The symptoms you describe is that of Botulism poisoning also known as "limberneck" as the swan cannot hold its neck in an erect position. The toxins affect the neuromuscular system which will eventually cause paralysis and the inability to breathe or swallow. Other poisons can also affect the neuromuscular system and poisonings usually cannot be treated through medication, only supportive care such as I.V.'s, respirators, charcoal, etc.

Once the swan becomes so weak that it cannot eat, then the toxicity affects are so far gone that any intervention may be too little too late and the inevitable occurs.

Botulism is usually seen after long periods of drought in which the birds can extend their necks to feed in areas that they normally cannot reach. These areas are usually rife with anaerobic (non-oxygen) bacteria and they produce the toxins. If there has been a drought and then major flooding, the bacteria can also be spread throughout the habitat and the swans, fish and other wildlife can also get a dose of the toxin. The best way to prevent Botulism is to vaccinate the birds to mitigate their deaths from the affliction, keep the pond or lake aerated using a fountain or aerator, and keep water levels high. If the poisoning is from insecticides, fertilizers, pesticides or other toxins, run-off and cause of the poisons from entering the habit must be addressed and prevented.

If the poisoning is from something in the habitat such as plants, then the offending plants, flowers, weeds, grasses must be identified and removed. If the food is left to mold and feeders not properly cleaned with food changed on a frequent basis, then this can also poison the birds. The Regal Swan