Context-Over the last 10 days, thousands of migratory birds have been found dead at Sambhar Lake, about 80 km southwest of Jaipur city. Officials have buried over 18,000 carcasses so far

Cause of death

• The evidence points to avian botulism, but this has not been officially confirmed.
• On the basis of history, epidemiological observations, classical clinical symptoms and postmortem findings, the most probable diagnosis is avian botulism.
• The clinical signs exhibited by affected birds included dullness, depression, anorexia, flaccid paralysis in legs and wings, and neck touching the ground. The birds were unable to walk, swim, or take flight.
• There was no rise of body temperature, no nasal discharge, no respiratory distress or any other sign.

What could be other possible reasons for the bird deaths at Sambhar Lake?

• After a Division Bench of the Rajasthan High Court led by Chief Justice Indrajit Mahanty took cognisance of the deaths, the Rajasthan government listed likely reasons:

  * Viral infection;

  * Toxicity, as a new area has been filled up after almost 20 years, and there could be higher concentration of salts along the edges;

  * Bacteriological infection; and

  * Higher temperature and high water levels due to a good monsoon. This might have led to an increase in competition for resources. The weaker individuals, exhausted from the long journey, perhaps were unable to compete, and may have succumbed to stress emanating from the shortage of food, susceptibility to disease/pollutants/toxins and other habitat-related factors in the wintering grounds, the government suggested. If that is the reason, the government said it is expected that with fall of temperature and lowering of water levels, incidence of such mortality will go down.

But why is it taking so long to establish the cause of death?

• The government is waiting for reports from various sources to establish the exact cause. It has so far engaged eight institutions and agencies, but has received complete reports from only two: RAJUVAS and the National Institute of High Security Animal Diseases (NIHSAD) in Bhopal. While RAJUVAS has suggested avian botulism, NIHSAD has ruled out bird flu.

Is there a concern for human health?

• Humans are primarily at risk from avian botulism only if they eat infected fish or birds. 

How common is avian botulism?

• There have been several waterfowl botulism outbreaks. Between 1995 and 1997 in Canada, an estimated 1,00,000 birds died in Alberta, 1,17,000 in Manitoba, and 1 million in Saskatchewan. In 1997, another 5,14,000 birds died due to botulism in Green Salt Lake, Utah, US. In 1952, an epizootic outbreak killed 4-5 million waterfowl across western US.

About botulism

• Botulinum is a natural toxin produced by a bacteria known as Clostridium botulin. It produces the toxin when it starts reproducing.
• The bacteria is commonly found in the soil, river, and sea water. There are around eight types — A, B, C1, C2, D, E, F, and G — of botulinum toxin and they are distinguishable when diagnosed. But all types of toxins attack the neurons, which leads to muscle paralysis, states a study.
• Botulinum affects both humans and animals but the type of the toxin varies — botulinum C in birds and A, B and E in humans.  The toxin has been recognised as a major cause of mortality in wild birds since the 1900s.

What happened at Sambhar

• The avian botulism that caused the mass die-off at Sambhar was caused by the climate, according to the IVRI report.
• Water levels were fluctuating throughout the year. Locals reported that due to a good monsoon this year, the water level reached the lake bed after a gap of 20 years.
• The good monsoon provided a favorable environment for the bacteria to spread. The bacteria needs anaerobic (absence of oxygen) conditions and does not grow in acidic conditions.
• The temperature of the water was about 25 degree Celsius. Its pH ranged between 7.4- 9.84.
• It also requires a nutrient-rich substrate, like areas with large amounts of decaying plant or animal materials. The monsoon brought with it a large population of crustaceans (like shrimps, crabs, and prawns), invertebrates (snails) and plankton (like algae).
• These living organisms are capable of hosting the bacteria for a long period of time. According to reports, the bacteria is also found in the gills and digestive tracts of healthy fish.
• It reproduces through spores and these spores remain dormant for years. They are resistant to temperature changes and drying. Under favourable conditions, the spores are activated.
• The IVRI report noted that after the monsoon, when the water levels receded, there might have been an increase in salinity levels which could have led to the death of these living organisms. At this point in time, the spores could have been activated.
• According to another theory, ‘a bird-to-bird cycle’ could also have led to the tragedy. In such an event, maggots feeding on dead birds can concentrate the toxin. Birds feeding on dead birds can get affected.
• This was observed in Sambhar too as researchers found only insectivorous and omnivorous birds affected and not herbivores.
• The IVRI report discounted external factors like water pollution and eutrophication (a body of water becoming overly enriched with minerals and nutrients, in turn inducing excessive growth of algae) as no farming was being carried out in the vicinity and the runoff from the same was not possible.

Sambhar lake

• Sambhar Lake is India’s largest inland saltwater lake at 230 sq km, spread mostly across Jaipur and Nagaur districts and also a part of Ajmer. It has a catchment area of 5,700 square km, with the water depth fluctuating between 60 cm in the dry season to about 3 metres at the end of the monsoon.
• Every year, the lake attracts thousands of migratory birds. A total 83 species of water birds have been recorded at the lake, the most abundant of which are little grebe, great crested grebe, great white pelican, little cormorant, black stork, and darter, apart from various species of plovers, egrets, herons, and geese.

What are the reasons that make salt concentration a concern?

• In a 2016 directive, the National Green Tribunal had noted the impact of the salt industry — including “unauthorised salt pans” — on the ecosystem of Sambhar Lake and asked the state government to cancel allotment of salt pans. Over the last week, the Wildlife Institute of India, the State Pollution Control Board and Sambhar Salts Ltd have collected samples to test for water quality. Part of the lake has been leased to Sambhar Salts, a joint venture of Hindustan Salts Limited and the state government. Sambhar Salts produces 196,000 tonnes of clean salt every year, which is around 9 per cent of India’s salt production.
• The lake was recognised as a wetland of international importance when it was designated as a UNESCO Ramsar Site in 1990. Today, as per NGO Wetlands International, it has the worst possible Wetland Health Score at E.