The Gambia declared a "public health emergency" on Wednesday after two cases of poliovirus were found in environmental sewage samples.
A new sampling policy was put in place in the Gambia in May, with the support of the World Health Organization.
"Two samples taken from sewers in Banjul and Kotu (a tourist town near Banjul) were positive for poliovirus," said Gambia's Minister of Health, Ahmadou Lamin Samateh.
"It is important to emphasize that this discovery reveals that the poliovirus is circulating, but does not mean that cases of paralysis have been detected among the population," he said.
In declaring this public health emergency, the authorities will launch an awareness campaign and increase epidemiological surveillance.
The government is also planning "at least two mass vaccination campaigns", targeting children under five.
Wild poliovirus was endemic throughout the world until a vaccine was discovered in the 1950s.
The Gambia was declared polio-free in 2004, according to the ministry.
In August 2020, WHO, citing a "historic moment," announced that wild poliovirus had been "eradicated" from Africa after four consecutive years without reported cases and massive child immunization efforts. Wild poliovirus is now found only in Pakistan and Afghanistan.
But another form of poliovirus, derived from a vaccine strain, can spread in its stool-derived form in areas with poor sanitation.
This attenuated form becomes more dangerous after a mutation that occurs when vaccine coverage is low and gives the virus the opportunity to multiply, according to WHO.
Polio, which is vaccine-preventable, is an infectious disease caused by a virus that invades the nervous system and can cause irreversible paralysis within hours. It mainly affects young children.