Leading Royal & SunAlliance Chase contender Lord Sam will miss today's race at the Cheltenham Festival.
The Victor Dartnall-trained eight-year-old, winner of all three of his outings over fences, was expected to give favourite Our Vic most to do this afternoon.
But Dartnall said: "He was off his grub this morning, so we ran a few tests on him, including a blood test and he is running a slight temperature.
"It's desperate for everyone at the yard but I suppose it is better he doesn't run than he comes last and finishes distressed.
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"Logically, Aintree must be the next target if I can get him right again in time.
"I can't believe our bad luck - this kind of thing has never happened before with him and hopefully will never happen again."