It is better to prevent illness and with ducks this is easy to do because they are naturally very resistant and hardy animals provided they have the following:
The most common health problems with ducks are ingestion of toxic or dangerous objects, unsanitary water and injuries. Here is a list of common problems and what to do.
If your ducks are displaying health problems but no obvious sign of injury or disease then it may be their diet that is causing problems. Firstly check that they aren't eating any poisonous plants. Secondly, you can try changing to a different feed. Thirdly, check their gullet - as ducks scoop up food from the ground they are at risk of picking up unpalatable things like nails which can get stuck in their gullet. This can result in gradual weight loss, lethargy and eventually death.
Ducks have quite fragile legs and feet so you should never pick a duck up by its feet. They can get cuts on the underside of their feet which you should treat by cleaning and disinfecting the area and then keeping the duck on clean straw until the cut has healed. Occasionaly a duck may strain a leg muscle which can also result in a limp. In this case it is good for the duck to exercise by swimming to help the muscle to heal. Sometimes a deficiency of Vitamin B3 (niacin) can cause limping. This is a water soluble vitamin and can be added to the drinking water or by feeding Marmite on toast until the limp has cleared.
Round and Tape. These are the most likely types of worms that your ducks may experience. Symptoms: Drop in egg production with an increase in hunger. Birds can also have diarrhoea as a symptom, although diarrhoea alone does not mean worms are the culprit. You should contact your vet who will give you medication to be included in your bird's feed. Flubenvet is a commonly available drug for this condition.
Symptoms: White build up around the feather base near the vent and in a bad case there could be a build up on feathers as well. The whiteness is lice eggs. If you do find lice eggs around the vent when checking the duck's health, brush them off and rub vaseline around the area. If the problem persists then you can use Sulphur ointment on the affected area. Prevention is the best option though, so dust the nestbox with louse powder every week or so. Finally, don't panic, they will not spread to you or other animals!
Symptoms: Egg production will go down. In a bad case the bird could look a bit pale from blood lost to the little suckers. You will not be able to find any by inspecting your birds during the day as they only crawl onto the birds for a nibble at night. Have a look every week or so for mites in any crevices. There are mite-sprays available to prevent this for certain periods of time.
Symptoms: Drop in egg production and thin, rough and wrinkly egg shells. Sneezing and gaspy birds with a discharge from the nose. Your ducks will be very unlikely to catch this airborne disease but it is still possible. The illness should only last for a couple of weeks. Egg production will improve again but will probably never be as good as before, with more occasional dodgy eggs.
Symptoms: Thirsty, wheezy, lethargic. Avoid this by making sure you clean the dropping tray regularly. Young birds are most vulnerable but adult birds can be infected. It is caught by breathing in spores from mouldy matter. Unfortunately there is no cure. The best thing for the duck would be to put it out of its misery, or get a vet to do it for you.