MacDonald was riding the Micky Hammond-trained Russian Royale in the Welcome To Hexham 2021 Conditional Jockeys’ Mares’ Handicap Hurdle on March 18 when falling three flights from home.
While he went to hospital on the day and was later discharged, he has been readmitted after being taken ill on Wednesday.
In a statement released on Thursday, the Injured Jockeys Fund said: “Conditional Jockey Aidan MacDonald had a fall at Hexham racecourse on March 18 and went to the RVI (Royal Victoria Infirmary) in Newcastle, from where he received extensive testing and was subsequently discharged.
“Yesterday, after being taken ill, he went in an ambulance to the James Cook Middlesbrough Hospital. He is in a critical condition and receiving active treatment for a suspected carotid artery dissection.
“Aidan’s parents kindly ask for privacy at this difficult time.”
MacDonald, 21, based in North Yorkshire, has ridden three winners from 45 rides. His most recent winner was aboard Ex S'Elance, for Hammond, at Catterick in February.
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