Roaches, various kinds of flys and small rodents could easily contaminate the food and water of the parrots. Almost all parrots are susceptible to this disease in one way or another. The Austral-asian and African species seem to be the most susceptible and the Eclectus is often a victim of this disease. In any case it is a disease that is only dealt with by prevention, since diagnosis and treatment in the clinical situation are so difficult. Here is a list not in order of importance.
Screen habitats to keep insects off parrot food and water.
Keep dogs to keep possums away. Use electric perimeter fencing to keep possums away. Remove all wet food before nightfall and hose out each cage leaving only dry food and fresh water for overnite. Keep chickens or quail under cages to keep insects down. Keep Tokay Geckos in area to keep roaches out at night.