Islamabad, Dec 30 : Polio viruses have been detected in the environmental samples from four districts in Pakistan, according to the country's Ministry of National Health Services Regulations and Coordination. Polio viruses were found in four environmental samples from Karachi and Hyderabad districts of southern Sindh province, southwest Chaman district and northwest Peshawar, the ministry said, Xinhua news agency. The ministry said the country has the most extensive and sensitive polio surveillance system in the world, adding that polio viruses quickly and easily target children with weakened immune systems. Polio vaccination is the only way to protect children from the virus and disability, it said. The health ministry underlined that parents must administer polio drops to children under five during every polio campaign in the country. According to the World Health Organization, Pakistan is one of the two polio-endemic countries in the world along with its neighbor Afghanistan. Pakistan has reported six polio cases so far this year, said the health ministry. /IANS