The death toll from AES, a fever that affects the central nervous system, has risen from 97 reported last Monday and medical and government officials are facing mounting criticism for struggling to contain the crisis in the town of nearly 350,000 people.
AES, known as 'brain fever' is caused by any one of a number of viruses. Symptoms include high fever, vomiting and, in severe cases, seizures, paralysis and coma. Infants and elderly people are particularly vulnerable.
"I received my last update in the afternoon and according to the latest report that I have, 129 children have died so far," Shailesh Prasad Singh, the top state medical official in Muzaffarpur told Reuters by phone. The town lies about 80 km (50 miles) from Patna, the state capital.
The precise causes of AES are not known, though a majority of medical professionals say it is linked to a ferocious heat-wave.
Some studies have blamed toxins in lychees, a fruit grown in abundance in orchards around Muzaffarpur. But many families told Reuters last week that their children had not eaten them in recent weeks.
Singh said a team of experts, including doctors, paramedics and other government officials were working "round the clock" to contain the spread of AES, which killed more than 350 children in a previous outbreak in Bihar in 2014.
Doctors from other parts of the country had been dispatched to Muzaffarpur to help staff at the Sri Krishna Medical College Hospital which had to evict a group of sick inmates from a ward to accommodate the surge in AES patients. Most of the children who died were being treated at the hospital.
Nearly half of all children in Muzaffarpur are underweight, and a similar number are stunted, making them vulnerable to AES, which grips the town almost every year when summer temperatures surge.
As the death toll mounts, both the Bihar and the federal governments have come under criticism for their inability to save children, and doctors and government officials have faced angry protests.
On Sunday, the Bihar government suspended a doctor in Muzaffarpur for dereliction of duties, news agency ANI, a Reuters affiliate, said.India's Federal Health and Family Welfare Minister Harsh Vardhan, who is himself a doctor, said last week that health officials have finalized plans for construction of a 100-bed pediatric intensive care unit in Muzaffarpur, but gave no timeframe for when it would be built.