Watch: "Low-grade infection" in yard says O'Brien

Aidan O'Brien has revealed that his stable has been suffering a "low-grade infection" and that Christmas Day coughed going to the start of the Betfred St Leger at Doncaster on Saturday.

His stablemate, Pierre Bonnard, was also a disappointing sixth in a race where the focus, understandably, was on Saffie Osborne becoming the first female jockey to win a British Classic aboard Highwayman.

Speaking to Kevin O'Ryan on Racing TV at the Curragh on Sunday, the Master of Ballydoyle said: "I suppose [it is] an unusual thing - Christmas Day coughed going to the start yesterday and it's one of them things that you're never prepared for.

"For the last couple of months, we've kind of been fighting a very low-grade infection through the whole place, through all the horses, two-year-olds and three-year-olds.

“It pops up every now and then and disappears. It pops up in their bloods, it pops up in their scopes, and you can't be sure where it is. Obviously before we run any horse they're always blooded [have their blood tested for signs of illness] and scoped to make sure that they're perfect, but you don't know.“Sometimes when you travel them it brings it out and obviously he [Christmas Day] was a little bit under the weather. He won't run anymore this year. He'll be put away and Pierre Bonnard [it] is probably the same thing. It’s one of those things that's been there for a couple of months.”

Many of O’Brien’s leading lights have continued to shine, with Darkness Falls and Sun Goddess being stunning Group One winners at this weekend’s Irish Champions Festival. He remains on course to break his own world record of Group One winners in a year, having achieved 28 winners at the highest level in 2017.But there have also been many disappointments, especially in July when he trained only six winners from 62 runners in Ireland, leading to speculation about the health of the yard.That was his worst haul on home soil for 21 years and included all 22 of his runners being beaten at the Juddmonte Irish Oaks meeting. He had the dubious distinction of being responsible for the last four home in the headline race.

O'Brien added: "We're dealing with it, like you have to keep it going. Sometimes when they travel it brings some little things to the fore, never any harm to them after, but obviously there's a little thing lurking there and we just have to be careful with it and that's just the way it was and obviously the horse yesterday was the same.

"We ran him back early after a tough run in York and you're always conscious of those little things. I wouldn't be taking anything away from any horse that has beaten any of them in the last couple of months but it's just there and that's the way it is and we just have to deal with it and keep moving forward but no, it's the way it is.

"He'll be put away and he's going to be a beautiful four-year-old and that's what he is really. He was trained hard early in the season for the Classics which are vital but I'd say obviously that was the reason and he'll be lovely to look forward to next year.

"Anyone who has horses know about these things and we [test the] blood so much, you see when there's a spike in the blood and SAA [Serum Amyloid A, a marker for inflamation or an underlying condition] starts to rise a bit and goes right up and then disappears down for no reason.

"Obviously there's a little thing there. We can't really put our finger on where it is or what it is, and it will work through and it always does but it's just there at maybe a delicate time.

"Every week we can go by and help them through, but obviously the three-year-olds are usually the ones at this time of the year are more prone to it because they have been trained for Classics and they would  have had a tough year up to then.

'We try and get them to compete in the middle [of the season] and against their older horses as well. Like I say we'll put him away and he'll be fine next year."

Speaking about Constitution River, who was sent off the 11-8 favourite for the Royal Bahrain Irish Champion Stakes but could only manage third, O'Brien said: "He'll have an easy couple of weeks. We will probably give him a precautionary antibiotic for a week or two weeks just in case there was something there that wasn't that we didn't see.

"Obviously he's a very fast horse and it was always possible that he could be a miler that we were stretching out to a mile and a quarter.

"He went through the race, he dreamed through it and that's what he always does. He has a lot of speed and like we always felt that he'd have never any problem going back to a mile.

"If the lads decide to go to Ascot with him he'd have a choice at a mile and a quarter or a mile.

"What can I say? There's nothing to prove but there obviously was some little thing [that was wrong]. The way he ran to the front and he didn't get the last half a furlong, so there was probably some little thing there. Hopefully a couple of easy weeks and a little bit of treatment there and hopefully he'll come back."