At least 31 children have died in northern India in the past 10 days from a brain disease believed to be linked to a toxic substance found in lychee fruit, health officials have said.

The deaths were reported by two hospitals in Muzaffarpur in Bihar state, famed for it lush lychee orchards, officials said.

The children all showed symptoms of acute encephalitis syndrome (AES), senior health official Ashok Kumar Singh said, adding most had suffered a sudden loss of glucose in their blood.

At least 40 other children complaining of similar symptoms were being treated at intensive care units.

“We are trying our best to save them,” said S P Singh, the chief medical officer of Sri Krishna medical college and hospital.

The outbreaks of the disease have happened annually during summer months in Muzaffarpur and neighbouring districts since 1995, typically coinciding with the lychee season.

“The health department has already issued an advisory for people to take care of their children during the hot summer when day temperature is above 40 degrees,” Ashok Kumar Singh said.

Known locally as chamki bukhar, the disease claimed a record 150 lives in 2014.

In 2015, US researchers had said the brain disease could be linked to a toxic substance found in the exotic fruit.

They also said more study was needed to uncover the cause of the illness, which leads to seizures, altered mental state and death in more than a third of cases.

Outbreaks of neurological illness have also been observed in lychee-growing regions of Bangladesh and Vietnam.