Botulism is a bacterial toxin produced by Clostridium Botulinum, types A and C. Rotting animals produce the bacteria as well as rotting plant matter, even mud can have the bacteria and the toxins. Most domestic fowl can be harmed by botulism, it does not spread like a viral disease as the birds have to consume the bacteria or the toxin. Mortality is high and the spores and toxin are stable and can survive a long time outside of a rotting carcass or rotting plant matter. Dead poultry carcasses buried in the litter or meal worms that have consumed a rotting carcass could be the causation.
Signs of botulism.
Nervous system problems, weakness, progressive paralysis of the legs, wings, then the neck, followed by death. Chickens tend to lay in the litter with eyes closed, the beaks are sometimes dirty from resting on the litter more than usual. The body will be extremely limber and a bird will not be able to hold up its head. Eventually the paralysis spreads to the heart and lungs and causes death.
As with most diseases, prevention is better than a cure. Try to keep rotting food away from the birds and clean up any stagnant water. Remove all dead birds as soon as possible. Feeding road kill by hanging and allowing the maggots to drop into the pen is not a good idea due to botulism risks.
Treatment, there is an antitoxin available from vets. Bactracin and streptomycin sometimes have been said to work. It is possible to flush the bird at the onset of symptoms, flushing out the rotting food starting with Epson salts or molasses flushes but that itself can kill the bird and is considered a last ditch effort. You can also use the selenium treatment, one part potassium permanganate to 3000 pars of water.
Past the anti-toxins and flushing all you can do is nurse the bird and try to get it to eat so the old rotting food gets flushed out. Some people claim that if caught early, burned black toast or medicinal charcoal, apple cider vinegar, mixed with water in a blender will help flush the toxins out of the bird’s guts.