Health officials said the children died at two hospitals in Muzaffarpur, a region of the state that is well known for its many lychee orchards harvested throughout May and June.
The state government is yet to confirm the cause of the outbreak but is attributing most of the deaths to hypoglycaemia - low blood sugar level.
But doctors said that more than 150 children under the age of 10 had been admitted with symptoms of acute encephalitis syndrome (AES) since 1 June, and that of these 43 had died. They said hypoglycaemia is one of the features of AES.
The link to lychees has been proposed in a 2014 study, among others, in which US researchers found AES could be linked to a toxin found in the fruit.
Outbreaks of AES have occurred each year in Muzaffarpur since 1995, and typically coincide with the lychee harvest season. The disease claimed a record 150 lives in 2014.
A similar association between the lychee harvest and outbreaks of AES has also been observed in other regions of Asia, including in the Bac Giang province in northern Vietnam.
May and June are also the hottest months of the dry summer in India, and senior health official Ashok Kumar Singh said parents should take preventative measures to protect their children.
“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,” he told the AFP news agency.
The 2014 study, published in Current Science, admitted the lychee connection remained unconfirmed but said there were close similarities between the outbreaks in Muzaffarpur and outbreaks of another acute encephalopathy disease called Jamaican vomiting sickness (JVS) during the ackee harvest in Jamaica.
Both unripe ackee and lychee, the study found, contain a toxic substance called MCPA, which has been shown to induce encephalopathy and hypoglycaemia in animal experiments.
The researchers said they did not know whether the toxin was present only in lychee seeds or in the flesh of the fruit, and whether ripe or unripe fruit were more likely to be harmful.
Either way, they noted that “well-nourished children are not affected” because their bodies could maintain normal glucose levels in spite of the toxin. The authors recommended further study and “ensuring adequate nutritional status in young children”.
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