The $5 million half-brother to champion racehorse Black Caviar is fighting for his life after being bitten by a white tail spider.

The horse, known as Jimmy, is being treated at the Werribee Veterinary Clinic outside Melbourne.

He was bitten about a fortnight ago but he suffered a reaction to the antibiotic administered after the bite.

This caused Jimmy to suffer diarrhoea, which in turn has led to him developing laminitis, which is a disease of the hoofs which can prove fatal.

Craig Cameron, chief executive of the horse’s owners BC3 Thoroughbreds, says the next 48 hours will be critical.

“It’s awful. It’s not just his racing potential, he’s a horse that’s got a great personality, he’s well-loved by his owners and everyone at the stables,” Cameron said.

“Any horse in the condition he’s in is shocking to see.”

Jimmy developed major complications during treatment. Prognosis 50/50 but he is receiving great care, including overseas consultants

— BC3 Thoroughbreds (@BC3thoroughbred) November 13, 2013

If Jimmy does survive, experts suggest it would be highly unlikely that he would make it to the racetrack.

The horse, by champion stallion Redoute’s Choice out of mare Helsinge, was bought amid much fanfare at April’s Easter yearling sales, just a few days before his famous relation was to race for the final time.

His price of $5 million set a new record for a yearling bought in Australia.

Jimmy is also related to now-retired champion three-year-old All Too Hard, and has a half-sister named Belle Couture who is expected to make her racetrack debut in the coming months.

The team from BC3 Thoroughbreds bought Belle Couture in 2012 for $2.6 million.