Nearly 80,000 birds die at Rajasthan’s Sambhar Lake

Find out what happened at India’s largest saltwater lake.

On November 7th, 2019, Sambhar lake in the Indian state of Rajasthan witnessed the death of nearly 40 demoiselle cranes. A few days later on November 30th, 2019, the lake witnessed the death of more than 8000 migratory birds including shovelers, stilts, snipes, pintails, avocets, and larger birds including common, demoiselle and sarus cranes.

Initially, a Bhopal based laboratory suspected this to be a work of avian flu. However the post-mortem of some birds concluded that Clostridium botulinum (Avian Botulism) had entered the soil and therefore affected the diet of the birds.

Avian botulism is a neuromuscular illness of birds caused by a toxin that is produced by the bacterium Clostridium botulinum. Avian botulism has been recognised as a major cause of mortality in wild-birds since the late 1900s. It paralyses the birds, most of the time also killing them eventually. Nearly 65000 birds of 36 species died at Sambhar Lake.

1. 50% Ducks: Mostly Northern shovelers and few common teal, gadwall, garganey, Spot-billed duck, ruddy shelduck, Common pochard and northern pintail
2. 40% Waders and other Waterfowl like: Coots, Ruffs, Plovers, Sandpipers, Shank, Moorhens, Pied Avocets and Black winged stilts
3. 10% Miscellaneous: Gulls, Indian Eagle owl, Flamingo, Black kite etc.

Some Important Points to keep in mind as on how the infection may have spread:

• Dead marine life creates perfect anaerobic conditions for C Botulinum to grow and produce a neurotoxin. If the bird happens to eat them, there are chances that the neurotoxin travels to the bird’s brain through blood.
• Botulism is transmitted between two organisms
• Even a little amount of this toxin is fatal for the birds.
• Neurotoxins are toxins which eat away at the nerve tissues, creating a nervous system disorder which leads to paralysis and sometimes death
• The carcasses of the dead bird can become a wonderful medium for this bacterium to multiply as long as anaerobic conditions persevere.
• Blow Fly lays its egg in such rotting carcass. Maggots feeding on these carcasses accumulate/store this toxin in their body. Maggots are immune to the effect of this neurotoxin.
• Scavengers which consume these maggot infected carcasses die because of the toxic compounds present in the maggots.
• Most of the birds that died at Sambhar Lake are not known to feed on the dead/maggot infested carcasses. It is safe to assume that they were victims of direct poisoning.
• This means they could have been one death which triggered an epidemic which affected the diet of the birds, therefore directly poisoning them.

However, not a single crow has died in the epidemic

• Scavengers have been eating dead and rotten flesh for millions of years and thus may have developed immunity to the assortments of potentially fatal compounds.
• This theory sounds logical but it required corroboration.
• There are studies which talk about the Immunity of Birds and Mammals to the effect of Botulinum toxin.
• One study showed that 90% Turkey Vulture, 42% Crow, 25% Coyote and 17% Norway Rats in the test sample had antibodies against Botulinum Toxin. This study provides scientific backing to our theory.