I don't know, @percypea, you may not be so keen on the aqueous fenbendazole once you see the price It is very highly concentrated, and I think it is being marketed towards the commercial agricultural industry. Here, it requires a prescription and the smallest quantity I could order through my vet was a liter -- for about $300 after the discount he gives me I'm not sure if the manufacturer got approval to use it in poultry here in the US.
Since I don't have a big barnful of peas, mine continue to get the 10% chalky white liquid (non-aqueous) version that is sold at Tractor Supply under the "Safeguard" brand name in an affordable quantity. I think (from what someone said the other day), you may find it as "Panacur" where you live. My peas get it soaked into bread chunks, which works fine for treating my small flock.
We often get questions from folks around the world about medications -- I find if I type either the brand name (of the other stuff) or the generic name (e.g. fenbendazole) into the web browser, I can often figure it out.
Yes, differential diagnosis is hard. Peas don't have lots of ways to show they are ailing, and they are often quite ill by the time they show signs. Looking at poo is one way -- @casportpony has a thread on the forum with samples of poo pictures. Patient observation and a careful eye help a lot. History of who's had what, such as worming and with what, how recently, can help. Knowing what diseases tend to happen with some kinds of exposures helps, based on knowledge acquired by reading lots of posts... Some things tend to recur in certain situations, such as cocci in young chicks, or blackhead after wet, rainy weather. Sinus infections and respiratory illnesses have some additional clues, of course. Some things get learned the hard way, on post mortem. A good avian vet is a real plus, but few of us are that lucky. In a pinch, many non-avian vets can either figure stuff out, or improvise, which is sometimes enough to save a bird. The fact that folks come here to BYC and freely share info is a tremendous resource for the peafowl community of owners worldwide.
If you are game to learn, @KsKingBee bought a microscope and the rest of the equipment to run fecal samples at home. He put up a thread about it, with his equipment and how-to information. His vet taught him how to run fecal float exams. It's not difficult at all, and I think he recouped his equipment costs quite quickly. That gives him really solid information about what is (or is not) ailing his birds. That way he knows what to treat for, or can eliminate what is not causing the problem. I think the next big step would be learning how to identify bacteria, if we can figure out how to do that at home, lol.
I'm not sure if I said it earlier, but welcome to the pea forum! Glad you are here!