conflict-affected or otherwise fragile can have weakened immunization services and access to vaccines, and the virus could spread if it enters regions where people are unprotected.
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    "Other countries need to be alert," Bari said. "So much of the world is polio-free that any polio anywhere poses a risk."
    After the declaration, an international response was urged to stop spread of the virus beyond the three countries listed as endemic for the disease. Nigeria was removed from the list in 2015.
    No cases have been reported through international transmission this year, but the risk posed by low vaccine coverage and fragile states keeps polio classified as a public health emergency of international concern. Officials want to make sure the world is ready and to truly end polio by the start of the next decade.
    "If Nigeria has taught us anything, it's that we can't be complacent," Bari said. "(But) our hope is that we can stop transmission this year."