Often, ducks get botulism from living, drinking, and eating in stagnant water. The water grows the bacterium, and the ducks ingest it.  Ducks can also get botulism from eating from eating small, dead invertebrates, as well as maggots that feed on the carcasses. Bad food and dying plant life can also give botulism to ducks. Limiting flies limits the number of maggots in the area, which can carry the bacterium. Flies can spring up for a number of reasons, especially if the ducks are being raised around other livestock.  Control manure. Manure is one of the biggest fly attractions. Make sure to remove manure at least 2 times a week. It's also important to dry out manure, as the moisture attracts flies. To dry it out, spread it out thinly in a sunny area, and then scoop it up once it's dried.  Clean up spills quickly. Both feed spills and manure spills can attract flies. Clean them up to help keep flies at bay.  Avoid having drainage ditches with weeds. These murky areas can attract flies.  Introduce fly-eating species. For instance, a small form of wasp, the fly parasitoids, has young that eat the pupa of flies, and they don't bother humans. If several ducks have died of botulism, it's important to remove the animals quickly. Other ducks can catch it from the dead animals, and it can infect the water even more.  The best solution is to burn the animals or bury them away from other ducks. Dead fish can cause some of the same problems as dead ducks. If dead fish are in a pond with the ducks, it's best to remove them if possible. Shallow water tends to be stagnant, and in warm weather especially, it can breed botulism. It's best to either drain the water off or flood the area to avoid these shallow breeding grounds.

TL;DR in Vietnamese:
Hiểu biết con đường nhiễm khuẩn botulism ở vịt. Kiểm soát ruồi. Dọn xác chết. Dọn cá chết. Dọn dẹp khu vực trũng thấp.