Viral diseases are only treatable by the fishes natural defenses. There are no effective medications. The most severe cases do receive antibiotics incorporated into the food. Koi which receive too much pellet food in the winter can become congested and acquire bacterial dropsy symptoms. Food is not being digested at low temperatures and begins to ferment.
Heat therapy gets the digestive enzymes working.
Feeding with a medicated food in warmer water has cured some cases of dropsy. However, depending on the actual disease organism, heat therapy can speed its growth and kill the Koi sooner. Parasites will be treated more quickly in warm water. Weak Koi will gain vitality and strength, and will have an enhanced ability to cope with the parasitic infestation. Some parasites are encrusted on the Koi and chemical treatments are ineffectual.