Pippa, a 4 year old female, died of avian botulism toxin a few days ago. She was a sibling of Susie, and lost out to Susie for the affection of Sully. She had been solo for almost a year.
I found Pippa in the water about an hour before dark, thrashing strangely, continuously dipping her head deeply in the water. When she came close to the shoreline, I was able to grab her neck, pull her from the water, and secure her wings and feet before carrying her back to my garage. She was becoming weaker by the minute, and began gasping. Flushing her system through a tube was not helping, and she passed before midnight.
Avian botulism occurs naturally in the environment, and can reside in submerged decaying matter, a food source for swans. This same thing happened to Gracie in 2007, also in the month of April. Once established in a swan's system, there is little hope for survival, so the ability to prevent or remedy this type of situation in the future is unlikely.