I have no idea. However, since you say what you've tried hasn't helped, I'll tell you what I'd do.
Aloe Vera juice in the water, 1 tbsp per gallon every other day for 7 days. Yogurt mixed into their feed and/or kombuca scobies and/or kefir added to their feed every day for 7 days and oregano oil, 3 or 4 drops per large container every day for 14 days. See what happens.
Why those items? Aloe Vera Juice flushes the intestinal track, yogurt/scoby/kefir builds positive gut flora (good gut flora = major assistant for immune system) and oregano oil is a powerful anti-biotic and pain killer. If you have Wormwood (russian herb, absinth) in the area, you could also give them that as a fresh plant supplement with the AVJ as they do similar things.
This is my general treatment for EVERYTHING when it comes to all my birds, the only thing it hasn't staved off is Marek's, however it does help with birds that are have a limp or slight mobility problem during flare ups.
Good luck, keep us informed!