High temperatures and reduced salinity in Sambhar lake may have created conditions conducive to avian botulism, resulting in the mass deaths of migratory birds in Rajasthan. At least 600 migratory bird deaths were reported by the Centre for Avian Research Institute earlier this month.
According to reports, the deaths began on October 26, 2024 and continued for approximately two weeks.
Avian botulism is caused by the ingestion of a toxin produced by the bacterium Clostridium botulinum, which leads to paralysis and can be fatal for birds. These bacterial spores are widespread in wetland sediments and are found in most wetland habitats, including invertebrates such as insects, molluscs, and crustaceans, as well as in many vertebrates, including healthy birds.
However, the United Kingdom-based Wildfowl & Wetlands Trust (WWT) has noted that conditions like high temperatures, low oxygen levels, and an abundance of organic nutrients may contribute to outbreaks among birds.
The India Meteorological Department (IMD) reported that Jaipur district, about 70 km from Sambhar lake, experienced above-average temperatures throughout October.
According to official records, maximum temperatures in the region were consistently between 1 and 5.1 degrees Celsius above normal from October 1 to 31. Minimum temperatures also rose above normal, with increases ranging from 1.9 up to 7.1 degrees Celsius during the same period.
As a salt lake, Sambhar may have experienced reduced oxygen levels due to a lack of rainfall in October. The expected rainfall for October, which averages around 13.7 millimetres, was not recorded at all, an IMD official stated.
“However, no rain has been received, which is also the reason why day and night temperatures were high,” the official added.
P Sathiyaselvam, deputy director and head of the wetlands and flyways programme at the Bombay Natural History Society (BNHS), suggested that a sudden change in water quality could have triggered the outbreak.
“The Sambhar lake is a salt lake and there seems to be an inflow of fresh water changing the quality of water,” he explained.
According to the IMD, Jaipur district alone received a large excess of rainfall during the monsoon, with 986 mm recorded against an estimated average of 524.3 mm between June and September — an 88 per cent excess.
“The fresh water may have reduced the salinity in the lake. Another factor is that migratory birds when arrive are exhausted from months of travel,” he told Down To Earth.
Birds weakened by travel or low immunity are more vulnerable to diseases like avian botulism, Sathiyaselvam said. As birds die and decay, maggots form, spreading contamination in the water body, further affecting other birds or animals that consume the infected birds.
“There is no treatment for the disease, but it is recommended to immediately remove and dispose of affected birds,” he said.
Similar cases have been reported in Australia and the United States when salinity levels drop, he added.
The disease was previously reported at Sambhar lake in 2019 as well, causing the deaths of nearly 18,000 birds in a mass die-off.
Predicting such outbreaks is challenging since they occur when environmental conditions align, the BNHS expert said. “In this situation, the shift from high to low salinity coincided with the arrival of migratory birds, which favoured bacterial growth,” he said.
“Spores may survive for years but only develop into bacteria that produce toxins under certain environmental conditions,” WWT noted.
Sathiyaselavam said there are about 57 diseases reported among wild birds across the world.