Health workers in western Niger are racing to contain an outbreak of Rift Valley fever that has killed at least 21 people over the past month, an aid agency said on Wednesday.

The highly contagious disease, which is transmitted to humans by mosquitoes or close contact with contaminated animals, has infected 52 people in Niger's western region of Tahoua since late August, the country's health ministry said.

The Alliance for International Medical Action (ALIMA) and Niger's health authorities have opened an emergency treatment center, in the region's hardest-hit district of Tchintabaraden, to look after the infected and stop the disease from spreading.

"Unfortunately, the 52 severe cases officially registered at present only represent the tip of the iceberg," ALIMA's medical coordinator Oumarou Maidadji said in a statement.

With no specific treatment or effective human vaccine, Rift Valley fever can cause blindness and severe haemorrhaging, leading the victim to vomit blood or even bleed to death.

Herders and farmers are deemed at higher risk of infection from the disease, which can devastate livestock.

Niger's health ministry said people in the Tahoua region, especially pastoralists, should avoid handling meat from infected animals, boil raw milk before consumption, and ensure that the corpses of dead animals are buried carefully.

ALIMA is also working with local partners and doctors to provide a mobile clinic which travels the region to inform the public about the disease and how to prevent it from spreading.

"Awareness is an essential step to contain the spread of the epidemic," said Maidadji. "As the annual festival of farmers approaches, vigilance is crucial."