I'd guess the addition of one new disease to a long list wouldn't constitute a substantial amendment.
I was wondering about the copyright rules a few months ago when I noticed one of the documents we issue to shipping companies has a copyright date. The problem is that the date in the small text is some time back in the Eighties (probably 1988), while the regulations have been amended several times since then, as has the content of the document.
To be honest, that's the only document of ours that I can ever recall seeing a copyright on and it makes no material difference as we give copies away free of charge to the shipping companies. My guess is that it was copyrighted with new regulations in 1988 and that has been forgotten about since or ignored when new versions have been proof-read.