The most expensive yearling sold at public auction in Australia is fighting for his life after developing laminitis.

Black Caviar's unraced two-year-old half-brother, known as Jimmy, was taken to the Werribee Veterinary Clinic two weeks ago for treatment for a suspected white-tailed spider bite.

During the treatment the colt had a reaction to antibiotics which has led to an onset of the hoof disease laminitis.

"Where we are at right now is Jimmy is in a critical stage of the development of laminitis," BC3 Thoroughbreds' Simon Marshall told Melbourne radio station RSN.

The colt was purchased by BC3 Thoroughbreds for $5 million at the Australian Easter yearling sale in April.

Besides being a half-brother to the champion racemare Black Caviar, retired after winning all 25 of her starts, the colt is also closely related to multiple Group One winner All Too Hard.