Symptoms include nausea, anxiety and loss of consciousness – and all patients have tested negative for COVID-19.
At least one person has died and 200 others are in hospital in India due to a mystery illness.
The first cases were detected on Saturday evening in the ancient city of Eluru, in the state of Andhra Pradesh.
Patients are suffering from symptoms ranging from nausea and anxiety to seizures, convulsions and loss of consciousness, doctors said, but they have tested negative for COVID-19.
A 45-year-old man who was in hospital with symptoms similar to epilepsy and nausea died on Sunday evening, the Press Trust of India news agency reported.
In total 380 have fallen ill and officials are trying to determine the cause of the new illness.
So far, water samples from affected areas have not shown any signs of contamination, although the sick are thought to have collected their water from the same source.
According to reports, a note released by Andhra Pradesh’s health department said initial blood tests did not find any evidence of a viral infection, such as Japanese encephalitis, dengue or chikungunya – diseases which are caused by mosquito bites.
State chief minister YS Jagan Mohan Reddy visited a government hospital and met patients who were ill on Monday.
Opposition leader N Chandrababu Naidu tweeted to demand an “impartial, full-fledged inquiry into the incident”.
It comes as India continues to be badly hit by rising coronavirus cases.
According to Johns Hopkins University, which is tracking the pandemic worldwide, India has recorded more than 9.6 million cases, second only to the US, and more than 140,500 deaths – third to the US and Brazil.
Andhra Pradesh state is among those worst hit by the virus with more than 800,000 detected cases.
The health system in the state, like the rest of India, has been frayed by the resulting pressure.
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