Polio that has crippled at least 13 children in Syria has been confirmed as being caused by a strain of the virus that originated in Pakistan and is spreading across the Middle East, the World Health Organisation said.
Genetic sequencing shows the strain found in Syrian children in Deir al-Zor, where an outbreak was detected last month, is linked to the strain of Pakistani origin found in sewage in Egypt, Israel and Palestinian territories in the past year.
"Genetic sequencing indicates that the isolated viruses are most closely linked to virus detected in environmental samples in Egypt in December 2012 (which in turn had been linked to wild poliovirus circulating in Pakistan)," the United Nations agency said in a statement on Monday.
Closely-related strains of the wild poliovirus of Pakistani origin have also been detected in sewage samples in Israel, the West Bank and Gaza Strip since February 2013, it said.
Polio virus has been confirmed in 13 of 22 children who became paralysed in the northern Syrian province of Deir al-Zor. Investigations continue into the other 9 cases. It is Syria's first polio outbreak since 1999.
No children in Egypt, Israel or the Palestinian territories have been hit by polio thanks to high immunisation rates and a strong response to the alert, WHO spokeswoman Sona Bari said.
Polio virus is endemic in Pakistan, Afghanistan and Nigeria despite a 25-year-old campaign to eradicate the disease, which can paralyse a child in hours.
Islamist fighters from countries including Pakistan are among groups battling to oust President Bashar al-Assad, leading to speculation that they brought the virus into the country.
The WHO says it is unlikely that adults, who generally have higher immunity, carried the virus into Syria and that its mode of transmission will probably never be known.
Syria's immunisation rates have plummeted from more than 90 percent before the conflict to around 68 percent. Polio mainly affects children under five and cannot be cured, only prevented.
"All the children (paralysed) are under two years old, so they were all born after immunisation services fell apart," Bari told Reuters. "No doubt the outbreak will be large."
Children living in unsanitary conditions are especially vulnerable to the virus, which spreads via faecal-oral transmission and contaminated food and water.
More than 20 million children, including 1.6 million in Syria, are to be vaccinated in Syria and neighbouring countries over the next six months, U.N. agencies said last week.
Reporting by Stephanie Nebehay; editing by Andrew Roche