Try treating for BOTH niacin deficiency AND for malnutrition. Its unlikely that you have a parasite. Most of us go through this. Its the pain of having waterfowl or chickens. We tend to go through this phase of having many of the youngest birds get sick. And usually its from malnutrition and niacin issues.
Also its no secret that a lot of the mill feeds, and pet feeds for fowl in the stores are deficient and basically not enough to support them. After I started augmenting the nutrition on my waterfowl I had a very noticeable down tick in sickness and deaths of ducklings, etc. Its all about nutrition.
You can also tell this in that its always the chicks/ducklings/goslings that get sick and not usually the adults. The weakest ones are more prone to problems from poor nutrition. With the adult birds what you see instead is that they'll stop laying eggs, or the numbers will go down a ton. You can also look for, if the egg yolks are pale, instead of dark orange, etc.
The higher the nutrition, the darker the egg yolks will be. This is an amazing built in thermometer for your fowl nutrition that lets you know if they have enough nutrition or not. The paler and lighter it is, the more its bare bones. The darker it is, the better their nutrition gets up to the point of dark orange and almost red. This holds true for ducks, and chickens. It should work for geese also, though the problem with geese is that you only get so many eggs; you get way less eggs (Although you might not have adult birds yet??)
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Do you only have 1 gosling? fowl do better in packs. They don't do well solitary. It affects mental health a lot. They do much better with a buddy or 2. (Although with some fowl you can have problems if there's too many males per female.)
