Worming Ducks

Ducks should be wormed, just like other poultry. Worming ducks is not difficult using a suitable product and usually simply involves mixing in a given quantity with their normal food ration. Many health problems can be avoided by worming twice a year. This is usually done after they have stopped breeding and again before they start laying, i.e. over the winter months since ducks will naturally find more earthworms during the wetter months and it is the earthworms that are the intermdiate hosts, carrying the worm eggs.

Worms cause lots of damage to the digestive system which can sometimes lead to death but certainly leads to increased feed consumption, fewer eggs and loss of weight if nothing else. If ducks carry too many worms, you can have a situation where impaction occurs - this is where a blockage occurs and stops the digestive system from working, killing the bird. If a very high worm load is being carried by the duck and worms are killed, they can also release toxins into the body that kills the bird.

This article on Worming Poultry should be read in conjunction with this (there's another link at the bottom of this article), but this article is intended to give some slightly more specific advice with regard to worming ducks.
It is proven beyond doubt that worm loads in birds that are kept in free range systems are far higher than birds kept in confined spaces. A classic sign of ducks having worms is that they will go off their legs or go lame. Duck faeces, containing worms are picked up by ducks dabbling and feeding and  transferred to intermediate hosts (such as earthworms) in the soil. Ducks eat infected worms and the cycle is complete. It is important to worm ducks but also to try to break this cycle through good management, resting ground when possible and keeping grass short so that sunlight kills worm eggs in droppings on the ground. If your birds are carrying a lot of worms or your ground is well used then it is a good idea to repeat the wormer dose since birds will pick up more eggs from the ground. Do this at an interval when the eggs will be hatching and but before they reach sexual maturity. For most types of worms, 3 weeks is a good time to repeat this.

Products for Worming Ducks

There are no products to my knowledge that are licensed for use with ducks in the UK. This is not to say there aren't products available that will worm ducks!

Testing of veterinary products in order to get them licensed is both very time consuming and very expensive. When there isn't much of a market commercially for this sort of thing, companies won't find any reasons to get their products licensed.
Vets (in the UK) can of course prescribe products they feel are suitable under their own ‘clinical judgement' and I think waterfowl breeders have passed on knowledge of the products they use for worming over the years.
Here are some of the most common wormers that I have heard vets are prescribing and are used by other waterfowl enthusiasts:

Flubenvet - There is no official dose for ducks. Most people will use the dosage given for Chickens and Geese. Note the dose rate will be different according to the pre-mixture you buy.  (one is stronger than the other). If you are in any doubt, you should of course seek the advice of your vet.

Ivermectin - Has been prescribed to people by their vets for use on ducks. This is usually a ‘spot on' product and it kills external parasites such as lice. According to my books, Ivermectin will not kill tape worm.
Verm-X - This is a 100% natural product that many people are now using. I have been using this for the last year and have had reduced egg count when testing. For a confirmed case of worms, I would use a chemical wormer such as Flubenvet.