doi:10.1038/nindia.2017.88 Published online 24 July 2017
It is not the toxin in the litchi fruits but exposure to powerful pesticides sprayed on the trees that is mainly responsible for the reported mysterious sudden deaths of children living around litchi growing areas in south Asia, a new study1 suggests.
A team of US-Bangladesh researchers reached this conclusion after investigating an outbreak in 2012 of Acute Encephalitis Syndrome (AES) among children living near litchi orchards in northern Bangladesh. AES is characterised by a deadly swelling of the brain.
Litchi, a small, red-peeled fruit with a sweet white pulp is cultivated across China and South Asia. In India, AES outbreaks are reported almost every year since 1995 in Muzaffarpur district of Bihar – the country's largest litchi cultivation region Bangladesh reported nine outbreaks between 2008 and 2016.
Recurrent AES outbreaks in areas around litchi orchards – particularly during the harvest season –led scientists to suspect an association between litchi and AES. But the causes and risk factors were not known.
The team led by Saiful Islam at the International Center for Diarrheal Disease Research in Dhaka investigated the 2012 outbreak in Bangladesh that affected 14 children 13 of whom died within 20 hours of the onset of AES symptoms. Based on epidemiological and clinical analysis, they concluded that it was triggered by liberal application of multiple, highly toxic pesticides – including endosulfan that is banned in 80 countries – in orchards where children used to play and pick up fallen fruits to eat without washing.
The finding that pesticides are the culprit refutes the conclusion of another Indo-US study2 that investigated an AES outbreak in 2014 in Muzaffarpur in India. It ruled out pesticides as a contributor and blamed the outbreak on naturally occurring toxins – hypoglycin-A and 'methylene-cyclo-propylglycine’ (MCPG) – in litchi seeds and pulp. Its authors had claimed it to be "the first comprehensive confirmation that this recurring outbreak illness is associated with litchi consumption and toxicity from both toxins."
However Islam, in a statement, says the epidemiological study by his team of the Bangladesh outbreak found "strong association" between illness and exposure to areas where pesticides were routinely used. "If consumption (of litchi) were the cause (of deaths) we would expect cases from other parts of Bangladesh where it is consumed," he says. Furthermore the observation that the disease outbreak ended with the onset of monsoon rains "suggests that toxins were on the surface of the fruits (and got washed away by the rains) and not inside (as implied by the Indian study)," he says.
Why did the Indian study miss the role of pesticides? Islam notes that "Pesticide poisoning is notoriously difficult to diagnose as metabolites to pesticides remain in biological samples for only a few hours following onset of illness."
Jacob John , Vellore based epidemiologist who was the first to link Muzaffarpur outbreak with the litchi toxin MCPG, says there is a difference between the two disease outbreaks.
While in Muzaffarpur the death rate was 50-60% and deaths occurred a few days after the onset of symptoms, the death rate was much higher in the Bangladesh outbreak where children died within 20 hours of onset, he told Nature India. "Also, pesticide use in Bangladesh is uncontrolled in contrast to the much restricted use in Muzaffarpur where a Litchi Research Station advises farmers."
Considering all this, John says the outbreaks in India and Bangladesh are "two different diseases both associated temporally and spatially with the litchi harvesting season."
1. Islam, M. S. et al. Outbreak of sudden death with AES among children associated with exposure to lychee orchards in Northern Bangladesh. Am. J. Trop. Med. Hyg. (2017) doi: 10.4269/ajtmh.16-0856
2. Shrivastava, A. et al. Association of acute toxic encephalopathy with litchi consumption in an outbreak in Muzaffarpur, India, 2014: a case-control study. Lancet Glob. Hlth. (2017) doi: 10.1016/S2214-109X(17)30035-9