Johnston, who has saddled more winners in Britain than any other trainer, said he began isolating last week and that a test on Wednesday confirmed he had Covid-19.
In an interview with www.horseracingplanet.com, Johnston, 60, said: “I didn’t really want any publicity because I didn’t want to make a meal of it.
“I’ve been isolated since last Tuesday but other people are a lot worse off than I am; other people have bigger problems so you don’t want to blow it up.
“It started with a cough and I tried to play it down. I didn’t really think I had it but after four days my temperature was off the scale and then I had no choice but to accept it.
“I did one of those tests you have to pay for. We were fairly sure, but we got the result of that yesterday.”
He added: “Hospital is the last place I wanted to go given what you see on the news. I’ve had an online consultation and the doctor keeps telling you that as you’re continuing to breathe OK, then stay at home and don’t go to hospital, so that’s what I’m working on.”
Johnston, who trains at Middleham, is also a qualified vet and been closely involved in the debate about when it is safe for racing to resume behind closed doors. He regularly saddles over 200 winners a year.
In the meantime, his wife Deirdre, and their youngest son Angus, have been taking part in special musical sessions that have been broadcast on Facebook to popular acclaim.
Meanwhile British Horseracing Authority chief executive Nick Rust has reiterated that racing will be ready to resume as soon as it is safe to do so.
Fixtures last took place in Britain on March 17, behind closed doors at Wetherby and Taunton, before meetings were initially put on hold until May 1 at the earliest.
That suspension has now been extended, to an as yet unspecified date, and crowds will not be expected on racecourses until at least June, with a phased reintroduction on the Flat taking place behind closed doors.
Rust said: “As soon as we are clear in our daily discussions with Government that we are able to return we will be ready to do so.
“When we have the right conditions we will be able to resume racing in as good a form as we can, which will be behind closed doors, will mean social distancing and is likely to mean there will be some restrictions on tracks and runners and where we race.
“We have planned for a number of scenarios and we are ready to go. Sport has to listen to what Government is signalling and like any other business it needs to follow the advice from Government, but we are talking to Government every day and making the point that when conditions allow, we are ready to resume.”
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