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

The deaths were reported from two hospitals in Bihar state’s Muzaffarpur district, an area known for its abundant lychee orchards that flourish in the region’s tropical climate in the summer.

All of those affected showed symptoms of Acute Encephalitis Syndrome (AES), a deadly viral infection carried by mosquitoes and characterised by high fever and inflammation of the brain.

Outbreaks of the disease have occurred in Muzaffarpur and its neighbouring districts for more than 20 years, with 2014 seeing the biggest loss of life when over 150 deaths in India were linked to the illness.