As per a report released by the Indian Veterinary Research Institute (IVRI), Bareilly, on November 21, 2019, over 18,000 birds in and around Rajasthan’s Sambhar lake were killed due to avian botulism, a neuro-muscular illness, which affects the nervous system of birds resulting in flaccid paralysis in their legs and wings and neck touching the ground. The report cited climate as the reason behind the mass die-off.
What is Botulinum?
Botulinum is a natural toxin, produced by a bacteria known as Clostridium botulin, which produces toxin while reproducing. There are about eight types of this bacteria, namely, A, B, C1, C2, D, E, F, and G, each distinguishable when diagnosed and generally found in the soil, river, and sea water. One thing is common among all types—they attack the neurons, leading to muscle paralysis, as per a study.
This bacteria can affect both humans and animals, but the toxin is of different type. For example, Butolinum C in birds while A, B, and E work in humans. It has caused a great number of deaths in wild birds since 1905.
Avian Botulism at Sambhar
Sambhar lake is India’s largest inland saline water body and is designated as a wetland of international importance, which attracts thousands of migratory birds during winter.
The avian botulism at Sambhar was caused by the climate. The good monsoon provided a favourable environment for the bacteria to spread. Water temperature was about 25 °C with pH ranged between 7.4–9.84 and there were large population of crustaceans, invertebrates, and plankton. These living organisms host the bacteria for a long period of time. Besides, gills and digestive tracts of healthy fish also contain bacteria. So, the conducive conditions for propagation of bacteria resulted in its spike.
Moreover, this bacteria reproduces through spores and these spores remain dormant for years and changes in temperature or drying does not affect them and are activated under favourable conditions.
The study also states that when the water levels receded after monsoon, there might have been an increase in salinity levels, resulting in death of these organisms and the spores could have got activated. There is yet another theory, i.e., a ‘bird-to-bird cycle’. In this condition, maggots feeding on dead birds can concentrate the toxin. And thus, birds feeding on dead birds got infected. In Sambhar lake too, researchers found only insectivorous and omnivorous birds died. So, it could be a possibility as well.
The scientists found the samples infested with maggots of third stage which proved that avian mortality occurred over a period of time. As per Animal Husbandry Minister, the rescue centres treated many birds of which many are alive and were released to their natural habitat.
Earlier Examples of Botulism
Botulinum has been causing the deaths of migratory birds around the world. In 2007 and 2008, about 7,000 water birds died in Lake Michigan, followed by 4,000 in 2012 and about 183 Laysan Ducks in 2008 in Hawaii. As per a study, botulism outbreaks may be more frequent as climate change alters wetland conditions, giving birth to bacteria and pathogens. Such outbreaks generally occur when average temperatures are above 21 °C and during droughts.