That’s so strange. I believe you when you say you’re cleaning like mad (no one likes repeat trips to the ER!) but the only common thread I can think of is that she’s at home when the symptoms hit!
Does she have a regular vet & are you in contact with them? Did they do any blood tests? Does she have any other symptoms besides vomiting (weight loss, undigested food in stools, lethargy, etc)? Is the ER vet avian certified?
I assume this is true vomiting and not regurgitation. So puzzling.
Right, are you familiar with behavior regurgitation? When they pump and try to feed you or an object..usually creamy white.
True vomiting they shake head and spray. Any color often very runny. But can have food chunks
Or non behavior regurgitate ( as in something is wrong) can have it fall out of mouth or lean over and have fall out , can be tan and little more liquidity, or have food items in it.
Besides cleaning products, scented candles, scent diffusers, sprays and things like that could be possible.
While Teflon poisoning is usually death. I have heard of a few cases of vomiting and listless...any nonstick products that might contain those deadly chemicals?
Was the food offered your parrot at the vet the same as you feed at home? What do you feed?
And as mentioned weight changes? How's poop look? Any other symptoms, Xtra naps , fluffed, lethargy, weakness, tires quickly after flight? Sneezing, swelling, drainage, discoloration of feathers above nares, plugged nostril?, changes in feathers?
Chronic zinc toxicity is a slight possibility. Often bell clapppers in toys ( sorry, but made in China ones have ben a culprit) are zinc coated or not safe metal. Zinc builds up over time in birds exposed to galvanized metal, earrings, and other sources. Symptoms vary, itchiness, weakness, neurological, feather plucking, vomiting...lot of symptoms unfortunately many health issues share the same symptoms.
Mega bacteria or called avian gastric yeast..see if I can link info on that. This has most likely been ruled out already in your bird because you had a veterinarian exam fecal. But because it may help others searching about vomiting birds. PDD a virus can have similar symptoms, mentioned in first link.
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https://veterinarypartner.vin.com/default.aspx?pid=19239&catId=102911&id=10158446
Common diseases by species
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Chlamydia is a pretty common infection and can effect different systems and have different symptoms. Common symptoms include weight loss and bright green stool or liquid around stool. Other than first morning poop , as first morning poops are often bright green due to bile acids and fasting overnight . So no freaking out parronts if first morning poop is green. Often respiratory symptoms, sneezing, clear draining intermittent from nostrils, watering eyes, swelling around eyes. , red eyes. Can cause vomiting but rare usually other symptoms with. Fluffed
Doxycycline is the only antibiotic that will treat it and takes 45 days of treatment, relapses and second treatment common. Other antibiotics won't treat , but might symptoms dissappear temporarily.