Terry Pratchett, who announced his diagnosis with early onset Alzheimer's seven years ago, has pulled out of a Discworld convention later this summer, saying "the Embuggerance is finally catching up with me".

Pratchett made the announcement with what he described as "great reluctance" on the website of the International Discworld Convention, where he had been set to appear as guest of honour in Manchester in August. "I have been putting off writing this announcement for quite some time and on good days thought I wouldn't have to write it at all," wrote the author. "I am very sorry about this, but I have been dodging the effects of PCA and have been able to write for much longer than any of us ever thought possible, but now The Embuggerance is finally catching up with me, along with other age-related ailments."

Pratchett was referring, in the "embuggerance", to the memorable description he initially gave to his posterior cortical atrophy, a rare form of early onset Alzheimer's he announced he had been diagnosed with in late 2007.

He told fans on the Discworld convention website on Wednesday that "this is the first time ever that I have been unable to attend a UK convention and I really am very sorry".

"They say time marches on, and it does, even though I have been running very fast to keep one step ahead of it. I really was looking forward to seeing your smiley, happy faces. Have fun everyone. Yes, on this occasion, have lots of fun," said the author, who has sold over 75m copies of his comic fantasy novels, the majority of which are set in Discworld, a realm held up by four elephants balanced on the back of a giant turtle.

The convention's chair, John Hicks, said that Pratchett would still be answering some questions from fans on video, that his business manager Rob Wilkins would be "bringing The Black Hat" – Pratchett's trademark – "to the Convention to represent Terry in absentia and we will, of course, welcome it with all due honours".

Pratchett has been as prolific as ever since he announced his Alzheimer's, publishing well-received titles including the Discworld novels Snuff and Raising Steam, the Victorian London-set Dodger, and a collaboration with science fiction author Stephen Baxter, The Long Earth. As recently as Tuesday, he tweeted: "Just to let you all know, the book that's on the screen in front of us – and is well underway – will be the 5th Tiffany Aching novel". Tiffany Aching is the young witch star of a series of young adult books. He added: "After the exciting news regarding Tiffany V – a little announcement that doesn't make me quite so happy", and pointing to his withdrawal from the Discworld convention.

The novelist – knighted for his services to literature – has been a fierce campaigner for Alzheimer's research since his diagnosis. He donated £500,000 of his own money for research, and is a patron of the Alzheimer's Research UK. Pratchett has also spoken out in favour of a euthanasia tribunal.

"If you did not know there was anything wrong with me, you would not know there is anything wrong with me. The disease moves slowly, but you know it's there," he said in his Richard Dimbleby lecture in 2010.

The lecture saw Pratchett go on to admit his vow that "rather than let Alzheimer's take me, I would take it. I would live my life as ever to the full and die, before the disease mounted its last attack, in my own home, in a chair on the lawn, with a brandy in my hand to wash down whatever modern version of the 'Brompton cocktail' some helpful medic could supply. And with Thomas Tallis on my iPod, I would shake hands with Death."