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Botulism is a disease found around the world. It affects a wide variety of wild and domestic birds, including chickens, turkeys, pheasants, ducks and other waterfowl. In Maine we encounter it primarily in waterfowl.
It is sometimes called limberneck, but a weak or paralyzed neck can have other causes besides botulism, so this is best used as a general term for the symptom and not a specific term for botulism.
THE CAUSE AND DISEASE PROCESS
The cause of botulism is a toxin produced by the bacteria Clostridium botulinum. C botulinum grows best and produces the most toxin in very warm decayed plant and animal matter; in Maine, this usually means decayed vegetation in stagnant water and animal carcasses in the summer. Maggots are often found with the carcasses, and they will sometimes contain a large amount of toxin. Ducks and other waterfowl will ingest the bacteria and toxin when the eat this vegetation, meat from the carcasses, and, since they love to eat insects, the maggots.
(Another botulism disease process involves birds without symptoms having C botulism in their digestive tracts, where it either suddenly produces enough toxin to make the bird ill, or is shed into the environment where it will be encountered by other birds.)
The toxin is absorbed from the bird's digestive tract into it's blood and lymph fluid and travels to the muscles and other tissues, where it blocks nerve impulses by interfering with the neurotransmitter acetylcholine.
Heart failure or respiratory failure are the most common causes of death. Some birds may drown (see below).
The signs of botulism are an ascending flaccid (loose rather than rigid, hence the term "limberneck") paralysis - weakness and paralysis that starts in the feet and progresses upward toward the head.
Birds are first reluctant to move, then are lame, and then are lying down.
Their heads may drop to the floor where they are supported by their bills.
The neck may be extended.
Some clinicians consider paralyzed eyelids a definitive sign that the bird's problem is botulism.
Breathing difficulties, quivering feathers, and ruffled hackle feathers may occur.
If the bird is in water, its head may drop under water and it will drown.
Sudden death without other signs is possible.
When a bird encounters C botulinum and its toxin, whether or not it becomes sick, how quickly it becomes sick, and how sick it becomes depend directly upon the amount of toxin it ingests. If a large amount is ingested the bird will become ill within an hour or two; if a small amount is ingested signs might not appear for a day or two.
Because birds will often share food sources, more than one bird in a flock will often become sick.
There are no practical tests that can be done to definitively identify botulism as the cause of a bird's signs. Diagnosis is usually tentative, based on the bird's signs, known access to decayed matter, and exclusion of other potential causes for the signs.
There are a fairly large number of other possible causes for the same signs, including congenital and problems, nutritional deficiencies, internal parasitism, bacterial and viral infections, trauma and neoplasia. It is not safe to assume that every bird that has "limberneck" has botulism.
The only practical treatment for botulism is supportive care: assisted (tube) feeding, trial therapy with antibiotics, and trial therapy with nutritional supplements, in particular niacin and vitamin D3. Treatment with antitoxin is not a practical consideration for birds, for a number of reasons.
Most birds will die, with or without treatment. Infrequently a bird will spontaneously recover, or will recover in apparent response to supportive care; if this happens it is usually within a day or two of the onset of signs. (If a bird remains ill for a longer time, then the diagnosis of botulism becomes very doubtful.)
Prevention of botulism is best accomplished by preventing access to decaying food, and decaying plant and animal matter. Botulism is a rare problem in poultry flocks that are well-cared for with regular provision of clean and fresh water and food and clean, dry bedding. Poultry flocks are not vaccinated for botulism.
Zoonosis (spread from birds to people) has been reported in rare, isolated cases but is not a practical concern.
Yarmouth Veterinary Center
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