At least 28 children have been killed by lead poisoning from illegal gold mining in a remote village in west-central Nigeria, health officials said.
Dozens more are sick in the region where doctors are still treating children from a 2010 mass poisoning in Zamfara state that killed 400 children and left many paralysed and blind because of delays in government funding for a clean-up.
Doctors Without Borders said it has cured half the 5,500 infected there and has started closing clinics.
Junior health minister Fidelis Nwankwo said all those newly infected in neighbouring Niger state are under five with a 43% fatality rate. He added that they have started treatment.
Michelle Chouinard, Nigeria director for Doctors Without Borders, said action must be taken quickly if the children in the Rafi area of Niger state are not to suffer irreversible neurological damage.
Ms Chouinard said doctors have cured 2,688 of 5,451 people infected and hope to complete treatment next year. They have had most success in the worst-affected village of Bagega, where all but 189 of 1,426 people have had the lead removed from their bodies.
Mr Nwankwo said: "The devastating impact of this outbreak is associated with new mining sites which were found to contain more leaded ores which are often brought home for crushing and processing."
Previous government efforts to forbid artisanal mining have failed as poor villagers make up to 10 times as much from gold than from farming.
In Zamfara state, where the processing area was found to contain over 100,000 parts per million of lead - the United Nations considers 400 parts per million safe - US-based TerraGraphics International Foundation took five and a half months to clean up and also trained villagers in safer mining.
"This (training) is working fairly well and I think it's one of the contributing factors to why the number of patients is decreasing so much and so quickly in Bagega," Ms Chouinard said.