Avian Botulism is a strain of

botulism Botulism is a rare and potentially fatal illness caused by a toxin produced by the bacterium ''Clostridium botulinum''. The disease begins with weakness, blurred vision, feeling tired, and trouble speaking. This may then be followed by weaknes ...

that affects wild and captive bird populations, most notably waterfowl. This is a paralytic disease brought on by the

Botulinum neurotoxin Botulinum toxin, or botulinum neurotoxin (BoNT), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromusc ...

(BoNt) of the bacterium ''

Clostridium botulinum ''Clostridium botulinum'' is a Gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the ability to produce the neurotoxin botulinum. The botulinum toxin can cause botulism, a severe flaccid paralytic disease in humans an ...

''. ''C. botulinum'' can fall into one of 7 different types which are strains A through G. Type C BoNt is most frequently associated with waterfowl mortality. The Type E strain is also commonly associated with avian outbreaks and is frequently found in fish species which is why most outbreaks occur in

piscivorous A piscivore () is a carnivorous animal that eats primarily fish. The name ''piscivore'' is derived . Piscivore is equivalent to the Greek-derived word ichthyophage, both of which mean "fish eater". Fish were the diet of early tetrapod evoluti ...

birds. Avian Botulism occurs all over the world and its understanding is important for wildlife managers, hunters, bird watchers, and anyone who owns wetland property as this disease can account for over 1,000,000 waterbird deaths in a year.

Prevalence and distribution

Avian botulism occurs all over the world and is especially predominant in North American wetlands. The degree of avian botulism outbreaks in populations is largely determined by how favorable conditions are for ''C. botulinum''. Ideal conditions for the presence of the BoNt carrying bacterium consist of low-oxygen, high-protein available substrate. This is common of shallow and stagnant waterways. Other factors such as pH and temperature, as well as a likely number of unknown factors contribute to the prevalence of outbreaks. Geographic locations with great amounts of this habitat can be assumed to have a greater number of outbreaks. Notable locations include the

Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...

, North Dakota, Japan, South Korea, and Saskatchewan. While this is frequently the trend there is also evidence of BoNt outbreaks in well-oxygenated lakes with low temperatures. Furthermore, ideal habitats like those described do not all feature avian botulism in their waterfowl populations which supports there are still unknown factors at play. In recent time, nearly 18000 migratory birds had fallen victim to this disease in India. The mass mortality has been reported in 2019 from Sambhar Lake, Rajasthan, which is also a Ramsar site.

Transmission

Avian botulism is not contagious in that it is not spread from bird to bird. Instead it is spread to birds through their consumptions of maggots infected with the toxin. Maggots become infected by feeding on substrates and organic material that host the Type C BoNt. Huge die offs caused by BoNt are the result of this maggot cycle. When an infected bird dies the maggots that feed off of it become infected themselves. These maggots are in turn consumed by additional birds. Thus with every infected carcass brings several infected maggots which increases the number of birds that can contract the disease. In this way massive outbreaks can occur.

Clinical signs

The botulinum neurotoxin is lethal because it causes paralysis. Field identification involves locating birds showing

flaccidity Flaccid paralysis is a neurological condition characterized by weakness or paralysis and reduced muscle tone without other obvious cause (e.g., trauma). This abnormal condition may be caused by disease or by trauma affecting the nerves associat ...

in the legs, wings and neck, as well as the presence of protuberant

nictitating membrane The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. All ...

. The presence of several dozen, or even hundreds, of fresh waterbird carcasses is the stereotypical sign an outbreak has occurred. In this case the specimens need to be taken to disease laboratory to determine the cause of mortality. Most commonly, detection of ''C. botulinum'' in carcasses during lab work is accomplished through analysis of

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) t ...

s (PCR) and is often the most successful method.

Prevention and control

The presence of avian botulism is extremely hard to detect before an outbreak. Frequent surveillance of sites at risk is needed for early detection of the disease in order to take action and remove carcasses.

Vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifie ...

s are also developed, but they are expected to have limited effectiveness in stemming outbreaks in wild waterbird populations. However may be effective in reducing mortality for endangered island waterfowl and small non-migratory wild populations. Field tests are needed.

References

{{reflist Bird diseases