We worm our peafowl 3-4 times a year.
I also regularly do stool analysis about once a month on our various pens/areas.
Parasites ARE the most common killer of peafowl. They are brought down by the infestation and a respiratory (or whatever) infection can set in. But it is SECONDARY to the PRIMARY, which is parasites.
Do I think t hat a wormer can be detrimental to peafowl who is ill with something else such as renal disease which has been mentioned here? Yes I do think it could. The answer to that is have a fecal analysis run and know for sure.
But there are many things that can cause renal failure such as viral, bacterial, fungal infections, coccidiosis, mycotoxins, IBV infection, the list is long. And is it acute or chronic renal failure? That is easy enough to diagnosis as well. Have the vet draw the blood or get a purple top tube for the CBC with 1 mL of blood and a red top (tube for spinning down the blood to serum) and a mL in it. And take it to your vet and have them run a CBC and chem panel to check kidney function (BUN and creatinine), calcium, phosphorus. It will reveal their liver function, any anemia. Infection will be detected, CBC can tell much about your bird.
You can also have uric acid levels checked which can reveal gout or kidney disease.
There is a lot to be learned from diagnostic testing on peafowl. And you can save their life by early detection and by knowing what you are dealing with so you can treat appropriately.
Stool analysis is about $15-20, the bloodwork about $75.
Some people can learn the stool analysis themselves. But it is a cheap test and worth it.
I have no problem with boosting immune systems and being cautious about worming.
However I am a realist and I know often times parasites are always ready to take hold and they will kill peafowl.