The  World Health Organisation, WHO said on Monday that Nigeria has reported its third polio case in the northeastern state of Borno while warning that more cases could appear in a major setback for the country.

Nigeria was on track of being certified free of the virus before two other polio cases were discovered in the same area in Borno State in August.

“A third child has been paralyzed by wild poliovirus type one (WPV1) in Borno state,” WHO said in a statement.

“It’s all linked to the same outbreak,” WHO said, adding: “detection of new cases is not unexpected and can be anticipated, particularly as disease surveillance is being strengthened including by conducting retrospective case searches.”

The three cases appear to come from the same strain of the disease circulating in the northeast for years, said Stephanie Mucznik, spokesperson for Rotary International, which is working with the Nigerian government to eradicate polio.

The latest case, Mucznik said, concerned a two-year-old boy suffering from the onset of paralysis on August 6 in the Monguno area.

“Genetic sequencing of the isolated viruses suggests they are most closely linked to WPV1 last detected in Borno in 2011, indicating the strain has been circulating without detection since that time,” Mucznik said.

Polio is a highly infectious viral disease which mainly affects young children and can result in permanent paralysis.

Though there is no cure,  it can  be prevented through immunisation.

Nigeria has already responded to the new discovery a wide scale immunisation campaign.

The campaign is expected to continue until November and includes the neighbouring areas of Chad, Cameroon and Niger.

Professor Isaac Adewole, Nigeria’s Health Minister warned in August that the affected areas “have had their health facilities destroyed by insurgents” and “accessibility was a barrier to service provision.”