Avian botulism killed over 18,000 birds in and around Rajasthan’s Sambhar lake, the Indian Veterinary Research Institute (IVRI), Bareilly, said in a report released on November 21, 2019.
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.
Other instances of botulism
Sambhar lake is not the first instance where deaths due to botulism have been recorded. For over a decade, botulinum has been causing the deaths of migratory birds around the world.
According to reports 7,000 water birds died in Lake Michigan in 2007 and 2008, followed by another 4,000 in 2012. In Hawaii, the toxin killed around 183 Laysan Ducks in 2008.
A study conducted by Spanish researchers shows that not all birds will be affected by avian botulism unlike avian flu.
It also states that botulism outbreaks are likely to become more frequent as climate change alters wetland conditions to favour bacteria and pathogens.
Previous studies have found that outbreaks tend to occur when average temperatures are above 21 degrees Celsius, and during droughts.
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